[sdiy] AC coupling caps on MS20 clone

[Harry Bissell]

Consider a 'guitar effect' that runs on a 9V battery, the DC operating point
is usually 4.5V internally, set by a voltage divider. Its a quasi-ground point for the circuit.
The input signal ground is usually desired to be 0V.  If you didn't include a pull-down resistor on the input
side of the blocking cap, the source impedance is what allows the input side to go to 0V (making the
typical nasty thump when the cable is connected.  If you put a low value resistor, the input impedance becomes
quite low, make it too high and it takes forever to settle...

For the filter case, it is probably impractical to attempt to pass what cannot be reproduced by the sound system...
so maybe 40Hz is a reasonable compromise. Or 20Hz ???

I consider signals like envelope generators to include true 'DC' levels... such as sustain.

H^) harry


----- Original Message -----
From: cheater cheater <cheater00 at gmail.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: David G. Dixon <dixon at interchange.ubc.ca>, Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 06 Aug 2010 10:22:49 -0400 (EDT)
Subject: Re: [sdiy] AC coupling caps on MS20 clone

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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Harry Bissell & Nora Abdullah 4eva