Odp: [sdiy] DAC for MIDI2CV

[Seb Francis]

Tony Allgood wrote:

> > I've seen in Tony Allgood's MIDI DAC that he uses 10n polyester caps
> with precision opamps (at least for the pitch), although I've read that
> polycarbonate and polypropylene caps have lower leakage.
>
> I used polyester because they didn't show any measurable improvement
> over polypropylene. The refresh rate is 31.25KHz in the midiDAC and the
> sample window is comparatively large. There's only eight channels and I
> think we use pretty much all the time allocated to that channel to
> charge the cap. Trevor Page could give you more details here. There is a
> small dead time before we open the 4051 to each of the caps to let the
> DAC output op-amp stabilise.
>
> Having said that I think that polypropylene would offer an improvement,
> although by how much its hard to tell. Rapid sell low voltage
> polypropylene at around 40p each.
>

I have some 2.2nF polystryrene caps kicking around, so I'm going to try with these first.  I'm thinking I might have to go up to 10nF, depending on how much current the MXL1014 opamp buffers take.  In general (because there will be 15 channels, and there will be software LFOs + ENVs) I wanted to go for as fast as possible S&H refresh times, but without the frequency being so high that it injects too much digital noise into the outputs.

Seb