Keyboard circuit (was Stretch tuning a resistor string)

[Harry Bissell]

The PAiA 2720 keyboard did. Check out the schems (on some body or bodies web
site on list... 'fess up !!!)
:^) Harry

Tim Daugard wrote:

> Does anyone know where I can find the schematic for this?
>
> > On a related note, someone mentioned never having seen a temperature
> > compensated constant current source in a commercial synth keyboard. The
> > Arp 2600 ( If I remember correctly) had a diode as part of the constant
> > current transistor's base bias divider. This was in there to help cancel
> > the VBE drift of the transistor, and thus give greater stability under
> > temperature swings.
>
> I have been using an led to set the base bias point for the current source
> (V+ to Led (V+ -1.6V)  to [base connection] to Gnd. The emitter is Vcc to a
> resistor (fixed or variable) to emitter. Transistor is PNP which gives a
> good current regulation. I haven't checked actual temp or load regulation of
> this circuit yet. I was using the circuit to provide a constant current
> source for a 555 to get a ramp voltage instead of a capacitor curve. If any
> one has an interesting current source I'd be interested. I'm trying to build
> a low cost synth with each module under $5 so far the only two modules that
> have broke the budget is a power supply (transformers cost too much) and a
> distortion effect/mixer (8 pots).
>
> Tim