[sdiy] Best topology for very low frequency resonant filter (Jewelrybox Ring Modulator revisited)

[anthony]

Today I decided to work someo more on my Japanese Girl-toy musical 
jewelrybox RING modulator. I was going to have just a simple transistor 
based phase-shift oscillator along with 2 buffer amps and the 
diode/transformer ring modulator. Then it hit me, "You know... a resonating 
filter is pretty much the same thing as a phase-shift oscillator..." So I 
decided that a filter alongside the ring modulator would be way cooler and 
more versatile and I can still use the filter to modulate the signal either 
when it's not resonating for a lot of mix-mashed tones and when it IS 
resonating as if it's being modulated by more or less a sine wave.

Questions:
What is the lowest practical cutoff frequency limit for a low-pass (or 
band-pass) filter?
What would be the best topology for the lowest possible frequency? (12-30Hz 
would be good lower limits for me)
What would be the best topology for a resonant filter to be used like a PSO?

I have a CGS Synthacon Tribute Filter on a breadboard that I thought might 
work well. I have a feeling a diode or transistor ladder filter with a 
24dB/oct response would be best. (I think the Synthacon is only 12dB?)

But I think of the synth circuits that I've seen for very low frequency 
filtering they've all been Sallen-Key I think...

I'm not looking to use this as anything like a normal tremolo since I'm 
pretty sure I'll have almost all of my tremolo bases covered with the 
Vibrobox clones I'm making and the wild Gate Sequencer/Lowpass Gate combo 
I've just about finished up, But I'd like for this to be able to get really 
slow. I know that this lower limit is actually going to be dictated by the 
transformer in my ring modulator and that an analog multiplier or balanced 
modulator chip is the way to go for ring modulator/tremolo, but I'm hoping 
to be pleasantly surprised by my results (which I will promptly report back 
here...).

cheers,
aa