In my Tonewheel Organ turorial, I recommended using gain compensated filters but didn’t say much about them. When increasing a filter’s resonance, you not only create an amplified peak and increase the cutoff slope but attenuate the pass band. Gain compensated filters, as the name suggests, are engineered for less attenuation of the pass band.

In the tutorial, that quality allowed me to use filters for both subtractive and additive effects; in general use, gain compensated filters just sound different. In subjective terms, I find gain compensated “creamier” and “punchier,” which I like. For reference, the classic Yamaha and Oberheim synths used gain compensated 12 db/octave filters. (To my knowledge, the only new analog synthesizer with gain compensated filters is the Futursonus Parva, which I think sounds great!)

The cost of gain compensation can be clear harmonic sweeps, though that varies from filter to filter. The distinctive sound of very high resonance levels in a non-gain compensated filter is out of reach, as well, though I’ve used gain compensated filters to mimic non-gain compensated filters but never the reverse. If you’re unsure of whether or not a filter is gain compensated, trigger a sine wave ( pitched well into the pass band) and raise the resonance; how much is the sine wave attenuated?

I made a little demo, comparing (Reason’s) Subtractor’s (gain compensated) and Thor’s (not) filters in their respective 24 db/octave low pass modes. (Subtractor first, with each example played twice.) I used Subtractor’s filter envelope to control both filters and two cutoff frequencies and two resonance levels. I hope it illuminates the different sounds of the two filter types for those hadn’t yet encountered it.

I made some basic adjustments to the main fader to keep the levels in the same range for each example but you’ll have to tolerate some variance – I didn’t have much time and it was difficult!