Fixed Filter Bank Pedals

[le lambin]

What's the advantage of a fixed filter bank (vs not fixed)?

It’s a different way of filtering- instead of sweeping the cutoff frequency of a variable filter, you’re adjusting gain of a bunch of non variable filters. It’s just a different way of subtracting frequencies, compared to a traditional lpf or eq.

The q of the filters is pretty narrow and I think they are 12db/oct for the band pass filters- since the incoming sound is split among the filters, when you lower the gain of a filter to zero you actually remove that frequency entirely, which is kind of cool. You can get really cool resonances especially if you put it in a feedback loop of some sort. I think it was originally designed to shape oscillators to mimic acoustic instruments but I’ve never used one for that.

I’m no expert though I’m sure someone else here can shed more light on the differences between fixed vs. variable.

[manymanyhaha]

Thanks for that, I had no idea. Been messing with VST filters for awhile now and had not thought about fixed v variable at all. That's very cool, gonna have experiment with that!

[Zork]

Isn't the Moog MF105 a fixed filter bank pedal?

[le lambin]

Isn't the Moog MF105 a fixed filter bank pedal?

Dang you’re right! I always wrote this off as a sequenced filter, but you can turn the sequencing off and voila, fixed filter bank! Although you’ll have to choose between two sets of frequencies. Thanks for mentioning it, I’ll look into it further.

[le lambin]

Yeahhh so the Moog is still super expensive- even the non-Midi murfs are $500 and up- still cheaper to cobble together a guitar to eurorack solution. Plus I can swap out filter modules as I like, which is potentially fun.

I predict a few things in plugging a guitar into one of these, however:
-I’ll need significant boosting/overdriving of the guitar signal going into the filter bank to get interesting results- I don’t think there is enough information in a vanilla guitar signal for the filters to “filter” in an interesting way
-a dirt circuit of some kind after the filter bank would probably also be fun
-feedback control of the filter bank circuit would likely be the logical next step for me to try, as it would allow the guitar signal to excite the filters more

Anyway…excited to try this.

[Zork]

Yes, prices for the moogerfoogers are insane unfortunately. If you go Eurorack, maybe you can track down one of the Pittsburgh Modular pedal enclosures: https://pittsburghmodular.com/patch-box
They are rare, so possibly expensive, too, but totally awesome.

[le lambin]

Yes, prices for the moogerfoogers are insane unfortunately. If you go Eurorack, maybe you can track down one of the Pittsburgh Modular pedal enclosures: https://pittsburghmodular.com/patch-box
They are rare, so possibly expensive, too, but totally awesome.

Man I really wish I was bridging the euro-pedal gap back when those were available- it looks freakin amazing. I'm probably going to go cheap 4MS powered case - cheapy I/O module (used Strymon or something else) - fixed filter bank. I was going to start with the Behringer one and then go from there- Doepfer looks cool too.

[retinal orbita]

I had an EHX Interstellar Orbiter for a while but it never quite worker for me exactly the way I want, I sold it and regretted it - is that in the same realm? It was hella fun.

[le lambin]

I finally cobbled together a tiny modular thing with Strymon AA.1 -> Behringer Fixed Filter Bank Moog Clone. As i suspected, vanilla guitar into the fixed filters sounds pretty meh. I put a Fairfield barbershop before the filter bank and instantly it was more interesting- the filters had some nice harmonics to latch onto. I then put a Greer Southland clone after the filter bank and it also sounded pretty good. I turned both the barbershop and the southland clone on, and it was fucking NASTY. I think thats the key here- having gain before and after the filter bank simultaneously. I got all kinds of cool flavors of distortion/fuzz with really intense EQ settings. I think there is something here- I'd like to see how the filters are constructed, because I know the Behringer doesn't use inductors. I'd love to put together a circuit with dirt -> inductor based fixed filter bank -> dirt, and tweak the individual filter values for guitar & bass. Using wah inductors would be crazy expensive, maybe there is a way to reduce the number of fixed filters, but make a few of them parametric, so you can choose which frequencies you want? I don't know- but I'm excited to try out some stuff!

I'm also going to put the filter bank in the loop of a clean delay and try to get some cool "screening" effects, see what happens.

[le lambin]

Okay- I’ve put the Behringer ffb into the feedback loop of a Boss dd500- with some gain and compression thrown in there, you can get some very cool tuned feedback going. This got me thinking- what if you could tune the filters to musical frequencies instead of the ones on the panel? Then you could play a riff/chord in C, tune maybe 3-4 filters to the notes in C, and have a really cool feedback “chord” behind your playing.

There are no such filter banks with multiple variable bands- but the Source audio eq2 has 10 completely customizable parametric filters- I think tuning the bandwith just right, I can get similar results to a ffb. I’m gonna order one, throw it in the loop and see what happens.

[le lambin]

The filter bank odyssey continues-

I got the SA EQ and via Nuero app, created 3 presets, each having 10 bands of parametric EQ tuned to a C major scale. I basically just set it up so each preset is an octave apart, to see how the different octaves behaved in the loop of the DD-500. I also put an overdrive and a compressor after the eq in the loop to try and control things a bit.

The results were very cool but requires further tweaking- the tuned feedback is very reactive to what frequencies are being pushed, and the gain of each frequency- even a 1db change makes a massive difference in the resulting feedback. The compressor I was using wasn’t nuanced enough to do what I wanted- I needed pretty heavy compression to keep the feedback from going out of control, but it was too heavy handed- I need something with a side chain insert so I can selectively apply it to certain frequencies. I have an empress compressor around here somewhere, I should try it…

Anyway, the experiment overall was a success. Setting up the eq via the neuro app is obviously not a quick or easy solution to tune each frequency band- you could make a bunch of presets and recall them with midi, but I think it would be more fun to have each frequency tunable, like a parametric eq with 12 freely assignable frequency bands. It would no longer technically be a fixed filter bank, but who cares.

I think it would also be fun to have a vca connected to each frequency, for tremolo effects, etc.