[quote author="Svart"]Can someone answer this question..? Why do we use smallish values for the film coupling caps that bypass 'lytics? (in parallel with the lytic)? why not use larger values? I can't seem to find the answer to this.[/quote]If you are talking about passing audio, electrolytic caps generally pass lower frequencies faster than higher frequencies, so the entire frequency response is sort of phase-skewed. In other words, the higher the frequency, the slower and more out of phase it gets. This causes the audio to ?smear? a bit. A small film cap in parallel helps correct this by passing the higher frequencies in phase with the lower ones.
There does seem to be a drawback with this approach, however. Even though you are by-passing the HF around the ele cap, the slower, phase-shifted HF is still going to come through the ele?s. So, even though the sound is a little clearer, it is still prone to some high-end ?smear?. This is probably why some respected designers just use electrolytics or tantalums and leave the film cap off altogether.
An alternative is to just use big films and place them in parallel to equal the value you need. The trade-off here is that films are considerably more expensive (10uF MFs are approx $5ea at Digi-Key).
