[sdiy] Electrolytic Caps for Audio Decoupling

[Harry Bissell Jr]

Hi Seb
  
  (put your flamesuit on...)
  
  A spanking for Seb.  :^P
  
   We need to separate the term "Decoupling"
  from the term "Coupling" in regard to capacitor usage. They are not the
  same.
  
  Decoupling caps are used on power supplies to prevent unwanted
  coupling between stages... so that they do not interact. Usually they
  are not critical in performance, ceramic, tantalum, electrolytic or combinations
  of the three are common and accepted.
  
  COUPLING caps are used to pass AC signals from one stage to another,
  usually across points that differ in DC potential. These caps ARE very critical
  to the quality of the signals passed.  
  
  (and this is what Seb meant to say :^)
  
  Capacitors for coupling should be chosen to suit the characterictics of the
  signal.  Most critical factors in coupling caps would be... Low leakage,
  Low Dielectric Absorption, low change in capacitance vs applied voltage.
  
  The clear winner would be polystyrene... but that would be impractical in
  values much above .1uF (and anything larger that .01uF is really hard to find
  and quite expensive).
  
  Mylar (polyester) has poor dielectic absorption... they ARE commonly used in
  values up to about .47uF.   Maybe you care, maybe you don't. They could
  change the sound.
  
  Values above 1uF are almost always electrolytic.  ESR is not usually a problem.
  Leakage, while fairly poor... is ot excessive in smaller uF sizes.
  
  Polarity is a big problem. If you know that one DC potential is always higher than
  the other, normal POLARIZED electrolytic caps may be used.
  
  Some people use two polarized capacitors in series, with the positive ends
  facing "out" for signals of unknown polarity.  I don't recommend this personally.
  They can degrade over time.  Some OTHER people will tie the center tap of those
  two caps to the most negative circuit point, through a hiigh value resistor
  to keep the DC bias correct. This could introduce noise from the negative
  supply INTO the circuit.
  
  If the polarity is unknown, a non-polar electrolytic would be a good choice.
  
  Some people like to parallel that electroytic with a film or ceramic cap to 
  overcome the self-inductance of the electrolytic cap and increase the
  high frequency performance.
  
  Now you are going into audiophile land...
  
  H^) harry

Seb Francis <seb at burnit.co.uk> wrote:  Hi,

I'm building a compressor from a THAT design note ..
http://www.thatcorp.com/datashts/dn115.pdf

The input decoupling capacitors are 47uF and I was wondering if there's 
a particular type of electrolytic which would work best for 
(high-quality) audio decoupling duties.  Are certain attributes 
desirable in this situation?  Or will any electrolytic do the job?

Thanks in advance,
Seb

P.S. If anyone has any experience building this circuit I welcome any 
input...



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20051207/f3575837/attachment.htm>