[sdiy] AC coupling caps on MS20 clone

[Harry Bissell]

The CV 'feedthrough' can be an artifact of a DC bias at an input. Just consider
a VCA... (even a very good one). If you have no DC on the input, you can move the
CV input at any speed (fast transient). You will hear nothing at the output. Add a DC bias
and you will hear a thump at the output. There is ~no~ (audio) signal at this time so you'd
call that output 'feedthrough'.  The DC blocking capacitors will keep the input at or near ground
if you have a well designed circuit.  (it would keep the circuit at the DC operating point at least,
whatever that point is)

The blocking cap would in effect, make sure the input signal is polite (has zero vole DC bias, eventually...)

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... 

H^) harry

----- Original Message -----
From: David G. Dixon <dixon at interchange.ubc.ca>
To: 'Mattias Rickardsson' <mr at analogue.org>, 'Synth DIY' <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 06 Aug 2010 09:47:20 -0400 (EDT)
Subject: Re: [sdiy] AC coupling caps on MS20 clone

> Did you just look & listen to an applied waveform, or did you VC
> modulate it with a quick envelope or LFO?
> CV bleedthrough is what I'd suspect to be the main problem here.

CV bleedthrough with 2164 is pretty small, especially if the right bias
resistor is used on the mode pin.  I use 560k, which should (if I'm reading
the figure correctly) virtually eliminate CV bleedthrough.  This would be a
much bigger problem with an OTA-based circuit.

Also, how would AC coupling caps block CV bleedthrough from a quick envelope
or LFO?

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-- 
Harry Bissell & Nora Abdullah 4eva