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Current-Domain Electrostatic Amplifier for Stax Omega II Headphones

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DIY Headphone Amplifier With Natural Crossfeed

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Headphone Monitoring Switch

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Low-Voltage Class-A Tube Headphone Amplifier

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Passive Preamp/Switchbox for the Desktop

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Portable DAC with Headphone Amplifier

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Simple Tube/Opamp Hybrid Amplifier

A
Single-Ended OTL Amplifier for Dynamic Headphones

A
Soundfield Simulator for Stereo Headphones

A
SRPP-Input Amplifier For Headphones and Loudspeakers by Tony Frazer

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Tube Headphone Amplifier/Preamp with Relay-Based Input and Power
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A
Zero-Feedback SRPP-Input Headphone Amplifier

All-Triode
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An
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An
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An
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Blue
Hawaii Hybrid Electrostatic Amplifier for Stax Omega II Headphones

Brute
Force In A Line Stage

Buffered
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Build
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Build
This High Quality Headphone Amplifier

Class A MOSFET Headphone
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Creating
a 2nd Headphone Feed When Only One Is Available

Designing
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Designing
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Designing
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DIY
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How
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Measuring
the Effective Impedance and Sensitivity of Headphones

Modifying
the Sennheiser HD500 Headphones

No-Compromise
OTL Tube Headphone Amplifier

Not
Just Another Headphone Amp

Notes
on DIY Electrostatic Headphones

Poor
Man’s Surround Headphones

Revisiting
Koss “The Plug” Headphones

SDS
Labs Headphone Amplifier

SDS
Labs Headphone Amplifier

The
6N1P OTL Headphone Amplifier

The
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The Kumisa III Headphone
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The
Lindesberg Portable Headphone Amplifier with Crossfeed

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Morgan Jones Mini Tube Headphone Amplifier

The
Zen Headphone Amplifier

Thoughts
And Processes As Relating To Mixing With Headphones

Tips
on Repairing Electrostatic Headphones

Top-Level
OTL Tube Headphone Amplifier

Using
A Headphone Console As A Balanced Line Distribution Amplifier

A
Pure Class A Dynamic Headphone Amplifier

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50mW Class-A Headphone Amplifier

A JFET-MOSFET Headphone
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An
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Chaotic
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Convolution
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Creation
Manipulation And Playback Of Soundfields

Digital
Dharma of Audio A/D Converters

Dolby
Pro Logic Surround Decoder: Principles Of Operation

High
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In-Ear
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Setting
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Signal
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Sound
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Surround
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Technologies
for Surround-Sound Presentation in Headphones

The
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The
Psychoacoustics of Headphone Listening (HeadRoom White Paper)

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Virtual
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A Pure Class A Dynamic Headphone Amplifier

by
Kevin Gilmore

Grado 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.

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 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

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

Figure
1

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.

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 unbelievably 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).

Figure
3

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

Download high resolution
images of PCB pattern and layout

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.

Figure
4

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.

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.

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.

c. 2001, Kevin Gilmore.

From The Homepage of Kevin
Gilmore. Republished with permission.

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