Keyboard circuit (was Stretch tuning a resistor string)

[Harry Bissell]

The keyboard circuit of the PAiA 2720 synth had such a compensated current
source.
I've used it for years. (well I replaced ALL the components but it occupies
the same physical space as the original...)
:^) Harry

Mike Granger wrote:

> I believe that I remember seeing a circuit that used a three terminal
> voltage regulator as a constant current source. It was wired up in a
> strange way to make this work. The instrument was a temperature
> measurement circuit that used a platinum RTD as the sensor, and it needed
> to be driven by a constant current for most accurate operation.
>
> 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.
>
> Don Tillman wrote:
>
> >    From: "Tim Daugard" <daugard at sprintmail.com>
> >    Date: Tue, 18 Jan 2000 14:40:20 -0600
> >
> >    Some one said use a voltage regulator for the keyboard resistor
> >    string.
> >
> > Who said that?
> >
> >    There are some benefits to useing a regulated current source
> >    instead of a regulated voltage source
> >
> > I'll say!  The major benefit is when the player hits more than one
> > note.  With a current source the output is the lowest or highest
> > of the notes played.  With a voltage source you'll average in some
> > unmusical way unless each of the key switches is double-throw.
> >
> > The only synth I know of that feeds the keyboard resistor string with
> > a regulated voltage is the Moog Taurus I.  And that uses double throw
> > keyboard switches.
> >
> >   -- Don