[sdiy] AC coupling caps on MS20 clone

[David G. Dixon]

> > "Yes, but what if the filter has a three-input mixer?
> > (You see,
> > that's my dilemma...)"
> >
> > Good! Single-input audio modules - requiring standalone
> > mixer modules
> > which means both extra hardware, scattered controls, and
> > reduced
> > ergonomics - is also *not* good design imho. :-)

> Audio modules with not mixer input are NOT a *not* good
> design if you, for example,  are not planning *always*
> work with mixed signals. It remembers me the worry
> subtraction patch again, sorry but it is more interesting
> generate complex waveforms by frequency modulating them than
> just mixing them. IMHO.

I put a three-input mixer on my very first filter, and I used it constantly.
In fact, there were very few times when I didn't have at least two signals
coming into it.  Often, I wished I'd had more than three inputs
(particularly after I built my analog shift register).

However, having said that, I did something slightly different with my latest
filter (the 2164 4P LPF).  I added a three-input mixer, but I put a fourth
panel pot in the mixer's feedback loop to give independent mixer output
control (very handy) and I brought the mixer output to the panel on a
separate jack.  This way, I can use the mixer as a standalone unit, without
using the associated filter.  Now, I can build other filters without a
mixer.

However, it is easy enough to add this capability to any filter board.
Indeed, with the 2164 Korgasmatron filter PCB I just laid out, by bringing
all four connection points on the mixer summer to an off-board connector
(input resistor junction, virtual ground, opamp output, and signal
destination), I have all the flexibility I need to do whatever I want on the
panel -- several AC coupling options (with or without switches), a single
signal input (with or without attenuation), or a two- or three-input mixer
(standalone or dedicated) with a fixed resistor or a panel pot in the
feedback loop.  That way, both Oscar and Mattias can be happy, AT THE SAME
TIME!

As Martha would say, "It's a good thing!"