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VCO Octave range
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author VCO Octave range
gtxdude
Over what octave range can the standard Oakley 2U VCO be expected to stay in tune when properly calibrated?
Synthbuilder
gtxdude wrote:
Over what octave range can the standard Oakley 2U VCO be expected to stay in tune when properly calibrated?

The VCO, once warmed up and at a stable temperature, should track at better than +/-5 cents across the whole audio range.

The biggest inaccuracy will be down the bottom (below 110Hz) and then again as the VCO goes over 20kHz.

Because of this it is always best to trim for 1V/octave in the part that interests us most. That is between 110Hz and 2kHz. I normally trim 1V/octave between 220Hz and 880Hz and HFT between 7kHz and 14kHz. This seems to give the best results at all frequencies. I have managed with the current issue 6 VCO to get the VCO to track at +/-2 cents from 110Hz to 14kHz - this is possibly more of a measure of inaccuracy of my CV source rather than the accuracy of the VCO. It's difficult to tell at that level.

However, the biggest enemy to perfect tracking is temperature. The 1K temp co resistor has a temp coefficient of 3500ppm/K but this value has both a tolerance (absolute error) and a temp co if its own. In other words the ability of the circuit to exactly compensate for any temperature drift is limited and is likely to be slightly different from VCO to VCO. However, unless you have wild swings in ambient temperature you probably won't notice any loss of scaling accuracy. It is advisable not to put your VCOs directly over your power supply though and keep them away from draughts.

Scaling is also slightly affected by patch lead, jack plug and jack socket resistance. And also the accuracy of the source CV.

Tony
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