[sdiy] AC coupling caps on MS20 clone

[David G. Dixon]

> I agree: separating signals based on purposes is *not* good design,
> and needing AC connection in a VCA is *not* good design either. :-)

Hear, hear!

> Speaking of "capacitors in the patchbay" etc, simpler solutions would be
> 
>  1) dual jacks - one DC and one AC coupled,

Yes, but what if the filter has a three-input mixer?  (You see, that's my
dilemma...)

>  2) toggle switch besides the jack,

That's probably the best option.  Then the cap goes right on the switch, and
doesn't even clutter up the PCB.  The cap becomes part of the panel
hardware.

>  3) level pot with pull-switch, to select AC or DC coupling.

Too complicated/expensive for my tastes (which are simple and cheap).

> However, these solutions are good for caps at the input or output, not
> the between-filter-stages cap in the MS-20 that David mentioned in his
> second email.

Based on the previous discussion, I don't believe that the caps are required
anywhere except at the input (and I'm even dubious about the need for it
there as well).

> The pull-switch solution is my personal favourite, but it might
> interfere a bit with another clever thing discussed here some weeks
> ago when it came to "bipolar" level pots... where the true null point
> can be hard to find, and a pull-switch could come in handy when the
> signal is supposed to be totally off.

See above re: complicated/expensive.

> A fourth solution then comes to mind, it should be possible to
> 
>  4) use a "stereo" jack to choose AC/DC by having the plug in/half-in.

This seems too fiddly to me, plus I'd probably forget which position was
which.  The switch is best (and I've even done that one before)!

> I expected the plug tip to be connected to either the jack's tip
> connector or the jack's ring connector, respectively, but I realized
> that it wasn't true - at least not on my Switchcraft stereo jack.
> - With mono plug half-in, BOTH jack tip and jack ring goes to plug tip.
> - With mono plug fully inserted, jack tip goes to plug tip, and jack
> ring goes to ground (via the plug's sleeve).
> 
> I guess you could use this stereo jack as an AC/DC filter input jack
> by connecting jack tip via a capacitor to the VCF, and the jack ring
> via a resistor to the VCF. A fully-inserted mono plug would then give
> its signal to the VCF via the capacitor as an AC path (while the
> resistor sits between ground and VCF), and a half-inserted plug would
> give its signal to the capacitor and resistor in parallel, essentially
> providing a DC path.
> 
> I haven't made up my mind yet about what solution or combination of
> solutions would be the most desirable on, say, the audio input of a
> filter... the ergonomics/liveplayability should definitely be
> considered, and perhaps both AC/DC and on/off switching is not even
> desired on the same input. :-)

Oh gawd!  Just use the switch, Mattias!