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SVCO HFT calibration
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author SVCO HFT calibration
Just finished calibrating a recently built SVCO-B. It is my third SVCO. It is calibrated beautifully and tracks accurately across the whole keyboard, but, and I am being pedantic, I can't quite get the HFT interval because I run out of trim pot adjustment when I'm within a whisker of achieving it. E.g with an A at 7040Hz the smallest interval I can get to for an octave above is 14106 and the 10k trim pot is at one end of it's travel (the same place it was set to provide a zero ohms reading prior to calibrating the SCL trimmer). Given the direction of travel I have observed it suggests I need a lower value than 0 ohms to get to the correct interval.

Whilst there is no perceivable audio issue (I did say I was being pedantic) I am wondering what might be causing this anomaly as I was expecting to end up with a successful setting somewhere in the middle of the trimmer range.

I think I may have also observed the same issue on my other SVCOs but would need to check.

I am using 1% metal film transistors throughout.

Any thoughts/feedback would be appreciated.
My first thoughts would be that the SCL trimming is not quite right. For it to be running sharp at 14kHz would normally suggest it to be overcompensated at say 7kHz. However, there may be other issues at work - accuracy of the log conformity of the NPN pair (in U6) or switching speed within the VCO core (U8 & Q3). I've not heard of any issues before though - Paul Darlow aka Krisp1 builds a fair few of these things and he normally flags up issues before the rest of us get to hear of them.

But if tracking is good at lower frequencies and you really need that level of tracking at 14kHz then you could try lowering R44 to 3k3. R44 actually compensates for the fixed switch time of the FET and comparator but maybe in your build they are running faster than I've anticipated.

Let us know how it goes.

Thanks for the feedback Tony, I also wondered whether it might be a really small calibration error in the output from the midiDac, i.e a tiny voltage increase in the output going up the keyboard. I will re-check the midiDac calibration at some point in the near future, I have a bit of a cabinet/module re-shuffle planned soonish.
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