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Old 8th July 2003, 05:34 AM   #1
PGW is offline PGW  United States
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Default Class A headphone amplifier

As my power amp has no headphone outlet I decided to build a little Class A discrete headphone amplifier, just to see (hear) what it would sound like.

The circuit design is attached. It's quite rudimentary, based on classic op-amp topology: mildly degenerated differential input with current mirror loading; cascode voltage gain stage, and emitter follower output biased into enough class A for driving headphones. The input diff amp current source is controlled by a green LED - I read (about twenty years ago) that green LEDs sound better than red ones. The small signal transistors are Toshiba 2SA1015/2SC2547 complementaries which I had lying around. The output transistors are BD139/140 running at 100 mA bias, so dissipating about 2W apiece. This is OK for driving hi-Z 'phones to very loud - I have a pair of Sennheiser HD600 which are 300 ohms load - without leaving Class A. The supplies are regulated with LM317/337 at 20V. No safety circuitry employed at this low power.

The amp. has open loop gain of about 80 with -3dB at about 40 kHz so I used dominant pole compensation and feeback for stability. The domininant pole cap I used is 1000 pF, with 47 pF across the feedback resistor of 22k for a gain of about 19 and a phase margin of 40 degrees according to the simulator. I used dominant pole compensation for its simplicity, I haven't tried any lead-lag compensation methods.

I built this on some cheap commercial prototyping board, which I would probably not do again as it was a pain getting all the connections sorted out. The grounds were tedious too: I made a few errors which were identifiable on test. The layout is quite compact, though. Most of the components I had laying around and are not especially 'hi-fi'. Resistors are regular metal-film, caps a mixture of electrolytic and ceramic, with mica and polystrene in feedback and signal paths.

Simulated performance looks OK: bandwidth to about 70 kHz, harmonics below 100 dBc, intermods at around -98 dBc. Sounds fine to me.

The circuit could probably be 'beefed up' to give a higher output power: the VAS could be uprated, or the output followers made into 2 stages: you could use a BD135/136 follower pair to drive the 139/140. In fact the BD135/136 were my original choice of output transistors, but I couldn't get them anywhere so used the 139/140 combo. Being lazy, you could probably use the BD139/140 biased at 100 mA to drive another BD139/140 biased at higher collector current, though I haven't tried this out.


-- John
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Old 8th July 2003, 05:35 AM   #2
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Default Here's the Bode plot

Here's the closed loop frequency response
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Old 8th July 2003, 05:36 AM   #3
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Default and the harmonic distortion

Simulated harmonic performance
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Old 8th July 2003, 05:37 AM   #4
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Default and the intermods

simulated IMD performance with 7kHz and 11 kHz fundamentals
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Old 8th July 2003, 05:57 AM   #5
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Default and finally, what it looks like

here's a picture of the amplifier. Sorry the picture's so small, I'm not such a wiz with the digital camera (borrowed).

The toroid is a 120VA 20-0-20 I had lying around and is far too big for this job. Now that I know the amp sounds OK I suppose I should rebuild it properly.

If anyone wants to lay out a PCB, be my guest.


-- John
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Old 8th July 2003, 07:06 AM   #6
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Default Simulation software.

Hi John,
That's quite a bit of work you did. Hope the circuit sounds fine.
Did you consider using a power boosted opamp ? Some Opamps could possibly drive the 300 ohm phones to ear bleeding levels even without a power bosting output stage.

What software did you use for your simulation? It looks interesting.
Cheers.
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Old 8th July 2003, 07:26 AM   #7
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John, have you tested to connect the compensation capacitor directly to signal ground instead? Maybe you get a little bit smaller load of the current mirror of the input staget?

If you have an OK sound card in your PC you can test the distortion in real life. Check out the russian freeware for this
http://audio.rightmark.org/
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Old 9th July 2003, 02:29 AM   #8
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Hi Ashok,

I did consider an op-amp based circuit - there are several on the 'Headwize' web-site that look very good, and an elegant one in a recent issue of Audioexpress. I just felt like trying to design a transistor amp, and see what it turned out like.

