Keyboard circuit (was Stretch tuning a resistor string)

[Don Tillman]

   Date: Wed, 19 Jan 2000 09:09:49 -0500
   From: Mike Granger <mgranger at greenville.infi.net>

   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.

Yeah, the three-terminal-voltage-regular-as-current-source circuit can
be found in the National application notes.  I can't believe this is a
high performance circuit because the current it supplies also powers
the regulator chip itself.  For instance, it doesn't work well for
lower current settings (like you'd use driving a string of resistors
for a keyboard).

   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.

That's on the original ARP2600 keyboard, yeah.  It's a very cheap
current source.  The ARP3620 two-voice keyboard improves on this
substantially with an opamp current source.  

  -- Don