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

by Kevin Gilmore

gilmore1_1.jpg

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

gilmore2.gif
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.

gilmore3.gif
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.

gilmore1_3.jpg

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.

gilmore4.gif
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).

gilmore5.gif
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

gilmore6.gif
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

gilmore1_2.jpg

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.

 

A Simple Tube/Opamp Hybrid Amplifier.

by Alex Cavalli, Mark Lovell and Bill Pasculle

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Introduction

After seeing many of the excellent and eye-catching tube-solid state amplifiers of others, we’d like to present a slightly different topology of hybrid amplifier design, using the same two basic components of a tube and an opamp. This amp is simple and has less than $50.00USD worth of parts and no lethally high voltages. It makes an ideal tube headphone amp for those who like the sound of tubes but worry about the safety of high voltage equipment.

Bill was looking for an amp that sounded good and also was a good companion for his Rio Carbon portable player. He also suspected that others might like something similar as well. With high bit-rate MP3 or WMA files, the Carbon can produce excellent sound, but as with many other portable players, benefits by the addition of a decent headphone amp. He also wanted to use the amp in his cubicle at work – requiring good sound without taking up too much space.

The design of the amp started when Bill suggested that he wanted to build one of the YAHA (Yet Another Hybrid Amplifier) amps , but using a different design. Alex, being mostly interested in tube amps, was not initially interested but as Bill and Mark began to suggest requirements for the amp and some options for the input tube, it was obvious that there was room to improve the typical hybrid topology. After more thought, Mark provided a list of some requirements for the design that he felt were necessary to meet in order to improve on the hybrid topology.

Alex went to work taking the design discussions and turning them into a draft design. Our first name for the amp (as a joke) among the team was Stoopid Opamp Headphone Amp (SOHA). The name, as so often happens with skunk-works project names, eventually stuck and finally we are just calling this amplifier the Stoopid or the SOHA. Another name for the amp, with the same acronym, might be the Simple Opamp Hybrid Amplifier.

The original prototypes assured us that it is possible to make a surprisingly good performing amp utilizing a tube at relatively low voltage and while still keeping the build cheap, easy, and reasonably electric shock-free. After altering some of the power supply and circuit values and testing the prototype, we ended up with something that was stable and fun to listen to for extended periods. It applies some compression but that’s part of its charm, and it would be a rather sterile sounding amp without the 12AU7/ECC82 altering the sound the way it does with its 40V plate voltage.

Amplifier Circuit

Most of the hybrid amps that have appeared in HeadWize threads and elsewhere (such as the Millet hybrid) have used the same B+ for both the tube and the opamp. Some of these amps are designed to be portable enough to run from a battery, but most are really constrained to a low voltage DC supply of some kind plugged into the line and so are not truly portable.

In addition, it is generally true that tubes that are not designed for low voltage use will not perform well at 12-24V (which is why the Millet uses special low voltage tubes), so we decided to try to provide the tube with higher B+ to get better performance, while still keeping the voltages fairly low. This meant that the amp could be small and portable although requiring AC power. Like the other hybrid amps, the SOHA is designed to give the sound of tubes while avoiding the high voltage risk that some builders don’t like. Still, providing a higher B+ permits us to get excellent sound from a more commonly available tube like the 12AU7/ECC82, which is in good supply from NOS and current production sources and which gives a wide variety of choices for tube rolling. Having this wide selection also makes it easier for the amp to be constructed in any part of the world.

The way in which most other hybrids use a common B+ for both tube and opamp has two detrimental effects on a hybrid amp:

  1. It puts the opamp in a single ended configuration where it needs an output cap to block half the B+
  2. It forces the B+ on the tube to be low so as not to exceed the maximum opamp rail voltages.

The first design decision was to decouple the B+ for the tube and opamp. Doing this makes it possible to use a standard bipolar supply for the opamp, eliminating the large output cap (as in the Chu Moy’s pocket amp for example) and requiring only a small coupling cap between stages (see power supply discussion below).

The tube is loaded with a constant current source (CCS) for two reasons:

  1. The resulting non-linear performance associated with low voltage operation is partially offset by the high dynamic impedance of the CCS
  2. A CCS has a much better PSRR than a simple resistor making it possible to have more ripple in the B+ and, therefore, simplifying the B+ PS.

The original amp is designed to run with FET input opamps. See note 1 (the BJT opamp section) for a version using BJT input opamps.

After extensive prototyping by Mark and Bill, and after posting the first design to the HeadWize forums and receiving feedback from several builders, we modified the original design. The most notable change was to the heater circuit. Originally the heater voltage was supplied directly from the AC secondary with voltage dropping resistors. This approach was implemented initially to maintain simplicity. However, because there is so much variation in transformer regulation, line voltage, and heater characteristics, the simple resistors were replaced with a regulator circuit. A side-benefit is the elimination of power wasted as heat since the dropping resistors reached 105°C – 115°C under normal operating conditions.

