[sdiy] AC coupling caps on MS20 clone

[Mattias Rickardsson]

On 6 August 2010 18:09, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> iirc, the original PAiA 2720 synth had some capacitors in the patch bay
>> that could be used when needed to knock out a DC offset...
>
> Funnily enough, whilst in the shower in my room here at "Ho-Jo's" in
> Roseburg, Oregon, I was envisaging a small "utility module" where one could
> plug a signal in and get an AC coupled signal out.  This seems like a more
> elegant solution for the problem than putting blocking caps (with or without
> switches) on every module.  From what I gather, any module a
> voltage-controlled gain cell (OTA or VCA) can suffer from this problem, but
> one wouldn't expect to find blocking caps on all these modules.  Indeed,
> putting them on a VCA would be quite useless (unless, like Buchla, one made
> a strict separation between CV signals and audio signals -- a philosophy to
> which I object vociferously).

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

Speaking of "capacitors in the patchbay" etc, simpler solutions would be

 1) dual jacks - one DC and one AC coupled,

 2) toggle switch besides the jack,

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

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.

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.
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.

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. :-)

/mr