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Class
A Headphone Amplifier

Class A Headphone Amplifier

This
class A headphone amplifier can output up to 0.5W into a
32-ohm headphones. I built this headphone amplifier for
dynamic headphones based on my rules of proper audio
design. People who know my designs will realize that
this headphone amplifier is much more than just a
headphone amp. It is a pure class A design containing a
new never-before-seen servo loop that is not part of the
audio signal chain in any way. The sound of this class A
headphone amplifier is just amazing.

Capacitors in the audio signal path are BAD. Even
the best silver-mica or poly caps exhibit
non-linearities at low voltage levels. Capacitors belong
in power supply sections and nowhere else. Capacitors
used to compensate an amplifer generally mean that the
amplifier is otherwise unstable, with poles in the right
half plane and is therefore a bad design.

Transformers in the audio signal path are even worse:
non-linearities in the gain structure, parasitic
capacitance between windings, impedance problems....
Transformers belong in linear power supplies and nowhere
else.

Ultra high open loop gain: REAL, REAL
BAD!!! That basically means anything with an opamp in
it. Opamp circuits with open loop gains of 10,000 or
more require large amounts of feedback to make them
usable. While this reduces THD, the intermodulation
products, and especially the transient intermodulation
products are much higher than they should be.

Servo loops MUST NOT be in the audio feedback loop.
This rule is also very important. Two of my favorite
high-end audio electronics manufacturers put servo loops
into the minus input of their amplifiers. Most other
manufacturers that use servo loops do the same thing.
opamps used for servo loops do not have an output
impedance low enough to make them suitable for this
purpose. Furthermore the dynamic output impedance of
opamps adds non-linearities to the audio when put in
series with the gain resistor on the minus input.

Kind of makes designing ultra high quality audio
stuff tough. My design goals in this amplifier were:
keep the gain per stage down, keep all stages in class
A, keep the differential front end from coming even
close to clipping by the use of a current source.
Because the amplifier has a low overall gain and little
feedback, a servo helps to prevent DC voltages at the
output. In general, if the open loop gain is kept down
to eliminate all or most of transient intermodulation
distortion, then the amplifier circuitry has to be
extremely linear and low distortion. Otherwise, you end
up with something that measures and performs like crap.

The Circuit

The schematic for the
standard (non-bridged output) version of the headphone
amplifier is shown in figure 1. The open loop gain of
the amplifer is about 35. Even with the feedback
removed, the THD is less than .01%. That's important
because the more linear an amplifier is without
feedback, the more the THD, IM and TIM distortions are
reduced to unmeasurable levels with feedback added.

Stage 1 is a dual FET fully-differential
fully-balanced front-end. The idling current is 2mA
total per dual FET (1mA per FET) and 4mA for the
complete front-end stage which consists of both dual
FETs. The dual FETs generate the bias which runs the
second stage, and keeps it and the resulting output
section in class A at all times. The FETs are ultra low
noise dual units specifically designed for audio uses.
The total voltage gain of the first stage is 50.

Stage 2 is the driver stage. It is a standard
class A voltage amplifier - in this case used as a
voltage shifter. The voltage gain is 0.5 and the idling
current 4.3mA.

The push-pull class A output
stage is a series of paralleled emitter follower,
current buffers. The voltage gain is 0.9 and the current
gain is 75. The idling current is 15mA per transistor
(or 60mA for the 8 transistors off the +16VDC rail and
60mA for the 8 transistors off the -16VDC rail). I have
designed the output section to run at what I have
determined is the sweet spot for these transistors,
which is 15mA each. Yeah, it gets hot; its supposed to
get hot (but not hot enough to require heatsinks). It's
not possible to make an amplifier with an output
impedance less than 0.1 ohm without throwing around a
fair amount of current.

The servo circuit is
new: most of the servo designs (Mark Levinson and Krell,
for example) put the output of the DC servo back into
the - leg of the amplifier. I just do not like this.
That puts the noise and non-linearities of the opamp
inside the audio loop.

