Modularizing DIY Modular Synth Modules

Most synthesizer module circuits have much of their real estate occupied by a few common functions:

• Power supply regulation and decoupling components
• Input buffers, using op-amps, perhaps with AC coupling components
• Output buffers, using op-amps
• Signal converters: dozens of kinds of voltage-controlled resistor-ish functions (for example, using transconductance amplifiers, diode or transistor configurations, vactrols, etc.), and voltage to current converters which may also perform linear-to-exponential conversion (usually transistor-based)
• If there is to be computer control, then DACs and analog switches

Indeed, the functional core (what is left after the common peripheral functions) can be quite small: the oscillator, waveshapers,filters and logic proper may be less than half the module's circuits. I see Ken Stone at Cat Girl Synths has this week started releasing some utility PCB designs for some of these common sub-modules.

Its not an entirely new idea: Tellun have their Multi-Use Universal Buffer board.

It is, in a sense, the opposite approach from that taken by ARP in their 70s synths: they bundled their functional core circuits into sub-modules that plugged into a product-specific carrier board which had much of the periphal circuitry: CMS has act-alikes for exponential converter, op-amp, VCO, VCA, VCF, ADSR, noise, balanced modulator, and sample and hold. I am not sure what the design reason really was for these: the sub-modules could be shared between products which would help agility and lower design costs, the sub-module could be swapped out which would help maintanance and upgrades, the sub modules could be potted in acrylic or resin which would help keep competitors eyes out and perhaps have some temperature coupling property, and some components that required matching transistors/diodes/resistors could be built and tested independently of the ultimate product which may help manufacturing and QA.