Blue Hawaii Hybrid Electrostatic Amplifier for Stax Omega II Headphones.

by Kevin Gilmore
(Project Editor: Chris Young)

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The Blue Hawaii amp is my latest design in my search for the perfect amp to pair with my Stax Omega II headphones. The genesis for this hybrid electrostatic headphone amplifier occurred when I was in Hawaii on vacation, at a fancy hotel on Maui. Sitting at the bar on the beach, drinking “Blue Hawaiis,” I drew the schematic for the amp on a placemat. The design is my conception of the mysterious and rare Stax T2 amp, which I have never been able to find at anything resembling a rational price.

I searched out any information I could find on the T2 in an attempt to create my own version. I was able to determine that it used EL34s as output tubes in a grounded grid configuration, which is the lowest distortion tube output circuit known. It also used 6DJ8s as input tubes with some solid state in the second and third stages. My design uses the first and second stages from my solid state electrostatic amplifier coupled with a third FET stage and then the final grounded grid stage.

My design ended up with a fairly large amplifier pulling significant amounts of power which results in a very smooth and extended frequency range from DC to over 200khz (-3dB at 400khz). Of all my electrostatic amps, this one has the largest output voltage swing. This is not an amplifier for the timid, nor is it a good idea to build this as your first project, though some, however, have actually done so.

The Circuit

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Figure 1
(Click here to see single-image schematic of amplifier.)

Figure 1 is the amplifier schematic. The entire amplifier has a differential topology from input to output to get a balanced input and for lower noise, less ground loop problems. The first stage is a differential amplifier with feedback directly from the output stage. It works equally well with both balanced and unbalanced audio input sources. The step attenuators from Goldpoint make good volume controls for this stage. The JFET device (Q1) is a dual JFET all on one wafer. It is known for extremely low noise and excellent matching, and is used in a number of expensive designs, such as the Nelson Pass amplifiers. Q17 is a current source that sinks 3mA.

Because the amp is totally DC coupled from input to output, drift in the input stage is a bad idea. Since the first two stages run in current mode, the JFET input is more linear than a pair of bipolar transistors. Dual transistors all on one wafer suitable for audio use are hard to find these days. The FETs steer current away from the current sources Q2 and Q3. Together Q2 and Q3 each supply 2mA or a total of 4mA. The Q17 current source takes away 3mA leaving 0.5mA in each of Q4 and Q5, but some of the sink current is coming from the output feedback, so each FET is actually using somewhere between 0.5mA and 1mA.

The approximate voltage gain of this stage is 5; this stage really runs in current mode. The unit was designed to work equally well in both balanced and unbalanced mode. For single-ended signals, ground either the + or – input and apply signal to the other. The much higher impedance of the JFET works better when one side is grounded for unbalanced inputs.

The second stage starts with a constant current source (Q2 and Q3). The current source feeds a common base amplifier (Q4 and Q5). The common base amplifier feeds a modified Vbe multiplier. I believe a famous designer is now calling this circuit a current tunnel. It’s the most linear way of translating the voltage down to the bottom rail. The voltage gain of this section is about 4. The basic idea of the first two stages is to supply the third stage with a very fast low impedance drive signal that is referenced to the bottom rail.

The currents flowing into the common base amplifier (Q4 and Q5) are the difference between what Q2 and Q3 are supplying and what the FET is taking away. The rest of the current goes down the tunnel to the vbe multipliers (Q6 and Q7) that convert the current back to voltage. The current sources in the second stage supply 2 mA each. With no signal, the FETs take 1 mA, leaving 1 mA going through the common base amplifier into the bottom transistors, which are wired as Vbe multipliers (like a zener diode in series with a resistor, except a lot less noisy). This generates the 13 volts (referenced to – rail) necessary to properly bias the third stage.

The third stage is another differential amplifier (Q13 and Q14) being driven via another constant current source (Q10 and Q16). The voltage gain is about 200. Q11 is the power supply for this stage and makes a 100 volt power supply with -400V as the reference. The power supply voltage for this stage is kept down to 100 volts to reduce the Miller effect and keep the frequency response up. The higher output impedance of this stage is lowered by the use of 2SJ79 transistors, which are used as zero voltage gain emitter followers. The use of FETs in this stage coupled with the current source further reduces the distortion and provides for a solid low impedance drive signal for the output stage.

The 4th and final stage is a tube in grounded grid configuration (V1/Q8 and V2/Q15), similar to the common base amplifier in the 3rd section of my solid-state current-domain electrostatic amp. Q9 and Q12 are high compliance current sources and supply 25mA of bias current. Think of them as linear pull-up resistors for current (in fact, one builder has replaced the current sources with large resistors). The use of a current source here instead of load resistors acts to further linearize the output stage and reduce output distortion. V1 and V2 are the equivalent of common base amplifiers and do the entire rail-to-rail output voltage swings.

With feedback, the overall voltage gain of the amp is exactly 1000. The frequency response is kept high due to the low impedance cathode drive. The EL34s are biased at 10 watts and have an 800V voltage swing (by comparison, the output tubes of my original DC-coupled electrostatic amp are biased at 2 watts with a 600V swing), resulting in a frequency response well in excess of 100kHz into a 150pF load. (+0/-0.1dB).

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Figure 2

A regulated power supply design is shown above. The ±15V supply is made with the standard 7815/7915 regulators. The high voltage supply is a pair of 400 VDC supplies, glued together at the output (P-channel MOSFETs are a lot more money than the equivalent N-channel MOSFET). In each section, beginning with a 460V raw supply, a PNP transistor (2SA1968) is used as a current source to feed the 400V zener reference. Then a N-channel FET is used as a high impedance, input voltage follower and outputs 400VDC. By the way, the same exact supply with a 350V zener reference string instead and a slightly smaller transformer (without filament windings) is what I use now for the solid state current domain headphone amp.

The bias supply is a voltage doubler with an adjustable reference. It has a range of about 350VDC to 650VDC. For low bias Stax headphones, put a 10M resistor to ground at the end of the 4.7M. to make the output voltage .66 times the voltage before the 4.7M, which puts it in the range for low bias.

Construction

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(Click here to download pc board patterns in pdf format. 1)
(Click here to download pc board patterns in pdf format. 2)
(Click here to download pc board patterns in pdf format. 3)

Caution: This project involves working with high voltages, so be extremely careful! Keep one hand behind your back at all times. 800VDC across both arms might possibly stop your heart.

This amp was assembled on three printed circuit boards (two for each channel of the amp and one for the power supply) and housed in separate enclosures. A complete set of pc board patterns (pdf format) can be found above. They could be sent to just about any circuit board manufacturer to have boards made. The top of the board is almost all groundplane. All the parts, including the tubes, are mounted on these boards – the tubes are installed in pc-mounted ceramic tube sockets from Parts Express. The tubes must be exposed through the chassis. They dissipate 20W each (actually 10 to 12 watts of plate dissipation plus another 6.3V * 1.6A = 10 Watts of filament power).

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It was so much easier to do a pc board for this amp, but if I were to make a prototype, I would again use the 0.1mm perf board; the layout look much like the circuit board. (Note: For a layout in a single chassis, see the interior view of Headamp.com’s Blue Hawaii amp below.) 99% of the wiring would be on the bottom, and it would be, therefore, rather flat. Mounting the tubes would be trouble though, and would cause mechanical problems. The tubes are fairly heavy and get stinking hot. Each chassis measures 12″ x 10″ x 3.5″. (Note: Headamp.com is selling the Blue Hawaii design in a single chassis measuring about 16.5″ W x 16.0″ D x 3.5″ H and may sell the Blue Hawaii pc boards. Please contact Headamp for more information.)

I have Mullard EL34 tubes, but keep them put away due to what I could sell them for if I wanted. I actually used the National Union tubes from Richardson Electronics which cost $11.50 US each.

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All of the parts except the 2SA1968 have lots and lots of sources such as Digi-Key, MCM Electronics and Mouser Electornics. Only B&D; Enterprises has the 2SA1968 in the United States. In Japan and Canada, they can be ordered from Sanyo direct – the minimum order is 100 at a time, but then they are $1.25 each or so.

Q9 and Q12 are each made of six 2SA1968 transistors in parallel with one 2SA1968 as the driver. Matching the transistors is not required – unless one of the 2SA1968s is way off compared to the rest in which case it might get way too hot.

All resistors are 0.25W except where labeled. It is important to have all the pnp current source transistors correctly mounted to a large heatsink with silicon impregnated washers. If any one pnp transistor gets too hot it can short out the whole current source.

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Standard tab heat sinks will do for the 2SK216 and 2SJ79 transistors, but the 2SA1968 and 2SC3675 transistors must be mounted a big heatsink (one for each channel), capable of dissipating 20 Watts of heat. I obviously fabricated them, but otherwise they can be obtained from Conrad Heatsinks cut to length. The IRFBC30 MOSFETs in the power supply must be heatsinked too: Mouser part number 532-529902b25.

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The Stax SRC-5 headphone jack came from AudioCubes.com. Since the price has gone up to $19 each (I paid $10), it may be more cost effective to use the Allied jacks (see the current domain amp project article). Allied has a $25 minimum order, the cost of three pieces. Then they must be filed down on a lathe. Actually I am buying the male connectors from Allied, because no one else sells them. The male connectors are much easier to convert to standard Stax plugs. The power supply-to-amplifier connectors are the Amphenol military 12-pin connectors. The 4 connectors (two male and two female) were $130.

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The custom Victoria Magnetics power transformer has these specs: 2 x 330VAC/150mA, 36VCT/100mA, 2 x 6.3V/5A (filament supplies). Everything with Victoria Magnetics is custom. I paid $110 for the transformer with shipping. They know about the Blue Hawaii design and will supply the correct transformer on request. For safety, I recommend a 2A/110Vac fuse located in the input line to the power supply.

