SSM2040

The SSM 2040 is a 4-pole filter from SSM. The device's cutoff frequency can be exponentially voltage controlled over a 10,000 to 1 range. It is an extremely flexible circuit building block which can be used in virtually any active filter design including low-pass, high-pass, band-pass and notch. Roll-off characteristics can be selected to be Butterworth, Chebyshev, Cauer, Bessel or any other filter type. Regarded as one of the best sounding filters, because of the way resonance and small amounts of sof distortion when overdriven, can add harmonics to the original signal.

History
Designed by Dave Rossum of E-mu Systems and used in the first 2 versions of the Sequential Circuits Prophet 5. Early SSM chips were often defective, and having to pay royalties led Sequential to switch to Curtis ICs in later versions of the Prophet 5.

Clones and schematics
In chronological order:
 * discrete SSM2040-style filter by Jürgen Haible, 1997.
 * SSM2040 Style 4 pole Filter by Jürgen Haible, 1998. A discrete version that reproduces the original sound. A lot of component selection is needed (matching transistor pairs). The circuit is not a pin for pin replacement, and it has a voltage controlled resonance path added. There'a little error in the drawing: the pnp current mirrors have C and E mixed up.
 * SMD SSM 2040 filter by René Schmitz, 1999
 * 4-pole VCF by Osamu Hoshuyama, 2002
 * The SSM 2040 Integrated Circuit, by James Gideon, 2006
 * SSM 2040 style VCF + discrete VCA (AMORE), 2011. Both circuits by René Schmitz.
 * Discrete Transistor 2040 SVVCF by Scott Stites, electro-music.com forum, Apr. 2013

Synthesizers etc. using the SSM 2040 or clone

 * Digisound 80-7. In 1982 this was replaced by the CEM 3320 based 80-7A which offered the same features but with lower noise and improved resonance.
 * Division 6 Filtare SEIII
 * Hearn Morley HM2040 potted module.
 * Mad Science Lab SSM2
 * Mattson Mini Modular FV1-B, based on the SSM 2040 designed by Jim Patchell
 * Radio Junk Box 2040 VCF
 * RSF Kobol Expander I
 * Sequential Circuits Prophet 5 Rev. 1 & 2
 * SMB-1 Project
 * SRM Synthesizer Octave Cat
 * SSM2040 VCF by Ryan Williams, March 2007.
 * Steiner EVI
 * Synthesis Technology MOTM-440
 * Tiptop Audio Z2040
 * VCF/Phase Shifter by Craig Anderton, AMS-100 guitar processor, Device 6, 1979
 * Vedic Scapes Model 2040, uses the HM2040
 * Voltage Controlled Phase Shifter module by Craig Anderton, AMS-100 guitar processor, Device 4, 1979