Keyboard circuit (was Stretch tuning a resistor string)

[Mike Granger]

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