Setup and Results

Test voltages (with the amp at idle) are shown in red on the schematic and are with respect to ground. To set up the amp, adjust the two pots in each channel of the amp. P1 controls the differential output voltage. Put a voltmeter between the 2 stators of one channel of the headphone and set this pot for zero. P2 controls the voltage with respect to ground. Put a voltmeter between any stator and ground and set for zero. Then repeat both adjustments a few times. The plates of both tubes should measure 0 volts with respect to ground when the pots correctly adjusted. Once the pots are adjusted, that’s it – there’s no change from headphone to headphone.

Setting the bias voltage depends on the headphones. For Stax headphones that can accept a high bias voltage, adjust the pot for 560V. I do not think that the Omega II headphones can be damaged by this amp unless the bias is set way too high. If the bias is set right, the outputs are close to 0V at idle, and all the LEDs are lit, then the amp pretty much has to be working correctly. Now if one or more of the outputs is stuck at +400V or -400V, then something is seriously wrong and needs to be fixed. An oscilloscope really helps.

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Adjust the pot to 580 volts for Sennheiser HE-90 and HE-70 headphones or leave it at 560V. For Koss headphones, adjust the bias for 600V. To use the Sennheiser HE60 headphones with this amp, I made the adapter shown above. Those are RS-232 female connector pins that fit the HE60 pins perfectly. By hand I cut a circuit board with lands exactly 3.5mm apart put the pins on the HE60 connector, lay them down on the circuit board and solder. Then attach wires and a standard stax plug.

The amp can output 1500 V p-p measured stator to stator. At 800Vp-p, THD is less than 0.004% from 20Hz to 20kHz. The actual frequency response is 0 to 100kHz (-3dB at 150kHz) into an Omega II load. Compared to the sound of my previous tube amplifier, the bass is no longer tubby; it’s very sharp and tight. The high end is no longer rolled off, so female voices sound much more real. If the bias supply is reduced to 280V, the amplifier will drive all electrostatic headphones. I tried it last night on a pair of SRX’s. I never ever heard them sound so good.

Previously with a standard dummy head, I measured the SPL in Omega 2 headphones driven by this amplifier. With a drive signal of 800Vp-p per side, the resulting spl is 106dB. THAT’S LOUD! The amp can put out 1500 volts peak-to-peak, and thats louder! I just ordered a pair of Stax SR-001 MkIIs, which can reach up to 120dB. My ears distort before the amplifer/headphones do. It is quite loud at clipping, but the clipping is a hard clip with no oscillation or ringing. To use the amplifier with electret headphones, delete the bias voltage. And probably keep the output swing under 200V. Electrets phones when driven with this amplifier can probably get very very loud.

Several of my previous electrostatic designs are available in the Headwize Projects section. Comparing the Blue Hawaii to my all solid-state current domain amplifier, they really are more the same than they are different, but in general, the differences are the differences between tubes and solid state, such as a much smoother high frequency response, which in the case of the Blue Hawaii goes well beyond 500kHz. Additionally, the four times power consumption of the Blue Hawaii means a much stiffer and tighter bass response. Even though both are flat to zero and test similar, the BH bass is much more apparent and tighter.

c. 2004 Kevin Gilmore.

All-Triode Direct-Drive Tube Amps for Electrostatic and Electret Headphones.

by Kevin Gilmore

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Electrostatic headphones, like electrostatic speakers, work on a push-pull arrangement. When one of the stators is going up in voltage, the other stator must decrease in voltage so that the diaphragm can move due to the static charge. These two direct-drive electrostatic tube amplifier designs will drive any existing electrostatic headphones without step-up transformers. The first is AC-coupled. The second is fully DC-coupled from input to output. These amps can also drive electret headphones – which are just permanently biased electrostatics – as long as the voltage swings are not too high.

I have built a total of 4 of the first amp, and one of the second. These are not beginner projects by any means. Since I have access to a chemistry electronics shop, I used lots of expensive and wonderful parts. The coupling caps I used are .22uF mica caps (probably 30+ years old) from military surplus (which today go for about $25.00 USD each). Due to the Apex op-amp DC regulators, the second design is a whole bunch more money. Both amps sound better than the Stax SRM-T1S unit, and virtually the same as the Stax Omega (if you could find one). And it definitely sounds better than the Sennheiser Orpheus.

BACKGROUND

Why I decided to design and build these headphone amplifiers. There are lots of reasons. First let me say that there are some things transistors do better than tubes, like low output impedance, which works better with low efficiency speakers like the Wilsons.

Then there are things that tubes do better than transistors. Like high voltage. Electrostatic headphones are very high-voltage devices. With a maximum voltage swing of 550 volts peak-to-peak, transistors or MOSFETs just are not good enough to do the job. The idea here was to design and build something good enough to make it into Stereophile’s Class A category.

Here is what is available in electrostatic headphone/amplifier combo’s:

Stax Omega and tube amp (approx. $10,000 a pair).
With the re-organization of Stax, this system is no longer available. The amplifier in this system is an all tube DC-coupled unit. When it was available, it was absolutely the best thing in the world!

Stax Nova Signature and either SRM-T1S or SRM-T1W amp ($1500/$1700).
The SRM series amplifier has tube output, but MOSFET input and MOSFET gain stages. The sound is nowhere as good as the Omega, but strictly due to the amplifier used.

Sennheiser HV60/HEV70 combo ($1500).
The solid state drive amplifier is horrible. Headphones themselves are excellent.

Sennheiser Orpheus ($12,000).
A wonderful system, although the inboard D/A converter is awful. Otherwise excellent.

Koss 950 (about $500).
The headphones are fairly decent, but the original transformer drive box is horrible. And the latest solid state drive box is also awful.

There are lots of good old headphones out there: old Stax units – both low bias and high bias (about 7 different models), the original Koss headphones, and the Jecklin Floats. The tube amps described in this project will drive any of them (but I recommend using only the AC-coupled amp with the low-bias Stax phones – see below).

THE DESIGNS

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Figure 1

Both amps are OTL and will output 500 volts peak-to-peak (measured from front to back) for an input of 1 volt peak-to-peak. The AC-coupled amplifier (figure 1) uses a series of single-ended pure class A gain stages (the second tube is a phase splitter). The first two tubes make up the first gain stage with a gain of about 20 and the second two tubes make up a gain of about 30.

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Figure 2

The DC-coupled amp (figure 2) uses two differential gain stages (the first stage is also a phase splitter). The output stage is single-ended pure class A. The first stage gain is about 7, the second stage gain is about 5 and the output stage has a gain of about 25. Decreasing the overall gain is easy – add an input pot. Increasing the gain is unnecessary as virtually any preamp can put out 1 volt, and that’s quite loud (about 100dB).

The reason for a fully direct-coupled amp is that interstage coupling caps, regardless of quality (even the silver mica units I use), are not perfect. The transformers used in some electrostatic speakers and early Stax drive boxes are also far from perfect. So in this amp, from input to output, only tubes touch the audio signal. It’s absolutely the best thing I have ever done.

I believe in feedback. Not a lot of feedback, but enough to fully stabilize the circuit. In both designs, the first two tube sections generate a stable pair of signals perfectly matched, but 180 degres out of phase. Then each of these signals goes to a voltage booster once again with local feedback to drive the diaphragm. I have listened to these units for literally thousands of hours, trying various kinds of tubes, ranging from NOS Mullard, RCA, GE and Raytheon to current-made junk. The feedback makes all these tubes sound virtually alike. Not at all like some tube preamps that sound dramatically different depending on the tube you use.

The DC-coupled amp is also rather expensive. The 6 Apex op-amps alone are $240 in parts. It goes without saying that the build quality will ultimately effect the sound. I built several of these units on a pure copper chassis, with ceramic silver tie points, just like Tektronix used to do in their tube oscilloscopes (the prototype shown in the pictures used an aluminum chassis and standard tie points). It’s insanely time consuming, but the result is worth it.

CONSTRUCTION

This project involves working with lethal high voltages, so be extremely careful! Keep one hand behind your back at all times. 600VDC across both arms might possibly stop your heart. The author accepts no responsibility for any harm caused by the construction of this project!

An electrostatic element has one diaphragm in between two fixed stator elements. Each amp has front and rear stator outputs. Call the front and back whatever you want, so long as the left and right ear pieces are wired the same, so that they are in phase.

The PA42 is a 350-volt opamp from Apex Microtechnology Corp. (I use a lot of Apex products in research applications). It’s a high voltage monolithic MOSFET unit. Since the opamps are strictly used for DC bias control, they do not influence the “TUBE” sound. They keep the output voltage at +300 volts. This is so that the output voltage can swing to zero and then to +600 volts. Note: the Burr-Brown 3583 high voltage opamp is not pin-for-pin substitute for the Apex; however it should work.

The Headphone Cable Connectors

All of the low-bias Stax units use 6-pin plugs (Amphenol microphone plug). All high-bias units are the same plug with 1 pin missing, and both diaphragms tied to the one wire. That way a high-bias headphone can plug into a low-bias driver, but the low-bias phones cannot plug into a high bias driver.

The Stax plug wiring scheme is as follows:

left front: pin 2
left rear: pin 5
bias: pin 1
right front: pin 3
right rear: pin 4
bias: pin 6

The high bias headphones do not have a pin 6. Instead, the bias for both elements is tied to pin 1.

The jack I used is an Amphenol 78-S6S. I am not sure where you get them. I have a bag of them that’s probably 30 years old. The were used as speaker connectors when speakers did not have permanent magnets.

It looks like this from the pin end:

  o
o o o
 o o

except that it’s not that regular.

  3
2 6 4
 1 5

Above it is shown wired.

The Sennheiser HV60 uses a 6-pin inline plug. Here’s the Sennheiser plug wiring scheme:

pin 1 (the corner notch): left front
pin 2: left diaphragm (bias)
pin 3: left back
pin 4: right back
pin 5: right diaphragm (bias)
pin 6: right front

The Chassis

I built the prototype shown in the pictures on an aluminum chassis (later units on a solid copper chassis that I bent up from .075″ thick copper myself). The first amp is built on a 13″ x 15″ x 2″ chassis. The second amp is built on a 15″ x 17″ x 2″ chassis. I spot-welded the corners together and cut out holes for tubes and other parts before painting the chassis to avoid scratching it.

