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Old 15th July 2012, 04:47 AM   #1
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Default Class A preamp: please critique my circuit and give me helpful suggestions.

Hi everybody, I designed this preamp using principles I learned from Self's Audio Power Amplifier Design Handbook 5th edition, assuming the same principles will translate to small signal amplifiers also.

I've bread-boarded the design and it seems to work well, there appears to be a smidgen of high frequency instability but that could be due to the fact that:

a) it's on a breadboard
b) I tested it using a portable oscilloscope (1MHz). (I'm potentially getting a good benchtop one next week)

Any suggestions for things I should do differently or component values I should change will be taken on board, tested and reported back on.

I appreciate any help offered.

Included are:
zipped CircuitMaker file
Simulated 1kHz FFT
Simulated 50kHz FFT
Simulated 10kHz pulse/square wave
Attached Images
File Type: jpg CCT.jpg (134.2 KB, 1020 views)
File Type: jpg 1kHz FFT.jpg (90.0 KB, 979 views)
File Type: jpg 50kHz FFT.jpg (89.3 KB, 761 views)
File Type: jpg 10kHz pulse.jpg (123.8 KB, 709 views)
Attached Files
File Type: zip Class A (3.9 KB, 67 views)
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Old 15th July 2012, 06:59 AM   #2
Calvin is offline Calvin  Germany
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basically the circuit looks ok. Just minor issues. I would always add a lowpass filter at the input, the base of Q2, a 1k/220pF could be a start.
Probabely rather a matter of personal taste, IŽd make the coupling caps smaller and raisen the resistor values instead (if thereŽs no noise penalty to pay or similar). C7 could probabely be reduced to 1”F film cap if R24 raises to 47k.
If using global feedback circuits IŽd waste a thought to exploit the principle even further. Means, that IŽd think of replacing R3 and R4 by a current mirror.
This would give more immunity against powersupply disturbations (PSRR), double the available current to charge/discharge the compensation-cap C5 (ŽspeedŽ) and more symmetry and lower distortion of the input difference amplifier. And/Or You could reduce the bias current of the difference amp, resulting in lower noise (the BC556 has a noise optimum around 200-300”A).
I assume that Q9 doesnŽt improve things here. Q10 has a quite high hfe (use a higher hfe class like the B- or C-types) . So it hardly loads the difference amp down, especially not if a current mirror would be used instead of R3,4.
If heat dissipation allows for IŽd also increase Q7/Q10s bias current a bit ap to maybe 10mA.
C4 might probabely be omitted with. The stage should work stable without C4. If it does, then C4 is of no positive use.
Was there a special reason to use a ccs-loaded (Q8) SE output stage (Q11) instead of a complementary pushpull stage? You know that the maximum signal current into the load is defined and limited by the class-A bias of Q11. A complementary PP (Q8 2N2222, Q11 2N2907) stage would allow for twice the Q11s bias current. This is hardly a issue if the load impedance remains high enough, but You never know what comes next.
IŽalso rather use TO220 devices for the output stage for easy mounting of a cooling fin or to the casing and to allow for higher bias currents/dissipation.
The MJE340/350, 2SD669/2SB649 or KSC2690/KSA1220 are fine candidates.
Any amp profits from clean stable supply lines. The values of C9 and C12 are quite small. IŽd suggest at minimum 100”F.


ps: Sonically wise You also might think of circuit wo gloabl feedback
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Old 15th July 2012, 07:42 AM   #3
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Hi Calvin, thanks for the response! There's a veritable treasure trove of helpful information here. I'm just trying a few of your tips out now and I'll report back later. Thanks again
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Old 15th July 2012, 10:46 AM   #4
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Ok. time to report back in. 1st off I should point out that the transistors in the circuit are indeed bc546C and bc556C, the labels in the schematic above were purely due to me being limited to what the software has. After checking, it seems that it has BC547C & BC557C models in it. These will be used for further simulations.

Changes made so far:

Input stage:

Filters - The input high pass filter has been changed to include a 1uF film & 47k i/p impedance. this was actually my original intention and what I had used in the past for opamps but the book suggested the other values so I figured I'd try it. A low pass filter comprised of a 1k//220pF has been added.

Current Mirror - After an initial failure trying to make a current mirror work (gave me bad bad bad distortion) I figured it might be a good idea to beta match the transistors first - after this the current mirror worked perfectly.

VAS Stage:
Q7/Q8 Bias - upon measuring it, it seems that it's already 11.6mA. I'm not sure why the simulation is different.

C4 has been removed, no observable difference. This can't be a bad thing.

Output Stage:
I'm not sure why I went for a SE output, it just seemed right at the time.

I'll test a push pull output stage tomorrow after work and report the results, but for now it's bed time.

I don't have at hand any of the t0-220 transistors you've listed. For testing purposes will BD139, BD140 suffice?

Danke mein deutschen Freund
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Old 15th July 2012, 11:21 AM   #5
Calvin is offline Calvin  Germany
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hereŽs a LTSpice sim of two versions.
Note the differences in input-LP-filter, current mirror, omittance of a transistor and dominant pole compensation cap in the driver stage (RC-filter at the collectors of input stage transistors instead), different output transistors with increased Bias current of 50mA and the added cap in the feedback leg to gnd for a dc gain of 1 (less output offset).
Click the image to open in full size.


ps. BD139/140 should work fine in a first test.
If You can handle LTSpice I could send You the sim-file

Last edited by Calvin; 15th July 2012 at 11:29 AM.
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Old 16th July 2012, 09:57 AM   #6
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Hi again

So I've attempted to make a a push pull amp using bd139, bd140 transistors and I'm failing horribly it seems. Here's the circuit I've come up with and all that seems to happen is a slightly triangular wave is produced at first followed by a distorted one when the transistors warm up (which are mounted on a heatsink).

Also, I have LTSpice and I would love for you to send me the sim file of the circuits you produced.
Attached Images
File Type: jpg Push Pull Mk2.jpg (131.1 KB, 398 views)
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Old 16th July 2012, 10:36 AM   #7
DUG is offline DUG  Canada
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shouldn't you have something connecting the bases of Q8 & Q11...a couple of diodes or something?
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
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Old 16th July 2012, 11:24 AM   #8
Mooly is offline Mooly  United Kingdom
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Originally Posted by DUG View Post
shouldn't you have something connecting the bases of Q8 & Q11...a couple of diodes or something?


What kind of load is this preamp going to drive ? You show a 47K load which can be driven via a single ended output rather than a push pull and I'm sure it would sound better.

But don't let anything put you off experimenting

How about equalising the input currents to the LTP ?
Do you really want the feedback return DC coupled ?
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Old 16th July 2012, 12:47 PM   #9
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Do you really need 4dB of gain? Most DAC's put out enough voltage to clip a power amp, so either a DAC needs a volume control, or a preamp with volume control should have unity gain. If you're using analog sources, then yeah I guess you'd need some gain in a preamp.

I'd recommend replacing the output stage with a diamond buffer. You connect it directly to your VAS.
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Old 16th July 2012, 12:53 PM   #10
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Also of concern is the signal applied to the base of Q7. It looks to me like there isn't any music signal being applied there. It appears that a small signal is being applied to Q7 due to common-mode error from the currrent-source feeding the input diff. pair.
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