Respectable THD levels for double EF Amp - diyAudio
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Old 23rd March 2013, 07:54 PM   #1
mcd99uk is offline mcd99uk  United Kingdom
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Default Respectable THD levels for double EF Amp

Hello,

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.

Paul
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Old 23rd March 2013, 08:02 PM   #2
Bensen is offline Bensen  Belgium
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Something must be wrong there! Your THD numbers divided by 100 are normal numbers for SS amplifiers.

Greetz
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Old 23rd March 2013, 08:08 PM   #3
mcd99uk is offline mcd99uk  United Kingdom
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Thank you. Did think they were a bit high.
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Old 23rd March 2013, 11:58 PM   #4
jaycee is offline jaycee  United Kingdom
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if youre using ltspice dont forget
.options plotwinsize=0
.options noopiter

or the compression will cause artificially high THD
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Old 24th March 2013, 12:45 PM   #5
mcd99uk is offline mcd99uk  United Kingdom
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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?
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Old 24th March 2013, 02:13 PM   #6
djoffe is offline djoffe  United States
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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 good...so...how 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 test...drive 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.


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Old 24th March 2013, 02:14 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by mcd99uk View Post
the slew rate looks very slow.
Figure?
Schematic?
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Old 24th March 2013, 04:04 PM   #8
mcd99uk is offline mcd99uk  United Kingdom
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Default 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.
Attached Images
File Type: jpg Amp2 20Khz Sine FFT.jpg (89.9 KB, 282 views)
File Type: jpg Amp2 square wave.jpg (86.4 KB, 281 views)
File Type: jpg Amp2.jpg (243.8 KB, 280 views)

Last edited by mcd99uk; 24th March 2013 at 04:11 PM. Reason: Found mistake on schematic
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Old 24th March 2013, 05:10 PM   #9
djoffe is offline djoffe  United States
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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.


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Old 24th March 2013, 05:23 PM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by mcd99uk View Post
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

Quote:
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.
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