[sdiy] Polyphonic tonal disorder!

[harrybissell at copper.net]

My bet would be that the 3080 were selected for
low input offset voltage. This varies with Gm
bias so it cannot be trimmed out easily

H^) harry


>
>
>
>---- Original Message ----
>From: lenham at clara.co.uk
>To: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] Polyphonic tonal disorder!
>Date: Fri, 20 Apr 2007 08:44:16 +0100
>
>>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.
>>
>>
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