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The outside of the chassis is painted black. I put it in an oven for two days to get the paint real hard (set the oven to 200 degrees Fahrenheit for at least 4 hours bakeout). The chassis should be degreased with alcohol before painting it. I used a special primer (do not remember which) and then sprayed on Krylon flat black. If an aluminum chassis used, have one copper sheet on the inside to make sure that everything has a really good ground.

The 1 inch thick EBONY wood side panels make it look cool. I only used ceramic tube sockets (once again military surplus). For added effect, I built these units in a style similar to that of 40+ years ago. Everything is on ceramic tie points. No circuit boards of any kind. Building things to silly standards is part of the fun.

The Power Supply

Since the majority of people will be scrounging parts, and some still like to build power supplies with tube rectifiers, I did not make elaborate power supply schematics. For me, solid state power is just fine. The diodes and capacitors I used were the best that I could lay my hands on. For the DC-coupled amp, standard tube rectifiers cannot generate 860V without breaking down. Solid state here is definitely the way to go.

The 12AX7 is either a 6-volt filament or a 12-volt filament, depending on how you wire it. In the schematic, I show it wired for 6 volts. The 6S4 is strictly a 6-volt filament tube. I use AC for the filaments. You could certainly run DC for the filaments, if you wish. It might result in less hum, which is probably already unmeasurable if not inaudible.

The transformers: I have a lot of scrounged parts and actually used two transformers, one for the high voltage, and another dual unit for the filaments. I just looked through a bunch of catalogs for transformers available. It’s no longer a tube world. Allied (Hamilton Avnet) still sells the right transformers. Model 6K7VG is a 75-watt unit with 750 VCT and 6.3V. Model 6K94HF is a 25-watt unit with 12.6 VCT. The first unit is certainly overkill power-wise. The outputs run class A and are set at 4 watts, meaning 16 watts on the 600-volt line. The 300-volt line draws less than 2 watts.

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Figure 3

The AC-coupled amp supply: Any high voltage transistor can be used for the 600 volt series regulator. I used a Motorola MJ16018 (they are still available). MJ12005’s work better. All zener diodes are 1 watt units. All electrolytics are the best I can find (typically Black-Gate).

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Figure 4

The DC-coupled amp supply: In the DC-coupled amp, the stators are at 300 volts nominal for a signal swing of 0 to +600 volts. The bias voltage is 860V, but it is still effectively 560 volts between the diaphragm and the stators, because EVERYTHING is lifted 300 volts. (In the AC-coupled amp, the stators are at 0 volts nominal with a signal of -300 to +300 volts, and the bias is at 560 volts.) The diaphragm is a capacitor – all it cares about is the differential voltage, which is still the same. However, I do not think that it would be a good idea to use the DC-coupled amp with the old low-bias Stax phones. Since the stators sit at +300 volts, bad things might happen with the low-bias phones.

The 100V reference regulator can be built in two ways. The simplest is the same sort of arrangement as the 600-volt regulator: a 100-volt, 1 watt zener diode, a resistor and a pass transistor. A 100K, 1/2 watt resistor will be fine. The reference draws virtually no current.

A better arrangement, which as yet I have no schematic for, is another Apex amp rigged as a x10 multiplier, and a precision 10 volt reference like the Burr-Brown REF10. This is what I use. The output of REF10 goes to + input of opamp. The output of opamp goes to – input with 99K resistor. Then connect the – input of opamp goes to ground through a 1K resistor. You have to rig yet another power supply to come up with +15 volts to light up the REF10.

SETTING UP THE AMPLIFIERS

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Figure 5

Biasing the headphone diaphragms: The diaphragm bias network for the AC-coupled amp is shown in figure 5. The AC-coupled amps will work with any electrostatic headphone, if the bias voltage is correctly set. Older Stax units are low-voltage bias (~330V). Most of the newer Stax are high-bias (560V). I do not know what the Koss units use (check the schematic or owner’s manual). The Sennheiser HV70 uses 580V. Anything about right, but not over the recommended voltage is fine. The DC-coupled amp should drive only high-bias electrostatic headphones (do NOT use the DC-coupled amp with low-bias Stax phones).

Balancing the gain for the AC-coupled amp: The gain adjustment control is Rp in the schematic. Put a 1kHz squarewave on the input. Then adjust the pot for equal amplitudes at the outputs. It is best done with an oscilloscope. If you use a 1kHz sine wave, most AC digital voltmeters will also work.

THE FINAL RESULTS

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How do these amps sound? Exactly what you would expect from an all-triode tube amp. The high end is liquid and sweet. No harshness of any kind, especially when compared with the Sennheiser unit. Clipping (you have to be listening quite loud) is virtually undetectable, whereas the Sennheiser unit overloads (lights the red light) and sounds crackly. I can literally listen for 4 hours at a time – something I could not do without extreme fatigue with any of the other amps.

The Stax SRM-T1S is actually a really good unit, but it uses 6FQ7’s which really cannot withstand the 600 volts. So as it gets louder and louder, the sound becomes more restricted, and because of the lower bias power, as the sound becomes louder the amount of high-frequency energy slowly disappears. I have heard of others complaining about this also. If you are in search of absolute excellence, the all-triode amps described here are the best electrostatic headphone amplifiers currently available, anywhere in the world.

Addendum

9/3/1998: The DC-coupled amp schematic (figure 2) was corrected as follows: R14, R22 are 5W resistors; R15, R23 are 1W resistors.

12/22/1999: The author writes: “I am using omega 2 headphones on my amplifier. The sound is so amazing especially with my new SACD-1.”

8/31/2001: Added high resolution pictures of the amplifier, including a new shot of the inside of the chassis.

2/4/2003: Corrected value of C9 in figure 1.

c. 1996, 1998, Kevin Gilmore.
The author’s website: The Homepage of Kevin Gilmore.

 

Notes on DIY Electrostatic Headphones.

by Chu Moy

Electrostatic headphones continue to capture the imaginations of audiophiles, although dynamic types comprise by far the largest portion of headphones sold. The sound of electrostatic headphones is often characterized as detailed, low distortion and spacious, owing to their large area, lightweight diaphragms driven over the entire surface. For all the mystique that surrounds them, electrostatic transducers are in many respects easier for the DIYer to make than dynamic ones. Although commercial electrostatic headphones are the product of sophisticated manufacturing processes, homemade versions are also capable of high performance.

From 1968 to 1979, the U.K. magazine Wireless World published three projects for DIY electrostatic headphones by J.P. WilsonPhilip D. Harvey and Neil Pollock. This article reviews the concepts of electrostatic headphone design with attention to the contributions of each of the above authors. Warning: the high voltages involved in these projects do pose a serious danger. Only advanced DIYers familiar with high voltage construction techniques should attempt to build electrostatic headphones. As the projects are over twenty years old, some of the materials and parts described may be obsolete. However, the resourceful DIYer may find some inspiration herein to use modern materials and parts.

PRINCIPLES OF ELECTROSTATIC HEADPHONE DESIGN

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Figure 1

For linear operation, an electrostatic transducer should be push-pull, and operate with a constant charge on its diaphragm. Figure 1 shows a cross-section of the basic assembly of a push-pull electrostatic transducer. It consists of two plates, two sets of spacers and a diaphragm. In a push-pull configuration, the diaphragm is suspended between two conducting, acoustically transparent plates which are fed high voltage audio signals that are 180 degrees out of phase. The diaphragm, which maintains a constant charge from a DC bias voltage, moves as the electrostatic differential between it and the plates varies with the audio signal.

In general, the requirements of a high performance electrostatic transducer are:

  • The diaphragm should be light and flexible. A lighter diaphragm will result in a transducer with a wider frequency response. Also, the thinner the diaphragm is, the easier it will be to damp.
  • A large transducer area that covers the ear will produce a more planar wavefront with more accurate localization cues. It will also provide a low diaphragm resonance frequency without requiring accurate control of the diaphragm tension. If the resonance frequency is too low, the transducer may become sensitive to subsonic signals or suffer a loss in sensitivity, because the bias voltage may have to be reduced.
  • The transducer area should not be so large that the interplate capacitance makes the unit difficult to drive.
  • In order for the diaphragm to hold a constant charge at low frequencies (when parts of the diaphragm can become unstable), it should have a very high resistance coating.
  • The rear of the transducer should be open to radiate the backwave from the diaphragm.
  • The spacers should be flat, insulating and of uniform thickness.
  • For maximum acoustic output, the spacer thickness should be as small as possible, but not restrict low-frequency movement.
  • The plates must be rigid, at least 20% open with a perforation spacing much smaller than the shortest wavelength to be reproduced. If the plates flex or are uneven, there could be variations in the signal field strength.
  • The transducer should have adequate acoustic damping to damp the diaphragm resonance and to prevent ringing on transients. The acoustic loading from the perforations in the plates provides some damping, but additional damping is usually necessary to even out irregularities in the frequency response.

Diaphragm Resistivity

The motion of the diaphragm is affected as follows: by the diaphragm tension at low frequencies, by the acoustic resistance of air at mid frequencies, and by the mass-per-unit area of the diaphragm at high frequencies. The resonance frequency of the transducer determines the lower frequency limit. If the resonance frequency is too low, it could limit the apparent sensitivity of the transducer.

To ensure that the diaphragm maintains a constant charge at low frequencies, no significant current should flow in less than half the time period of the lowest frequency to be reproduced. Given a diaphragm with perfect conductivity and a transducer having capacitance C farads, and low frequency response to 27Hz, then the diaphragm must be fed via a resistance R ohms, such that:

RC > 1/(2 x 27) (approx.)
If C = 150pF (C= eA/d)
then R > 1 x 108 ohms.

 

 

However, R cannot be so high as to prevent the diaphragm from charging.

The Transducer Frequency Response

electro2.gif
Figure 2

Figure 2a shows the mechanical-equivalent circuit of the transducer. m is the mass-per-unit area of the diaphragm, S is the suspension of the diaphragm in the transverse direction, and 2Rm is the damping in the mid frequencies from the impedance of the air. Fo is the peak force-per-unit area on the diaphragm.

