Respectable THD levels for double EF Amp


2008-03-21 10:13 pm

Would like some help. Have been designing an amp using Bob Cordells book and have finally got somewhere after many hours. It is a double EF output with darlington VAS and darlington Vbe multilier. It passes the phase and gain margin tests in Bob's book.

Have been doing some THD simulation and get the following results.

All tests have been into a 4 ohm load at +/- 20V output.

1Khz sine wave = 0.02%
10Khz sine wave = 0.8%
20Khz sine wave = 0.8%

Unloaded 1Khz sine wave = 0.0001%

I'm thinking I may have a slew rate issue.

Are these anywhere near respectable?

The only thing is that the the odd harmonics dominate.



2008-03-21 10:13 pm
Turned the compression off and increased the resolution of the simulation and now have the following.

0.0095% @ 20Khz +/-20V output into 2 ohms.

Done some square wave simulations no overshoot or ringing but the slew rate looks very slow. What is a good slew rate to aim for? Or am I missing something in LTspice?
one reasonable estimate of the required slew rate would be that which would allow a 20 kHz full power sine wave without strain. Assume 20 volt peaks, based on your +/-20 Volt rails, and that gives you 20,000*20*2*pi=2.5 Volts/us.

Now, that would leave no margin...e.g. the amp would be at the edge of slew rate limiting, which isn't much safety margin? 4x would say 10 v/us, 10x would say 25 v/us.

Perhaps a reasonable to extreme test would be to use an intermod the amp to a bit below full power output with a 20 kHz sine wave, and use some of the rest of the swing with a simultaneous 1 kHz sine wave, then look for the intermod sum and difference products in the FFT.

Update My Dynaco
Akitika GT-101 Audio Power Amplifier Kit


2008-03-21 10:13 pm
Please go easy on this...

djoffe, sound reasonable. Just trying to understand the test you suggested.

Elvee, here is the schematic, and a couple of figures...

There is some overshoot on the square wave. These appeared after removing the ideal current source.

The square wave is offset. After a number of cycles it centres itself. Is this slew rate again?

Please bear in mind this is my first attempt at designing an amp. Have been using the late Professor Leaches site and Bob Cordell's book only. Not trying to do anything ground breaking or flash. Just aiming for a solid design.

Thank you for the help. :)


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Last edited:
It looks like you have plenty of slew rate...It's hard to tell but it looks to be between 20 and 40 v/us.

The intermod test works like this. A high slew rate signal tends to max out the first stage current, pushing all the tail current into one or the other input transistor. If it does that to the extreme, it changes the gain of the amp, that even with feedback present, shows up as intermod distortion.

Given the slew rate that you showed, I don't think you'll have a problem.

I will say, that the schematic image was a bit hard to see.

Akitika GT-101 Audio Power Amplifier Kit
Update My Dynaco


2006-09-08 2:04 pm
There is some overshoot on the square wave. These appeared after removing the ideal current source.
That's a minor issue. It could be eliminated by fine-tuning a small cap across the main FB resistor

The square wave is offset. After a number of cycles it centres itself. Is this slew rate again?
No, that is caused by your servo running for ever in the pre-sim period with the square wave high or low.
Simplest fix is to disable the servo for this kind of test, it has little influence anyway.
At first sight, the SR is something like 20V/µs. Not huge, but sufficient.

If you want to improve it you have to determine through which capacitances the current flows during the transitions.
You can then try to reduce the capacitances in question, or increase their charging currents.


2008-03-21 10:13 pm
Intermodulation fft and hopefully larger schematic

Thank you for your replies. :)

Have tried to make schematic easier to read. These are the first images I've posted on this forum so need to learn.

djoffe, have tried to implement your suggested test and have attached the fft produced. Not sure how to interpret though.

Elvee, have tried disabling the servo and the result was as you said would happen. Going to try and improve slew rate by looking at the capacitor currents. "Gut feel" tells me stability may be affected by this though.

One thing I have noticed is that it clips very hard and sticks badly. Think maybe adding baker clamps would be a good idea.



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2008-03-21 10:13 pm
No free lunches. Just been trying to increase the slew rate and all I get is a reduction in stability and no increase in slew rate.

Thank you, djoffe, good to know as I can't find the limiting factor regarding slew rate (yet).

My latest little play has been implementing basic baker clamps. There is still sticking at the rails. The dip after sticking is about 3v. Both halves of the sine wave are identical.

More research and experimentation required. :D


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Mark Johnson

Paid Member
2011-05-27 3:27 pm
Silicon Valley
Are R23-C9 drawn as intended? Perhaps their purpose is to introduce an LPF with corner frequency around 8 MHz, to reduce RF interference?

I also wonder whether it may be a false economy to omit emitter resistors from Q31, Q32? If their VBE mismatch is ± 5 millivolts, is the resulting bias current acceptable to you?

Finally, isn't it more common to specify 3A or even 5A parts for catch diodes D11 and D12?
Not trying to do anything ground breaking or flash. Just aiming for a solid design.
It looks hellish complicated for such a modest goal. The very thought of soldering so many components offends my innate sense of laziness.:D

BTW, shouldn't the top of C2 and the bottom of C3 be connected to the positive supply and the negative supply rail respectively instead of ground? That should help the PSRR a bit. (These are the capacitors near the inputs of the two halves of the VAS).


2008-03-21 10:13 pm
Update on development

Update time... :)

Have taken note of the suggestions. Tried each to see the effectiveness.

Have made the following changes:

- The clipping problem was solved using feedback baker clamps as
detailed on Bob's book.

- Changed the design to a triple and changed the VAS and Pre driver

- Increased the gain slightly.

- Added emitter degeneration to the current source.

- Changed amp input network.

This has brought the thd figure for near full power @ 20Khz into 2 ohms down to 0.005%. Still appears to have adequate phase and gain margins. The slew rate has increased to 120v/uS.

Should I implement some sort of current limiting? May do this at the power supply though.

What do you guys think?

Am I mad??? :D

Godfrey - It's not the soldering that worries me its the pcb layout. And whether I can get it to work in reality.




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