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Old 18th September 2005, 08:33 PM   #1
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Question Small value caps (newbie alert)

So, having done a repair job on my old Cyrus I am interested in getting a bit more understanding of how it works. Looking at the schematic here I can understand mostly what's happening. One thing I struggle with though is the function of the various small value capacitors.

The large value caps all make sense, but the small caps like C33, C35, C41, C47/C49 and most of all C1 don't seem to do anything useful at audio frequencies. Can someone enlighten me as to what purpose they serve?
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Old 18th September 2005, 08:51 PM   #2
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C33 is for radio frequency input filtering. Very usefull unless you want to pick up AM radio station. It also prevents the amp from being driven to its slew limit by too high frequency components for example from dirty DAC.
C35/47/49 make a frequency compensation. They prevent the amp from oscilation caused by total 180 degrees phase shift at 100s of kHz/ several MHz. 180 deg. of phase shift is the same as invertion for a sine, so negative feedback becomes positive. Frequency compensation is a filter within feedback loop, which makes the frequency of 180deg. phase shift to have less than unity gain.
If you want to learn more the keywords are: frequency compensation, bode plots, nyquist stability criterion, dominant pole.

C41 is unusual, most designers would leave this 'cascode' without it. This is for preventing the cascode connection from local ocsillations/instabilities at many, many MHz. This sometimes occurs, for reasons I mentioned about a month ago in a thread about casodes.

However you are right that these caps don't do anything useful at AUDIO frequencies themselves.

regards
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Old 18th September 2005, 09:26 PM   #3
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Thanks. Very helpful.

It's been more than 10 years since I last did any control systems so all the stuff about bode plots is a dim memory! Still I can see that the -ve feeback would turn in to +ve feedback at high enough frequencies.

What about C1 on the output?
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Old 18th September 2005, 10:12 PM   #4
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So-called Zobel filter, also for stability, at some topologies at least.
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