SEM pot board

SEM pot board

Here’s my second go at the potentiometer board for my version of the SEM (previous post here).

I’m fairly happy with it, apart from the fact that my additions have pushed it over the original size a bit, meaning I won’t be able put two together in a 19″ box like the ObieRack.

Extra/different stuff:

  • second LFO (hence the wonky LM13700)
  • plus/minus one volt octave switches for the VCOs (hence a few references and a quad opamp)
  • LED indicators for the LFOs (couple of op-amps)
  • LED indicator for the second envelope (which is a bit pointless)
  • switches for oscillator shapes and modulation sources (due to the general lack of centre-tapped pots)
  • external connections routed one side of the board

There’s loads wrong with my paper and cardboard test front panel (it’s too small, for one), but this is some kind of idea of how it might look. The circles around the pots were just to work out maximum sizes for the pots and spacing while I was arranging it.

SEM with test front panel

The attack pot for the first envelope is something like 1mm too far to the right on the board, aargh. Forget doing the board again though.

Initially I was just going to make two, then I ended up making four of them, because why not.

potential four voice SEM

I mean, Mr Splitradix has five of the things, and Vince has bloody ten of them, so just keeping to four seemed fairly (almost) restrained.

This now means I’ve got to build some sort of panning mixer for the four voices. And also a MIDI interface and maybe some sort of flexible cv/gate assigner thing so I can round-robin play and hold the four voices from one or two channels of the MC-4.

And now I think about it a bit more, it’d be a good idea to have some common VCO pitch and cutoff tuning controls as well. And a common LFO. Let’s cover the world in LFOs.

(I hadn’t really thought this whole “let’s make a load of SEMS” thing through, really.)

I’m definitely not doing common envelope controls though. The envelopes on the boards aren’t voltage controllable as they are; it’s said that the FVS used early versions of the CEM3310 for this, and that just sounds like a massive faff. Even more of a faff.

One thing that I absolutely love about the SEM from a construction point-of-view is that the front panel just plugs into the voice board – here’s a photo of one of them balanced in-between some plants on a windowsill:

OK, so it looks strangely wonky in the photo but everything connects up so easily, and it’s just so compact.

I guess this’ll be no surprise to anyone building Eurorack modules, but after having wired up a System 100 model 102 clone all point-to-point, the relative lack of wiring is glorious.

Fuck wiring! It’s no fun!

There was good discussion on the Analogue Heaven mailing list recently (yeah! mailing lists! from the old days!) about the FVS versus the OB-X, and there was some thought that the OB-X sounded somehow nicer because of the OTA used in the audio summer (and the pre-distortion technique they used to counter-intuitively reduce the distortion on the super dynamic signal), so it’s tempting to try and implement that in the output section.

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