Ambika multimode filter- why 12 db?

Why are all multimode filters always 12 DB? i get it for bandpass and even highpass sounds but for lowpass isnt a lot of bass and deeper sounds lost when having only a 12 db curve?

Four pole filters take up more room on the board. With the small voicecards on Ambika you can’t fit a multimode 4 pole circuit on there. It may be possible with surface mount, but that’s not then DIY friendly.

i got idea - make “joined” filterboard - one bigger board which will contain 3 voices together, with all connectors on right places to fit current ambika MOBO layou… maybe then little bit bigger space will make possible to do 4P multimode filter… is just idea, maybe it is nonsense :slight_smile:

The simplest structure to build a multimode filter is the state variable filter (SVF). It has a 12dB curve. Pretty much all multimode filters found in synths are some flavour of this. It’s a nice circuit that doesn’t take much space…

You can get a steeper curve for the LP mode by chaining a second SVF after that (this is how some Roland 24dB LP filters are built!) ; but the problem is that you’ll get a strange resonance. What should happen when you increase resonance? Should this affect the resonance of the first section only, or of both sections? In both cases the result is strange, not as good as true 24dB filter.

The other approach is pole-mixing - from a classic 24dB cascade. This gives you the classic low-pass with 24dB curve, and all kinds of fancy responses… But it requires more parts to achieve, and then you don’t get a 12dB resonant HP as “radical” as a SVF. You know, the bass-boosting 12-dB resonant HP…

In the end, the best thing to do would probably be to have 2 circuits - one SVF for the BP/HP/LP2 modes ; and a cascade for the LP4 mode. I think this is what Elektron did on the A4.

None of these options are possible for Ambika given the size constraints…

i see…

Or, you can buck conventional wisdom, and use the oscillators to shape your bass sound. A 12dB slope will leave a nice shimmer on your bass frequencies. Also, you may want to turn off the key tracking for a “darker” tone.

@pichenettes The A4 indeed has a 4-pole low pass ladder followed by a 2-pole multimode filter. What’s interesting about the second filter is that it also includes even less steep 6 dB/octave LP and HP modes. Especially the HP1 mode is very nice for sculpting the low-end of bass sounds since you can very nicely reduce the lowest sub frequencies while boosting what’s immediately above that very precisely using the resonance parameter.

But the Elektron A4 would never have been the same had it been a DIY kit.