The Amplifier Circuit

The basic amplifier circuit uses a very small number of components, as shown:

cavalli2_fig1.png

R1 100k Log Pot C1 1000u 10V Electrolytic
R2 300R 1/8W C2 100n 100V
R3 560R 1/8W D1, D2 1N4148 or similar
R4 2k Trimpot U1 OPA2134 or similar
R5 1M 1/8W V1 12AU7/ECC82 or equivalent
R6 150R 1/8W

Figure 1 – Basic SOHA Amplifier

The topology of the amp is a simple grounded cathode gain stage coupled through a capacitor to an opamp wired in unity gain mode. The standard SOHA uses LND150 depletion mode MOSFETs for the CCS for reasons discussed below. The amplifier circuit is, thus, very simple. Trim pots are provided as part of the cathode bias resistors to adjust for variations in tubes to set the plate voltages to ~40V. Each CCS is set to regulate at approximately 1mA.

An advantage of this design over many of the other hybrid designs is that there is no large coupling electrolytic at the output. The required inter-stage coupling capacitor is small making it possible to use good quality film/audio capacitors here at nominal additional cost.

With a 12AU7/ECC82 the input stage has a gain of about 12. This is sufficient for almost any source driving almost any headphones which is why the opamp is simply operating as a unity gain current buffer. However, with such high gain it is possible to exceed the input voltage tolerances for the opamp with just 1Vp at the input. The data sheet for the OPA2134 (and many similar opamps) indicates that the maximum input voltage is (V-) – 0.7V to (V+) + 0.7V. This means that the input swing must not exceed the supply voltage by more than one diode drop. The diodes ensure that this does not happen.

If the diodes conduct, the excess current passes into or out of the bipolar power supply. What happens thereafter depends on the ability of the tube to source/sink current and the ability of the PS to sink/source it. In this case, the tube will source/sink in the range of hundreds of micro amps which will find their way to the output caps of the bipolar supply which are in turn supplying current to the opamp V+ and V-. The output of the regulators will fluctuate some, but at this point the amp would not be operating properly anyway.

The standard CCS for the basic SOHA uses a single LND150 MOSFET in this configuration:

cavalli2_fig2.png

R7 1k 1/8W R8 360R 1/8W

Figure 2 – Standard LND150 MOSFET CCS

For a discussion of why this was selected as the standard CCS, see note 2 (the CCS comparison section) at the end of the article. One limitation on the standard SOHA CCS is the uneven availability of the LND150 MOSFETs globally. To ensure that this amp can be built almost anywhere, we have created two variations that use other devices for the CCSs. The first uses J113 JFETs and the second uses 1N5297 current regulator (CR) diodes. Here are the schematics for both variations:

cavalli2_fig3.png

R8

1k5 1/8W

Figure 3 – Alternate CCSs using JFETs and CRDs

Care should be exercised when building the SOHA with the J113 JFETs. Their maximum Vdss is 35V. Under normal operating conditions they will see only about 15-20V, but if the plate voltage on the tube is too low it is possible to exceed this maximum and destroy them. To protect the JFETS, the minimum plate voltage should never be set below 20V (see below the warning about adjusting the trimpots). The 1N5297 CRD has a 100V maximum and should withstand all of the normal voltages in this amp. The J505, noted in parenthesis, will also work but has only a 50V maximum. This makes the J505 a little more robust in this circuit than the J113, but less desirable than the 1N5297.

Power Supply Circuit

The key to this amp is the power supply. Initially the amp used an easy-to-acquire 30VCT/200mA transformer. As noted above, during the development and testing process, including builds by several HeadWizers, we changed the heater supply from AC to regulated DC. To accommodate the 150mA DC drawn by the heater it is necessary to increase the current spec on the secondary to 400mA. This will also give some headroom for the amp itself. Eventually, we chose the Amveco TE70053 toroid to replace the original split bobbin transformer. Another benefit to using the toroid is less EM radiation in the box and, since the SOHA also designed to be small, this reduces or eliminates problems with PS buzz. Other transformer possibilities are in the Power Supply section below. You can use a higher current rating transformer without difficulty, but if you increase the voltage be careful about not exceeding the maximum input voltage for the regulators. The bipolar opamp supply is a conventional regulated supply using 78L12/79L12 inexpensive regulators. They have a maximum input voltage of 40V.

The trick to the power supply is the use of a 1.5x full-wave voltage multiplier to generate the B+ for the tube. To make the voltage multiplier, the entire secondary of the transformer is rectified through a pair of coupling capacitors and bootstrapped on top of the V+ of the bipolar supply. With a typical transformer with 25% regulation and with no load on the B+ for the tube, this generates over 80V (this is marginally dangerous and will give you a pretty good sting so be careful).

