[sdiy] Electrolytic Caps for Audio Decoupling

[harrybissell]

Hi Seb (et al)

Seb Francis wrote:

> Ok, I consider my wrist well and truly slapped !

:^P

> So continuing on the subject of AC-COUPLING caps ...
>
> >
> > 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.
> >
> In the THAT datasheets and design notes they show polarized caps with
> the negative end connected via a resistor to the IC inputs which are at
> virtual ground.  And in fact many times I have seen polarized caps used
> in places like this where in all likelyhood the polarity will be
> reversed about half of the time.

In most of these cases... it is expected that there will be NO DC potential
across the capacitor.  (almost) ALL audio circuits are designed to have net
zero DC in their outputs... so any capacitor in this case will probably not
be polarized
at all.

In this case, the capacitor will degrade over the years, its voltage blocking
might
go down over time... but the life will probably be much longer than the audio

equipment.  If you reversed the cap, probably no problem.

I did not study the THAT data sheet (the that... lol).  It might be that the
input is
not at ground potential. If its negative, their polarity of the cap is
correct.

> I'm guessing this may cause some distortion, and certainly degrade the
> polarized cap over time.

Distortion ??? who knows. Probably the effect of ripple in the control signal

will be WAY more distortion than that cap (not THAT ic, that cap :^)

> > If the polarity is unknown, a non-polar electrolytic would be a good
> > choice.
> >
> This sounds like good advice, but I wonder why this type of cap is not
> more commonly shown on schematics, especially featuring THAT ICs which
> are meant to be pretty high-end audio components.

Again, if you do not expect DC, the non-polar cap might not be a big
advantage.
At the low current levels... even a reverse-bised polar cap will live a long
time.

NOTE :  Analog Modular Synthesizers are an excellent place to use non-polar
electrolytics for coupling (if you need the really large values).  You don't
know WHAT
will be patched to an input...it could have very real DC offsets, and large
ones at that.
(not THAT ic... that offset :^)

> > 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...
> >
> No need to go there .. this will be a compressor for analog drums +
> analog synths :)  I have plenty of audiophile compressors in software if
> I want to compressor a whole mix or something.

I usually use a non-polar lytic by itself. I have never heard the difference
in my synths.
If possible, I use a high enough input impedance to make the cap small...
then I can
use a film capacitor.  The THAT ic (yes that ic :^) has in input of 20K ...
so you need
the large value.  If you had a buffer amp driving the input... maybe a .01uF
would be
enough to get the low frequency response.  They don't want to tell you THAT
:^P

H^) harry
(sorry, silly tonight... Like Porky the Pig used to say... th..th..th...
THAT's all folks :^)

>
>
> Seb