Analog Days book cover art

This month we’re really excited to publish this excerpt from a fantastic book by Trevor Pinch and Frank Trucco, to coincide with the release of the Moog Multimode Filter UAD Powered Plug-In. Bob Moog’s influence on the audio community changed music forever, and we honor his memory.

The Moog Ladder Filter

Moog’s voltage-controlled oscillators, amplifiers, filters, and envelope generators were initially based on standard circuits. Sometime during late 1965 and early 1966 Moog came up with a novel design for a filter, known as the low-pass filter or ladder filter (after the ladder of transistors in the circuit). This filter is the crown jewel of the Moog synthesizer — it is the “rich,” “fat,” “juicy” tone that nearly everyone refers to as the Moog sound. This is the filter that Moog’s rivals at ARP and EMS were most envious of and which they tried to copy.

Filters, which control the higher harmonics of sound, were not new; they had been used since the days of radio. Bob seems to have designed his unique low-pass filter from a combination of book knowledge and his usual tinkering. He presented the design in a paper delivered to the AES convention on October 11, 1965. A year later, on October 10, 1966, he filed for a patent, which was granted on October 28, 1969. It is the only item on the whole synthesizer that Moog ever patented.

A low-pass filter can be thought of as a gate in a stream. The higher it is raised, the higher the harmonic frequencies that can pass through it — or, more correctly, under it, since it’s a low-pass filter. If you speak into a long pipe, your voice will be muffled because the pipe is acting as a low-pass filter; talking into a pillow has the same effect. Where the cutoff occurs — that is, how high the gate is raised — depends on where the cutoff control is set. By varying the height of the gate, the timbre or tone color of the sound can be varied. (Timbre is what allows you to distinguish a pitch played by a clarinet from the same pitch played by an oboe.) By voltage-controlling the gate, the musician can sweep the filter through its range, changing the timbre of the sound by emphasizing some harmonics and attenuating others.

Bob found a novel circuit to do this, using pairs of transistors connected by capacitors arranged in a ladder. This makes the filter balanced, because the signals can go up both sides of the ladder at the same time. The signals enter the bottom of the ladder, and those with higher frequencies find it hard to make their way up the ladder because of the electrical properties of the transistors and capacitors. One of the main factors in the quality of the sound from the Moog filter is a characteristic called the cutoff slope. A filter doesn’t actually chop the high harmonics off completely but rather attenuates them. The cutoff slope refers to how abruptly the amplitudes of the high frequencies taper off. The Moog filter has a much sharper cutoff slope than almost any other synthesizer filter.

One of the main factors in the quality of the sound from the Moog filter
is a characteristic called the cutoff slope.

The filter has many other qualities, such as a sharp resonance around the cut-off frequency. When the filter is overdriven — which means that the amplitudes of the signals going into the filter are too large — it produces a rich form of distortion that is characteristic of the fat Moog sound. Jim Scott, a Moog engineer who worked extensively with the ladder filter, adapting it for use on the Minimoog, commented, “This filter defies analysis. There are lots of subtle things going on that almost defy mathematical treatment.” The best analog components in sound nearly always have this quality of not being quite understood and nearly always involve some not quite specifiable resonances and distortions that occur at high frequencies beyond audible range but that produce audible effects.

When the filter is used with an envelope generator in the bass range, the resonant deep sound is particularly appealing and was soon discovered by synthesists. Over the years it has become a staple of pop and rock music, as has the bass sound of the Minimoog (which uses a similar filter). Bob Moog was himself a witness to the power of his bass sound when he was invited to bring his synthesizer to a New York studio session where Simon and Garfunkel were recording their album Bookends (1968). Moog set up the bass sound himself for the track “Save the Life of a Child,” which opens with this sound: “One sound I remember distinctly was a plucked string, like a bass sound. Then it would slide down — it was something you could not do on an acoustic bass or an electric bass … a couple of session musicians came through. One guy was carrying a bass and he stops and he listens, and listens. He turned as white as a sheet.” The significance of the Moog bass sound was not lost on this session musician. The Moog not only sounded like an acoustic or electric bass, but it also sounded better.

Moog liked to repeat this story; he felt that at last he was “getting somewhere.” At last the Moog was finding a home among musicians at large, rather than being an instrument merely for the avant-garde.