[sdiy] Need a simple synth circuit for Hammond modification

[David Brown]

I just finished restoring a 1953 Hammond M-2 Organ.  It's a basic 
model without percussion which I'd really like to have.  I think most 
folks just find a M-3 amplifier and use it to get the percussion 
circuit.  It seems like with modern electronics, it wouldn't take 
that much to add percussion.

Percussion is where you take one of the drawbar tones and put a decay 
on it.  You use the low impedance characteristic of the key closure 
as a gate which triggers an envelope decay that controls the gain of 
an amplifier for that drawbar tone.  The decay will not retrigger 
until all keys are released.

I've got plenty of positive voltage but no negative so I want to 
build this out of a single sided supply if possible.  Seems like I 
just need a simple voltage controlled amplifier with an envelope 
generator that has a fast attack and slow decay.  I'm guessing the 
decay may be as much as 0.5 seconds.

As for the trigger, it seems like I could use the same low impedance 
characteristic or perhaps a simple envelope follower.  I've been 
trying to come up with an envelope follower that had a built-in decay.

Does anyone have ideas on simple circuits that might do the trick?  I 
think the signal level is quite low.  I'll need to do some 
investigation to determine some of the signal and time characteristics.

Dave