My servo feeds back to
the current sources for the dual FETs in stage 1. Like
all servos, it is an integrator. Due to the large
(relatively) integration capacitor and the 1 meg
resistor, the frequency of this filter is 0.05 Hz. With
even a decent opamp, the servo's noise is in the tens of
microvolts, and does not affect the operation of the
current sources significantly.

The servo opamp
in this amplifier measures the DC at the output, if any,
integrates it and applies it to the midpoint of the two
LEDs. The LEDs do have a slight change in voltage with
respect to current, about 3 or 4%, and that is enough to
make the servo work. Notice that if the transistors or
the resistors are very poorly matched, the servo will
not work because its total control range is at most 10%.
Most standard servos (such as the Mark Levinson or Krell
servos) have a much wider range.

For high
impedance headphones, a little DC will not hurt the
phones. With the low impedance Grados, even 0.1VDC over
a long period of time will definitely damage and/or
change the sound. If all the parts are hand matched, the
power supplies are exactly the same and all the
resistors are really good quality, the amp should be
stable and should not drift. In that case, the servo
could be omitted or replaced with a 20K trimmer pot
wired from +16VDC to -16VDC, with the wiper going to the
DC adjust pin. The prototype uses 0.05% tolerance
resistors, and I hand-matched the transistors. The
output DC is less than 6mV and has stayed absolutely
stable for the few months I have had the unit.

Schematic for - output for bridged version of
amplifier.
Figure 2

The amp gives about 0.5W
into 32 ohms. In the classical definition of class A,
the top transistors would be sourcing 120mA and the
bottom transistors sinking 0mA. However the two 3k
resistors in the second stage actually prevent the total
shutdown of either transistor bank by keeping the
opposing stage at an absolute minimum of 5mA. In any
case, 0.5W into Grados is unbelieveably loud.

The balanced bridge output version of the
amplifier (figure 2) is for those headphones that can be
wired as dual mono (see the addendum for instructions on
converting a pair of standard Grado SR-80 headphones
into dual mono headphones). It has twice the voltage
swing, twice the slew rate and 4 times the output power
(competes with the $2600 HeadRoom balanced Max
amplifier).

Schematic of power supply.

The ultra-regulation of the power supply (figure
3) is so over the top and unnecessary that most, if not
all, people building this amplifier would not even
notice the difference. However there are a number of
benefits to this design. First its a dual tracking
design. Because the open loop gain of the amp is low,
its common mode rejection due to the power supply is not
great. However if both the + and - voltage rails move up
and down the same amount, there is no bias drift.

Because of the pre-regulators, the total
line/load variations are under 0.0001%. The fast
capactors allow the power supply to react rapidly and in
a controlled manner with highly reactive loads like the
Grados. The opamp outputs are active in both directions;
they can push or pull to keep the power supply at
exactly the right point. The power supply design was an
attempt to come up with the absolutely best power supply
I could. It is even quieter than batteries.

Construction

The prototype was built
with point to point wiring, keeping it tight and tiny.
The layout is exactly as shown on the schematic, which
is also why the board is only one layer. Although the
amplifier is easy to build without a PC board, I
designed one for the amplifier. Each board (you need 2
for the standard amplifier or 4 for the bridged version)
is 3.3" x 3.8". In a few months, I may redo the board in
one of the systems where I can ship the file over the
internet and get boards back (like expresspcb.com).

The servo's total control range is at most 10%,
so the some of the servo-related parts must be closely
matched for optimal operation of the servo. The 500 ohm
resistors, 1.6V LEDs, bias transistors and second stage
transistors must be matched to within 0.5% or even 0.25%
for optimal operation of the servo (the dual FETs are
already matched). To match the LEDs, put one in series
with a 10k resistor, hook it to 15VDC and measure the
voltage across it. Do it to a few of them and pick the
closest match.

Circuits for matching the NPN and
PNP transistors.

