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A string synthesizer produced by ARP Instruments; it was one of the most popular of this type of synth in the 1970s. The Omni was actually a combination unit, containing a fully polyphonic (every note on the keyboard can play simultaneously) string section, a paraphonic synthesizer section, and a monophonic bass section. All three sections were based on top octave division circuitry.

The string section, as was typical of this type of unit, had specific circuitry to generate violin-like and organ-like sounds, and offered a few presets with very limited sound modification capability. The output was piped through a phase shifter circuit to thicken the sound. The synth section also played fully polyphonically but had a single VCF for all of the notes; most players used it to layer in with the string sound, rather than by itself. The bass section played only in the bottom two octaves.

ARP produced the original Omni model starting in 1975. Sales were slow at first. Mark Vail states in his book Vintage Synthesizers that a consultant suggested that ARP actually raise the price; when they did, sales took off and the Omni became a big profit item for ARP. The Omni's phase-shifted string sounds became an icon of 1970s music. In 1978, ARP replaced the original model with the Omni 2, containing improved synthesizer and bass sections; this remained in production until ARP went bankrupt in 1981.

The Omni is moderately popular with collectors today. Existing units that have not been restored usually need repair; ARP used many wet-slug tantalum capacitors in the Omni, most of which have dried out and need to be replaced. Also, some of the early production 4000-series CMOS logic ICs have failed due to metal migration. Workable substitutes are available for all of these parts.

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