bypass

[novalves]

I have recently read, seen, discussed the topic of by-pass capacitors for electrolytics. Some designers apparently by-pass the by-pass caps!
Pre3 does not have these things.

Why do some circuits include them ? Is it for noise reduction, more smoothing ??

[John Caswell]

Hi Novalves,
Bypassing electrolytic (elcos) capacitors with other smaller capacity different types helps the overall bypassing of unwanted signals, eg RF, to ground by reducing the impedance of the path as elcos can have very high imppedances to RF and the like so you need low impedances.
I generally tend to place .01/.02µF ceramic disc capacitors across elcos to improve the RF bypassing, but it is not always necessary or even desirable. It can be a very much "suck it and see" job as it will depend upon many interacting variables.

John - Dr John

[novalves]

ta.

I have also read of designers eschewing ceramics because they "ruin" the sound, in their opinion.

I have used polypropelene bypass caps to great effect in my phono preamp. Bass has improved tremendously. (Shunt feedback transistor design).

[John Caswell]

Hi Novalves,
First thing to remember is that whatever you do/recommend some one will trash it.
Ceramic disc capacitors are excellent at bypassing RF which is why I suggested them in the first place many years ago, based on first hand engineering practice.
I can only say try them and other types to see which you prefer, after all it is your ears doing the assessing not anyone elses.
I suppose if a designer said "stick your head in a gas oven and light it" someone would

John -Dr John

[Richard]

Yes indeed The bad press for ceramics is as coupling/signal caps where they are not stable in value with temp. Morgan Jones covers this I think. For RF bypass to ground this will not matter.

Rich