ARP 1005 ModAmp (1970–1981)

Two oscillators go in, a metallic storm comes out - and the 1005 lets you steer the wreckage with a VCA and a pair of preset tuning voltages.

Overview

The 1005 ModAmp is a ring modulator that does more than one job. Feed two audio signals into the balanced modulator and it multiplies them (A × B), then divides the result by ten so downstream modules don't choke on the output. That division tells you it was designed to live in a larger system. The 4014 sub-module at the core is a discrete transistor-based balanced modulator - a precision analog multiplier built around Wilson current mirrors and an LM301 op-amp - and it handles signals from DC to 20 kHz. The result is clean multiplication with minimal carrier leakage, so you hear the sum and difference tones rather than the oscillators bleeding through.

The modulator is only half of it. ARP packed a voltage-controlled amplifier into the same 5U panel, and it's the same OTA-based design used in the 1006 Filtamp. The VCA takes two CV inputs, each with its own level pot, and switches between linear and exponential response. Linear mode outputs Vin × Vcontrol divided by ten. Exponential mode outputs Vin × 10^((Vcontrol - 10)/2). Gate the metallic clangor with an envelope for rhythmic bursts, or open it slowly for organ-like swells. The VCA control input impedance is 100 kΩ.

The third piece is what separates the 1005 from a generic ring mod: a preset voltage section that generates two control voltages, CVa and CVb, derived from the difference and average of two input voltages - typically the VCOs feeding the modulator. Ratio and Tune pots dial in exact detuning offsets, so when you switch into Mod mode, the oscillators shift to a pre-calculated interval that complements the inharmonic spectrum. Front-panel illuminated push buttons switch between Unmod and Mod, with gate and trigger inputs available for automated control.

It weighs approximately 3.2 lbs and measures 1.25" H × 7.25" W × 6.5" D as a module. It draws ±15 V @ 35 mA regulated plus +12 to +15 V @ 20 mA unregulated. The 1005 was a specialist, not a standalone star.

Key Features

The 4014 Balanced Modulator Core

Early 1005s shipped with the Hybrid Systems 107C, a third-party potted module that got ARP to market faster. By 1971, ARP had replaced it with their own 4014, which offered better integration and trimmability. The 4014 is the same sub-module that later turned up in the 2600. Its design gives clean multiplication with minimal carrier leakage, a common flaw in cheaper ring modulators that lets the original oscillator signal bleed through. In Mod mode on the 1005, what you hear is almost purely the sum and difference tones.

Dual Inharmonic Preset Voltage Generator

Two control voltages, CVa and CVb, are derived from the difference and average of two input voltages, typically the VCOs feeding the modulator. The Ratio and Tune pots dial in exact detuning offsets, so flipping into Mod mode shifts the oscillators to a pre-calculated interval. The circuit uses LM301 op-amps and precision resistors to maintain stability. Service notes show grounding errors in early schematics - the emitter of Q7 should go to ground, not -15V - which can trip up modern clone builders working from the wrong documentation.

Integrated VCA with Dual CV Inputs

The VCA is the same OTA-based design used in the 1006 Filtamp, and it accepts two CV inputs, each with its own level pot. A toggle switch selects linear or exponential response. Linear mode gives precise gate-like on/off control, while exponential mode delivers smooth, organ-like swells. Having the VCA built in saves a patch cord and a module slot. The control input impedance is 100 kΩ.

Illuminated Push-Button Modulation Switching

Front-panel illuminated push buttons switch between Unmod and Mod, with gate and trigger inputs available for automated control. That means you can flip the effect in and out from a sequencer or keyboard gate without repatching. The lamps behind those buttons are a known weak point. They burn out, and replacement bulbs are non-trivial to source.

Historical Context

The 1005 came out when the 2500 system was the go-to for academic studios and avant-garde composers. Unlike Moog's more musical, performance-oriented designs, the 2500 leaned into complexity, patch programming, and voltage control as a compositional tool. The 1005 fits that approach. It's not a "fun" module, but a serious one, built for people who wanted to explore the edges of harmonic perception. Ring modulation itself was still relatively exotic at the time, used by pioneers like Karlheinz Stockhausen and Wendy Carlos. ARP didn't invent it, but they made it more accessible and controllable than most competitors.

The 1005's closest rival might have been the EMS VCS 3's ring mod section, but that was a simpler, less precise affair. The 1005 offered studio-grade accuracy and the ability to automate the entire process. It was often paired with the 1004 Oscillator, 1006 Filtamp, and 1027 Sequencer - the manual suggests using 1004 and 1023 VCOs. Sharing modules between the 2500, 2600, and even the 5000 was key to ARP's efficiency, and the 4014's move into the 2600 is the clearest example. The 1005 never became a flagship, but it was a quiet workhorse in high-end systems. The 2500 itself was ARP's inaugural product, manufactured from 1970 to 1981, with only about 100 custom-built units produced due to its high cost and technical complexity. A contemporary survey listed the 2500 system at £5,400 (Studio-Sound, May 1972, p.37). Contemporary surveys covered the 2500 system's full module complement, including sequencers and keyboard controllers, at a list price of £5,400 (Studio-Sound, May 1972, p.37).

Collectibility & Value

Finding a working ARP 1005 today is rare. It wasn't uncommon when new, but 2500 systems were often heavily modified, and modules got lost or cannibalized over decades. When one does surface, expect to pay between $1,800 and $3,000 depending on condition, with fully serviced units at the higher end. The real cost is in maintenance. Aging carbon composite resistors drift, tantalum capacitors dry out, and the potted 4014 sub-module can fail silently. The VCA section is generally sturdy, but the CV generation circuit is sensitive to power rail instability and grounding errors, especially if earlier repairs used incorrect schematics. The illuminated push buttons are another weak point; the lamps burn out, and replacement bulbs are non-trivial to source. The front-panel pots are high-quality Switchcraft units, but they can develop crackle if not exercised regularly.

Before buying, insist on a full functional test: verify that the ring mod output is clean with no carrier bleed, that the VCA responds correctly in both linear and exponential modes, and that the preset CVs track accurately. A unit that powers up but hasn't been tested under load might still have failing transistors or op-amps. For those who can't afford the original, the Behringer 1005 (B1050) offers a Eurorack-compatible version at a fraction of the price. It covers the core functionality but simplifies the CV generation and uses modern ICs, losing some of the original's nuance. The AMSynths 5U replica is closer to the original, designed by a builder who spent years reverse-engineering the ARP service notes, but it's a boutique item, not mass-produced.