SSM2040

The SSM2040 IC is a 4-pole filter from Solid State Micro Technology For Music (SSM). Regarded as one of the best sounding filters, because of the way resonance and small amounts of soft 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.

Circuit building block
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.

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 SSM2040 filter by René Schmitz, 1999
 * 4-pole VCF by Osamu Hoshuyama, 2002
 * The SSM2040 Integrated Circuit, by James Gideon, 2006
 * SSM2040 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 SSM2040 or clone

 * AMSynths AM8040
 * Digisound 80-7. In 1982 this was replaced by the CEM3320 based 80-7A which offered the same features but with "lower noise and improved resonance level".
 * Division 6 Filtare SEIII
 * E-mu Systems Blue Box and Audity
 * E-mu Systems SSM Voice Card
 * Hearn Morley HM2040 potted module.
 * Mad Science Lab SSM2
 * Mattson Mini Modular FV1-B, based on the SSM2040 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

Projects

 * Build the PGS Lowpass Voltage Controlled Filter, Common Mode, January 1983, Number 1, pp. 10–15
 * SSM2040 style VCF + discrete VCA (AMORE), 2011. Both circuits by René Schmitz.
 * Synthesizer Phase Shifter by Thomas Henry, Polyphony, October 1984, pp. 14–17.
 * Discrete Transistor 2040 SVVCF by Scott Stites, electro-music.com, Apr 2013