Juno filter for the XM8

UPDATE: This filter circuit has some limitations to its maximum cutoff frequency and high frequency tracking. I will post an updated version that adresses these problems.

I have simulated a Juno filter, based on various designs found on the net. I have made a few deviations from the original Juno filter:

- Frequency tuning happens before the frequency 1V/oct mixing point. The Juno has if after, which means that chaning the tuning also changes the tracking. My change may have messed up other things, no guarantees there. My version makes it easy to set the base frequency over several octaves. NB: The resistor and pot values in my schematics may not be ideal.

- Input audio is attenuated heavily. In my simulations, anything above 1V p.p gets distorted. I have chosen to allow +/- 10V audio. R29/R30 do the attenuation, and changed values for R27 and R26 make up for it in the output stage.

- Resonance control is 0-5V but highest possible resonance equals what we would get from 10V in the juno. This corresponds to a maximum control current of 0.25mA, approximately what I found in the specs in my post about the ba662. I still need to see how this works out i practice. The control curve is different too.

- Output is now in phase with the input

- Filter has unity gain at 5V CV (or slightly above it).

- Freq CV in the schematics is 0.5V/oct to get a 10 octave range from 5V CV, the summing point has 1V/oct when using 100k input resistors.

I have suggested two alternatives for R26, R27 and R38 - choose the one with the range you need for the input voltage. the 10Vp.p alternative will probably distort for higher amplitudes while the 20Vp.p is probably more noisy as it attenuates and amplifies more to run the core at the same amplitude.

R26 may be replaced with a 100k resistor and a 200k potentiometer (for the 20Vp.p version) to be able to tune the output VCA max amplitude.

U15 adjusts tracking, U14 sets initial frequency, U16 adjusts output symmetry.

PS: The filter may be tapped after the second pole (the output of U4) for 12dB instead of 24dB response.

Simulated circuit

0 - 10V resonance (original circuit from Juno 106). 1V/step

0 - 5V resonance, modified circuit. 1V/step

Still to do: check response curve for the VCA CV.