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RE: Cap Value

"isn't it true that to completely escape the effects of a first order high pass (or low pass) filter the frequency of interest needs to be one decade away from the -3db point?

So for a high pass filter (like a cathode bypass) if the caps is sized for a -3db point of 2Hz then at 20Hz there is no phase shift or amplitude loss?

So at 20Hz and up, wouldn't there be 0Vac across a cap? Unless there is some other "flaw" in the cap?"

I had been meaning to reply to this sooner, Tre. No, it wouldn't be 0Vac at 20Hz. Above the "corner frequency" of 2Hz, the AC voltage will fall off at about 20dB per decade, so roughly 20dB down at 20Hz.

It all comes down to "how small is sufficiently small to be inaudible," really. But clearly, for a sufficiently large capacitance, the audibility must drop below anyone's threshold of hearing.

But since this thread really began with a discussion of the breaking-in of capacitors, the issue really is not so much about the difference in sound between the given capacitor and an idealised "infinite" capacitor. The discussion should really be about what could be different in the electrical properties of the given capacitor after it has been used for 1000 hours, or whatever. I would suppose that the electrical properties of a competently constructed capacitor should not change significantly during its nominal lifetime. But in any case these things can be measured, and my own suspicion, skeptic that I am, would be that in most cases any reported sonic evolution is more likely to be due to psychological evolution in the mind of the listener than actual verifiable changes in the sonics. (Verified by suitable comparative listening tests, or by measurements.)

I know that talk of double-blind comparative listening tests is like a red rag to a bull for some people, so let me comment on the other option, of measurements. One possible approach, which gets around some of the objections by those people who like to portray the notion of an unimaginative guy in a lab-coat blindly measuring something without understanding what he should really be trying to measure, is something that I think Ralph was hinting at in a recent reply. Namely, to make a comparison between component A and component B in some sort of a bridge circuit, so that you can essentially null out everything except the difference signal between what comes out from component A and what comes out from component B.

One could, for example, set up two identical tubes fed with the same signal, one bypassed with a new capacitor and the other bypassed with a 1000 hour old capacitor, and then look at the difference signal between the two cathodes. (One could actually set up a much better test rig in which it wasn't necessary to depend on using two different actual tubes, of course.) It could be interesting to see what differences, if any, were apparent.

One would not need to know, in advance, exactly *what* difference in the two signals one was looking for, which was was responsible for any alleged difference in the sonic performance. But if the difference signal between the two was tiny to the extent of being indisputably below any plausible threshold of hearing, then one could be pretty confident that any reported sonic difference was all in the mind. On the other hand, if the difference signal was large enough to be within a plausible threshold of hearing, one could have some confidence that someone really was hearing a genuine sonic difference. On balance I would be quite surprised if that turned out to be the case, but it would be very interesting, informative and educational if it were true.

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