Through The Circuits

Compared with other Moog modules, the 901-C Output Stage is old, rare, and practically obsolete. When patched to a 901-series oscillator, it controls the oscillator’s waveform’s amplitude and polarity, rendering the oscillator more versatile as a control-voltage source. It has two control-voltage outputs, one of them 180 degrees out of phase from the other. The 901-C was a hand-wired pre-production model built in 1967, and only 15 were manufactured.

The 902 VCA is an essential module that changes the amplitude of audio and control-voltage signals passing through it. Its three control inputs allow an envelope generator, LFO, or any other control-voltage source to control loudness or signal strength. The Fixed Control Voltage knob determines its output level before a modulation signal is applied. The Control Mode switch determines whether its response to modulation is linear or exponential. The 902 has two Signal Inputs and two Signal Outputs that accept either audio or control-voltage signals.

This module generates two different types of noise—white and pink—and can be used as an audio source or a voltage source. White noise is a random distribution of all frequencies, each with equal amplitude, but the human ear tends to emphasize the upper frequencies. Pink noise is filtered white noise that sounds more evenly distributed; it closely resembles the sound of a waterfall.

Like all low-pass filters, this module passes audio signals below its cutoff frequency and attenuates them above that frequency. Practically every synthesizer has a low-pass filter. What made this module unique when it first appeared was its transistor-ladder design, which Bob Moog patented in 1969. The distinctive sound of Moog synthesizers is often attributed to Bob’s low-pass ladder filter design.


You can control the 904-A’s cutoff frequency either by turning its Fixed Control Voltage knob or by applying a modulating control voltage to one of its three Control Inputs, which raises the frequency set by the Fixed Control Voltage knob. You control its resonance—an increase in amplitude at the cutoff frequency—by turning its Regeneration knob. The 904-A’s resonance is not voltage-controllable.

This module is the companion high-pass filter to the 904-A Low Pass Filter. A high-pass filter does the opposite of what a low-pass filter does. It passes audio signals above its cutoff frequency and attenuates them below that frequency. A modulating control voltage lowers rather than raises the frequency set by its Fixed Control Voltage knob. It has the same inputs and outputs as the low-pass filter, but no Regeneration knob.

This module allows the 904-A Low Pass Filter and the 904-B High Pass Filter to work in tandem, functioning as a band-pass filter or a band-reject filter. A band-pass filter passes only frequencies within a specified band and attenuates frequencies higher or lower than that band. Conversely, a band-reject filter attenuates frequencies within a specified band and passes only frequencies that are higher or lower than that band.


Instead of a cutoff frequency, you dial up the 904-C’s Center Frequency and Bandwidth to specify the band of frequencies it either passes or rejects. Because both of those controls are voltage-controllable, you can use any modulation source to change their values dynamically.

Aside from the keyboard, an envelope generator is the most important source of control signals in any synthesizer. It fires each time it receives a trigger signal from a keyboard or sequencer. Like most envelope generators, the 911 module has four stages. Three are lengths of time (labeled T1, T2, and T3 on the 911), and one is a voltage level (labeled Esus). Most envelope generators label these four stages attack, decay, sustain, and release, which is why they’re also called ADSR generators.


When used as modulation sources for an amplifier and filter, a pair of 911 modules can shape the loudness contour and the harmonic content of every sound a synthesizer makes. For that reason, most synthesizers have at least two envelope generators and sometimes more.

Simply put, this module converts sound into electricity. By tracking changes in the loudness of any sound and turning them into changing control voltages, the 912 module lets you modulate any voltage-controllable parameter on any other module. You could use the sound of a saxophone, for example, to control a filter’s cutoff frequency or the sound of your voice to control an amplifier’s envelope. The louder the sound, the higher the voltage.

An oscillator is a synthesizer’s primary sound source. Dating back to 1971, the 921 is a voltage-controlled oscillator module that generates sine, triangular, sawtooth, and variable-width rectangular waves, the most essential waveforms used for analog synthesis. Each waveform has its own output, and you can choose from six additional waveforms from another pair of outputs.


The 921 has a remarkably wide frequency range, and it also provides a sub-audio range for generating control signals, making it a low-frequency oscillator or LFO. Three control inputs accept signals from any modulation source. The 921’s most unusual features are its Clamping Point selector and Clamp Trigger input, which can force a waveform to reset to any of several points in its cycle. (You may notice a black residue in the photo above on the metal edge of the module. The light residual residue that remains is due to this particular module being caught in a fire.)

This module is similar to the 921 oscillator, but it’s considered to generate better-quality sine, triangle, sawtooth, and rectangular waveforms. It requires a 921A oscillator driver module to function—the 921B has no pulse-width control or frequency inputs—and up to three 921B modules can share a single 921A. The 921B’s Synch. input makes it possible to phase-lock the waveforms of numerous oscillators. Its D.C. Modulate input accepts linear control voltages, and its A.C. Modulate input accepts exponential control voltages, providing greater flexibility than most oscillators.