With a 3.9k input gain feedback resistor I could easily pass a 20V p.p. wave through the filter without distortion. But I wanted to see if I could get a distortion similar to the moog filter, and yes, I could.
Swapping the 3.9k resistor with a 33k makes the filter overdrive close to 10V p.p, quite similar to the Moog.
This is of course pre-filter amplification. I have seen people talking about the minimoog doing feedback of the original signal through the external input jack, and this sounding better, so I'll try that next. I also need to come up with a good way to control overdrive, one that is not so dependent on input amplitude.
|Input (green) vs output (blue). 33k input resistor, 50k output pot|
This being a state variable filter means it can produce a multitude of filter variations at the same time - low pass, band pass, high pass and notch. It is also two filters after one another, which means we can get various falloff. I've played around with this and come up with 10 variations that are more or less usefull:
6dB BP + 12dB LP
6dB BP + 12dB HP
Notch (first SVF)
Notch + LP
Notch + HP
Notch + BP
I am not sure of the usefullness of all these but the cost to add them all is very little. Here is how I indend to wire them, with resistor values giving the following 'plateau' gain.
Constant current inputs
Just like with the Juno and Moog filters, I've swapped the resonance and vca gain CV circuits for my own, linear designs. The VCA gain control is exactly the same as for the Juno (but with slightly different part values), and has a similar deadband. The resonance on the other hand, is different. The resonance circuit works opposite of the one in the Juno, increasing the resonance OTA gain reduces the amount of resonance.
Because of this, increasing CV must decrease the output current. Also, when doing exponential conversion in software later, we must generate a negative exponentially decaying signal instead of an exponentially increasing one, which is too bad as it means that we cannot have a common control system for all filters. I will have to look closer into this.
|CV vs I_abc for one resonance OTA, original Jupiter 6 circuit. With a linear response, a similar CV curve must be calulated in software.|
Oh, and because of the way I did the linear control, we don't get a deadband.