Juno filter with higher possible cutoff

While trying to find a suitable 0-5V CV filter range I discovered that my design had some unfortunate limitations.

The maximum cutoff achievable was around 16kHz. I tracked this down to two factors: The 220k resistor from the VCA-OTA output to ground and the output filter capacitor (fixed low pass filter). Removing the output cap helped a lot, but I still could not get higher than 30kHz cutoff independent of control current.

Cutoff is max 30kHz and has very poor tracking at high frequencies. Current is the total current for all four OTAs, divide by 4 to get I_abc

I still had to change the resistor, which meant I could not get proper unity amplification at 5V CV. Compensating elsewhere in the circuit (input buffer or pre-OTA) distorted the signal at my wanted 10V p.p. amplitude (while such a distortion may be desireable, I want control over it and it was not something I planned now).

I switched back to the original design with a 47k output resistor. Then I thought, I can compensate for the loss using additional output amps. I added two inverting amps (to keep the output in phase) and this worked well.

From my previous experiments I knew that I could replace the resistor-to-ground feeding a buffer, into an inverting amp with the same resistor in the feedback loop. Doing this meant I would only need a single additional opamp to get phase correction and unity gain. But doing his also had a surprising effect - distortion and cutoff was immediately improved!

Using this and a constant current input to the OTAs let me run the filter up to at least 130kHz. This is completely unrealistic though, as it would require a higher control current I_abc for the OTAs than they can handle (max is 2mA), but past 60kHz could be achievable

Max cutoff is above 130kHz (though this would break the OTAs) and tracking is near perfect.

Increasing range

I then re-added the expo converter and got a surprise/reminder. I still couldn't push the filter past 24kHz. I simply could not get a control current higher than about 760uA. After trying a few adjustments in the exponential converter itself (changing reference current and emitter resistor) I turned to my exponential converter simulator and quickly discovered the problem - the current is controlled by changing the voltage drop across the resistor connected to the OTAs control pin. The pin is always around -13.3V in the simulator, and the transistor pulls its collector further and further away from this to generate a higher and higher current. But at about one diode drop above 0V it cannot go any further, it simply cuts off.

Plotting the voltage at the right collector of circuit A shows that it cuts off above at approx 0.63mV. 

Similarly, the plot of the current running through the 18k resistor shows that it cuts off at about 860uA. Note that this is slightly higher than the 760uA of the OTA control because the voltage difference is higher (-15.6V here vs -13.9 in the OTA control)

So, to get a higher current than this, I had to reduce the resistor value (remember, U=R*I, to get a higher I from the same U, R must decrease).

Doing this let me generate up to 1.38mA, putting the cutoff close to 40kHz.

Changing to a 10k resistor increases the maximum current.

Tracking using the expo converter

CV response (0.5V/octave) using expo converter. Blue line has 18k resistor to OTAs, red has 10k and no output filter cap, and has slightly better tracking. Also note that the response is the same even if the resistor value changed, the base frequency has not moved up/down.

Note that I have NOT tried to tune tracking when using the expo converter, it is possible that one could get a lot better tracking than this by adjusting U15. It is however also possible that there is something that prevents the expo converter from getting a perfect tracking over the full frequency range.

Output lowpass filter

I've added a 22pF output filter cap in, which seems to give a cutoff around 50kHz. This affects the tracking at high frequencies a bit too, so I'll have to do some testing.

New circuit

Final notes

I do not know if changing the max cutoff frequency or the output lowpass filter affects the sound. It is possible that this messes with the Juno sound. Maybe the cutoff should not go as high (though the manual says 5-50kHz or 4-40kHz). I like try it at least. One could also squeeze out an even higher cutoff by reducing the resistor even further. The OTA control pin is at -13.3V in the simulator but it could perhaps be even lower in the circuit, but never less than the negative supply (-15V). The transistor never gets past about 0.7V. This means the overall range will be below 16V. As maximum I_abc is 2mA we COULD use an 9.1kOhm resistor and get max I_abc 1.75mA (or an 8.2kOhm and get 1.95mA, but then we are really close to the