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Old 19th October 2013, 01:33 PM   #411
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Quote:
Originally Posted by AndrewT View Post
You have a 2channel scope.
Show both the input trace and the output trace.
Ok sr.
Here are in-out scopes.
first group.
Upper waveform is output.
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Last edited by thimios; 19th October 2013 at 01:42 PM.
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Old 19th October 2013, 01:39 PM   #412
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Second group.
Waveforms superimposed.
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Old 19th October 2013, 02:32 PM   #413
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Your generator should have had flat topped square waves ! They aren't so at LF. Should have been a perfect square. Try the generator loaded only with the scope probes at 20 Hz and see if you get a perfect square. If so ,then load it with the amp and see what happens. In both cases scope should be looking at the input signal.You need to get to the bottom of this!

By the way, skip the sine waves. We don't need to see them now. You need to look at only one or two square wave frequencies. Maybe 20 Hz or 50 Hz and 100 Hz. You are only trying to solve the issue of the LF rolloff.
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Last edited by ashok; 19th October 2013 at 02:36 PM.
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Old 19th October 2013, 02:52 PM   #414
AndrewT is offline AndrewT  Scotland
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post412
pics 2 to 4:
are the input and output on top of each other?
In pic 1 is the lower the input (nearly flat)? and the upper the output (LF roll-off)?
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Old 19th October 2013, 03:41 PM   #415
Bigun is offline Bigun  Canada
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Scope probes can distort square waves - they have a resistor/capacitor divider network inside and usually they have a small adjustment screw so you can balance things out. This is why the scope has a square wave generator. It would be best to check the square wave with a simple direct piece of wire from the generator to scope input.
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Old 19th October 2013, 05:44 PM   #416
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Quote:
Originally Posted by AndrewT View Post
post412
pics 2 to 4:
are the input and output on top of each other?
In pic 1 is the lower the input (nearly flat)? and the upper the output (LF roll-off)?
Yes all pictures in post #412 shown input and output on top of each other,except picture 1.
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Old 19th October 2013, 06:09 PM   #417
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Quote:
Originally Posted by Bigun View Post
Scope probes can distort square waves - they have a resistor/capacitor divider network inside and usually they have a small adjustment screw so you can balance things out. This is why the scope has a square wave generator. It would be best to check the square wave with a simple direct piece of wire from the generator to scope input.
Sure but i have adjust this trimer as required.
Scoping direct on generators output(with the same probe) square wave is flat.
This time i have examine many amplifiers.
I can't see any of them with flat square wave in these low frequencies.
Then i take care to increase the value of input capacitor.
22uf is the required value to came 22Hz flat.
The problem is that never i have seen this big value capacitor in commercial amplifiers.

Last edited by thimios; 19th October 2013 at 06:31 PM.
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Old 20th October 2013, 08:11 AM   #418
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I think that this little LF anomaly is direct consequence of bootstraped VAS. It is not unexpected and is not a reason for panic. Read Elliott article about Curren sources, sinks, etc., part about bootstrap ccs. In fact, it could be the reason for good subjective impression that P3A makes.
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Old 20th October 2013, 08:55 AM   #419
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Oops! May be I am wrong. I found Motorola application note AN485 and there is square wave response for 50Hz, 1kHz and 10kHz. 50Hz square wave has the same falling top as thimios's P3A while 1kHz and 10kHz are level. AN485 has active VAS ccs and 10uF input cap, so it seems that falling top line on the square wave amp output is caused by some other mechanism.

Fortunately there will never be any music signal resembling 50Hz square wave.

Last edited by ivanlukic; 20th October 2013 at 09:02 AM. Reason: addition
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Old 20th October 2013, 01:33 PM   #420
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by thimios View Post
Sure but i have adjust this trimer as required.
Scoping direct on generators output(with the same probe) square wave is flat.
This time i have examine many amplifiers.
I can't see any of them with flat square wave in these low frequencies.
Then i take care to increase the value of input capacitor.
22uf is the required value to came 22Hz flat.
The problem is that never i have seen this big value capacitor in commercial amplifiers.
I can't say I've ever tested an amp with a square wave at such low frequencies so I don't know if it is a problem or not. I tend to use the square wave from my scope because it's so clean whereas my variable frequency signal generator is not that great but does let me measure gain as a function of frequency. Once I did use a square wave at 20KHz and it fried the amplifier. I read a bit about the stupidity of what I did and that there may be little value in testing at such high frequencies with square waves so I don't do that anymore. From what you are saying, it sounds like it is not a problem. Does the amplifier sound good - yes? - then there's no problem
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