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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inevitable SEM clone testing

SEM testing box

Everyone’s done a SEM-clone, haven’t they? Or a SEM-type of thing. There’s a good reason for that, they do sound really nice.

I’ve been plotting this for about five years or so. I was stymied for a while by the designator-less PCB layout in the service manual, but finally got some boards made just before Christmas. I went through the pain of putting it all together in a PCB box just to test, seems like it works quite nicely.

SEM clone board

Maybe too nicely, I need to work up the motivation to make a panel PCB.

I went for multi-turn potentiometers for the presets, 0.1″ headers for the connectors rather than whatever freaky Molex connectors were on the original because I’m cheap, and kept the component spacing and track layout otherwise as far as possible.

I’m using 2N3906 and 2N3904 for the BJTs, and J112s for the FETs just because they seem to be available. The surface-mount adaptors for the LM301s seem to work alright (as long as you actually solder the legs…), and I kept with 741s. I’ve heard that the reissue uses TL062s, for whatever that’s worth.

I’ve bunged the usual CA3046 sub in for the CA3086. I was intending on using a surface-mount version in a DIP adaptor, but then found that the usual adaptor is too wide to fit in the standard socket. In retrospect I should’ve just actually used a surface-mount footprint on the board.

I stuck with the 723 regulator which was probably daft because they’re obsolete, but then I’m probably only going to make a couple of these. I found some probably-way-too-big silver mica caps for the filter capacitors, and some weird massive looking yellow film capacitors on the filter input, mostly chosen because they might be terrible. Maybe in a good way.

Mistakes that I made and discovered so far include swapping the designators for R73 and R75, a couple of transistors (Q7 and Q16) are flipped around the wrong way, and for some reason it seems like I just gave up when it came to the F connector in the middle of the board.

Let’s look at some wires, the horror behind my quick test setup. My wife thinks I eat them or something.

SEM temporary box wiring horror

I’ve deviated from my usual tactic of employing an Oakley PSU run off a Yamaha PA-20 (which works really well), and instead trying out a switching power supply from China off eBay, fronted up by an LM317/LM337 in a vague attempt to filter out any spiky noise. I haven’t put my oscilloscope on the rails, or made any great effort to try and reduce any noise, but the SEM seems to be behaving itself so far.

I made some recordings of the SEM-in-a-box, hopefully it sounds a bit like it should. It’s the usual MC-4-driven frenzied sequencer nonsense.


High pass!

Band pass!

Also I did a little recording with it along with the 606/Machinedrum for drums and System 100 for bass, and then thought it was boring so I started overdubbing a few more passes by hand. Like I say, this is just a temporary box for testing, it’s not going to have the Oberheim logo on it or anything when it’s done.

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