The Moog 921 is a standalone, full-featured voltage-controlled oscillator (VCO), whereas the 921A (Oscillator Driver) and 921B (Oscillator) are a two-part system designed to work together, allowing a single driver to control the core functions of multiple oscillators simultaneously.
Here are the key differences:
Moog 921 (Standalone Oscillator)
Integrated Design: The 921 is a single, complete module that contains all control functions and outputs.
Independent Control: Each 921 module has its own individual 1V/Oct input for pitch control and individual pulse width control, allowing for greater flexibility in creating separate melodic lines or independent LFO functions.
Features: It includes additional features like an "Auxiliary Output" with its own attenuator and a dedicated low-frequency oscillator (LFO) range switch with better stability at low frequencies compared to the 921B in LFO mode.
Master/Slave Architecture: The 921A serves as a "master" control module (driver) that sends common control voltages to one or more "slave" 921B oscillator modules.
Shared Control: The 921A's frequency and pulse width controls affect all connected 921B modules simultaneously. This setup is ideal for creating rich, unison sounds or stable oscillator banks that tune together, as all oscillators track from a single control source.
Module Functions:
921A Oscillator Driver: This module has no audio outputs. It provides CV control for frequency (pitch) and pulse width modulation for the connected 921Bs.
921B Oscillator: This module generates the audio waveforms (sawtooth, triangle, sine, pulse) but relies on the 921A for the main 1V/Oct pitch tracking and pulse width control signals.
Efficiency: The 921A/B system was designed in the original Moog modular systems to save panel space and potentially cost, as only one set of main controls (the 921A) was needed for a bank of oscillators.
In essence, the choice between the two designs depends on whether you need a single, versatile, independent oscillator (921) or a bank of oscillators that can be controlled from a single master source (921A/B combination).
If you most frequently use banks of oscillators in unison, the 921a/b is designed for that purpose.
If you need the flexibility to use oscillators in separate voices, the 921 will be the one you want.
A single Moog 921A Oscillator Driver can control up to 12 Moog 921B Voltage Controlled Oscillators.
The units are typically hardwired together via the power bus or a specific connector on the back of the modules, removing the need for extra patch cables for basic control.
For most practical modular synthesis setups, it is common to use banks of three oscillators per driver.
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