Help for a noob about low-pass filters

[Shroom]

So I have a filter I want to build that's a little complicated, but I'm not very knowledgeable about electronics. I know some basics about first-order RL and RC filters, and I did some reading about the Sallen-Key set up, but I'm still pretty confused, so I was hoping you guys could point me to some reading material, builds, or just advice that could help me out.

I want to make a low-pass filter with a steep cut-off at a frequency controllable by both knob and expression pedal, if one is attached. I'm not really interested in any resonance, because I want to use this as both a bass guitar effect, and a filter that I can send music through to use my bass amp as a sub during parties or whatever else, although I guess a resonance knob would take care of that issue. Active or passive is not important, unless either of these gives the circuit characteristics that are better-suited to my needs.

I guess the minimum I need is to know the best/simplest type of circuit to do this, so I at least have a direction to go in to do some research on my own. Thanks for anything you guys can give me 

[PRR]

> a steep cut-off at a frequency

Define "steep" and "range".

6db/oct is easy, but mellow. 36db/oct is very hard.

4:1 range of freq is easy. 1,000:1 range is hard.

In music, 12db/oct 2-pole is your first approach to "steep".

Steep passive low-audio filters need awkwardly large coils. Do active.

Manual adjustment is just the equal-resistor 2-pole filter with a 2-gang pot.

Remote control (pedal) is NOT simple. See if you can find such a thing ready-built.

[Shroom]

> a steep cut-off at a frequency

Define "steep" and "range".

6db/oct is easy, but mellow. 36db/oct is very hard.

4:1 range of freq is easy. 1,000:1 range is hard.

In music, 12db/oct 2-pole is your first approach to "steep".

Steep passive low-audio filters need awkwardly large coils. Do active.

Manual adjustment is just the equal-resistor 2-pole filter with a 2-gang pot.

Remote control (pedal) is NOT simple. See if you can find such a thing ready-built.

How difficult is 18 or 24 db/octave? As far as the expression pedal, I don't want to build that myself. I just want the filter to have an input for a store-bought one, if possible.

Thanks a lot, by the way

[R.G.]

As Paul mentioned, steeper is harder. Harder yet is one-knob control.

The closest fit to what you're defining is a voltage controlled double Butterworth.

The Butterworth filter is a setup with the least ripple in the passband. Other filters, notably the Chebyshev, have steeper rolloffs but cost you ripple in the passband.

Single knob control is harder. Unless you can get quad-, hex- and/or octuple-section pots with close tracking (and you can't, at any reasonable price), you're looking at doing a voltage controlled filter with multiple sections controlled from the same control voltage, and tinkering to get them to track. At least if you stay analog.

If you can accept going partly-digital, there are fourth-order (24db/decade) filters which can be driven by a clock that is 100x the cutoff frequency. Then all you have to do is use the single knob to track the clock frequency to where you want it, and pre-filter any audio going in so it has no frequency content more than about 1/3 to 1/2 the clock frequency to prevent aliasing with the sampling frequency. That filter needs to be analog for best results. And then you have to make the clock track the expression pedal to get the expression pedal input.

You may be guessing by now that your apparently-simple request has some hard-to-fulfill requirements hidden in it. Can it be done? Sure. But it's not a two-opamp circuit. It's probably a four-OTA circuit with a few other opamps to keep track. Or an MF-10 digital lowpass filter and some other stuff around it to make it work.

For a look at a some filter designs and background, go here: http://sound.westhost.com/articles/active-filters.htm#intro
For a look at some fixed frequency double-Butterworth filters (they're referred to as Linkwitz-Riley) go here: http://sound.westhost.com/project09.htm

So the task is converting those fixed frequency filters, with the selected ratios of 1% resistors, to track with an input voltage. That's the multi-pot way or complex resistor-faking way. Designing a second-order Butterworth lowpass that's voltage controlled, making at least two of them be identical, and running them in series is the OTA method.