ARP 3620 (1974–1980)

Name: ARP 3620
Brand: ARP

The keyboard that turned the 2600 from a lab instrument into something you could actually play

Overview

The 3620 is not a synthesizer. It's the remote duophonic keyboard controller built for the ARP 2600. The 2600's original keyboards were barebones controllers, and they treated a serious instrument like a breadboard exercise. Before the 3620 arrived in 1974, the 2600 was, in the words of the documentation, a sonic powerhouse trapped in monophonic purgatory. The 3620 was ARP's answer - a dedicated two-voice controller that gave the 2600 the front end it had been missing.

It launched alongside the 2600P v4.0 and was positioned as the premium controller for that revision and everything after it, at the top of ARP's keyboard hierarchy. It also replaced the 3604P keyboard for the 2600P synthesizer outright. The earlier 3601 and 3604 felt like afterthoughts. The 3620 was designed with the synth rather than bolted on afterward.

It is duophonic, with single or multiple triggering available, and it sends two separate memorized control voltages, gate, and trigger down the cable to the 2600. It draws its power from the main 2600 unit, so there's no separate supply to hunt down. The 3620 is the piece that made the 2600's later years playable, and it's the reason collectors treat a 2600 without one as incomplete.

Key Features

Two Memorized Control Voltages, Low Note and High Note

The heart of the 3620 is its two-voice CV architecture. CV-1 tracks the lowest note played and CV-2 the highest, and both stay locked until every key is released. That's not polyphony - the keyboard plays no more than two keys at a time - but it's a genuinely musical form of duophony. Route one 2600 oscillator to CV-1 and another to CV-2 and you can play intervals that hold their tuning, with each voice following its own control voltage. The keyboard generates four voltages in total: gate, trigger, lower-voice CV, and upper-voice CV. Gate and trigger and the lower voice travel to the 2600 over the main linking cable, while the upper voice connects with a standard patch cord.

Interval Latch

This is the 3620's signature trick, and it needs the foot switch to work. Depress the pedal while holding an interval and the keyboard memorizes the distance between the two notes. From then on, playing a single note produces that same interval until the pedal is released. The manual is explicit that both upper and lower voices are required. It's a practical tool for orchestral patches where an instrument holds a fixed interval for a stretch of a performance. Because you can release the switch, play a new interval, and re-latch, you can change the memorized interval on the fly. Major fifths and octaves are the obvious starting points.

Trigger Mode: Single or Multiple

The trigger mode switch decides how the ADSR envelope gets fired when more than one key is down. In multiple mode, the trigger output fires every time a key is depressed regardless of how many others are already held. That's the common setting for most playing. In single mode, a trigger is produced only for the first key depressed. The manual notes that trigger mode does not affect the behavior of the AR envelope generator, only the ADSR.

Onboard LFO with Delayed Vibrato

The 3620 carries a single built-in LFO offering triangle, square, and random waveforms. It's wired directly into the pitch path rather than left to consume one of the 2600's modulation buses. The vibrato is delayed, so notes start clean and the modulation eases in instead of hitting at full depth. Multiple vibratos for chorus effects are easy to set up. The LFO's sine wave can be delayed from zero seconds out to roughly 2.5 seconds, and the rate tops out around 25 Hz.

Pitch Bend with a Center Detent

The pitch-bend control sums an offset voltage with both voice CVs and is calibrated to a maximum of plus or minus one octave, so it doubles as a transpose for bends. The knob has a dead zone at center, which keeps you from warping your tuning every time your hand comes to rest on the panel. The transpose switch moves the playing range up and down by two octaves, summing a plus or minus 2 V offset with the upper and lower voice CVs.

Portamento, Footswitch Controlled

Portamento is switchable from the panel or via a momentary push button, and the slider stretches the glide time from 1 msec out to about 2 seconds. One quirk: when portamento is switched on, the upper voice memory is disabled. The portamento and interval-latch footswitch jacks accept only ARP foot switches - the manual is blunt about this - and both are there to keep your hands on the keys during performance.

External Vibrato Input

The keyboard takes an external vibrato input, so any waveform patched from the 2600 - triangle, square, sine, sawtooth - can modulate the keyboard CV in place of the internal delayed sine. Patch a square wave in and you get a trill. It adds range without adding a single control to the front panel.

Historical Context

The 3620 arrived when the 2600's playability was becoming a liability. In 1973, Tom Oberheim had already hacked the 3604 into a duophonic controller, which showed there was real demand for more expressive control than ARP was offering. ARP itself was pitching the 2600 line to keyboard players as an instrument that any pianist could pick up. It pushed that message in national advertising throughout the mid-1970s, with period ads in Modern Recording (August 1976, p.59) reassuring players that if they could play keyboards, they could play an ARP.

ARP's response wasn't a new synth but a smarter keyboard. The 3620 launched with the 2600P v4.0, just before the legal fallout with Moog forced ARP to abandon the 4012 filter. That timing made the 3620 the face of the 2600's later years.

While Moog stayed with monophonic controllers and Oberheim was still building discrete voice modules, ARP gave 2600 players a way to layer two notes without going full polyphonic. It wasn't as flashy as a polysynth. One point of tension in the record: the production span is given as 1974-1980, yet at least one account places the 3620 as the keyboard supplied with the 2600 from 1975 onward. Treat the 1974 start as documented but not universally corroborated.

Collectibility & Value

The 3620 is only as valuable as the 2600 it pairs with, but in that pairing it's close to essential. Standalone working keyboards typically sell between $1,800 and $2,800 depending on color scheme and condition, though no date is attached to that estimate and it should be treated as a rough range rather than a current quote. Gray-faced units from 1974-1976 command a slight premium for their association with the last of the 4012-filter 2600s. The orange-over-black models from 1977-1980 are more common but still desirable. The 3620 weighs in at 11.3 kg, which matters when you plan a shipping quote.

Failures cluster in predictable places. The Pratt-Reed keybed is sturdy but develops dead spots when contacts oxidize or misalign. The gate/CV buss wiring is delicate, and botched repairs - daisy-chaining resistors instead of grounding each key individually - cause runaway notes and stuck triggers. The LFO delay circuit is prone to capacitor drift, which shows up as uneven or dead vibrato. A full service by a tech who knows ARP's quirks typically runs $400 - $600 including recapping and contact cleaning, and slider replacement kits are still available.

Buying checks are straightforward but non-negotiable. Test every key across both CV outputs, confirm the interval latch holds cleanly, verify the repeat function triggers reliably, and make sure the pitch-bend knob sweeps smoothly with a clear center detent and no wobble. Avoid units with replaced sliders or drilled panels. If a seller claims the keyboard has aftertouch, they're almost certainly describing the Korg reissue - the original 3620 has none. The patch-sheet facsimiles floating around eBay for $49.99 - $99.99 are paper accessories, not keyboards.