Juno filter: VCA and Resonance CV circuit tweaking

I took a closer look at the Juno 106 circuit for VCA and Resonance CV to figure out what they did to the base-emitter voltage drop in the transistors in the circuit. While simulating it turned out that it actually uses a 10V resonance CV where I put 5V. This confirms my initial testing where I had to use 10V resonance CV to get a proper response.

Juno 106 VCA CV

Anyway, following the tuning instructions in the Juno 106 results in a response where the first 150mV of the CV does not output any I_abc current, because the transistor does not switch on until it sees a high enough voltage. This is practical as it will fully turn off the VCA. In my Xonik VCA I have assumed that 0V CV cuts off the output current, but if there are small differences in the supply lines or something we may still get a tiny output. The Juno does not have this problem because of the 'deadband'.

CV vs I_abc  (red) before calibration - notice deadband at the start of I_abc

CV vs I_abc after calibration, deadband ends at about 150mV

CV vs voltage at opamp output - notice that we get 10V at the start.

After calibrating the trimming I tested the current outputs:

VCA:
max -301uA

Resonance:
max -205 to -356 uA depending on the trimmer.

I wanted to add a mixing point for the VCA CV. I am not quite sure how to do this on the existing design - I could possibly add a resistor to the positive input of the VCA CV op amp - so I decided to use the current converter from the Xonik VCA.


Using standard component values, I ended up with this version

which has the following params:

VCA: 
max -320uA

Resonance:
max -147 to -405

If trimming the size of the deadband is not necessary, the trimmer and 1MEG resistor may be replaced with a 1.5MEG resistor to -15V. This will give a deadband of about 150mV.

Leaving out the 1MEG resistor will give 0 current at 0V but with the possibility of a tiny current leakage, slightly turning on the VCA.

Frequency CV

I also modified the Frequency CV mixer a bit, now the trimmer lets you add or subtract up to 5.6 octaves.

As a side note, the 390k resistor to -15V adds 3.8V to the output. Switching the resistor from the trimmer from 270k to for example 150k would give the same trimability and remove the need for the 390k, but makes the trimmer slightly more sensitive.


Here is the updated circuit. I still want to add linear FM of the filter cutoff, but other than this I am getting close to a version that can be tested.