A fully loaded 2720 cabinet. Courtesy of modularsynth.com
A modular synthesizer system sold by PAiA from 1968 until some time in the early 1980s. This was the more budget-priced of the two lines of modular synths that PAiA manufactured during this time frame, the other being the 4700 series. The 2720 was designed with simple circuits and short-cuts to keep the cost to an absolute minimum. Combined with the fact that the modules were sold only as kits, this made it possible for the finance-poor musician, who might be nowhere close to being able to afford a Moog or Aries modular, to get into modular synthesis with a 2720 (assuming that he or she was handy with a soldering iron).
This started out as the 2700 series. The backbone of the system was a base cabinet with a panel area to mount modules, plus an integrated keyboard and performance controls. One of the modules had to be a power supply, unless an external supply was available. At first the modules available were a VCO, an LFO, a non-resonant single pole low pass VCF, a bandpass VCF, a VCA, an AR envelope generator, and a buffer-inverter.
The keyboard built into the 2700 base cabinet was a dodgy affair involving levers with metal shirt buttons acting as the switch contacts. This was felt to be a cost-saving bridge too far by even the most budget-conscious customers, so it was soon superseded by the 2720 base cabinet, having a proper 49-key keyboard, and all of the existing modules were re-designated as 2720 series modules. As the market progressed and performers demanded more functions and improvements, additional modules were introduced, and revised versions, particularly for the VCO and low pass VCF, were offered.
The modules used a very small format, about 5 inches (12.7 cm) high, and with a horizontal unit of about 2 inches (5.1 cm). Original modules were all 1 or 2 units wide; later, some wider modules were introduced. As a cost-saving measure, the panels used black graphics on an unpainted aluminum background, giving the whole affair a very shiny appearance. (On some of the earliest production modules, the panel graphics were hand painted.) The 2720 did not have fully integrated patching; audio and control signals used different voltage levels, and audio signals were patched with 1/8" (not 3.5 mm) jacks, while control signals used a pin jack similar to a small banana jack. One of the shortcuts used in circuit design was that the VCOs and VCFs did not have exponential converters; they used Volts/Hz scaling. The resistor values in the keyboard's resistor array generated the proper voltages for this scaling, but it made it impractical to include a pitch wheel or similar mechanism, since there was no easy way to scale the modulating voltage for the note currently being played.
The 2720 modular line gave many performers their first experience with synthesizers. It is unknown how many base cabinets or modules were sold (only a few of the original 2700 base cabinets were sold), and how many kits were never completed by their buyers. Anecdotally, a great many were scrapped during the late 1970s and '80s as their owners moved on to newer and more capable equipment, and others broke down (or never worked properly in the first place) due to lack of skill by the customer who assembled them. Finding a working example is rare today, and they tend to actually be rather pricey in the collector market.