mandag 31. desember 2018

Juno/jupiter VCA and resonance CV

I just discovered something surprising.

First of all, my Juno CV runs the output VCA at very low currents, less than 0.2mA, while the LM13700 can be driven up to 2mA.

I have started researching this to see if I can change the CV to get unity gain at 5V on the JP6. For some reason I expected the control circuit to be converting a linear CV to an exponential one, because I didn't think the Juno had it in it to do this elsewhere. But it makes perfect sense I guess, this lets us generate both linear and exponential envelopes in software.

I took a closer look at the juno 106 schematics and discovered that the CV present at R37 in my schema is not 0-5V. It is actually an adjustable voltage, amplified right after the DAC. It is adjustable from min 0.2V/max 10.3V to min 0.32V/max 10.42V.

Now, this means that the max current at the control of the VCA is about 0.3mA, It is not a major change but it is something at least. And without having to worry about changing the response curve of the VCA CV control, I can change the max control current easily.

As for VCA gain adjustment, I can either chose to add a slightly higher than unity gain and adjust this in software, or add a adjustment pot. A CV summing point may be a good idea anyway

PS: Without adjustment, CV below 0.6V will give 0V out due to the diode drop of the transistor.

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