Introduction to ARP submodules


Analogue synthesiser circuits, being closely related to analogue computer building blocks, often rely on the interaction of several semiconductor devices to realise a particular function. In such precision circuits the temperature dependence of semiconductor parameters has to be taken into consideration, as device temperature is likely to vary during operation. A means by which functionally related semiconductor devices and compensation components can be held at uniform temperature will usually have to be provided.

In integrated circuits thermal coupling is inherent in the design as the semiconductor junctions are implanted onto a common silicon chip. Back in the days before integrated circuits were commonly available, critical discrete circuit blocks would often be encapsulated in potting compound to ensure temperature independent performance. Apart from the technical objectives, encapsulation provided a means by which proprietary ideas could be kept secret.

Circuit Modularization

Early ARP products such as the modules of the model 2500 synthesiser had some of the basic circuit blocks encapsulated. Typical examples of such circuits are the exponential converters of the oscillators, and the variable gain cells of the Multimode Filter. Also, early keyboard units made use of an encapsulated discrete FET-input op-amp module as key control voltage buffer.

For the model 2600, however, ARP changed to a more compact and cost-effective modularization scheme - complete synth module cores would be built into little encapsulated plastic packages. As a result, you only needed a minimum amount of support circuitry, mainly panel controls and trimmers, to make completely functional synthesiser modules such as oscillators, filters, and envelope generators. Incidentally, ARP (or Tonus, as they were called back then) made an effort to sell these circuit modules separately to experimenters and educational facilities, "Encapsulated Function Generators For Use In Electronic Music Systems" [Note 1], as they were marketed in the early 1970s.

Encapsulation Schemes

Early encapsulated circuit blocks typically consisted of a small circuit board enclosed in a plastic case filled with epoxy potting compound. The simple circuit blocks used in the model 2500 measured 1.125" square, while the 2600 submodules were 2" square. The obvious disadvantage to having circuits potted in epoxy is that they cannot easily be repaired, as the components are inaccessible through the hard potting material. There have been reports of people using special organic solvents to clear the potting material, but this is a very slow and potentially hazardous process - not to be recommended.

Sometime in 1972 or 1973, ARP changed from monolithic potting to a two-layer encapsulation scheme. Module cases were now filled with silicone rubber so that the circuit board and components were covered, then sealed with a layer of epoxy, see figure 1. These modules are repairable, although clearing off the silicone rubber around the components is tedious and time-consuming. See the section on opening encapsulated modules.

Figure 1. Cross-section of two-layer encapsulated module.

During the mid-seventies, ARP stopped encapsulating most of their modules. Putting special metal clips around transistor pairs fulfilled thermal coupling requirements for low-drift operation. Also, the increased use of integrated circuit transistor arrays made encapsulation schemes redundant. However, some of the voltage controlled filter modules did remain encapsulated up until around 1976 by which time they had been replaced with the newer 4072/4075 design. The reasons for maintaining encapsulation were probably more strategic than technical, as the core of some of the filter circuits infringed on a Moog patent.

While the 2600 and Soloist (2700) were extensively designed around submodules, later models omitted this scheme in order to keep costs down. The Odyssey (2800) had three large circuit boards that accommodated the main electronics and front panel controls -- only the voltage controlled filter circuit took the form of a submodule. Subsequent models would follow this scheme, and the smallest synths in the range, the Axxe (2300) and Solus, were practically single-board, submodule-less designs.

Note 1: Scan courtesy of Terry Michaels.

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