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

[John Mahoney]

At 04:43 AM 4/26/2007, Steve Lenham wrote:

>>>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.

Thanks! I'm saving this message for reference, because I just may do this.

While we're on the subject, do you have a favorite replacement 
trimmer for the OB-X?
--
john


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