onsdag 25. oktober 2017

Jupiter 6 filter clone - initial simulation

After the success with simulating the juno filter, I tried the same with the Jupiter 6 multimode filter today. I struggled a lot before I got things up and running - among other things I forgot to connect the power to the resonance OTA of the first cell, and I never got the resonance control from the Jupiter 6 schematic to work so I swapped it for the one from the Juno.

After that however, things turned out VERY nice! What's interesting is that the circuit is exactly the one from the datasheet, with the same component values (replacing the OTAs with LM13700 without the output transistors, and using separate opamp buffers) - and the frequency response tracks the one from the juno clone very well. I got a slightly better tracking by switching from 330pF caps to 323pF, but 330pF will work just fine.

Here are a few plots before I go into details about some discoveries. The output VCA has been tweaked to give us a 0dB output for pass frequencies of the 24dB low pass filter>

This first one shows the frequency response of four of the five possible variations of the two cells. Here the LP output of the first stage is connected to the second stage:

  • Red is LP (12dB) + LP (12dB), or a 24dB LP
  • Green is 12dB LP tapped after the first stage
  • Blue is 12dB HP tapped after the first stage
  • Turquoise is 12dB LP + 12dB HP, or a 12dB BP

In the second one I've connected the HP output from the first stage to the second stage

  • Red is 12dB HP + 12dB LP, or a 12dB BP, similar to the option from plot 1
  • Green is 12dB LP tapped after the first stage
  • Blue is 12dB HP tapped after the first stage
  • Turquoise is 12dB HP + 12dHP, or a 24dB HP

The -3dB cutoff (for the 24dB LP filter) is at 727Hz (measured)

As for the discoveries: For a long time I could not get rid of the resonance. I even tried changing the feedback resistors that tap the stages and send them to the resonance OTAs, but it just didn't want to go away. But then I realised that I had forgotten to connect power to one of the resonance OTAs.

So what did I learn from this? Well, without the resonance OTA, the circuit actually self-oscillates. By adding parts of the signal tapped after the first 6dB cell, we cancel out the feedback that leads to oscillation (or at least that's what I think happens).

This leads to another realization - the control signal for the resonance is reversed. In the Juno, 0V of resonance CV turns off resonance and it increases the closer to 10V we get (also, I think the transistor in the Juno resonance CV circuit acts as an exponential converter, but I have not checked this). In the Jupiter 6 filter, when using the same resonance control circuit, 0V gives a huge (80dB) spike!

Maximum resonance

At 10V resonance is fully turned off. I have yet to confirm that 10V is indeed the correct max voltage for turning off resonance, but with this setting the plot looks exactly like it does for the juno with resonance CV=0V

Oh, and as a closing note - turning up the resonance does NOT attenuate pass frequencies. In the Juno, frequencies below the peak are attenuated up to 8-10dB - but more of that in a separate post comparing the two filters. I'll also post the circuit and work out the necessary control circuitry to get 1V/octave responses for the cutoff, but that's for later.

Update: I just came across a Jupiter 6 filter clone, I belive it is by the guy that writes at polysynth.blogspot.com. It says that the clone is open source, but for my own sake and the fun of working things out by myself, I'll try as hard as I can to not look at his clone before I finish mine :-D

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