A name you often hear associated with classic analog filters is Doug Curtis, founder of Curtis Electromusic Specialties (CEM) and designer of dozens of ICs— VCOs, VCAs, and VCFs—that have been used in vintage and modern synths from Oberheim, Sequential Circuits, Linn, Moog, Simmons, and Doepfer, among many others.
The classic Curtis lowpass filter has attitude and is often described as brassy or aggressive, with a resonant frequency ranging from earth-shaking to ear-piercing—just what you need for percussion, bass, leads, and effects.
Dave Smith Instruments put new Curtis chips (not NOS versions) into its DSM01 Curtis Filter, a skiff-friendly 8HP Eurorack module with 2-pole (12dB/octave) and resonant 4-pole (24dB/octave) responses.
In addition to a direct filter output with CV inputs and knobs for frequency cutoff and resonance, the module has a VCA with dedicated I/O. Whatever you feed the VCA input (an EG, for example) controls the volume coming from the VCA output. Both outputs can be used simultaneously. A switchable boost of about 5.5 dB is also available at the filter input.
The onboard VCA lets you sculpt the sound right from the module and is especially handy when wringing out that juicy 4-pole resonance the DSM01 is capable of. You don’t even need an audio input: just crank the resonance and ping the VCA input.
Just for fun, I put the DSM01 up against my Sequential Circuits Pro-One, which has a CEM3320 filter: In 24dB mode, the module sounded identical to the synth, including its resonant behavior.
While that’s certainly impressive, I’m more interested in the fact that the DSM01 can be sweet or nasty, depending on what’s needed in a patch. Versatility and attitude is why you choose the Curtis filter sound, and the DSM01, with its built-in VCA, gives you plenty of both.