The power supply, including the heater circuit is shown below:

cavalli2_fig4.png

R9 2k2 1/8W BR1, BR2 100V 1A Bridge Rectifier
R10 1k3 1/8W VR1 12V Fixed Regulator 78L12
R11 11k 1/8W VR2 -12V Fixed Regulator 79L12
C3-C6 100u 100V VR3 Adj. Negative Regulator LM337
C7, C8, C11 47u 16V D3, D4 1N4002
C9, C10, C12 470u 35V T1 30VCT 400mA Transformer

Figure 4 – the SOHA Power Supply

When the B+ is loaded with the tube, with each triode drawing ~1mA, the voltage is pulled down to between +55-65V. This means that there is plenty of headroom in the B+ to run the tube at+40V while still leaving space for driving 7-10V into the opamp. And this seems to give very good performance. As noted above, using a CCS for the plate load relieves ripple requirements on the B+ so a much simplified and less expensive filter section becomes possible. For the components as drawn the B+ ripple is about 1mV. The capacitor values are kept low and, hence, the capacitors are small and inexpensive. Again, for CCS PSRR comparisons see below.

The heater supply uses a full-wave rectifier into a negative 12.6V regulated supply. Pay careful attention to the orientation of the rectifying diodes. The heater supply is attached to the negative half of the bipolar supply. This was done because the heater current will pull down the input to the filter section of whichever half of the bipolar supply to which it is attached. Since we are using the positive supply to bootstrap the B+ for the tube we don’t want the heater supply to pull this voltage down. Therefore, it is derived from the negative supply because if the negative input to filter drops by a volt or two the regulator will not be affected. Pay careful attention to the orientation of the rectifier diodes and capacitors in the heater circuit since it is a negative supply. A power indicator LED can be attached to the heater supply taking care to note the polarity. The negative regulator should be heatsunk to dissipate about 2W.

Construction

The full schematic for both channels and the PS is shown below with the complete parts list less some miscellaneous components such as enclosure, power switch, etc.

cavalli2_fig5.png
Click here to see full-size schematic.

R1 100k Log Pot C3-C6 100u 100V
R2, R12 300R 1/8W C7, C8, C11 47u 16V
R3, R13 560R 1/8W C9, C10, C12 470u 35V
R4, R14 2k Trimpot D1, D2, D5, D6 1N4148 or similar
R5, R15 1M 1/8W D3, D4 1N4002
R6, R16 150R 1/8W U1, U2 OPA2134 or similar, dual or single
R9 2k2 1/8W V1 12AU7/ECC82 or equivalent
R10 1k3 1/8W BR1, BR2 100V 1A Bridge Rectifier
R11 11k 1/8W VR1 12V Fixed Regulator 78L12
R7, R17 1k 1/8W VR2 -12V Fixed Regulator 79L12
R8, R18 360R 1/8W VR3 Adj. Negative Regulator LM337
C1, C13 1000u 10V T1 30VCT 400mA Transformer
C2, C14 100n 100V

Figure 5 – Full SOHA amplifier, both channels and PS

For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 1/8W. For the 1N5297 version simply replace the entire CCS with the single diode.

The amp has been built several ways by different HeadWizers [click here to see forum member Neurotica’s (Jim Eshleman) SOHA build narrative]. Mark and Bill initially built the prototypes using point to point wiring on perfboards and several others did so as well. Bill eventually also made a homemade PCB while Alex designed a PCB using the commercial package ExpressPCB (see below). Most builds to date have been like the prototypes with the PSU and amplifier circuits on the same board. Pictured below is a pictorial drawing showing how the SOHA can be wired point-to-point on a 4 x 6-inch perfboard.

cavalli2_fig6.png
Figure 6a – Bill’s SOHA constructed by point to point wiring on a perf board
Click here to see full-size layout.

Part of our purpose with the design and component specs is to keep everything as small and cheap as possible. The parts list shows the parts from the usual American suppliers. Mark was able to source similar parts from Farnell and RS in the UK.

