[sdiy] Polyphonic tonal disorder!

[Steve Lenham]

From: "John Mahoney" <jmahoney at gate.net>

>>I've just serviced an Oberheim OBX ..
>
> On my OB-X it sounds like there is higher filter resonance on 2 voices, 
> but there isn't (AFAIK) any trimmer for adjusting that. I'd like to get 
> all 4 voices sounding more alike (they will never sound the *same*, and 
> that's a good thing). Any ideas?

You're right - the only trimming on the OBX filter is scale and offset for 
the cutoff frequency.

I didn't try to change the filter timbre on the machine that I worked on, 
but would speculate that the variations in resonance might be down to 
differences between the CA3080s used in each voice's filter, in particular 
A14 which provides voltage control of resonance.

Many of the 3080s in 'my' unit were marked with a red dot, implying that 
Oberheim carried out some matching/selection at the time of manufacture, but 
maybe the matching wasn't all that tight or perhaps 30 years of drift has 
magnified their differences.

You could try matching some 3080s for...what? transconductance?...and 
fitting them to the voice cards as a set. Or just fit socket(s) to the 
offending voice cards and keep trying 3080s till you find one that sounds 
right...

Do report back if you try this - it would be good to know. It might not even 
be the 3080s, so we could rule them out.

Good luck!

Steve L.