It sounds real enough to me, but I haven't compared other headphone amps with my cans, so the things I appreciate like detail, reasonable freedom from coloration and a lifelike (lively) sound may be due to the headhones themselves. The sound is similar in tone and tune to my amp/speakers (Linn LK100/Keilidh) but with more detail.

The software is Agilent-EEsof 'Advanced Design System', which for me is freeware (otherwise $25k/year/license). It is an RF/microwave design software, and includes a nonlinear frequency-domain simulator called harmonic balance: this takes all the harmonics and intermod products present in the circuit and 'balances' the contributions of each frequency in the total signal, so it's great for distortion predictions. Surprisingly, it comes with all these libraries of low frequency transistors - the BD139/140 and the Toshiba txs were already in the library, so I just plugged them in and watched the result.

I didn't simulate the regulators, so the power supplies are ideal (zero ohms DC to daylight) and in reality will have an influence on the sound, though using a Calss A circuit should ameliorate practical non-idealities in this respect.

-- John
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Old 9th July 2003, 02:35 AM   #9
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Quote:
have you tested to connect the compensation capacitor directly to signal ground instead? Maybe you get a little bit smaller load of the current mirror of the input staget?
An interesting suggestion, P-A. I guess I was thinking the C_dom acted like a Miller cap around the voltage amp stage, augmenting the Cbc of the transistor and hence providing the dominant roll-off in the amplifier.

I'll check your suggestion out in simulator-land: I don't have a PC sound card, but it seems like that should be my next audio purchase (darn, I was hoping for a new cartridge).

-- John
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Old 9th July 2003, 03:41 PM   #10
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Classic design, classic arguments; since the topology is basically the same as most power amps many threads elsewhere in solid state cover many of the design choices

Some quick “armchair” engineering comments (which will probably all have vociferous opponents)

BD139/140 seem to be “don’t ask, don’t tell” with regard to speed, if you really have only 40 degrees phase margin with such a heavy dominant pole compensation then they must be < 10 MHz Ft – I would look for 50-100MHz Ft output devices which are readily available at your power/voltage level (MJE171/181)

Faster output devices should let you reduce Ccomp, which with your input stage current, gives a frighteningly low slew rate/power bandwidth limit – this slew rate limit also could be helped somewhat by increasing diff pair bias current (more than ~2X requires reconsidering current noise and input bias)

Ed Cherry continues to advocate moving Ccomp to the output, enclosing the output transistors in the VAS/Ccomp feedback loop; easy to try in sim, may have problems with separately mounted power transistor wiring parasitics

Slew rate/power bandwidth may be helped by 2-pole compensation, your low dominant pole frequency (from the slow output devices) doesn’t leave much room but I think a 2nd pole @~100-200KHz could help – reducing Ccomp or using 2 pole compensation also increases loop gain, reducing distortion

Following Cherry and Self’s analysis, i guess the output stage is still probably limiting gain/linearity due to the nonlinear load on the VAS; an intermediate buffer stage with AC bootstrapping should hugely reduce VAS loading/increase linearity – many power amps use “triple” Darlington/Sziklai ouputs but 2 stages should be enough for a 300 Ohm load

Baxendall’s Super Pair trick lets you have VAS input buffering and cascode output linearity with the same # of components as your cascode – if you add a buffer between the output stage and the VAS then you could consider using the super pair on the VAS current source as well (my reply in: Amplifier Rail Detector - Protection Circuit )

Your sim data looks too good to be true with the high Ccomp’s low selw rate limit, what is the reference level for your dB plots? What does a 40 KHz 30 Vpp out waveform look like? (yes you don’t want to listen to it but it should show that your slew limit is too low)

I am in favor of filtering but a single pole @70KHz is arguably audible with ~0.1 dB droop @7KHz, increasing above; I believe DBT advocates claim 0.1 dB level differences over an octave or more are detectable
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