[sdiy] Hacking the expression pedal input on a JD-800

[Ingo Debus]

Am Donnerstag, 19.02.04 um 19:54 Uhr schrieb Cornutt, David K:

> I've been contemplating that expression pedal input on my JD-800...
> I'd like to figure out a way that I can drive a CV input into it.
> The output is intended to take a Roland expression pedal.
> It uses a TRS 1/4" jack.  From looking at the schematic,
> the tip is the input, the ring is a +5V output, and the sleeve
> is ground.
>
> I've tried just connecting a CV source to the input, but that
> doesn't work.  I'm not sure, but I'm guessing that this is
> some kind of current-loop interface; it expects to see a
> current flowing from the ring to the tip, so if you just hit the
> tip with a CV it doesn't draw current from the ring and
> therefore doesn't do anything.

Don't know about the JD-800, but I have the schematics of the Roland 
A-80. The tips of the pedal jacks (the A-80 has four) are just plain 
voltage inputs, with a 6.8k pullup resistor to +5 volts. The voltage 
sources connected to the rings are tricky: if these are shorted to 
ground the voltage is turned off; so a switch pedal with mono plug can 
be used here as well.

Ingo