R9 Oakley VCO

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welhamc
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R9 Oakley VCO

Post by welhamc » Sat Jan 25, 2020 10:42 am

I was wondering the significance of the value of R9 for the issue 7 Oakley VCO. For older MidiDACs (4 and below) R9 should be 180K but for newer versions/MU/Eurorack it should be 220K. My MidiDAC is issue 4, but I also sequence using a beatstep pro unit, what value should I plug in? Does it affect scaling for 1V/Oct or more of an impedance thing

Many thanks!

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Synthbuilder
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Re: R9 Oakley VCO

Post by Synthbuilder » Sun Jan 26, 2020 5:00 am

R9 only affects the absolute pitch of the VCO. That is, what note is playing when you have 0V on its 1V/octave input. Scaling, thankfully, is not affected.

Previously there was no standard amongst different modular systems that defined what pitch corresponds to what CV input. Therefore some 20 years ago I chose to make my VCOs produce middle C (C4) at their typical settings when the 1V/octave (KCV) input is at 5.00V. Thus, I would expect the VCO to be producing 261.6Hz when its KCV input is 5V, the coarse tune pot is set to slightly above the minimum, and the fine tune pot is in the middle. To do this, R9 on issue 7 boards, and R28 on issue 6 boards, should be 180K.

These days with the popularity of Eurorack modular systems a sort of standard does now seem to be in place with a KCV of 0V being represented by midi note number 36 and thus any connected VCOs would be working at a basic pitch of C2 (65.4Hz). To allow at least one sub-octave the Oakley VCO should then produce C1 (32.7Hz) when its 1V/octave (KCV) input is 0V or unconnected, the coarse tune pot is set to slightly above the minimum, and the fine tune pot is in the middle. This is two octaves higher than the traditional Oakley way of doing things. To do this, R9 on issue 7 boards, and R28 on issue 6 boards, should be 220K.

Since you have both the older Oakley and the newer Eurorack (although I am unsure what the Keystep actually produces) you have a bit of a conundrum. Ultimately, you need to try one value and see if that works for you. If not, try the other.

There's also a cludge, involving one resistor, you can add to the issue 4 midiDAC to bring it to the newer standard but I'm not sure how easy that will be to do. You need to add an 18K resistor between the +REF (the +5V reference output and found on one of the pins of the INIT trimmer) and pin 6 of U1. You will then need to recalibrate your midiDAC INIT trimmer so that the Pitch CV output produces 0.000V when a C2 note (midi note number 36) has been sent to the module.
Tony

welhamc
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Re: R9 Oakley VCO

Post by welhamc » Sun Jan 26, 2020 9:29 am

Thanks for the reply Tony.

From how you've described it to me, I agree with you in that it makes most sense for 5V (middle value) to equate to C4 (middle key) on the keyboard, so have gone for a 180K resistor

I'm not too fussed about having to retune oscillators on a patch by patch basis to account for different offset values so don't think this will be a problem with other pitch cv sources

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