Part # Mouser Catalog Number Description Qty. Price Total
R9 270-2.2K-RC Xicon 2.2k 1/8W

10

0.11

1.10

R3, R13 270-560 Xicon 560R 1/8W

10

0.11

1.10

R11 270-11K Xicon 11k 1/8W

10

0.11

1.10

R10 270-1.3K-RC Xicon 1.3k 1/8W

10

0.11

1.10

R5, R15 270-1.0M-RC (regular CCS) Xicon 1.0Meg 1/8W

10

0.11

1.10

270-100K-RC (mu follower) Xicon 100k 1/8W

10

0.11

1.10

R2, R12 270-300 Xicon 300R/1/8W

10

0.11

1.10

R6, R16 270-150-RC Xicon 150R 1/8W

10

0.11

1.10

R4, R14 652-3306K-1-202 Bourns 6mm 2K pot

2

0.56

1.12

C7, C8, C11 140-HTRL16V47 Xixcon 47uF/16V

3

0.07

0.21

C3, C4, C5, C6 140-HTRL100V100 Xicon 100uF/100V

4

0.42

1.68

C9, C10, C12 140-HTRL35V470 Xicon 470uF/35V

3

0.3

0.90

C1, C13 140-HTRL16V1000-TB Xicon 1000uF/16V

2

0.25

0.50

C2, C14 1429-1104 Xicon 0.1uF (100nF)

2

0.44

0.88

CCS Options
512-J113 J113 JFET

4

0.24

0.96

R8 270-1.5K Xicon 1.5k 1/8W

10

0.11

1.10

OR
689-LND150N3-G LND150 MOSFET

2

0.55

1.10

R8, R18 270-360 Xixon 360R 1/8W

10

0.11

1.10

R7, R17 270-1K-RC Xixon 1k 1/8W

10

0.11

1.10

  OR
610-1N5297 1N5297 CC Diode

2

4.29

8.58

BR1, BR2 821-DB102G 1A 100V Bridge

2

0.33

0.66

VR1 512-LM78L12ACZ LM78L12 Regulator

1

0.27

0.27

VR2 512-MC79L12ACP LM79L12 Regulator

1

0.34

0.34

VR3 512-LM337T LM337 Regulator

1

0.5

0.50

567-273-AB Wakefield Heatsink

1

0.38

0.38

Power LED 351-3310 Xicon Blue Led 3mm

1

1.5

1.50

271-560-RC 560R 1/4W LED Resistor

10

0.09

0.90

D1, D2, D5, D6 78-1N4148 1N4148 Diodes

10

0.03

0.30

D3, D4 512-1N4002 1N4002 Diodes

5

0.10

0.50

R1 313-1240-100K Taiwan Alpha 12mm pot, 100k

1

2.84

2.84

575-393308 IC Socket

1

0.36

0.36

J3 161-3502 3.5mm Headphone Jack

1

0.92

0.92

J1 161-1052 RCA Jack Black

1

0.82

0.82

J2 161-1053 RCA Jack Red

1

0.82

0.82

Total

38.04

OR
DigiKey
U1, U2 OPA2134PA-ND OPA2134PA

1

2.63

2.63

T1 TE70053-ND Amveco 30V CT 500mA

1

12

16.22

Total

18.87

Optional Sources
Newark
U1, U2 75C4624 OPA2134-PA

1

2.37

2.37

18C6948 J113 JFET

2

0.2

0.40

VR1 34C1091 LM78L12ACZ Positive regulator

1

0.28

0.28

VR3 34C1076 LM337T Regulator

1

0.67

0.67

R4, R14 46F1092 Bourns 6mm 2K pot

2

0.27

0.54

Antique Electronic Supply
V1 T-12AU7-JJ JJ 12AU7 Tube

1

8.95

8.95

V1 P-ST9-511 Tube Socket

1

1.95

1.95

Miscellaneous
Knob
Power Switch
Wire
Fuse holder and Fuse (0.25A)

The Amveco toroidal transformer (30VCT/500mA) is available from Digikey. Remember with 15-0-15 VAC (nominal) secondaries and the poor regulation of these inexpensive transformers, you will see over 21V with no load at the inputs to the bipolar power supplies and ~85V with no load for the B+. Because of the poor regulation make sure to use capacitors with voltage ratings that meet these off-load conditions. Note that the PS parts table shows 100V capacitors for the B+ section. If you use a higher voltage transformer make sure that the input to the regulators does not exceed their maximums (typically about 37V).

Some other possible split bobbin transformers are: Dagnall D3019 (0-240 pri), D3023 (0-115,0-115 pri). Both are 12VA. Other possible toroids are: MULTICOMP MCTA015/15 (0-115,0-115 pri), MULTICOMP MCFE015/15 (0-230V pri), or MULTICOMP MDCG015/15 (0-230V pri).

This design is optimized for 12AU7/ECC82 and its exact equivalents (5963, 6189, and 6680) rather than a close equivalent (or other types of tubes such as 6922).

All three flavors of CCS provide a degree of PSRR and some immunity from power fluctuations. They differ in availability worldwide and in maximum voltage ratings. The best overall CCS uses the LND150 MOSFET which is not available everywhere. The J113 FET is widely available but its maximum DC voltage is only 35 volts. Normally the FET wouldn’t see more than 15-20V unless the plate voltage gets too low. The 1N5297 CRD has a maximum voltage of 100 VDC but is not as widely available and is also expensive. Nevertheless, working amps have been built using all three types of CCS. Trim pots located at the cathodes are used to adjust the plate voltage. In order to prevent burning out the CCS FETs the cathode trim pots should always be turned to their highest resistance when swapping in a new tube.