The electrical-equivalent circuit in figure 2b converts m to an inductance of M = jwm henries, the suspension S to a capacitance of S-1 farads, the damping to 2Rm ohms, and the force to a voltage Fosin(wt)V. The high frequency response of this circuit is constrained by the inductance M, which is directly proportional to the diaphragm’s mass-per-unit area. Therefore, the lighter the diaphragm, the smaller the inductance and the wider the frequency response.

The transducer acoustic output

The acoustic output of an electrostatic transducer is limited by the breakdown of air and diaphragm stability, but can be improved by coating the plates with insulation. At low frequencies, the output is constrained by the diaphragm touching the plates, and so is affected by the spacing between the diaphragm and the plates. The force acting on a diaphragm in a push-pull transducer is calculated as:

electro3.gif

where e is the differential plate voltage, E is the polarizing potential on the diaphragm, A is the diaphragm area, and d is the spacer thickness. This equation suggests that output can be maximized by increasing e or E. However, the maximum value of E for diaphragm stability is

electro4.gif

where T is the diaphragm tension. The maximum bias voltage, then, is limited by the plate spacing. The maximum voltage between the diaphragm and plate is limited by arcing in the airgap, which is also a function of d. Thus, the construction criteria for a high output electrostatic transducer are: a high diaphragm tension, a small diaphragm area with the highest acceptable diaphragm resonance frequency, the smallest plate spacing with good low frequency response.

CONSTRUCTION OF ELECTROSTATIC TRANSDUCERS

The following sections describe the electrostatic headphones built by J.P. Wilson, Philip D. Harvey and Neil Pollock. All three designs are similar and construction techniques for one can often be used with the others. When assembling the transducers, all drilled holes should be deburred, and all dirt or lint between the diaphragm and the plates should be carefully removed; otherwise, there might be arcing or a loss of sensitivity. All exposed high voltage areas on the transducers must be insulated to avoid shock.

Because the diaphragm bias supplies for all of the headphones have a high impedance (see the section on Electrostatic Headphone Amplifiers below), there is no danger of lethal shock – although DIY electrostatic headphones should be inspected periodically to ensure proper and safe operation. As a further precaution, Mark Rehorst recommends that bias supplies be built from low voltage DC step-up converters (available to 5000VDC or more), instead of using high-voltage AC transformers. These converters are sold in IC packages by companies such as Emco High Voltage Co.

The Wilson Electrostatic Headphone

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J.P. Wilson built a pair of electrostatic headphones as an experiment to eliminate the “in-head” characteristic of headphone sound fields. He reasoned that normal hearing of sound involves the processing of head motion cues and the acoustic diffracting properties of the outer ear. The wavefront of a sound source arriving at the ear is nearly planar and could be simulated with large-area headphone transducers that covered the ears. The transducers had to be poor acoustic reflectors to prevent sound reflection between the transducer and the head and ears, and to be free-air mounted so as not to form a semi-enclosed resonant cavity with the ears.

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Figure 3

The Plates

Figure 3 is a detailed illustration of the Wilson transducer. The size of the plates was chosen to be large enough to overlap the ears and give the desired bass resonance frequency to the diaphragm. Wilson used paxolin boards (4.75″ x 4″) with holes that comprised at least 20% of the total area and spaced closer than the shortest wavelength to be reproduced. .

The smoother side of each plate was made conductive by first applying a stripe of high conductivity paint over the surface to the edge of the plate to be the connection point for the brass shims that carry the signal source (the shims could also contact the graphite directly, without the paint). Wilson then covered the perforated area of the plates with a thick coat of colloidal graphite (Aquadag), being careful that none of the paint or Aquadag would spill into the holes. Once dry, the surface was polished with a dry cloth and blown clean. A flame was applied to the surface just before assembly to remove any remaining dust or lint.

The Spacers

The spacer material should have uniform thickness and be an excellent insulator. Wilson used acetate sheeting with a 0.04″ thickness, which produced 90dB SPL (free field – the level is about 10dB higher in an artificial ear) from 30Hz onward. The output increased to nearly 100dB by reducing the thickness to 0.025″, but only above 60Hz due to restrictions on the diaphragm excursion. For each plate, Wilson cut four rectangular pieces of acetate that formed a frame with the same outer dimensions as the plate. After gluing the spacers to the plates (Evostick adhesive), he drilled the assembly holes around the spacers and applied a stripe of high conductivity paint to the inner edge of the spacers for connecting the bias supply.

The Diaphragm

The factors affecting the resonance frequency of the diaphragm are size, shape, mass and tension. Wilson’s diaphragm was formed from a sheet of 0.0005″ thick plastic foodwrap (Vitafilm), which gave the assembled transducer a free-air resonance of 50Hz (damped to about 30Hz). When the transducer was placed against the ear, the resonance frequency was further reduced due to the increased effective mass of the trapped air.

Wilson put a conductive coat of colloidal graphite to both sides of the diaphragm, so that it would hold a charge. The resistance of the coating had to be in the range of 100M to 10G ohms (as measured between two parallel electrodes 1″ long and separated by 1″). Too low a resistance would result in charge migration during signal movements and a loss of linearity.

To apply the graphite, Wilson first pressed the plastic sheet onto moistened glass with a rubber roller. When the top of the plastic surface was dry, he used a piece of cotton to rub on colloidal graphite, rubbing until nearly all of the graphite was removed. When the coating reached the proper resistance, he turned over the plastic sheet and coated the other side (rubbing carefully to avoid damage to the first coat).

The diaphragm was then glued to the spacers (Evostick adhesive) on one of the plates and was pressed between two flat surfaces to dry (the glue should not touch the connection between the diaphragm and the conducting stripes on the spacers). Wilson put the diaphragm assembly under a hot grill for a few seconds to tension the plastic (a fan heater with restricted air flow would also work). The Vitafilm tensioned in a consistent manner, so Wilson concluded that the resonance frequency was constant for same size diaphragms.

The two halves of the transducer were joined together with 6BA nylon nuts and bolts and brass shims inserted to make contact with the conducting stripes. Note: both sides of the diaphragm are charged by the bias voltage. Wilson used a single brass shim for bias supply by “nicking” the edge of each diaphragm so that the conducting stripes on both sides were in contact.

Acoustic Damping and Final Assembly

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Figure 4

Without foam acoustic damping, the frequency response of the Wilson transducer was not very flat (figure 4). Wilson sandwiched the transducers between two layers of 4mm foam, which served as acoustic damping and were held in place with rubber bands. Adding additional layers of foam did not improve the response. He fashioned a 12″ x 0.5″ strip of 16-gauge aluminum into the headband with a slight twist at each end to hold the transducers flat against the ears.

Wilson set the bias voltage for each transducer, so that when he blew the diaphragm in the direction of one of the plates, it returned to the central position. The source of the bias voltage was from the high voltage power supply of a 15W tube amplifier (375V), which had been converted to drive the electrostatic headphones directly (see below more details about the amplifier connections). Note: to avoid excessive capacitance, Wilson bundled the transducer wires neatly in parallel, not twisted. The capacitance of each transducer is about 80pF.

The Harvey Electrostatic Headphone

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Figure 5

Philip D. Harvey designed an electrostatic headphone that featured a small frame around the transducer to extend the low frequency response (figure 5). He called the frame a “transmission tunnel,” which was bolted to the transducer with plastic screws. The tunnel was molded from polystyrene plastic and lined with latex rubber foam for acoustic damping.

The Plates

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Figure 6

The plates are made from singled-sided copper-plated fiberglass board. Harvey drilled 3mm holes, spaced 5mm apart, in the plates, such that the holes comprised 30% of the total plate area (figure 6). He removed a 2mm copper border around the edge of each plate and the bolt holes to prevent charge leakage if the diaphragm tore.

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Figure 7

To form tags on the plates for making safer high voltage connections, Harvey clipped a corner on each plate (an alternate corner on the second plate) and both corresponding corners on the spacers (figure 7). When the spacers and plates were stacked, each plate had an exposed corner for soldering a connection. The connections were insulated with Plasticine.

The Spacers

The spacers are cut in a single piece from a sheet of 0.37mm thick polyvinyl acetate (see figure 7). Harvey experimented with different spacer thicknesses. With thinner spacers (0.18mm and 0.25mm), the acoustic output increased, but the airgap tended to ionize on humid days (the ionization manifested itself as low frequency clicks).

The Transmission Tunnel

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Figure 8

The transmission tunnel is a type of earcup for mounting the transducer assembly (figure 8). Harvey cast the tunnel from polystyrene with a two-part wooden mold that was overwaxed for easy removal. Tunnels made from colored polystyrene turned out to be more sturdy than the clear polystyrene versions. He did not specify the depth of the tunnel, but denoted it as “short.”

The Diaphragm

For the diaphragm, Harvey selected a thin, light film made by the Borden Chemical Co. (15u thick p.v.c. sheeting with a resistivity of 109 ohms). To prepare the diaphragm, he constructed a wood frame (200mm x 250mm) over which he stretched and taped the plastic film (smoothing any creases). The frame was placed over a slightly thicker piece of glass (240mm x 190mm) to support the film. Harvey rubbed colloidal graphite (Aquadag) over the plastic surface until it had a resistivity of 108 ohms – (he used the same method as Wilson for measuring resistivity). Unlike the Wilson diaphragm, the Harvey diaphragm appears to be coated with graphite only on one side.

Acoustic Damping and Final Assembly

electro12.gif
Figure 9

To connect the bias supply to the diaphragm, he enlarged a bolt hole in one of the plates and the corresponding hole in one spacer (the lower plate and spacer) to accommodate a brass bush, which when inserted into the plate-spacer, made electrical contact with the diaphragm (figure 9). Harvey assembled the transducer by first taping the diaphragm (graphite side up) over the the upper spacer with double-sided tape. The lower spacer and plate with the brass bush went over the diaphragm. Then, he positioned the transmission tunnel over the lower plate, clamping the bias supply wire in between and in contact with the brass bush.

