[sdiy] AC coupling caps on MS20 clone

[David G. Dixon]

> I have often read that the PCB traces at the inverting input (aka
> summing node) shold be kept as short as possible. I think the idea
> is, if the summing node itself is viewed as an additional voltage
> input, it's a very high-gain one (input resistor zero ->
> theoretically infinite gain).
> On the other hand, as long as the opamp does its job properly, the
> input impedance of this "input" is zero (hence it's called virtual
> ground). In other words, it's a current input, not a voltage input.

The impedance of a summing node is easy to calculate, and never really that
large.  On that basis alone, I don't think the length of the trace matters
at all.  I never worry about it, and it never seems to be a problem.
Indeed, some summing nodes (such as CV inputs on oscillators and filters)
can have many inputs and the traces can get quite long, and even involve
flying wires to the panel.  I don't think this ever matters.