OPA2134 and its relatives are fairly common opamps for audio. This was a good place to start. The authors would like to know how other FET input opamps perform and welcome feedback from builders. The OPA551, for example, is a FET input opamp that drops right into the Stoopid. However, it only comes in single packages so you will have to account for this with the build.

FET input opamps are preferred because there is a risk that BJT input opamps may tend to excessively load the tube and defeat the effect of the CCS. To use BJT input opamps see the section below for modifications to do this. The authors welcome feedback on the performance of the SOHA with BJT input opamps.

A BUF634 could easily be put into the unity gain feedback loop of the opamp to give super high output. One change that might be necessary if really trying to pull 200mA is to increase the size of the input capacitors in the bipolar PS to more like 2200uF. Even larger values may be required to get full bass.

Mark added 150 Ohm resistors (R6) at the outputs as this enables the amp to drive low and high impedance headphones without experiencing a large change in volume. These can be left out of the circuit at the builder’s discretion, however, their use is recommended. Likewise the pairs of 1N4148 diodes connected to the non-inverting inputs of the opamps are optional, but serve to protect the opamp inputs from overload and their use is recommended.

Here are a few details to pay attention to during construction and double check before applying power to your SOHA:

  • Wiring the Triad transformer is not intuitive; the pins are not numbered consecutively. Study the datasheet carefully.
  • The 78L12 and 79L12 do not share the same pinout.
  • The capacitors in the heater supply (as well as those in the negative half of the bipolar supply) have their positive leads grounded.
  • Use of a star-ground is highly recommended.
  • Use of shielded cable from the input jacks to the pot, from the pot to the tube grids, and from the opamp to the output jack is highly recommended. Attaching the safety ground to the star ground is optional. Most builds have worked fine with the star ground floating but an occasional unit has been quieter with the safety ground connected to the star ground.
  • Grounding the pot body is usually required to eliminate static/hum.

Wire dress is important in this amp to avoid hum. Keep all signal wires away from the transformer; keep the filament wires as far away from the audio circuit as possible.

PC Boards

We’ve created Express PCB boards for the SOHA. These are related to the full schematics with part numbers shown.

For the J113 version eliminate R7, R17 and change R8, R18 to 1k5 1/8W. For the 1N5297 version simply replace the entire CCS with the single diode.

ExpressPCB and PDF files for both the amp and power supply are included below. The tube socket on the amp board is in the center of the board. Note that the tube socket mounts on the foil side of the board. With this configuration you can easily mark a hole in the center of the standoffs and punch it out to pass the tube through so that the tube can stick up through the chassis while the components are sticking downward.

The copper layer in these PDF and ExpressPCB files can be used for home etched boards.

SOHA Amplifier Board (PDF)
SOHA Power Supply Board (PDF)
SOHA Amp and PS boards (ExpressPCB)

Techniques for making home PCBs were suggested by HeadWizer Bill Blair. Here are some links that Bill used to make his own SOHA boards using the single layer PDFs:

EasyPCB Fabrication
HomeBrew Printed Circuit Boards

The boards can be jumpered to use all three versions of the CCSs and to operate as standard plate drive or as source follower drive. This is the stuffing guide for these possible configurations.

cavalli2_fig12
Click here to see full-size stuffing guide.

Figure 6b – Bill’s stuffing guide for the SOHA amplifier PCB

Setup

Wire everything up but don’t put the tube/opamp in yet. Measure the voltages at the B+, V+, V-, and heater. They should be >80V, +12V, -12V, and -12.6V respectively. If they are not then there is a problem that must be fixed before inserting either the tube or the opamp.

If voltages are good and nothing has fried, power down and then insert the tube and opamp. Before powering up again, dial your trim pots so that they are in the maximum resistance position. This will put the maximum bias on the tube. Measure the voltage at the plates (pins 1 & 6) and adjust the associated trim pot until the voltage comes down to 40V for each plate. After these adjustments, measure the B+ again. It should be between 55-65V. Occasionally you may find a NOS tube does not work well in this circuit. You may need to replace the tube to get good results. If so, the tube is probably outside of its published operating characteristics. Each triode of 12AU7/ECC82 draws only 1mA from the B+ supply and at these low currents there can be a wide variation in operating characteristics, particularly among tubes that may be marginally within spec.

Results

OK, how does it sound? Well, in short, stoopidly good. When first powered up, the prototype plate voltage was only 17V and the amp sounded decidedly solid state. Very “steely” and just tonally “off”. As the plate voltage was raised the sound became more lush and tube-like. At 40V the amp began to perform extremely well. The SOHA easily rivals the Cavalli-Jones/Morgan Jones which costs over three times more to build! It’s got decent amounts of bass, classic sweet tube midrange and plenty of top end extension. Also the soundstage is extremely wide and respectably deep. This thing is just plain stoopid fun to listen to!

