[sdiy] AC coupling caps on MS20 clone

[Harry Bissell]

iirc, the original PAiA 2720 synth had some capacitors in the patch bay that could
be used when needed to knock out a DC offset...

let's not even start with the "CV feedthrough"

H^) harry



----- Original Message -----
From: David G. Dixon <dixon at interchange.ubc.ca>
To: 'Harry Bissell' <harrybissell at wowway.com>
Cc: 'Mattias Rickardsson' <mr at analogue.org>, 'Synth DIY' <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 06 Aug 2010 11:03:41 -0400 (EDT)
Subject: RE: [sdiy] AC coupling caps on MS20 clone

> The CV 'feedthrough' can be an artifact of a DC bias at an input...
> The blocking cap would in effect, make sure the input signal is polite
> (has zero vole DC bias, eventually...)

Thanks, Harry.  I get it now.  So, really, coupling the input is all that
matters.

> The better question, how long does it take the blocking cap to settle to
> that 'zero volt' DC level given
> a change at the input, and how does that play with the overall frequency
> response  :^)

> I find that for most circuits, the choice of value for blocking caps is
> the most difficult choice to make, trading
> settling for low frequency response, and that there is an optimum trade-
> off point that still may leave you
> less than satisfied...

Yes.  The original Korg circuit used 33uF caps.  In my simulations, would be
pretty useless, as they would only block the most quiescent of DC signals.
However, anything less than about 1uF alters the input signal too much.  I
haven't used coupling caps on my other filters, and I've never noticed a
problem.  I think I'll either take my chances without it, or use a switch to
give the user a choice.


-- 
Harry Bissell & Nora Abdullah 4eva