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.
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.
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
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
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
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
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.
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.
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.
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.
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%.
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.
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.
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.
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.
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).
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.
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
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
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.
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.
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.
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:
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
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.
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".
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.
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.
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.]