LFO Filter Freq Generating Its Own Sound?

Hey everyone!

Alright, this might be a total noob scenario…Recently I picked up Saint Stink’s Project File and noticed that there’s a track making sound with no synth machine or sample enabled.

I haven’t been able to completely figure out how this is happening, but I think I’ve isolated it down to the LFO and filter. It looks like the LFO has a square wave and its dest is set to filter frequency. The filter itself has maxed out resonance at the very low end. There are red trigs for the sound it makes when playing the pattern. I’ve also noticed that turning off the synth engine disables the sound, even though the synth machine is disabled.

Am I somehow hearing the filter on its own without a sound source?

I’ve been trying to figure this out all day, questioning my sanity! Does anyone have any idea what might be happening here?

Yeah, filters can resonate on their own when you turn up the resonace and you’ll get something like a sine wave, some filter provide a very clean sine, some not so and with keytracking set to octaves you can even play them with a keyboard. You’d tune the filter by turning the cutoff, if you set it to a C note and have keytracking, you can play the filter like an oscillator.

You can also “ping” filters, that means to send a short pulse into the filter which is almost self-resonating that will excite the filter to ring out. Sine waves are good for fm, because they are so simple, you can modulate it a lot before it turns into noise. Try some fast filter modulation on a self-resonating filter for fm tones.


Alright cool, makes sense. Thanks for the heads up!

I’ve heard of the self resonating concept before, but since this is my piece of hardware, I’ve never been able to mess with it myself.

Would you happen to know how I could test this out with the AR? How do I trigger the filter to self resonate in this case?

Also, when you say “tuning the cutoff”, I assume you mean moving the slope to the frequency that matches the specific note on the keyboard?

Apply an envelope to both resonance and the volume of the amp and you should be able to “trigger” the self oscillation.
I just got a Rytm and did not tried yet so I might be wrong.

I wanted to try to “ping” the filter of the Rytm with the impulse machine, will try today if I can.

You just turn up the resonance until it starts to resonate on its own, then just turn the cutoff frequency knob. It’s like the tune knob on an oscillator. You can use a tuner or an analyser to help with tuning. Keytracking on the Analog Four/Keys set to 32 is octaves. Tune to a C note, set keytracking to 32 and you can play with the keys like an oscillator.
Experiment with audio rate lfos, since a self-oscillating filter emits a sine wave, it’s well suited for fm.

I usually use an impulse machine on the Rytm, because I need the lfo for fm, amp decay set so it nicely rings out, but you can also ping it by using an lfo targeted at the filter frequency.
You probably can also ping it with an lfo on resonance…

On the A4/AK you can use a narrow pulse to excite the filter, similar to the impulse machine on the Rytm. Send a short burst (short note length) square wave with narrow pulse width into the filter with resonance short before self-oscillation (turn the resonance up to self-oscillation, then turn it back a bit).