ARP 2500 Modules

A modular system built around a pin matrix instead of a patch bay, and the modules that made it worth the trouble.

Overview

The ARP 2500 was a chassis full of design decisions, and the modules are where those decisions live. The biggest one is architectural: ARP chose a switch matrix over patch cords. Studio Sound (June 1972, p.16) records the reasoning. ARP weighed patch cords, pin matrix and switch matrix, and picked the switch matrix for the 2500. Owners who have lived with both arrangements tend to defend the matrix on setup-time grounds. The trade is that you route by pin position rather than by cable, which rewards muscle memory and punishes the occasional visitor.

The module lineup follows a house habit of bundling. ARP's practice, as described in the 2500 documentation, was to package together and internally connect functions that a user would otherwise wire up externally anyway. The 1045 VC Voice is the clearest example. It holds a voltage-controlled oscillator, voltage-controlled filter and voltage-controlled amplifier with envelope generators in one unit, internally connected so that only the keyboard and output need to reach the switch matrix. That saves eight matrix connections and a good deal of setup time. The individual components are still separately usable, and each carries its own input attenuator.

Then there is the 1047 VC Filter/Resonator, the module collectors talk about. It is an unusually complex filter offering, per the documentation, all the functions possible from one package. Its resonant circuits are capable of very high Q, a maximum of 512. At those Q values the filter will oscillate, and a single short pulse through it can produce percussive sounds. ARP added a "keyboard percussion" panel area in the top right corner to make the most of the keyboard's trigger pulse. It also added a warning. Push the Q control too far and the filter veers toward self-oscillation and overloads. A red indicator lamp flags the condition, and a slight adjustment of the self-contained input level attenuator corrects it.

The envelope and noise side of the system gets similar attention. The quad envelope generator has delay controls on two of its four generators, letting the start of an envelope be postponed relative to the command signal. The documentation notes this is useful for synthesizing double and triple string instruments. The 1016 noise generator supplies two slow random noise outputs, meant to be injected in small amounts into oscillator control functions so that pitch wanders slightly the way a real trumpet or cello does.

According to users writing in Canadian Musician, the 2500 offers routing flexibility without extra hardware. Patch connections can be made from any control-voltage or audio source into the 2500 and back out again, which largely eliminates the need for multiple patch bays to split a signal. That same account describes driving 2500 modules from an Odyssey's CV, gate and trigger outputs while the Odyssey stayed free for other work. It also describes pairing the system with a Sequential Circuits programmer for 64 groups of programmable preset voltages across three VCOs, a VCF and a VCA.

The modules are also a maintenance proposition. Period documentation says plainly that at least one 1045 was skipped in a review because it was malfunctioning at the time. These are decades-old modular circuits, and a full 2500 system is a collection of individually serviceable units rather than a sealed instrument. Buyers chasing a single rare module should expect to evaluate it on its own merits.

ARP's own owner's manual for the Series 2500 was sold separately at $5.00, with a data sheet catalog of module and system specifications at $3.00 — and the fact that ARP printed a standalone data sheet catalog tells you something about how modular this system really was. The modules were the product; the chassis was the frame.

Similar items on eBay

No exact ARP 2500-modules listings are live right now — these searches show current ARP and similar vintage synth gear on eBay:

As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.

Related Models

• ARP 1004-T
• ARP 1004
• ARP 1005 ModAmp (1970–1981)
• ARP 1006 (early 1970s)
• ARP 1023 Dual VCO