All about it
The MM317 VCO is a precise, clean and accurate VCO with a split personality....
Traditionally, analogue VCOs have been divided into two camps.
Triangle-core VCOs track accurately over a wide range and have pure-sounding triangle waveforms, but don't necessarily do oscillator sync all that well and their sawtooth waveforms need to be stitched together like Frankenstein's monster and can sound like it.
Sawtooth-core VCOs have gutsy sawtooth waveforms and evil oscillator sync, but require extra circuitry to maintain any semblance of accurate pitch tracking in their upper range, and their triangle waves (let alone their sine waves) can be even less convincing than a triangle-core oscillator's sawtooth.
The MM317 answers the question - what if you could have the best of both worlds?
Firstly, it contains a precise triangle core which takes care of triangle (natch) and pulse waves as well as sine.
Secondly, it contains a sawtooth generator which tracks the triangle core and is hard-synced to it, giving a powerful sawtooth wave that can track accurately-any "flattening" of the response at the high end instead results in a very small drop in amplitude rather than pitch deviation.
Thirdly, the sync/reset circuitry is quite sophisticated and resets both the triangle and saw cores to their start point- so it gives a gutsy sawtooth-style sync sound. (glad I didn't say that while I was eating!)
Us being us, we have added the optional Linear FM modulation capability from the original datasheet along with Dave Brown's suggested AC-coupling circuit.
We are also working on some surprises within the circuitry and controls, so stay tuned!!!!
The original specifies two CA3080 OTA chips. We have not been tempted to replace them with an LM13700 as one of them is specified as a CA3080A, which would have been most likely due to its better output current leakage specs, which would prevent "flattening" at low frequencies. As the Aries designers knew their stuff and did not "gold-plate" their designs unless it was justified, we have taken this requirement seriously. There are no longer LM13600s or LM13700s guaranteed to A specs in production, so we will be using new production CA3080A chips from "afterlife" manufacturer Rochester Electronics.
Also like the original, the expo converter (the MM-1748) is an epoxy-potted module. There is no official documentation of the circuit and indeed it was blocked out on schematics, presumably because it is almost identical to the ARP 2500 VCA's expo converter. I managed to successfully reverse engineer it by deduction from the AR317 schematic (Aries designers did their maths!), and Dave Brown independently reached the same conclusion by testing circuit connections on an actual module. Woohoo!!!!
Tech Specs (all subject to change)
+5V: Not required
Eurorack standard, 10HP width
40mm depth below mounting rails with connected power cable
Watch this space