headphones, for me, are terribly uncomfortable, but my amp sure makes
them sound nice. (The ones in the picture are the SR225; I also have a
pair of HP-1.) They are the only low impedance headphones out there
that I know of. Most if not all of the standard opamp-only headphone
amps have trouble driving Grados with any decent bass, because they
want a high current output. This amplifier can output up to 0.5W into a
32-ohm load - which is unbelievably loud for the Grados.
this headphone amplifier for dynamic headphones based on my rules of
proper audio design. People who know my designs will realize that this
amplifer 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. I should patent it, but in any
case, it is "copyright 2001 Kevin Gilmore."
Kevin's Rules of Proper Audio Design
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
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
- 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
- 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.
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.
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.
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.
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.
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.
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.
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
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.
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).
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.
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.
Download high resolution images of PCB pattern and layout
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
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.
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.
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).
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
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).
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.
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.
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
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