[sdiy] AC coupling caps on MS20 clone

[Oscar Salas]

But when capacitor is bypassed, the inverting input node will be a long cable that goes and comeback to the front panel switch. Will be it good?

--- On Sun, 8/8/10, David G. Dixon <dixon at interchange.ubc.ca> wrote:

> From: David G. Dixon <dixon at interchange.ubc.ca>
> Subject: Re: [sdiy] AC coupling caps on MS20 clone
> To: "'Synth DIY'" <synth-diy at dropmix.xs4all.nl>
> Date: Sunday, August 8, 2010, 12:10 AM
> > If you want switch in front
> panel, this solution implies connect a cable
> > in a high impedance part -the inverting opamp input-
> when ideal design pcb
> > wants short traces at that points.
> > I don't see the need.
> > 
> > It would be better place the capacitor on the input or
> between two stages.
> > So I would say between the sum inverting amplifier and
> the 30K input
> > ssm2164 resistor. (I'm supposing the schematic)
> 
> I believe that the impedance at the inverting input of an
> inverting opamp is
> R1 || R2 (where R1 is the input resistor and R2 is the
> feedback resistor).
> Hence, if a blocking capacitor C is inserted between R1 and
> the inverting
> input, then this impedance becomes (-j/wC + R1) || R2
> which, at DC (w = 0),
> becomes simply R2.
> 
> If this were a non-inverting opamp, then you'd be spot
> on.  (Indeed, that's
> the issue with a sample-and-hold cap-and-follower, where
> the high-impedance
> trace should be kept as short as possible, and even
> surrounded with a guard
> trace).  In this case, a resistor to ground would be
> required to shunt off
> any current, and the impedance would then be that of the
> shunt resistor in
> parallel with the cap.
> 
> You are confusing the impedance into the inverting input
> (which is very
> large indeed) with the impedance of the trace connected to
> the inverting
> input (which is a parallel combination of all of the
> impedances leading away
> from the trace: the input resistor (or cap), the feedback
> resistor, and the
> inverting input (which can be ignored because it is so
> large compared to the
> other two).
> 
> (If I'm smoking crack here, then someone please correct
> me!)
> 
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