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

by Kevin Gilmore


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.


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).


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.

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.


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

except that it’s not that regular.

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.


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.

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).

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.


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.



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.


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.


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