Patents

Patents are also useful for their descriptions of some musical instrument circuits and the approaches the designers took which may not be described to the same extent by other publications, although the variety of patent laws between countries can lead to contradictory information.

How patents work
A patent is the granting of exclusive rights to a novel product or process which must be disclosed to the public in a patent application. These rights lasts for twenty years but only in the country in which the patent has been filed and granted. "For most individuals and small scale startups, any involvement with the patent system is almost guaranteed to cause a net loss of time, money, and sanity."

Cloning vintage gear
If you clone a circuit for distribution your PCB artwork and schematic will have to be different to the original.
 * Novel circuits can be patented but not copyrighted.
 * Schematics and PCB artwork can be copyrighted

Specific patents

 * Music Synthesis Patents Collection collated by Aaron Lanterman 2005 - archived
 * Mellotron/Chamberlin patent reviews and analysis collated by J. Donald Tillman, September 2006
 * ARP patent reviews and analysis collated by J. Donald Tillman, September 2007
 * Moog patent reviews and analysis collated by J. Donald Tillman, September 2006
 * US3518578A, Signal compression and expansion system by Alan V Oppenheim and Thomas G Stockham Jr, 1967 - a diode version of the Serge gain cell.
 * US3714462A, Multiplier circuits by D Blackmer, 1971 - a transistor version of the Serge gain cell.
 * US3969682A, Circuit for dynamic control of phase shift by David Philip Rossum, 1974 - covers the SSM 2040 filter
 * US4404529A, Lowpass filter with electronic control of cutoff and resonance by David P. Rossum and Ronald N. Dow, 1980