[sdiy] AC coupling caps on MS20 clone

[cheater cheater]

On Fri, Aug 6, 2010 at 16:32, Harry Bissell <harrybissell at wowway.com> wrote:
> 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.

Why? I'm not following

> 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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>
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> Harry Bissell & Nora Abdullah 4eva
>