Nylon nuts and bolts completed the assembly. The diaphragm was tensioned with warm air and the tunnel’s inside walls lined were with latex foam rubber (polyurethane foam) to dampen cavity resonances and insulate the ear. Harvey constructed two amplifiers (tube and solid state versions – see the section on electrostatic headphone amplifiers) for driving his headphones. Harvey tried bias voltages from 200V to 600V, which gave increasing acoustic output, leveling off above 600V. The finished units had a bias voltage of 300-350V.

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Figure 10

The Pollock Electrostatic Headphone

Neil Pollock sought to make an electrostatic headphone that could produce more realistic sound pressure levels. Most electrostatic headphones of that time (1979) had a bias voltage of 400V or less and a minimum spacer thickness of 0.5mm. These could generate a maximum free field r.m.s. sound pressure of about 93dB, whereas some musical recordings sounded best when the headphones could reach 100dB or more. Pollock designed a transducer with a bias voltage of 800V to 1300V (as well as a matching amplifier, discussed below) that could output over 100dB SPL.

The Plates

electro14.gif
Figure 11

The four plates (figure 11) are cut from a sheet of 3mm thick Perspex acrylic. Pollock stacked the plates and drilled a matrix of 3mm holes through all four plates. He drilled a countersunk hole in one corner of each plate so that a M2.5 or M2 brass screw would lie slightly below the surface. All of the holes were deburred and the plates were masked before applying the Aquadag. The brass screws were attached with connecting tags and then the resistance between the tag and any point on the plate measured. If the resistance was more than 10K ohms, he applied additional coats of Aquadag. Finally, he covered the Aquadag with a coat of clear polyurethane varnish to prevent ionization in the airgap during periods of high humidity. The dry varnish was sanded slightly to reduce the surface gloss, which tended to stick to the diaphragm.

The Spacers

electro15.gif
Figure 12

The spacers (figure 12) were built by layering sheets of drafting film, laminated to a 0.8mm thickness with rubber cement. After cementing the spacers to the Aquadag side of the plates, Pollock drilled countersunk holes for the diaphragm. Each pair of plate-spacers was positioned face-to-face, and the assembly holes drilled. He painted the spacers with Aquadag to the inner edge of the spacer and into the countersunk hole to provide electrical contact with the diaphragm (taking care that no Aquadag should spill into the assembly holes). After installing the diaphragm contact screws, Pollock checked that there was a resistance from the connecting tags to all points on the Aquadag.

The Diaphragm

Unlike the other diaphragm designs, the Pollock diaphragm is not coated with graphite, but is used without any coating. The diaphragm material is 0.0127mm thick food wrap film, similar to Vitafilm. So long as the diaphragm contacts are made as shown, the film’s own high surface resistance is adequate to hold a charge.

Pollock began assembling the transducer by stretching and taping the plastic film (smoothing out any wrinkles) over a piece of rigid cardboard that had a cutout larger than the plates. Then he pressed the diaphragm between the two plate-spacer assemblies and bolted the plates together with nylon nuts and bolts. The excess film around the plates was trimmed with a razor blade.

Acoustic Damping and Final Assembly

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Figure 13

To test the transducers, Pollock connected the bias supply (set to the minimum of 800V) to the diaphragms, and checked them for centering. If a diaphragm moved to one side or oscillated, he heated it with a light bulb to tension it. The transducers were then wired into headphones, and silicone rubber applied to insulate the exposed contacts. He made acoustic dampers from 6mm foam plastic, glued (or sewn) into pouches that enclosed the transducers. With the dampers, the frequency response of the transducer is 30Hz to 40kHz ± 5dB and down to 20Hz when measured in an artificial ear (figure 13).

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The headband came from an old pair of headphones. Pollock attached an acrylic bridge between two of the assembly screws that protruded from the foam damper of each transducer to mount the headband. Each transducer has an approximate capacitance of 100pF. To minimize capacitance, he loosely bundled the wires between the headphone and amplifier (as opposed to twisting them together) and kept the resulting cable less than 1.5m in length.

Once the headphones are assembled and connected to the amplifer, the bias voltage is increased to the point just before the diaphragm collapses or the airgap ionizes. Pollock was able to set a maximum bias voltage of 1,300V on his units.

ELECTROSTATIC HEADPHONE AMPLIFIERS

The impedance of an electrostatic headphone is primarily capacitive. Step-up transformers that can provide sufficient voltage to a capacitive load are difficult to manufacture to manufacture and to find. Direct-drive amplifiers may be a more practical solution, if they can handle capacitive loads and have high voltage gain. Although the following electrostatic amplifier designs are more complex than those for dynamic headphone amplifiers, they are not exceedingly so – the requirement of high voltage power supplies notwithstanding.

electro16
Figure 14

Wilson drove his headphone with a commercial 15W push-pull tube amplifier (a Radford STA 15) that had been converted for electrostatic headphones (figure 14). The audio signals for the plates were taken from the anodes of the amplifier output stage. The Radford had a power supply of 375V, which Wilson tapped with a 10M resistor for the bias supply. It ran without a load connected to the output transformer for maximum voltage swing, but adding an output load may be preferable in some cases for amplifier stability. Further, some tube amps may balk at providing maximum output continuously to an electrostatic load in this manner. Harvey tried the circuit with his electrostatic headphone and could not eliminate the intermodulation distortion, which was audible even at low levels.

electro17.gif
Figure 15

Instead, Harvey built two dedicated amplifiers to drive his headphones, one tube-based and the other solid state. The spacer thickness used in his transducers was 0.37mm, which limited the voltage between the diaphragm and either plate to about 1000V. With a diaphragm bias voltage of 300V, the plate voltage could go as high as 500V and still have a safety margin against ionization on humid days or from signal surges.

electro18.gif
Figure 16

The amps are designed to output between 300V to 400V peak and have a similar topology: a differential input stage feeding a push-pull output stage. The tube version (figure 15) can generate a 400V peak signal; the transistor version (figure 16) a 300V peak signal. The transistors in the differential stage (Q1/Q2) are matched (they can be any NPN silicon unit with an hfe > 50 at 1 ma. and a VCE > 35V). P1 is adjusted to match the base voltages of Q1/Q2. The collector voltages of Q3/Q4 are set to an average of 115V using P3. Then the collector voltages are balanced with P2 and the process repeated until the collector voltages of Q3/Q4 are at 155V.

electro18a.gif
Figure 17

Pollock devised two electrostatic headphone amplifiers based on the National Semiconductor LM3900N current-input opamp. The standard bias version (figure 17) is for an earlier electrostatic headphone that he built based on the Wilson transducer and can be driven to full power from the standard headphone outputs of a stereo receiver or preamp. It produces a 300V peak-to-peak signal with 1.0% harmonic distortion at 1kHz. The amp has a frequency response of 10Hz-40kHz (-3dB) and a power bandwidth of 10Hz-15kHz.

The output stages are class A. IC1a is an inverting stage to produce an opposite phase output. R1 compensates for the small signal resistance of a diode in the non-inverting input of IC1b. If the headphone capacitance is greater than 150pF, R2 and R3 should be reduced to maintain the power bandwidth and the transistors should be heatsinked. The value of Rx is missing in the original schematic, but is estimated to be about 2.4K ohms from the similar topology of the high-bias version of the amplifier. P1 and P2 are adjusted so V1 and V2 are at one-half the supply voltage.

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Figure 18

The high-bias version of the Pollock amplifier (figure 18) can generate 1400V peak-to-peak at 5kHz with an input sensitivity of 2.8Vrms for maximum output. The circuit topology is virtually identical to the standard-bias amplifier. Of special note are the output transistors, which must have VCE ratings in excess of 1000V. Pollock chose transistors that are used in television horizontal deflection circuits. These have peak VCEs in the range of 1.5kV. The specified device is a Matsushita 2SD200. Substitutes include the BU206, BU209A, MJ105, PTC146-RT and SK3115-RT.

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Figure 19

Pollock built the high and low voltage sections on separate circuit boards to reduce the risk of instability and damage from building errors. Q1 and Q2 were heatsinked. The signal ground was taken from the source to avoid ground loops. If the amplifier shows instability, C1 can be increased to compensate. The power supply was installed in a ventilated, grounded, metal case. The high voltage section employes a half-wave voltage doubler (figure 19). The low voltage supply is decoupled at the amplifier with 0.1uF ceramic capacitors next to each opamp.

Addendum

9/10/99: Added new figure 17. Also made various corrections and additions to text.

References
Harvey, Philip D., “Electrostatic Headphone Design,” Wireless World, Nov. 1971, p. 527.
Pollock, Neil, “Electrostatic Headphone Amplifier,” Wireless World, July 1976, p. 35.
Pollock, Neil, “Electrostatic Headphones,” Wireless World, Nov. 1979, p. 51.
Wilson, J.P., “High Quality Electrostatic Headphones,” Wireless World, Dec. 1968, p. 440.

c. 1999, Chu Moy.

Troubleshooting Electrostatic Headphones.

by Mark Rehorst

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[Editor: Electrostatic headphones are high voltage systems. Please observe extreme caution and follow high-voltage safety procedures when working with these circuits!]

Most of my experience is with electrostatic loudspeakers (Quad ESL63 and my own designs). I have some Koss ESP9/50 phones, but on the two occasions when they have had problems, I took advantage of the lifetime warranty and let Koss fix them. Electrostatic headphones are basically the same as electrostatic loudspeakers speakers – same operating principles, same basic circuits.

To understand ES headphones, start by studying the readily available material on ES speakers. Be very careful when working on the interface/adapter box and headphones, because the ac voltage that drives the phones can go as high as 2,000V. The internal circuit should consist of a high voltage power source (some phones derive the bias power from the audio signal) and probably a couple of step-up transformers that raise the output voltage from the amplifier to the drivers. [Editor: Modern electrostatic headphones often use high voltage amplifiers instead of step-up transformers. The author advises to first check power supply voltages, then check circuit voltages when troubleshooting these headphone systems.]

