Keyboard circuit (was Stretch tuning a resistor string)

[Harry Bissell]

Why don'cha try the LM134/234/334 Current Source... the app notes show how
to make a zero tempco current source. Its in a TO-92 package and has a max
output of 10mA (fine for keyboard strings...). I've used it in a 4-20mA
current loop circuit sucessfully for a number of years (5 generations of
loops actually...)
:^) Harry

Don Tillman wrote:

>    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