Freebase Dali

Look for an effect pedal that uses an LFO to automatically control your LP filter and allows you to set the frequency of the oscillations. Obviously, a manual resonance control will also come in handy if you want your sweeps to be defined.

What this basically is, is a wah pedal... except you're not manually controlling it. I know you're thinking 'auto-wah', but auto-wah uses additional parameters like threshold and other things. If you can get an auto-wah pedal that actually lets you change all its parameters extensively, you'll have what you're looking for.

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Roivas

Unless I'm mistaken, the wah effect utilizes a band-pass filter (which will roll off the low end, too!). Not what I'm looking for, but you're right about the LFO and resonance settings.

Maybe guitar pedals aren't the way to go?

Freebase Dali

There are most certainly wah pedals sweeping with low pass. There are also wahs that let you select between LP and BP.

Anyway, forget wah pedals.
this is probably something like what you're looking for:

Electrix Filter Queen
Electrix Filter Queen - Vintage Stereo Filter | DiscoFreq's Effects Database




If you don't like that one, here's a crapload more:
envelope-controlled, low-pass filter | DiscoFreq's Effects Database

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