Juno filter: cap values and expo converter musings

After spending the last few weeks experimenting with the Juno filter, I have some thoughts. It seems the exact compnent values along the signal path are not terribly important to how the filter works. A lot of the filter is tweakable as we've seen in my previous posts - the input CV ranges, the VCA gain etc.

I have tried various cap values as well, and today I made a comparison of both cap values and changes to the expo converter.

Expo converter

The expo converter core is dependent on a reference current. Right now that is set up using a 1.5MOhm resistor, but changing this does not change the output curve, it only changes WHAT linear CV gives what output.

I tried swapping for a 1MOhm resistor, and using the base octave trimmer I could still get the necessary output range. Here are the currents through a single I_abc resistor:

Using 1.5MOhm resistor, the output current is:

Full trim and 5V CV range: 3.2nA to 1.377mA
Center trimmed: 165nA to 168uA

Using 1MOhm resistor, the output current is:

Full trim and 5V CV range: 4.9nA to 1.379mA
Center trimmed: 248nA to 238uA

The center trimmed range is well within the total range independent of resistor, with room to spare for additional octaves.

Filter caps

The Juno 106 uses 240pF caps, other Roland synths use 330pF, and we use 270pF. Here are the responses to a center trimmed CV using a 1.5MOhm reference resistor:

240pF
0V = 7.7Hz
5V = 6.4kHz
10V = 42kHz

270pF: 
0V = 7Hz
5V = 5.7kHz
10V = 39kHz

330pF
0V = 6.1kHz
5V = 4.7kHz
10V = 34kHz

Note that at 10V all of the outputs are at their max (I_abc is 1.38mA) so tracking is off. But all filters reach well above 20kHz. If one aims for a 40k cutoff 240pF and 270pF seem equally suited.