Hey I’m wondering of there’s a schematic or way to make the ms-20 filter (based on the LMNC ms-20 filter design) output the high pass signal at the same time with the low pass filter. I know the basic design and schematic only outputs the low pass signal, and there is a version that switches to high pass by using a switch. However, I’m curious if it’s possible to have it where the input signal can be filtered by the high and low pass filters at the same time and the cutoff for both low and high pass can be controlled individually and have the result only come out of one output, or because of the lm13700 design, the only way to achieve this is by putting the same signal into two separate filters, one and high and the other on low, and mixing the results on something like a mixer? Does anyone know?
You can apply the same signal to both the low and high pass inputs. The result will be a notch filter I believe. (I can’t find where it says this but a simulation of a similar filter design suggests it.) However you will not have individual control of the low and high cutoff frequencies. There’s only one cutoff knob! In principle you could replicate the CV section and send different CVs to the two OTA stages, but you would not want that for normal LPF operation, and in any case I think the low cutoff (at least) would be affected by both CVs.
Kassutronics design has got a fader between the LPF and HPF.
Thanks for the link. It says
It blends from a two-pole low pass, via a resonant allpass, to a one-pole resonant high-pass.
I went back to my simulation and discovered I had the input wrong on the second OTA — with that corrected then indeed with the same signal going to both inputs you get an all pass filter. So not much of an effect on the sound when resonance is turned down.
Until you put a bunch of them series and you’ve made yourself a phaser 
Well, put a bunch in series mixed with the input. One APF mixed with input will give you a notch.
Hi everyone. I having trouble understanding the Hi Pass resonant section of those MS-20 style filters.
In order to have independant control on HP et LP would it be possible to duplicate the second filter stage ?
I would like to achieve something like this this : Audio in → Hi Pass Filter → Low Pass filter → Audio Out.
To keep CV control and resonnance on the HiPass Filter, would I have to duplicate both stages of the filter ?
Thanks
Yes. It’s in a feedback loop with the first stage, the whole loop would have to be duplicated.
That would be equivalent to a band pass filter. You might consider something like this filter which resembles the MS-20 design but has LP, HP, and BP outputs:
There’s no clipping in the resonance feedback but maybe that could be implemented if that’s what you want.
Thanks a lot, that design is really interesting.
If I duplicate the circuit, and HP and LP have independant cut off knob, it wouldn’t act like a band pass right ?
Right, I mean it’d be a band pass of sorts, but different shape than a single cutoff one.