The amp drives headphones of any impedance between 16 Ohms and 300 Ohms without problems, which covers most that are currently available.

The compression applied by running the 12AU7 with 40V at the plate allows an unexpectedly refined sound with no sharp edges, yet without being mellow. It will reproduce transients when required and has a respectable dynamic range. The overall result is something than can be listened to for extended periods with no “listening fatigue” and providing a pleasingly wide and reasonably deep sound stage.

As is the case with tube amps, a warm up time is required and in this respect the authors agree 20 minutes is required for it to sound its absolute best, but of course it’s up and running after 30 seconds.

Mark has compared this amp to three other headphone amplifier designs available at HeadWize having built them: namely the CJ, the CL MkII, and the BCJ MkI, (the BCJ MkII was unavailable). All of the alternative designs used for comparison tests are more expensive to build, all require potentially lethal voltages, and all are optimized to ensure the tubes are working at their optimum.

Clearly, the SOHA would be the worst of the bunch? Not so. It proved itself to equal the CJ and gets closer than expected to the CL MkII. That’s pretty impressive stuff for a tube amp deliberately designed to be cheap and not use lethal voltages.

Tube rolling in this amp is also a lot of fun. Mark and Bill, who listen mostly through Sennheiser HD-600’s, found that grey-plate 5963’s from GE and RCA and Brimar sounded better than other tubes. Some other Headwizers with low-Z cans seemed to prefer black-plate versions of these tubes. Among the new production tubes, the Electro-Harmonix 12AU7 seemed to approach (but not exceed) the performance of the NOS tubes while the JJ 12AU7 was a somewhat distant second. Differences between tubes seemed to be in the clarity of the top end and the amount and quality of the bass.

Here are some photos of Bill’s SOHA in its final home.

cavalli2_fig7.jpg
Figure 7 – Bill’s SOHA Top Side and Figure 34 – Bill’s SOHA The Guts

Note 1: BJT-Input Opamps

As noted above the SOHA was designed to use FET input opamps. However, to permit opamp rolling, we’ve created two minor variations that permit the use of BJT input opamps.

Bipolar-input opamps like the TSH22IN, NE5532, or NE5534 can substitute for the 2134. But bipolar opamps will have lower input impedance than the FET input opamps. This will increase the loading on the tube and increase the distortion.

One way around the increased loading is to configure the CCS as an active load source follower. This variation requires only one change in wiring at the CCS and will only work for the LND150 and the J113 versions. An active load source follower is a variant of a well-known tube topology where the CCS that is acting as a plate load is also utilized as the output device in a follower configuration. With this topology the output impedance of the gain stage drops considerably and its ability to supply current increases in proportion. With both FET topologies we can wire the FETs as source followers to make a hybrid follower configuration for the first stage.

If you’re using the LND150 CCS you can convert the CCS into a source follower by simply changing the point where the coupling capacitor is connected. The FET then becomes a source follower with low output impedance and the ability to drive higher currents into the load.

cavalli2_fig8.png
Figure 8 – Changing the LND150 CCS for BJT-input Opamps

If you’re using the J113 CCS you can covert it to a source follower using the same technique:

cavalli2_fig9.png
Figure 9 – Changing the J113 CCS for BJT-input Opamps

Although the 1N5297 CRD is actually a JFET wired as a CCS we cannot access the source of the device so the CRD cannot be used when driving BJT opamps.

For example, a full amplifier schematic for the standard LND150 CCS with BJT opamp is:

cavalli2_fig10.png
Figure 10 – Driving BJT input opamps

If your amplifier exhibits high DC offset with BJT opamps, you can decrease the value of R5 from 1M to 100k or even 50k without overloading the gain stage. Note that decreasing the value of R5 while leaving C2 at 100nF also reduces the low frequency response of the amplifier. To correct for this, increase the value of C2 so that the product of R5 x C2 is the same as 1M x 100nF. For example, if you decrease R5 to 100k, then to maintain the same low frequency response, increase C2 to 1uF.

For BJT input opamps, the full schematic is this:

cavalli2_fig11.png
Click here to see full-size schematic.

