ARP 3604 (c.1972–1980)

Name: ARP 3604
Brand: ARP

The keyboard that turned a modular powerhouse into a stage-ready beast—49 keys of vintage ARP attitude, wired to command the 2600 like a conductor with a grudge.

Overview

The ARP 3604 doesn't make a sound on its own. Plug it in, though, and the 2600 plays like an instrument instead of a lab experiment. It's a heavy-duty analog controller, built to give players real-time control over one of the most volatile, expressive synthesizers ever made. The keys are slightly springy and medium weight. The build is industrial, almost military-grade, with a chassis that laughs at road cases and a keybed that's seen more control rooms than cocktail lounges. It's monophonic, so no chords. That suits leads, basslines and tight sequences just fine.

The 3604 was originally sold as an optional companion to the ARP 2600. It wasn't the only keyboard in the family - there was the smaller 3601 and the later 3620 - but it quickly became the preferred choice for players who wanted reach without compromise. Forty-nine keys gave enough range for both low-end growl and high-frequency shriek. The integration with the 2600's patch bay meant you could route gate, trigger, and CV signals with surgical precision. The point was wiring into the synth. The 3604 never pretended to be a standalone instrument.

This isn't a "musical" keyboard in the modern sense. There's no velocity, no aftertouch, no expression wheels or assignable knobs. What it has is a direct connection to the 2600. You get pitch bend from a spring-loaded lever that feels like it was salvaged from a jet cockpit, a dedicated portamento switch, and a gate time knob that stretches notes like taffy. That's it. No menu system, no digital layer, just the circuitry. It's like driving a car with no power steering: exhausting at first, then satisfying once you learn to work with the resistance.

Specifications

Manufacturer	ARP Instruments, Inc.
Production Years	c.1972–1980
Original Price	$595 (approx. $4,300 today)
Keyboard Type	Monophonic analog remote keyboard
Keys	49 (F to F)
Key Action	Spring-return, mechanical linkage
Pitch Bend	Spring-loaded lever (up/down)
Portamento	On/Off switch with variable glide time knob
Gate Time Control	Adjustable gate duration knob
CV Output	1V/octave
Gate Output	Positive trigger
Trigger Output	Positive pulse
Connectivity	18-pin cable to ARP 2600 (proprietary)
Power	Powered via 2600 (no internal power supply)
Weight	22 lbs (10 kg)
Dimensions	35.5" x 11.5" x 4.5" (90 x 29 x 11.5 cm)
Compatibility	ARP 2600, ARP 2600P, ARP 2600C
Finish	Black tolex with brushed aluminum faceplate, Tonus or G-clef logo (varies by era)

Key Features

Designed as a System, Not an Accessory

The 3604 was engineered as a core part of the 2600 experience. Other manufacturers treated remote keyboards as optional extras, but ARP designed the 3604 to feel like a natural extension of the synth. The 18-pin cable carries CV, gate, trigger, and power in one thick, coiled tether. The layout mirrors the 2600's no-nonsense approach. The portamento knob and gate time adjustment sit right next to the pitch lever, within thumb's reach. There are no hidden functions, and everything is labeled in crisp, utilitarian type.

Pitch Lever with Purpose

The 3604 doesn't use a pitch wheel. It uses a stiff, spring-loaded lever mounted on the left side of the keybed. It fights back, so you don't accidentally bend a note. The lever moves in a short arc, gives precise control over pitch modulation, and returns to center with a snap. It's not as smooth as a modern ribbon or wheel, and each nudge feels deliberate, almost mechanical. Players who've used it on stage report that it holds up under aggressive playing, though the spring can weaken over decades of use if not maintained.

Gate Time Knob: The Secret Weapon

Most vintage keyboards give you a fixed gate time or nothing at all. The 3604 has a dedicated knob that adjusts how long the gate signal stays open after you release a key. Turn it up, and you get legato lines that blur between pitches. Dial it back, and you get staccato precision, good for tight sequencing or percussive effects. Patched into the 2600's envelope generators, it lets you shift from punchy attacks to slow swells on the fly.

Historical Context

In the early 1970s Moog had the market cornered on modular systems, but they were expensive, fragile, and notoriously difficult to tour with. ARP rethought the workflow instead of copying. The 2600 was a self-contained, semi-modular synth that didn't require a PhD to operate, and the 3604 was its mobile command unit. Released around 1972, the same time as the 2600P revision, the 3604 answered a real need: musicians wanted to play these machines, not just patch them. Studios were adopting synthesizers for pop and jazz, and artists like Herbie Hancock, Stevie Wonder, and Edgar Winter were pushing them into live settings.

It wasn't the first remote keyboard, but it was one of the first designed specifically for a semi-modular synth with performance in mind. Competitors like Oberheim and EMS offered their own solutions, but they were often clunky or limited. The 3604 stood out for its integration, durability, and sheer presence. ARP never marketed it as a standalone product (it was always sold alongside or as an add-on to the 2600), but collectors now treat it as a crucial piece of the ecosystem. Losing the original 3604 can drop a 2600's value by thousands.

Collectibility & Value

A complete 2600 system without its original keyboard feels incomplete, like a car without a steering wheel. Standalone 3604 units rarely appear on the used market, and when they do, they run $1,800 - $2,800 depending on condition, provenance, and whether the original cable is included. Units with the rare "Tonus" logo (used from 1972-1974) are especially sought after, often fetching premiums over the later "G-clef" versions. Cosmetically, these keyboards age like vintage amplifiers, with scuffed tolex, oxidized trim, and yellowed labels. Major damage to the keybed or chassis can be a dealbreaker.

Functionally, the 3604 is tough, but not immortal. The biggest failure points are the pitch lever mechanism, which can wear out or lose spring tension, and the internal wiring, which may degrade over 50 years of coiled-and-uncoiled abuse. The 18-pin connector is proprietary and no longer manufactured, so broken cables are a real headache, though some techs now fabricate replacements. The keyboard has no active electronics. It's purely mechanical and passive, so it doesn't "go bad" like a synth voice board, and repairs are about precision mechanics, not circuit tracing. A full refurb - cleaning the key bushings, lubricating the linkage, replacing worn springs - can run $400 - $600.

When buying, test every key for consistent triggering and check that the portamento and gate time knobs respond evenly. Make sure the pitch lever returns to center without sticking, and verify that the 18-pin cable isn't frayed or cracked. If the keyboard is being sold separately from a 2600, ask for proof it's been tested with a compatible synth. Some units sit in storage for decades and develop oxidation on the contacts. If you're pairing it with a 2600, confirm the model: early 2600s (pre-1976) use the same 4012 filter and 4027 oscillators as the best-sounding 3604-equipped systems, and collectors care about that synergy.