[sdiy] AC coupling caps on MS20 clone

[cheater cheater]

Hi Harry,

On Fri, Aug 6, 2010 at 16:11, Harry Bissell <harrybissell at wowway.com> wrote:
> 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)

what do you mean with "well designed" here? what is the "dc operating
point"? Excuse my lack of knowledge :)

> 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  :^)

An interesting way to look at it is: no signals that you get have DC,
they just have very very very low frequencies in them.

> 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
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