R1 100k Log Pot C3-C6 100u 100V
R2, R12 300R 1/8W C7, C8, C11 47u 16V
R3, R13 560R 1/8W C9, C10, C12 470u 35V
R4, R14 2k Trimpot D1, D2, D5, D6 1N4148 or similar
R5, R15 100k 1/8W D3, D4 1N4002
R6, R16 150R 1/8W U1, U2 BJT opamp, dual or single
R9 2k2 1/8W V1 12AU7/ECC82 or equivalent
R10 1k3 1/8W BR1, BR2 100V 1A Bridge Rectifier
R11 11k 1/8W VR1 12V Fixed Regulator 78L12
R7, R17 1k 1/8W VR2 -12V Fixed Regulator 79L12
R8, R18 360R 1/8W VR3 Adj. Negative Regulator LM337
C1, C13 1000u 10V T1 30VCT 400mA Transformer
C2, C14 1u 100V

Figure 11 – Full SOHA with BJT opamp, both channels and PS

Note the part changes shown in red. These are the only changes necessary to use BJT opamps in the SOHA. The layout of amp does not change.

Note 2: CCS Comparisons

The choices for standard CCS and acceptable variations are derived from three criteria:

  1. maximum breakdown voltage of the CCS
  2. current regulating ability
  3. PSRR

These comparisons were done using PSpice simulations. These simulations are not likely to give absolute accuracy, but they are good at providing a relative comparison among the various CCSs.

Simulations were done for the following CCS types:

  • Single LND150
  • Single J113
  • Single PN2907A
  • Single 1N5297
  • Single PN2907A with CRD bias string
  • Cascoded J113
  • Cascoded PN2907A
  • Cascoded PN2907A with CRD bias string

This table shows the current variation, PSRR, and breakdown voltage (BV) for these various configurations:

Current Variation

Ripple at Plates

BV

300mVp 1kHz Input

1mVp 120Hz Ripple from PS

Delta I (uV)

Delta V (mV)

PSRR (db)

Topology

Cascoded JFETs (J113)

0.01

0.002

-54

35

Single MOSFET (LND150)

0.9

0.0025

-52

500

Cascoded BJTs with CRD

0.26

0.0095

-40

60

Single BJT w/ CRD

1.2

0.0135

-37

60

CRD (1N5297)

6.6

0.017

-35

100

Single JFET (J113)

13.3

0.034

-29

35

Cascoded BJTS (PN2907A)

0.34

0.058

-25

60

Single BJT (PN2907A)

1.2

0.06

-24

60

The cascoded JFETs have the best current regulation, followed by the MOSFET. The cascoded BJTs with CRD and without CRD have the next best current regulation. This might make these the next best choices. But, we must look at the PSRR and BV tables too.

The cascoded JFETs also have the best PSRR but they have a low BV. The LND150 has nearly the same PSRR (indistinguishable as a simulation result) but a very high BV. The LND150’s current variation comes in fourth behind the cascoded BJTs. However, the PSRR for the cascoded BJTs is 12db and 27db less than the LND150. Furthermore the BV for the BJTs is on the margin of where the voltages in the amp may be, and the BJT CCSs require many more parts than the either the JFETs or the MOSFET.

Taking all of these results together, the LND150 rises to the top for the standard SOHA because of its good current regulation, excellent PSRR, very high BV, and low parts count. The cascoded JFETs come in second because of their excellent regulation, PSRR, and low parts count. The CRD comes in third because of its good regulation, high BV and extreme simplicity (only one part).

Appendix: Simulating the Amplifier in OrCAD PSpice

Alex Cavalli has provided the project files for simulating this amplifier using OrCAD Lite circuit simulation software. The simulations will run in OrCAD Lite 9.1 or 9.2 only (later versions of OrCAD Lite and OrCAD Demo are more restrictive and will not run the simulations). The installation files for OrCAD Lite 9.1 or 9.2 can be downloaded from various educational sites on the internet. Search for them using the keywords OrCAD or Pspice and 9.1 or 9.2. OrCAD 9.1 is the smaller download (27MB). If you have trouble finding these files, email a HeadWize administrator for help.

There are 4 programs in OrCAD Lite suite: Capture, Capture CIS, PSpice and Layout. The minimum installation to run the amplifier simulations is Capture (the schematic drawing program) and PSpice (the circuit simulation program).

Download Simulation Files for SOHA Headphone Amplifier

After downloading cavalli2_soha_sim.zip, create a project directory and unzip the contents of the cavalli2_soha_sim.zip archive into that directory. Move the .lib and .olb files into the \OrcadLite\Capture\Library\PSpice directory. These are the component libraries containing the SPICE models for the vacuum tubes, MOSFETs and opamps used in the SOHA. (Note: heater connections are not required for any of the triode models.) In OrCAD’s Capture program, open the stoopid.opj project file.

The two basic types of simulation included are frequency response (AC sweep) and time domain. The time domain analysis shows the shape of the output waveform and can be used to determine the amplifier’s harmonic distortion. They both run from the same schematic, but the input sources are different. For the frequency response simulation, the audio input is a VAC (AC voltage source). The time domain simulation requires a VSIN (sine wave generator) input. Before running a simulation, make sure that the correct AC source is connected to the amp’s input on the schematic.

cavalli2_sim.png

The following instructions for using the simulation files are not a complete tutorial for OrCAD. The OrCAD HELP files and online manuals include tutorials for those who want to learn more about OrCAD.

