Here's a trick I stumbled across-- if you send the same source into the two halves of a ring modulator (in this case, a Frostwave Blue Ringer), it acts a band pass filter.
In the video above, I alternate between modulating a tone with itself and modulating it with a carrier oscillator, as you would in a typical ring modulator arrangement.
To begin with, a Voice of Saturn Sequencer is sending control voltage to a Plan B Elf LFO and gate pulses to a Doepfer A-140. The triangle out from the LFO and the envelope from the 140 are combined in the top half of a Doepfer 132-3 and sent out to a mixer.
(An old, noisy Akai Headrush is on the mixer's Aux send, and all four of its outputs are returning to their own channels on the mixer, as well.)
0:39 the VCA output is re-routed to a multiple and sent to both inputs on the Blue Ringer.
1:06 The mix knob is swept from Dry to Wet and Input 2 modulates Input 1.
1:42 The modulator source knob on Blue Ringer is swept from a mix of Input 2 and the unit's internal oscillator, to the internal oscillator only and finally to signal in Input 2 only.
2:25 Changing the volume on Input 2 and Input 1 changes the frequency of the band pass output (slightly).
2:35 Mix knob is swept from fully wet to fully dry.
I apologize for the horror movie lighting-- we had a green light bulb left over from Halloween and I couldn't resist.