AC coupling questions

Eric at Svetlana Electron Devices svetengr at earthlink.net
Fri May 8 21:46:09 CEST 1998


>   On this subject the FAQ says, "Use 0.47uf metallized polyester or
>   polyproplyene.  Values less than this will attenuate bass frequencies."
>   Mind you it makes no mention of impedances.  Regardless, a .47uF
>   polypropylene cap is the size of a bumblebee.

Tube electronics usually use coupling caps between 0.1 and 0.47 uF.
(this is because circuit impedances in tube circuitry are usually
much higher than in solid-state circuits.)
It is common to see very small, low-quality electrolytic caps
used in solid-state audio electronics for coupling--often 10 uF
or thereabouts. Cheap, cheap, cheap, and small--quality never
even enters into it!

I would recommend film capacitors in the 1 uF to 10 uF range
for typical synth circuits. Even the cheapest films are
VASTLY better than any electrolytic in terms of leakage,
hysteresis, parasitic inductance, dielectric linearity,
temperature sensitivity, etc. etc.

I can refer list readers to makers of super-premium film
capacitors if desired. (Be warned--values above 2 uF are
usually somewhat bigger than a bumblebee......more like a
giant rhinoceros beetle......)

Eric Barbour
Svetlana Electron Devices
Portola Valley CA USA




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