Is there another filter mode hiding in the FX25?

[Mark Hammer]

Flipping through an old ETI projects issue this morning, I stumbled on a chapter about OTAs, and found a couple of schematics for VC filters.  This piqued my interest so I flipped through some other schematics for voltage controlled filters using OTAs.  It occurred to me that the DOD FX25 that Alex Petrini has a project for at topopiccione might actually have another filter mode hiding in there.  From the tone and output tap point, it would seem that taking the output after IC2a yields a bandpass output.  I can't absolutely confirm it but there might be a lowpass filter output avalable if the output comes from IC2b.  As near as I can tell, this involves redirecting C11 from the junction of R14/R16 to the junction of R19/R20.

I probably won't get mine finished until tomorrow so I can't confirm just yet, but I hope to tomorrow (Sunday).  In the meantime, if anyone can confirm on the basis of theory, that would be nice.  I don't expect it to sound anything like a nice fat 4-pole lowpass filter but hopefully it would retain some bottom and be more suitable for bass as well.

One of the nice things about lowpass filters is that with lots of resonance they retain the "focus" of a bandpass filter with emphasis at the cutoff, while not forfeiting the bottom the way a bandpass does.

A second feature I hope to explore is the variation of the recovery with a decay control.  I'm assuming that R9 behaves very much like the 150k resistor in the Dynacomp, by slowing down the charge-up of C7, and slowing down the falling back of the filter.  I'll try a 1M pot in series with a 220k fixed resistor instead of the fixed 1M resistor.

Got my fingers crossed.  This could be nice if it works out.

[hank reynolds 3rd]

I'm not that up on the theory, but I'm pretty sure the vcf's at the EFM site used a similar/same method for getting a lowpass/bandpass output ...they also had an opamp with a bunch of diodes for a resonance control (I think a lot of them are based around the Ms20's filter)



Sam