I used a Tektronix curve tracer
to match the transistors. Figure 4 shows two circuits
for matching the beta (gain) of the NPN and PNP
transistors using just a voltmeter. Simply measure (and
match) the voltage of each transistor's collector with
respect to ground. It should be in the range of 10V to
11V for the NPNs or 5V to 6V for the PNPs.

There
are no substitutes for the FETs. The PNP and NPN
transistors in the prototype were the MPS8099 and
MPS8599. Onsemi has discontinued them, but there are
still lots available. I am buying all my transistors now
over the web from MCM Electronics. I have had too much
trouble with everyone else. The 2SA1015 PNPs are $0.46
each; the 2SC1815 NPNs are $0.41 each. The 2SJ109
p-channel dual FET is about $6.50; its n-channel
counterpart is $5.90. By the time you add it all up,
it's still under the minimum MCM order, so you've got to
buy something else.

Stuffed PC board.

The high-speed 5uF capacitors in the power
supply are not critical. They satisfy the "lunatic
fringe" in audio. These capacitors are rated for a slew
rate (dv/dt) of about 4 times a standard capacitor. They
cost $7.50 from the Illinois Capacitor Company.
Similarly, the opamps in the power supply are not
critical. In the prototype power supply, I used the Apex
PA09, which has a 400V/uS slew rate and costs $167 each
(remember, I am crazy). Normal people should use the
Texas Instruments OPA549 (10V/uS) that cost $11 each.
The LM3xx regulators are heatsinked (dissipating at
least 3W each, 6W for the bridged version of the
amplifier).

The enclosure is the Mod.U.Line by
Precision Fabrication Technologies Inc. (part number
03-1209-BW), available from Newark Electronics. They are
about $85 a piece these days. The aluminum enclosure is
easy of use, easy to punch, and keeps a decent paint
job. The headphone jack is a cheap Radio Shack one.
Although it works fine. I have since gotten some Neutrik
connectors which I will put in at some point. In theory,
there is a ground loop between the RCA jacks on the back
plate and the headphone jack on the front plate.
Although the 60 Hz hum is about 110 db down, the Neutrik
jacks are isolated and will make this problem go away.

Interior of amplifier chassis.

The
amplifier's output is voltage limited (not current
limited) because the second stage runs out of voltage
swing - typically about 6V rms. For a 32 ohm load, the
maximum output power is 1.125W (0.1875A). For a 300 ohm
load such as the Sennheiser HD600, the maximum output
power is 0.125W (0.02A). To increase the maximum output
voltage to, say, 10V rms, try increasing the power
supply to ±20VDC and change the 500 ohm resistors in the
current sources/sinks to 600 ohms (or ±24VDC and 700
ohms - but careful not to fry the output transistors).

The bridged version will output 4.5W into the
32-ohm Grados and 0.5W into the 300-ohm Sennheiser
HD600. At full blast, the amp does drop out of class A,
but at levels that will fry your ears anyway.

The Result

Adjustments: In the power
supply, adjust the top 20K trimmer pot for +24 volts at
the tap after the LM317. Then adjust the bottom 20K
trimmer pot for -24V at the tap after LM337. (Note: the
±48V taps are shown for reference only; they are not
actually used by the amplifier.) If the servo has been
replaced with a 20K trimmer pot, adjust the trimmer so
that the output measures 0VDC. Anything under 25mV is
fine.

I listened to the balanced HeadRoom Max at
The Home Electronics Show. The low frequency bass slam
is absent from the unit. When I listened to the
BlockHead ($3333) and compared it to my
unbalanced/balanced amp, the same thing happened.
Without the kaboom, it's just not as much fun to listen
to.

This amp gives the Grados and Etymotic
Canalphones a fuller and much more upfront sound than
the built-in headphone jacks on various players - the
bass has a snap to it that it never used to have. And
the image moves from around your head to the center of
your nose. The Etymotics tend to sound kind of thin and
distant with other amps. In general, I have to say that
this amp produces bass that is much more solid - similar
to putting the microphone in front of a bass violin
instead of inside it.

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Class A Headphone Amplifier - Link

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