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1 MU Multimode Filter
MUFF WIGGLER Forum Index -> 5U Format Modules  
Author 1 MU Multimode Filter
A while ago, I was working on a dual multi-mode filter in a two MU panel. Since making it a dual was adding to the packaging and cost issues and since the current preferences seem to be for a 1MU package for filters, I decided to concentrate on a single channel filter in a single width module.

The GMS-214MU is a four pole OTA based filter with a mixing matrix and analog switch that allows the filter to function as any one of fifteen possible pole and passband combinations.

Here is a list of the mode profiles:

  • 1 Pole Lowpass
  • 1 Pole Highpass
  • 2 Pole Lowpass
  • 2 Pole Bandpass
  • 2 Pole Bandnotch
  • 2 Pole Bandnotch + 1P LP
  • 2 Pole Highpass
  • 2 Pole Highpass + 1P LP
  • 3 Pole Lowpass
  • 3 Pole Highpass
  • 3 Pole Highpass + 1P LP
  • 3 Pole Allpass
  • 3 Pole Allpass + 1P LP
  • 4 Pole Lowpass
  • 4 Pole Bandpass

The profiles are selected using two front panel pushbuttons and LED displays as shown here:

Here are some frequency plots of a couple of the modes. The first is a 2 Pole Band Notch with 1P LP:

The second is a 3 Pole Allpass which is more commonly known as a "phasor":

Frequency control CV can be input through a 1V/Octave input or an FM CV input that has an attenuator. The filter resonance can be set using a front panel control and resonance CV input. This input has an attenuverter control. If resonance is pushed high enough, the filter will start to oscillate.

There are two audio inputs that are mixed with a passive X1 mixer.

The filter in MU is available for preorder now. This is a link to the website page for the filter: GMS-214MU The preorder page in the Synthstore is: GMS-214MU Preorder

The plan is to make the filter also available as a 12HP Eurorack module.
Any audio demos?
When you chance profiles does it make unwanted sounds (some filters make pop/click sound when you change the pole settings)?
Dave Peck
Well this looks really interesting! Reminds me a lot of the Oberheim Xpander filter - is there a connection?
Holy moly John, that looks enticing indeed.

I'm a bit shaky on the allpass settings - I was told that is how phase shifters are made? (with allpass filters that change ΓΈ as a function of frequency), so.....would those 3-pole allpass settings emulate a phaser?

And how might that be different than just sweeping the bandnotch settings?

Maybe you should send me one so I can see ;-) hihi
I should have some audio demos up within a couple of days. I have a killer bagpipe patch going right now.

It should seem very similar to the O-Xpander, only with a slightly different chip from the same chip family.

The allpass is similar to the notch in that a phase inversion occurs where the null occurs so frequencies above the null are inverted in phase. The null frequency in the allpass is twice the frequency of the knee. That is what gives a phaser the hollow sound. In a notch, the inversion occurs at the frequency of the knee. I just noticed I spelled phaser wrong too. Should have set it to stun.

The filter is, by necessity, AC coupled (no DC shall pass, if you need that, a state variable filter should be chosen) but the matrix mixer is on the output side of the coupling so switching between the various modes should be relatively noiseless as the signals are ground referenced.
this looks pretty wild, I'll grab one when it's out
I have posted an audio demo for the multimode filter. It isn't a song, just a bunch of sequencer loops processed in various ways with this filter. There is some FM modulation applied to the filter but resonance was controlled manually. The track was recorded with some hall reverb and no other effects. I think it will give an idea of the range of this unit.

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