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The purpose of my advice

Posted by Al Sekela (A) on June 16, 2004 at 19:12:48

In Reply to: Re: At least three, besides cost. posted by blahblah on June 16, 2004 at 11:00:39:

Sampsa is burning up the Internet with many audio questions. This is good, and he seems to be absorbing knowledge quickly. My answer to his particular bypass question was to get him to think about the possible downsides to installing bypass capacitors. I don't know what equipment this question is for, or how recently it was designed and built. Allowing for the possibility that it is recent, he could actually make it sound worse by following the traditional guidelines for bypassing electrolytic capacitors.

It is not the case that bypassing these capacitors is always a bad idea, as you seem to think. Especially older gear may have large electrolytic capacitors with high impedances at high frequencies, and older, noisy rectifier diodes that generate a healthy RFI level in conjunction with the power transformer. Bypassing these capacitors with appropriate film capacitors can lower the resolution floor.

You do understand that solid-state junction rectifier diodes store charge to support forward conduction, and that this charge is removed in a large and fast reverse current pulse every time the transformer output voltage falls below the capacitor voltage? This is one reason to bypass the electrolytic capacitors, and is why it may not be a good idea to increase the capacity. The RFI is generated by the rectifiers and affects internal and external circuits. HexFRED rectifiers have reduced stored charge and generate less RFI, and snubbers at the rectifiers may also help, but space constraints may prohibit these more direct approaches to the problem.

Another reason (most would say the main reason) to bypass is to reduce cross-talk between gain stages fed by the same capacitor. More elaborate designs include separate regulators or filters to reduce this cross-talk.

If Sampsa's equipment was designed recently and has these issues taken into account, then adding bypass capacitors may not do him much good, and could induce some mechanical resonance problems.

It is correct to call these capacitors 'filters,' because that is what they are. They exist to filter the ac component of the rectifier output waveform. Electrolytic capacitors have finite life in filter applications because the ac (ripple) current that passes through them heats them and drives off the liquid in the electrolyte. Capacitor vendor web sites contain design information to permit calculation of the maximum ripple current to maintain the rated capacitor lifetime.

The audio circuits supplied by the filter capacitors depend on them to be at fixed potentials (i.e. ac short-circuits) over the passband of the circuit and beyond. Your comments about slow time constants are incorrect. These time constants have to do with the design of the ripple reduction, not the need for near-instantaneous response to the audio circuits. As I said above, there will be cross-talk problems if the capacitors are too slow to respond to the audio frequency current demands.

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