The high voltage source connects between the diaphragms in the earpieces and the center taps of the transformer secondary windings.  The two end leads from each secondary go to perforated, conductive plates on either side of the diaphragm in each earpiece. The diaphragm is charged to a high voltage level, typically 500-750VDC in headphones, and the high voltage ac signal (audio) is applied to the plates on either side of the diaphragm.  The resulting electric field pushes/pulls the diaphragm back and forth producing sound.

If the headphones are dead, and you’ve tried the obvious stuff like checking the headphone cord, then the next thing to check is the fuse in the interface box.  If the fuse is blown, you probably have a short in the bias supply and need to fix it or the box will keep eating fuses. If the fuse isn’t blown, you may still have a problem in the bias supply.

If both channels are dead in the phones, then the bias supply is probably dead. If only one channel is dead, then there are a couple possibilities: one of the earpieces has developed a short, one of the resistors between the bias supply and the earpieces has opened up, or least likely, one of the audio transformers has failed.

You can check the earpieces with a multimeter. There should be three wires going to each earpiece. Check the resistance between all the leads.  They should all read as open circuits.  If they don’t, you’ve found the problem.

The most likely thing to die in the interface box is the diaphragm bias supply. The bias supply consists of three types of components.  There will be a voltage multiplier made up of a string of diodes and capacitors, and there should be one or two resistors that connect between the output of the supply and the diaphragms in the earpieces. The bias supply puts out so little current that it will be difficult to test.  The easiest thing to do is usually to just start replacing diodes until the thing starts working OK. If that doesn’t work, start replacing caps.  If that doesn’t work replace the resistor(s).

For further info on the operation of electrostatic speakers/phones, check out How To Build Electrostatic Loudspeakers.

c. 1999, Mark Rehorst.

DIY Electrostatic Headphones.

by Andrew Radford

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BACKGROUND

I first thought electrostatic headphones were a joke. It didn’t sound safe to strap high voltage transducers to your head. But after my work with electrostatic loudspeakers, I had the skill and knowledge to try building a pair and listening to them.

Basically an electrostatic headphone works just like an electrostatic loudspeaker, of course on a smaller scale. It modulates a diaphragm using electrostatic force, rather than electromagnetic force as in conventional speakers. They are usually a charged sheet of plastic film suspended between two conductive sheets called stators. The stators are ideally acoustically transparent but in reality are perforated metal.

Rather than build a pair from scratch, I decided to modify an existing pair of dynamic headphones to be electrostatic. A look around the house yielded the perfect candidates – an OLD pair of headphones, and they are of the 1970’s bulky construction, a simple circular design, like half a ‘barrel’ for each ear. Not pretty to look at, but a lot easier to retrofit with electrostatic panels than the modern curvy compact ones.

The transducers can be made any size and shape, just as long as they are flat! There needs to be at least 10mm of spacer so the glue has a nice bit of area to stick to. I would think that the minimum diaphragm size would be a circle of about 50mm in diameter. Less than that and the bass would suffer.

Unbelievably, the first prototype I built needed nothing more than mylar to get working, all other parts were adapted from the headphones. The headphones already had thin perforated metal grills crimped around the 95mm circular driver. There were also circles of card 0.5mm thick. So I used these parts along with mylar to make a monophonic earphone! Once I got that working I set to work making a proper stereo pair.

Here is an exploded view of what the panels are like; please note it is not to scale, the holes are actually only 1.5mm in diameter.

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PARTS

Stators

The stators I used were 1mm stainless steel perforated metal. The holes were about 1.5mm thick and about 30% open area. I found it in the junk pile in the garage. There is lots of scope for change here, any perforated metal can be used, so long as it has holes less than 2mm, and at least 25% open area. It can also be a lot thinner than the 1mm thick steel I used.

The most difficult aspect was cutting it into a 95mm circle, I used tin snips, which warped the metal, and I could not get it perfectly flat again. There is not much room for dishing when the gap between the stator and the diaphragm is only 0.5mm. Thinner metal that could be cut with ordinary scissors would be better.

Spacers

Spacers need to be good insulators so I used plastic. The trick is finding plastic of the right thickness (0.4 -1mm) that also takes glue. I eventually found a cover of a ring binder, it was a kind of textured plastic which holds epoxy better than it otherwise would. It is still marginal. Polycarbonate (lexan) or acrylic (perspex) are also good. Even cardboard worked in my first prototype, although I did soak it in polyester resin diluted with acetone.

Diaphragm

This needs to be thin film. The best is mylar. It is hard to find, and can usually only be got in rolls of a couple of kms. I can also be bought from the ESL Info Exchange, linked to in the links section. Try asking a Dupont distributor, they may sell you a single roll.

CONSTRUCTION

The best way I found to build these ES panels is similar to Roger Sander’s technique described in “The Electrostatic Loudspeaker Design Cookbook” which I recommend you reading if you are serious about making a useful and practical ES loudspeaker or headphones.

The stators

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The metal stators must be cut to the correct shape to fit the headphone cans. In this case, a circle. There are many ways to cut metal. The first time, I used heavy duty tin snips. These work quickly, but unfortunately they bend and war the metal, which is unacceptable when the spacing between the stator and the diaphragm is only half a millimeter. Jigsaws and nibbling tools are better.

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The stators should be painted. Unlike larger Electrostatic speakers, which utilise several thousand volts, the 500V can be insulated by a good coat of enamel paint, so it is a good idea. There must be a small area that is unpainted however, to provide an electrical contact. I used a piece of brass shim. You can see on the last photo above the area of bare metal that I sanded back. (The dark gunk around the edge of the metal in that photo is epoxy glue from a previous rebuild)

The four stator/spacer assemblies should then be assembled using epoxy.

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It is best to use a thin film of glue rather than uneven globs. Also, it is very important that the material you use for the spacers actually sticks to the glue. Even scuffed with sandpaper, the spacers I have are marginal in that regard and as a consequence the stator/spacer assemblies are quite fragile. This can sometimes work to an advantage, because if you make a mistake it is easy to rip it apart, scrape the dried epoxy off and start again.

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When the stators are glued, cover them with plastic, and lay a flat heavy object on top of them (like the heavy pane of glass) so that they set straight.

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The panels

The diaphragm must be under tension to work properly in the ES headphones. So the mylar must be stretched. This is probably the hardest part of the whole construction. First, a sheet of clean glass is laid down on something white, like paper. Then the mylar is laid down on the glass and stretched. I used a stretching jig made up with a wood frame and screws, but you might get away with just flattening out the mylar with tape and tensioning it up later by heatshrinking it with a heat gun. The stretching jig I used was detailed on Sheldon Stokes site, SDS labs. This jig was originally intended for stretching mylar for the refurbishing QUAD 57 Electrostatic loudspeakers. I used half the jig.

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The mylar must be stretched not so it is drum tight, but just free of wrinkles. It must also be held taut unto the glue has reached full strength and will hold it on its own. For the 5-minute epoxy glue that I used, this was a few hours to be safe.

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The jig is shown as a mess of tape and wood. It actually has a number of screws witch pull the mylar taut. Once the wrinkles are just out, it only takes another half turn to get it tensioned. That’s only a few millimeters. Mylar has incredible tensile strength for its thickness.

Once the mylar is tensioned, clean it with solvent (acetone, IPA etc) and lay it on a clean flat surface (glass again). Then the graphite must be applied. The graphite makes the mylar conduct electricity just enough to hold a charge, i.e. several mega ohms per inch. Consequently, only a small amount has to be ground into the surface. You can see by the picture the tiny pile of graphite powder, that is plenty. The powder should be rubbed in hard until the mylar has a grey tinge to it, then the excess must be wiped away. Any loose graphite means clicks pops and hisses in your earphones.

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In order to get electrical charge to the diaphragm, there must be another brass shim contact. I cut mine in a ‘T’ shape in order to get more contact area, while still maintaining a almost complete bead of glue for the mylar to stick to, which, BTW, it doesn’t stick to very well. Mylar doesn’t seem to stick to any glue well! You could also use copper tape or maybe aluminum foil – it would be fragile. I have also had success with just leaving a small tag of the diaphragm hanging out of the assembly and connecting to it by pressing a brass shim contact against the graphite treated side.

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Lay the diaphragm contact on the film of glue, and don’t get any glue on it, so that it will make a good contact with the graphite side of the mylar. Next, lay the stator/spacer that DOESN’T have the diaphragm contact under the mylar, and gently press down on the glue to make it an even film.

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Then the other stator/spacer with the diaphragm contact is placed directly on top, making sure it is aligned and that the all contacts suit the wiring on you headphone cans. Then cover them with plastic (or better still.. mylar!) and cover them with another pane of glass and a weight.

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When it is all cured, cut the assembly free from the mylar, and with a sharp blade, trace around the outside of the spacer to remove all trace of the diaphragm. This step is important – you must be diligent. It is also why my stators are of slightly smaller diameter than the spacers, so that there is more than the tiny 0.5mm spacer thickness between the stator and the ragged edge of the cut diaphragm. If a small piece of the diaphragm shorts the 0.5mm distance to one of the stators, charge will be lost and the results my be audible. The mylar should be skinned tight, with absolutely no wrinkles. If there are, rip it up and do it again. You can tell when the diaphragm is well tensioned, flick it with your fingernail. If you hear a nice tone then is OK. You can also hold the panel up and look through the perforated metal at a reflection on the shiny diaphragm surface, and get an idea of the tension that way. If you can see the diaphragm is wrinkled, rip it up, start over. Also, if for some reason there is a dead short between the stators and you can’t find it, rip it up, start again. Check that the brass shims are actually making good contact with the perf. metal also.

The headphone enclosure

This must provide complete support for the ES panels for they are most probably pretty fragile.

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The pictures show (what is left of) the old dynamic headphones which I butchered brutally and without compunction to accept a new pair of electrostatic transducers. The backs were cut off so the phones had only enough plastic to mount the ES panels. They resembled ‘rings’ with just a 20mm deep plastic cylinder supporting the ear cushion.

