I’ve posted some things about pole mixing filters. Here, finally, is a post about an actual pole mixing filter.
I don’t have the final front panel yet, but this module seems to be working:
It’s a multimode VCF based on a (partial) design by Electric Druid. I made a number of modifications but the basic design is not really changed.
It uses pole mixing to achieve different filter responses. The Shape rotary switch allows selection of two different low pass, two high pass, two band pass, and two notch filter shapes. The latter two can be altered by changing resistors on the PCB; enough footprints are provided to get just about any possible filter shape. In addition there’s an expansion header potentially allowing use of a daughterboard or expansion module for additional filter shapes or other features (though this has not been tested yet).
The design is based around the Alfa AS3320 filter chip.
Aside from changes I talked about earlier, some other changes I made:
The Shape rotary switch allows selection of two different low pass, two high pass, two band pass, and two notch filter shapes. The latter two can be altered by changing resistors on the PCB; enough footprints are provided to get just about any possible filter shape. Besides that, I added an expansion header potentially allowing use of a daughterboard or expansion module for additional filter shapes or other features (though this has not been tested yet).
The signal input section uses an inverting amplifier to sum the inputs. Usually you don’t care about audio signal phase, but occasionally you might, so I added a second inverting stage to restore the phase… except I apparently was overlooking the fact the output stage also is inverting, so I was making the correct phase incorrect. Oh well…
Also, a 5 V or -5 V input would result in filter stage outputs of about 10 V, which is too large. With Vcc = 12 V, maximum rated output without clipping is 9 V. So I reduced the gain on the first input stage by changing the 100k feedback resistor to 51k. Then on the output stage I increased the gain to compensate.
A minor thing: To do the pole mixing, the ED design uses resistors with values 30k, 15k, 10k, 7.5k, and… well, good old standard resistor values so far but not here: 5k isn’t one but 4.99k is, so they use that. It’s standard but not that standard — it’s E96 series, so is harder to find and more expensive when you do. But 2k and 3k are very standard (E24), so it’s cheaper and less trouble to use those in series instead.
ED uses resistances of 2 x 30k and 2 x 30k + 1 x 15k for the band pass filters. But those values result in an output amplitude that’s 6 dB and 12 dB below the input, respectively, at the filter peak. To get the peak output up to the input level I prefer using 2 x 15k and 2 x 7.5k + 1 x 3.75k. No, 3.75k is not a standard value, but 3k and 750Ω are, so, again, you can use those in series.
In my first iteration the mixing didn’t work because the mixers not selected by the rotary switch had their outputs floating. This polluted the LP1–4 signals going in to the mixer that was selected. To fix this I added op amp buffers between the AC coupling capacitors and the mixers.
Schematics, layout, Gerbers, documentation, and a Python script to generate Bode plots for different pole mixes may be found at https://gitlab.com/rsholmes/pmf.
Analog Output 