You can freely build on How Robert Moog Used Transistors to Shape Synthesizer Sounds
This patent expired in 1986. Every claim — 0 independent, 0 dependent — is now unenforceable. Anyone can use, reproduce, manufacture, sell, or offer for sale this technology without a license.
Original assignee
ROBERT A MOOG
Patent granted
1969
Expired
1986
Forward citations
7
What this patent covers
This patent describes a method for building electronic filters by utilizing the base-to-emitter diode resistance of bipolar transistors. By controlling the current flowing through these transistors, the engineer can change the filter's cutoff frequency, which determines which sound frequencies are blocked or passed. This mechanism allows for the smooth, voltage-controlled adjustment of audio signals, which is essential for creating the expressive, sweeping timbres found in electronic music.
What is now free to use
All 0 claims of US 3475623 are in the public domain. Specifically:
The 0 dependent claims add narrowing limitations and are also free.
What is NOT covered
Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.
Does not cover digital signal processing or software-based filtering algorithms.
Does not cover passive filter circuits using only resistors, capacitors, and inductors.
Does not cover vacuum tube-based audio filtering circuits.
Who is building on this today
Moog Music continues to manufacture instruments based on this core architecture. Many boutique synthesizer companies and DIY modular synth manufacturers also produce variations of this ladder filter circuit.
Products built on expired version of this technology
Moog Minimoog synthesizer
Moog modular synthesizer systems
Eurorack modular filter modules based on the ladder design
How to cite this patent in your documentation
ROBERT A MOOG. US Patent 3475623. Electronic high-pass and low-pass filters employing the base to emitter diode resistance of bipolar transistors. Granted 1969, expired 1986. Now in the public domain.
Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.
PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.