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The original cord and drivers were junked. The new cord is about 1.5m of computer IDC cable (10 strands). It has an insulation rating of 300V, and I only use every second strand to further reduce loss of signal in the wire due to capacitance, and reduce the risk of insulation breakdown. The plug is a 25 pin D connector, not ideal but all I had available at the time. Again, I soldered to every second pin.

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How exactly they are mounted depends on the headphones, but mine are just friction fit, held fast with the wires soldered to the brass tabs, and the odd drop of glue. 5mm foam for acoustic damping is recommended.

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The above pictures were taken with the ear cushion and foam rubber removed so you can see the detail of the stators. In the leftmost picture you can see the red spacer ring behind the stator. The stator is painted black.

Driving Electrostatic Headhphones

A Step-Up Adapter for Power Amplifiers

Of course ES headphones can’t run off normal headphone sockets, they need there own special high voltage amplifier and a high voltage bias source. The bias voltage is around 450V. This was achieved with a voltage doubling rectifier running off 230VAC, which was from two little transformers back to back to provide isolation from the mains. For lower voltage mains (such as 115V for the USA), the design can be used as-is, except have a 115:24 transformer then a 12:115 transformer to get 230V. Or else use 115V and add a few more steps to the ladder rectifier, i.e. a few more capacitors and diodes to get 115 * 3 or maybe 4.

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The bias voltage is not super critical. The higher the voltage, the more efficient the phones, cause there is more charge to attract. This is good because it means that a lower ratio on the transformer can be used. Go too high, and it will become unstable, and the diaphragm will ‘collapse’ into one of the stators even with no signal, i.e., the attractive force of one stator (the two stators are never exactly equally attractive) becomes enough to overcome the tension force holding the diaphragm in equilibrium. I have been meaning to try a variable bias with 500V and a pot. Since this is truly a no-load voltage, a high value pot could be used, so long as it is rated for the voltage.

The capacitors used in the voltage doubler were what I had laying around. So here is a guideline: The caps need to be non-polar, high voltage. So the cheap 630V polyester caps would be ideal for the job. The higher the capacitance, the more ‘grunty’ the bias supply, but since the only load is leakage diaphragm->stator, a VERY small value could be used. On an ESL speaker many times the area of headphones, 0.047uF capacitors are used. I believe the values could go smaller with phones. There would not be much to gain from going too small though. My prototype uses 2uF which is HUGE, bordering on unsafe! My bias supply can be turned off, and even the 2uF is enough to keep the phones going for 30 minutes, and I would consider my pair to be VERY leaky.

I used a 20Mohm resistor in line with the bias source, which is a current limiting resistor. The diaphragm needs to only be charged up to be affected by the stators, so the bias supply provides no current during normal operation (only leakage current, which is next to zero) so no current equals no voltage drop across the resistor. In a short circuit fault condition, The resistor limits the current to a minuscule amount as the full 450V is dropped across the 20Mohms. Since these headphones are around your cranium, this current limiting resistor is your friend. Don’t leave it out.

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The stators need around a 300V voltage swing, and need next to no current. So a high voltage, high output impedance amplifier is ideal. These can be built easily with signal tubes such as the 12AU7 and the 6SN7. Eventually I would like to build such an amplifier. In the mean time, I created a cheap way of operating the headphones using the 100V line to speaker level impedance matching transformers used in PA applications. These can be got from any electronics store, they are around 2-5WRMS and are only a few bucks.

They usually have 5K and 2K tappings on the primary, and 16, 8, 4 and maybe 2 on the secondary. But don’t be fooled, the 2.5K tapping is NOT the center tap for the 5K tap! (Remember – turns ratio = square root of ratio of impedances). However, in this application you can get away with making a ‘virtual’ center tap by tagging a couple of 1Mohm resistors across the 5K tap, this will not load the transformer at all.

The common and 5K taps go to the stators, the negative of the bias supply goes to the virtual center tap and the positive to the diaphragm contact. Hook the other side of the transformer to a power amplifier. It will not load the amp at all. I recommend a fairly powerful amp, just because they have a greater voltage swing than smaller ones. Try different tap settings eg 8,4 ohm, the smaller the value the higher the step up ratio and the louder the headphones. It is a good idea to start on a high value eg 16ohm and work down!

I run on the 2-ohm tap with a 50W amp that has 35V supply rails. You will almost certainly get different results because factors such as bias voltage and diaphragm/stator spacing determine efficiency and maximum sound level. Also make sure the amp likes having no load, some amps especially valve amps are iffy about loads and may like a dummy resistive load.

The resistors are all 1/4W, the diodes are 1N4007 (1000V) and the capacitors are 2uF, 250V, but these should really have a higher voltage rating and don’t need to be nearly that big (0.01uf would probably be ample). The case is made out of a car battery charger. Note how the headphones have been reduced to ‘ring supports’ for the ES transducer and ear cushion. There is 4mm foam rubber on either side of the ES panel. This is acoustic damping, and also keeps fingers off the metal stator.

The upper 2 transformers in the picture are the 4W PA transformers, the grubby little ones are the bias supply transformers. The diode/cap ladder is under the D25 plug.

A Tube Electrostatic Amplifier

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I have constructed a valve ESL headphone amplifier as detailed in the TubeCAD.com article Electrostatic Headphones by John Broskie. The amplifier uses 2 6SN7 tubes and 2 12AX7 tubes. Currently I have just built a prototype, and am working on reducing hum and increasing bass response. The sound is promising, though!

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A lot of the part selection was dictated by what I had on hand, as you can see the power transformer is ridiculously oversized. The switch next to it is the HT switch, this is switched on once the heaters are warm, to prevent cathode stripping since there is a solid state rectifier.

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THE RESULT

I built these headphones for fun, and to replace the awful cheap ‘earbuds’ that I was previously using. But these headphones do work exceptionally well. Having never listened to a high end commercial offering like Sennheiser or Stax, I cannot tell you how they compare. However they are clearly superior to any ‘affordable’ headphone, and for price you can’t really complain (especially considering Sennheiser Orpheus ES headphones are US$14,990!!).

Bass is very refined and DEEP. Not like most cheap headphones, which either have no bass at all or augment the bass and end up muddled (“Now with Bass Boost System”).

Midrange is clean and rich, very natural and not harsh at all, and the highs are especially clear. You can listen to these headphones for long periods with far less fatigue than earbuds. Cymbals sound like Cymbals, not like hissy-fits.

Volume is acceptable, when you turn it up too loud the diaphragm hits the stator and gives a very audible snap crackle and pop which is VERY annoying. The headphones still almost make it to what I would consider maximum listening level. They are by no means quiet. They could be made to go louder by increasing spacer thickness (and hence excursion limit), but that would also mean an even greater increase in voltage swing to achieve the same output… it quickly gets out of hand.

Since these ES phones are open-backed, other people can hear what you are listening to, but it is not intrusive. It is like when someone turns up there walkman REAL LOUD. But it also means any outside sounds go straight through the diaphragm and you can hear them easily. They don’t block out the outside noise like normal headphones, so they are best listened to in the dead of night….

c. 2000, Andrew Radford.
From the author’s website: diyAudio NZ. Republished with permission.

 

A Current-Domain Electrostatic Amplifier for Stax Omega II Headphones.

by Kevin Gilmore

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I bought the Omega II headphones without the amplifier ($1995 + shipping from EIFL Corporation in Japan). I would love to listen to the SRM-007t or SRM-717 amplifier, but really do not want to fork over $4000 to do so. I have been working on this solid state Stax headphone driver for a long time. It satisfies all of the design requirements. Of course it sounds absolutely amazing which is clearly the goal here. There are no capacitors in the signal path. Its fully DC coupled. No expensive parts, and can be built by just about anyone.

The amplifier operates primarily in the current domain. The first stage is a voltage controlled current sink. The second stage is a current-controlled voltage source. The fourth stage is a constant current sink. The main advantage of current domain amplifiers is speed. Standard voltage gain amplifiers with lots of gain are affected by the Miller Effect which prohibits extended frequency response.

This solid state amp is so much better than my tube amp that I no longer listen to it. I’m not a solid state snob; it’s just plain better. The people who have listened to this amplifier (some of whom were giants in the industry in their day) love it, much more than my tube amp. I love it too. I can’t stop listening to it. The tube amp has moved into a secondary position in my listening rack.

How It Works

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The first stage is a differential amplifier with feedback directly from the output stage. It works equally well with both balanced and unbalanced audio input sources. The step attenuators from Goldpoint make good volume controls for this stage. The JFET device is a dual JFET all on one wafer. It is known for extremely low noise and excellent matching, and is used in a number of expensive designs, such as the Nelson Pass amplifiers.

Because the amp is totally DC coupled from input to output, drift in the input stage is a bad idea. Since the first two stages run in current mode, the JFET input is more linear than a pair of bipolar transistors. Dual transistors all on one wafer suitable for audio use are hard to find these days.

The approximate voltage gain of this stage is 5. But it really runs in current mode. The unit was designed to work equally well in both balanced and unbalanced mode. For single-ended signals, ground either the + or – input and apply signal to the other. The much higher impedance of the JFET works better when one side is grounded for unbalanced inputs.

The second stage starts with a constant current source. The current source feeds a common base amplifier. The common base amplifier feeds a modified Vbe multiplier. I believe a famous designer is now calling this circuit a current tunnel. Its the most linear way of translating the voltage down to the bottom rail. The voltage gain of this section is about 4. The basic idea of the first two stages is to supply the third stage with a very fast low impedance drive signal that is referenced to the bottom rail.

The current sources in the second stage supply 2 mA each. With no signal, the FETs take 1 mA, leaving 1 mA going through the common base amplifier into the bottom transistor (which is wired as a vbe multiplier). This generates the 13 volts (referenced to – rail) necessary to properly bias the 3rd stage. The bottom transistor acts like a zener diode in series with a resistor, except a lot less noisy.