Frequency Response (AC Sweep) Analysis

  1. Run OrCAD Capture and open the project file stoopid.opj, if not already open.
  2. In the Project Manager window, expand the “PSPICE Resources|Simulation Profiles” folder. Right click on “Schematic1-ac” and select “Make Active.”
  3. In the Project Manager window, expand the “Design Resources|.\cavalli.dsn|SCHEMATIC1” folder and double click on “PAGE1”.
  4. On the schematic, make sure that the input of the amp is connected to the V4 AC voltage source. If it is connected to V3, drag the connection to V4.
  5. To add the triode library to the Capture: click the Place Part toolbar button (orcad1.gif). The Place Part dialog appears. Click the Add Library button. Navigate to the triode.olb file and click Open. Make sure that the analog.olb and source.olb libraries are also listed in the dialog. Click the Cancel button to close the Place Part dialog.
  6. From the menu, select PSpice|Edit Simulation Profile. The Simulation Settings dialog appears. The settings should be as follows:
      • Analysis Type: AC Sweep/Noise
      AC Sweep Type: Logarithmic (Decade), Start Freq = 10, End Freq = 300K, Points/Decade = 100
  7. To add the triode library to PSpice: Click the “Libraries” tab. Click the Browse button and navigate to the the triode.lib file. Click the Add To Design button. If the nom.lib file is not already listed in the dialog list, add it now. Then close the Simulation Settings dialog.
  8. To display the input and output frequency responses on a single graph, voltage probes must be placed on the input and output points of the schematic. Click the Voltage/Level Marker (orcad2.gif) on the toolbar and place a marker at grid of U6. Place another marker above R9 at the amp’s output.
  9. To run the frequency response simulation, click the Run PSpice button on the toolbar (orcad3.gif). When the simulation finishes, the PSpice graphing window appears. The input and output curves should be in different colors with a key at the bottom of the graph.
  10. The PSpice simulation has computed the bias voltages and currents in the circuit. To see the bias voltages displayed on the schematic, press the Enable Bias Voltage Display toolbar button (orcad5.gif). To see the bias currents displayed on the schematic, press the Enable Bias Current Display toolbar button (orcad6.gif).

Time Domain (Transient) Analysis

  1. On the Capture schematic, make sure that the input of the amp is connected to the V4 sinewave source (VAMPL=0.4, Freq. = 1K, VOFF = 0). If it is connected to V3, drag the connection to V4.
  2. In the Project Manager window, expand the “PSPICE Resources|Simulation Profiles” folder. Right click on “Schematic1-signal” and select “Make Active”
  3. From the menu, select PSpice|Edit Simulation Profile. The Simulation Settings dialog appears. The settings should be as follows:
      • Analysis Type: Time Domain(Transient)
      Transient Options: Run to time = 80ms, Start saving data after = 40ms, Max. step size = 0.001ms
  4. To display the input and output waveforms on a single graph, voltage probes must be placed on the input and output points of the schematic. Click the Voltage/Level Marker (orcad2) on the toolbar and place a marker at grid of U6. Place another marker above R9 at the amp’s output.
  5. To run the time domain simulation, click the Run PSpice button on the toolbar (orcad3). When the simulation finishes, the PSpice graphing window appears. The input and output curves should be in different colors with a key at the bottom of the graph.
  6. To determine the harmonic distortion at 1KHz (the sine wave frequency), harmonics in the output waveform must be separated out through a Fourier Transform. In the PSpice window, press the FFT toolbar button (orcad7.gif). The PSpice graph changes to show the harmonics for the input and output waveforms. The input and output curves should be in different colors with a key at the bottom of the graph.
  7. The fundamental frequency at 1KHz will have the largest spike. The other harmonics are too small to be seen at the default magnification. In the PSpice window, press the Zoom Area toolbar button (orcad8.gif) and drag a small rectangle in the lower left corner of the FFT graph. The graph now displays a magnified view of the selected area. Continue zooming in until the harmonic spikes at 2KHz, 3KHz, etc. are visible.
  8. Harmonic spikes should exist for the output waveform only. The input is an ideal sine wave generator and has no distortion. To calculate total harmonic distortion, add up the spike values (voltages) at frequencies above 1KHz and divide by the voltage at 1KHz (the fundamental).

Note: simulations only approximate the performance of a circuit. The actual performance may vary considerably from the simulation as determined by a number of factors, including the accuracy of the component models, and layout and construction techniques.

c. 2006 Alex CavalliMark Lovell and Bill Pasculle (remove _nospam_).