How Robert Moog Used Transistors to Shape Synthesizer Sounds

A 1969 invention by Robert Moog that uses the internal resistance of transistors to create the iconic filters that define the sound of analog synthesizers.

What it 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 it doesn't cover

• —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.

The clever bit

Moog realized that the base-to-emitter junction of a transistor acts like a variable resistor that changes based on current, allowing for a precise, voltage-controlled filter that remains stable and musical.

Why it matters

This circuit is the foundation of the 'Moog Ladder Filter,' arguably the most famous component in the history of electronic music. It enabled the transition from bulky, unreliable modular systems to the expressive, musical synthesizers that defined the sound of 1970s rock, funk, and electronic music.

Real-world examples

• 1.Moog Minimoog synthesizer
• 2.Moog modular synthesizer systems
• 3.Eurorack modular filter modules based on the ladder design