The third stage is another differential amplifier feeding another common base amplifier. The simple differential amplifier has a voltage gain of about 100. The common base amplifiers are used to reduce the miller effect on the differential pair. Since the miller effect depends on both gain and output voltage swing, reducing the output voltage swing of the bottom differential transistors significantly improves the speed of this circuit.

The fourth stage is an emitter follower driven by a constant current source (gain = 0.99). This output stage dissipates 12 watts total (3 watts per transistor x 4 transistors). The main design goal was low output impedance. For example, my electrostatic tube amp has a 50K load resistor and thus has a 50k output impedance. This amp has a 25 ohm output impedance (actually a little less with feedback) The result is a much more extended high end. The slew rate of the solid state amp is more than 5 times that of the tube amp.

For the output stage, each 2SC3675 sources or sinks 9 mA at a quiescent output voltage of zero volts referenced to ground. For the driver stage, each 2SC3675 sinks 1.1 mA, resulting in 1 VDC at the collector (referenced to ground). The bases of the 2SC2705s sit at about 16 volts (referenced to – rail). The overall open loop gain of the amplifier is about 2000, but feedback reduces it to 1000. Even without any feedback of any kind the total harmonic distortion of the amp is still under .02%.

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My first prototype, the unit in the pictures, uses an unregulated power supply. Given the stiffness of the capacitors, and the fact that the amplifier is pure class A, there is absolutely no fluctuation in voltage when signal is supplied. Of course, a regulated supply is always better. A regulated design is shown above. The 2SC3675 and 2SA1968 are mounted on heatsinks (the small tab ones are fine). The transformer is a Thordarson 24R22U (Allied # 704-0952). Adjust the pot to get 580VDC for the bias voltage.

The ±15 volt supply is an encapsulated fully regulated power supply brick from Sola Linear (Allied part number 921-9215), which retails for $117. I used a 60 mA version, but thats overkill, because the total current drain is about 12 mA for both channels. Lots of companies make these. It’s the black brick in the picture. It is NOT a switching supply. I do not use switchers in audio stuff if I can possibly help it.

Construction

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Download full size PC board designs and component layout 1
Download full size PC board designs and component layout 2
Download full size PC board designs and component layout 3

This project involves working with high voltages, so be extremely careful! Keep one hand behind your back at all times. 600VDC across both arms might possibly stop your heart.

All resistors are 0.5W. Most do not need to be. The 300K resistors in the top of the 3rd stage need to be 0.5W. The 150K resistor in the current drive in the last stage needs to be 0.5W. I am trying to find 2SA1968 transistors, which are 900 volt PNP types. If they are fast enough, then the two 300k resistors can be replaced with current sources instead, making the amp 100% current source driven.

The LEDs in the amplifier circuit are voltage references (1.7 volts types in the prototype) which track changes transistor voltage with temperature (low voltage zener diodes have tracking problems). They also serve to show that the unit is running properly. If the LEDs are not lit, something is wrong. You could always replace each LED with 3 1N914 diodes in series, but the LEDs look so pretty (reminds me of the glow of a vacuum tube).

I am using standard regular brightness red LEDs. The blue and green ones run at different voltages (blue = 2.6 volts, green = 2.1 volts). Using LEDs with voltage drops greater than 1.7V can affect biasing. Higher LED voltage drops in the first and second stages will tend to cancel each other out, and the numbers will be the same. That is, a higher voltage diode will increase the current sources from 2mA to maybe 3 mA (each), but at the same time, the current sink in the first stage will go from 2mA to 3 mA (total), so the net result is zero.

However in the final stage, a higher voltage diode will increase the standing power. As long as the heatsinking is good, an increase from 12 watts per channel to 15 or so is just fine. The transistors are actually good for 10 watts each, so it is possible to increase the bias to 40 watts per channel.

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All 4 output transistors are mounted on one aluminum angle that bolts through the front panel to the heatsink. The mounting heatsink is 4″ x 5″ x 1/8″ aluminum plate, punched and then bent along the short axis. There are 4 holes that hold the transistors to the angle, and 5 holes that bolt the angle to the heatsink. The blue-finned heatsinks I found on some old power supplies. I used them because they were big enough and pretty at the same time. The 2 2SC3675 drivers have small standup heatsinks.

The two pots balance the output voltages to 0V referenced to ground. Begin the adjustment by putting a voltmeter between + output and – output and setting the first pot for zero volts. Then put a voltmeter between the + output and ground, and set the second pot for 0V. After the amplifier warms up for 30 minutes, adjust the pots again. I adjusted my unit once, and keep checking it every so often. The output voltages on my unit are less than ±200mV. Compared to the 580 volt bias, that is close enough to 0V. And that is over a 1-month period.

Assemble the output stage with care. The full output voltage swing exists between the bases and the collectors of the bottom output transistors. Poor soldering techniques combined with excess flux can cause an arc which may damage the transistors. It happened to me once.

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The Stax jack is Allied part number 719-4043. For all headphones except the Stax Omegas, the plug fits in all the way. On the Omegas, the plug is a little fatter and does not fit in all the way, because the plastic center of the jack is about 0.25″ below the base of the metal rim. So I put the jack in a lathe, and took 0.25″ off the metal rim so that it is flush with the plastic insert. This modification does not affect the fit of other Stax headphone plugs. For details on how to wire the jack, see All-Triode Direct-Drive Tube Amps for Electrostatic and Electret Headphones.

The 2SC3675 is made by Sanyo. The 2SA1968 and 2SA1156 are from NEC. The rest of the transistors are from Toshiba. Here are the current prices:

2SK389 1.90 each
2SC1815 0.30 each
2SC380 .37 each
2SC2240 .55 each
2SA970 .79 each
2SA1156 .82 each
2SC2705 .49 each
2SC3675 1.56 each

In the USA, all of the Japanese semiconductors are available from B&D; Enterprises. B&D; takes credit cards. The entire semiconductor cost not including the power supply is about $50 USD. The parts are also available from MCM Electronics, Farnell and Newark Electronics. Since they are all the same company, these parts can be purchased just about anywhere in the world.

There are no recommended substitutes. No American manufacturer makes 900V PNP or NPN transistors with a low Cob anymore. Neither does Phillips of the Netherlands. The only manufacturers of these transistors are Sanyo and Toshiba, and only because they are heavily used in dynamic focus applications for large CRT monitors.

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The enclosure is a Mod.U.Line by Precision Fabrication Technologies Inc. (part number 03-1209-BW) and is available from Newark Electronics, probably Allied too. It measures 3″ x 12″ x 9″.

The Results

I do not think that the Omega II headphones can be damaged by this amp unless the bias is set way too high. If the bias is set right, the outputs are close to 0V at idle, and all the LEDs are lit, then the amp pretty much has to be working correctly. Now if one or more of the outputs is stuck at +300V or -300V, then something is seriously wrong and needs to be fixed. An oscilloscope really helps.

The amp can output 800Vp-p or 1200Vp-p with headroom. At 800Vp-p, THD is less than .008% from 20Hz to 20kHz. The actual frequency response is 0 to 45khz (-3db at 45kHz) into an Omega II load. Compared to the sound of my previous tube amplifier, the bass is no longer tubby; it’s very sharp and tight. The high end is no longer rolled off, so female voices sound much more real. If the bias supply is reduced to 280V, the amplifier will drive all electrostatic headphones. I tried it last night on a pair of SRX’s. I never ever heard them sound so good.

Last weekend, I took home a standard dummy head, and measured the SPL in Omega 2 headphones driven by this amplifier. With a drive signal of 800 volts peak to peak per side, the resulting spl is 106db. THAT’S LOUD! The amp can put out 1200 volts peak-to-peak, and thats louder! I just ordered a pair of Stax SR-001 MkIIs, which can reach up to 120dB. My ears distort before the amplifer/headphones do. It is quite loud at clipping, but the clipping is a hard clip with no oscillation or ringing. To use the amplifier with electret headphones, delete the bias voltage. And probably keep the output swing under 200V. Electrets phones when driven with this amplifier can probably get very very loud.

[Editor: Contact the author to discuss the possibility of obtaining pre-etched PC boards for this amplifier.]

c. 2000, Kevin Gilmore.
From The Homepage of Kevin Gilmore. Republished with permission.

Addendum

2/21/01: Corrected mislabeled transistor part number: 2SC1815 (was 2SA1815).

9/5/01: Corrected mislabeled transistor part number: 2SC3675 (was 2SC367).

2/12/2002Richard Albers built the following version of the CDEA amp with some interesting modifications of the original circuit. He writes:

I have changed the 2SK389 FET for a MAT02 Dual Transistor in the first Stage of the CDEH-Amp. There were no problems, and it all worked fine from the start. It sounds much cleaner then with the Dual-Fets now, and I guess they add less harmonics to the music.

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In the third Stage of the CDEH-Amp, I have changed the Voltage-Divider 350K/20K, which sets the Bases of the SC3675 at ca. 20V. For the 20K Resistor i have put in a 20V, 1.3W zener. For proper working, I set the current through the zener at 7mA. Two 25K ohm, 5W Mills non-inductive wirewounds replace the 350K, dissipating ca. 2.2W of heat. To reduce the zener noise, I have put a 4.7uF tantalum together with a 47uF electrolytic capacitor in parallel with the zener diode. Noise is no problem.

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The Cabinet is a very simple construction, with the advantage of ease changing components or parts. There is only a wooden base with two side-panels. The front is the large heatsink together with an aluminium-angle. A suitable top-cover is under construction. The whole construction could be made way smaller, all parts on one pcb, with a smaller toroid-transformer, and all built in a industrial case, but for my own usage, it’s ok.

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The two smaller transformers under the wooden cover are the 10H-chokes for the high voltage power supply. The little transformer on the bottom generates the bias-voltage. The oversized big-one is a special-made 250W transformer, from Experience-Electronics in germany. The electrolytics are from EPCOS (Siemens).

This is a further way to tune-up this fantastic machine. Together with the MAT02 dual-bipolar input device and using only the best parts you can get, such as non-inductive Caddocks, very low ESR electrolytic caps in the high voltage section, and so on, there is no better electrostatic headphone amp in the world. It sounds just fantastic!