Since it’s a matter of occasional confusion, even amongst people doing synth repairs for a living, I thought I might try a simple description of how divide-down polyphony in analogue instruments works.
An RCA CD4024AE 7-stage frequency divider IC.
The basis of the technology is a circuit called a frequency divider. [1] This can be done differently with digital manipulation, but a typical analogue frequency divider responds to a (significant enough) change in input voltage by waiting until a similar change recurs one or more times before changing its output. The simplest and commonest division is the one-half or first suboctave, where the count is every two cycles. (A binary divider. n.b. divider circuits are often also known as counters.)
A simple binary divider triggering on the rising edge of an input waveform will switch its output state with every voltage rise (“edge”) detected. Here the falling input edges do not affect the output. Divider circuits which trigger on the falling edge can also be built.
In electronic instruments, input to dividers typically comes from an oscillator circuit. Originally, each of the notes in the top octave of divide-down instruments were generated by a separate individually tuned oscillator. The collection of oscillators this required is usually referred to as an oscillator bank.
I’ve been somewhat reluctantly acquiring Eurorack modules, and recently decided that to save a bit, I’d be as well to make my own case out of an old wooden box and some bits I could buy. This didn’t work out as, once I had racks and power distribution, it became apparent that the box would need rather more modification than I’d planned. But I still wanted to rack some modules. So with a bit of rummaging around I found some scrap plywood and some aluminium sheet I took off something a few years ago. [1] Careful measuring shows that there’s a section of the sheet that should do the job, in which there’s minimal corrosion and only one hole, and I might be able to get the power socket in that space. So let’s go.
I want this case to be as compact as possible, though I don’t really know quite how this is going to fit together. I’m thinking it should be possible to fold the aluminium sheet so that it clips into the middle screw groove in the top and bottom extrusions of the (captive-nut) racks, thus avoiding any exposed metal edges. If it works, it should also be possible to fold a front and back plate over a base and then hold them in with a small number of screws.
the plan
I cut the ends out of plywood with mostly a handsaw, but using an electric jigsaw for the curve (which I drew freehand on the wood since it wasn’t trying to be circular anyway). The curve didn’t quite work out thanks to problems with the saw and clamps, but some work with a flap sander improved things. [2] The racks have countersunk ⌀3·5mm holes at either end, presumably on the basis that minimising protrusion is good. I had to buy some bolts, but couldn’t get 3·5mm so I’ve drilled the holes out to 4mm. Washers and Nylock nuts on the outside.
At this point things are a bit rough; the plywood could use a little filler but the aluminium can just be sanded and primed. Both will be painted. Black, because, black.
I’m also intending to line the wooden parts with aluminium foil. I wasn’t entirely sure but eventually decided to go with a thinner plywood scrap for the base. The front and rear support strips are another offcut of something, cut longitudinally. In practice the plywood I had for the base turned out to be a bit curved so I used a larger offcut for the rear support strip to help straighten it. The ends screw into these, making the assembly quite solid already.
A little while ago I obtained a stack of old electronic instrument paperwork which I’ve been sorting through and am planning to scan. To start with, this is a single sheet giving what seems to be an update for the Armon “A61S-A492-K4009-ARMONPIANO”. I’m not clear whether that’s one or three separate models. The update is for a revision to a tone generator board, now AR-27. It’s undated but I’d guess late-70s. Perhaps this will be of interest to someone?
For some years I’ve been writing the Quasimidi 309’s full name as Rave-0-Lution with a zero in the middle rather than a capital O, which I’ve never liked but had somehow believed it was the original. But it struck me earlier that I can’t remember why I thought that, so I should check. And I realised that I can’t tell the difference at a glance between the O (presumably) in LUTION and the 0 (presumably) in 309 anyway, let alone decide which of them the middle character matches.
I do have a PDF of the manual but it’s a scan. As it happens I’ve had the font — Serpentine Bold Italic (PS Type 1) since about the time Quasimidi made this and it’s a good bet that they had the same font, as it was part of a Freehand package at the time if I recall. So I looked at them on screen to see if there’s a difference and couldn’t see any. So I put them one atop the other in Illustrator with colours mixed, and, yes, there’s a difference, but to be honest it’s so slight that it might as well not be there.
I never much liked this font though I probably used it for odd jobs around that time once or twice as we weren’t spoiled for choice. (Nor were Quasimidi.) But this isn’t what I’d call good design. Perhaps this package is traced from the originals, and the originals were identical? In which case I shouldn’t bother asking if it’s a zero and just use the O? I took close up shots of the middle character and the O as printed on the Q309 to compare, and put the one atop the other in Photoshop. There’s less of a difference here than between the font outlines, but still, there is a difference, and most of the differences are in the same places, which I think is a smaller difference than you’d expect from lithographic printing on brushed aluminium anyway. (In both images the majuscule O is in green and the zero (?) is in red.)
I’m not completely certain one way or the other, but I’m feeling free to write it as Rave-O-lution rather than Rave-zero-lution in future.
Posted under Musical Technology at . Last updated 2024-09-17 21:43.
I got this, ready built, because I needed a simple switching function for another Eurorack module and it was being sold cheaper than any actual switching module or combination that should do it, and hey another sequencer will always be handy. So here’s some notes:
There’s no start / stop button(s) or input(s). It can be stopped by plugging a dummy plug into the clock input but that’s yer lot. This seriously restricts its utility because you can’t easily get it to work in sync with other devices or even start it manually at a desired time. (But it occurs to me that you could mount a switch on the end of a dummy plug?)
The sequencer doesn’t seem to respond to V‑Trig signals at the clock input in the way I’d like; that is, it resets to 0V but most input sources I use start high. Inverting the input fixes that, but it then starts out of step so anything it’s controlling may need to be reset to the previous step.
The clock input is a bit finicky in other ways; not all clock signals will drive it; perhaps it needs a slightly higher voltage. The Doepfer dividers manage it, some other modules don’t. (This is independent of polarity.)
Sometimes it steps the outputs without stepping the LEDs but I haven’t worked out how that was happening, and by the time I got it to work it wasn’t happening any more.
There is a manual available from Erica but it’s a build-it manual and is very short on actual functional description of the finished module, so all the above points had to be worked out slowly and uncertainly rather than just read, and remain a little tentative.
With that worked out, it’s doing the job as intended but if I’m going to use it for the usual range of purposes one might use a sequencer, a useful mod might be to put a switch on the internal clock signal. And a reset switch to start it high if need be. For the time being I’m using an external inverter on the input.
My MS‑10 has been behaving a little poorly recently, with the EG release time fluctuating to a gradually growing degree. So I’m going to have a look at it. First, have a look at the service manual as this failure seems likely to be a misbehaving capacitor. And yes, there’s a 6µ8 Tantalum cap (C35) on the release stage, so that’s the main suspect. I hear some bad things about Tantalums, but I’ve never had one fail before. Anyhow, I did have the thing apart for cleaning when I got it but didn’t take the PCBs out as it involves a fair bit of unscrewing things. So this is my first look at the relevant board (KLM-126c).
(Picture taken after repair.)
Checking C35 in-circuit, difficult to be certain but nothing obvious wrong.
