[sdiy] OBX trimming (was Polyphonic tonal disorder!)

[Steve Lenham]

>>You're right - the only trimming on the OBX filter is scale and offset for 
>>the cutoff frequency.
>
> It can't be too hard to add a resonance trimmer to each voice card, can 
> it?

No, it isn't. The resonance control voltage (pin E7 on each voice card) is 
converted to a current by a 10K resistor. This current flows via a 
grounded-base transistor into the control pin of the resonance control 
'3080.

I would have thought that swapping that resistor (Oberheim don't give them 
numbers!) for a lower value resistor in series with a preset (say 7K5 fixed 
+ 5K preset, giving +/-25% adjustment) would do the trick.

HTH,

Steve L.