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Oakley VCO v4 and Slim VCO-B calibration
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Author Oakley VCO v4 and Slim VCO-B calibration
rjstreet
Hi all

I have been using a V4 Oakley VCO's without issue for some time and I have finally got around to putting together two slim VCO-B's. I want to get them equally calibrated using Tony's advice / rule of C4 = 261.6Hz and 5v CV input from the Oakley MidiDac (v4). I'm using a Roland A-49 keyboard into the MidiDac so previously, dropping down two Octaves was no problem.

I can calibrate the Slim VCO's perfectly but I cant get the two full VCO's to tune down to 261.6 at 5v. It seems to be two octaves above with no further travel in the TUN preset!

I have followed the calibration instructions carefully and all reference voltages and intervals are perfect. I've measure the midiDac and I'm sure it's 5v precisely. 'Coarse' is set to minimum and 'fine' to mid point. Is there are resistor value astray here somewhere? I'm thinking set for .com use maybe?

Any ideas would be welcome?

Many thanks

Richard
Synthbuilder
Interesting. It should be able to pull down to that frequency. I haven't changed anything in the CV summing circuitry from version 4 to the current version 6 VCO.

In the first instance check that pin 1 of U3 is reading -10.00V. If not, then look at either R4 and R6, both of which should be the same value of 22K.

If this is fine - maybe something is a little out elsewhere - I may have changed the odd resistor value over the years that V4 was active but I don't think so. If the VCOs are running fine then there is nothing to worry about. To get them to go lower simply change R28 to 180K if it is not already, and if it is, then change R28 to 150K (1% metal film or better).

Tony
rjstreet
Hi Tony

Thanks for your note and apologies for the delay in response. I've been away for a few days enjoying some sun!

I've checked all the voltages / resistors again for both of my VCO v.4's and all are correct as per your original BoM / schematics.

I also checked the schematics differences between V4 and V6 and as you said, all resistors remain the same. The only difference being the move from OPA2277's to LT1013's. Slim-VCO's are clearly very different.

I have now changed R28 to 120k as 150k would only get me to 523.2Hz (C5) and not down to 261.6Hz (C4).

So,all is working fine now and I can bring all my various flavours of VCO together!

Many thanks

Richard
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