[sdiy] Need a simple synth circuit for Hammond modification

[Bob Weigel]

Trek makes a percussion upgrade.  I think my friend Andy Seitz sells 
them.   Alternatively you could put a MIDI upgrade on the key movement 
and just mix in a synth with some percussion presets and then you could 
do a lot of other stuff too obviously.  -Bob

David Brown wrote:

> 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
>
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