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DIY sequencer box


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I had been thinking for a while how to integrate some sequenced stuff into our band's music. Initially I looked into an MBSEQv4 Lite, but there was no way with Lite to manage several sessions except by swapping micro SD cards. I realised quickly this would get both increasingly tedious as the song count would increase over time, and unnecessarily expensive, as the SD cards you can buy locally (i.e. easily) tend to be at least 16Gb nowadays, and the space would be mostly wasted when used for a single session. I guess one could buy a load of cheap no-brand 2Gb SD cards off eBay, but the juggling of microSDs and keeping track which one is which was not an appealing prospect. So I decided to design a custom version of the MBSEQv4.


I used to use Eagle to make my custom PCBs, but the PCB size limitation in the free version had irritated me already previously. Even the paid-for versions of Eagle didn't allow for very large PCBs; IIRC, there was a ridiculous jump from a 160x80mm board from the second most expensive version to something like 4x6 METERS in the industrial version (which was also the most expensive). And nowadays Eagle is for annual subscription anyway.

Nevertheless, already before the news of Eagle's subscription scheme broke, I had decided to teach myself Kicad. The annoying size limitation would be gone, which was doubly important to me, as I wouldn't be having the PCBs done at OSHpark etc., but I would isolation-route them on my own small Smoothieboard-controlled tabletop CNC (with a working area of 289x375mm).


Initially I thought of designing a stripped-down version with only the bare essentials for band rehearsals use (basically a playback machine for sessions made at home with my main MBSEQv4). But given that the two LCDs would determine the minimum width, one that wouldn't really be "stripped-down", and that a few extra rows of buttons would't really take up that much more space considering the panel width, I decided to go all the way :-) The working area of my CNC was just about enough to make the PCB in one piece. If you look carefully at the front panel images, the end bit on each side of the seq PCB are made out of separate pieces, which was dictated by the CNC's working area limitations.



In addition to the two-sided PCB for the panel itself, I put all the necessary DIN-DOUT boards into a single PCB as well. (So there's a mighty mess o' wires underneath the panel.) I first thought of the matrix IO, but quickly realised that with min. 1.27mm traces (isolation routing you know), two layers and all those diodes it would be if not impossible within the given constraints, then at least a lot of work. The DIN-DOUT way was certainly dirtier, but more transparent as well, in terms of which input and output goes where.


After the seq PCB was finished and in working order, I drew a front panel / 2-sided PCB for an MBSID as well. The DINs are mounted on the bottom side, no DOUTs. There's also some extra circuitry I thought I'd use for bank memory and a small amplifier, but the banks fit under the hood in the end, and the amp was a terrible idea to begin with, so I didn't do it in the end. I had one extra SSM2044 filter PCB which I had drawn (or rather re-drawn) years ago based on Seppoman's design (the Gerber files are not published, but the image of the circuit is in the wiki), so I drew a custom version of the AOUT-LC board with two 12bit channels for cutoff (and two 4bit channels unused), plus a normal AOUT-LC for resonance, using two 8bit channels.

The encoders under the LCD are for cutoff, resonance, LFO rate, LFO depth and Evn1 depth. The two pots on the right-hand side panel are for noise gate sensitivity and volume.


Next I drew a stripped-down version of the LPC1769 PCB. I had three LPC1769s, one of which was a leftoever after migrating my main seqv4 to STM32F4, and another one was used in a sample player. The "official" LPC1769 PCB, containing all possible headers, was unnecessarily large for a humble sample player (which doesn't require even LCDs), so I could shrink it down considerably: the resulting PCB is hardly bigger than the LPC1769 footprint (MIDI in and TDA1543 circuits are under the LPC1769).


I built two sample players, one for playing mellotron and SID drum samples, and another one to be used as a drummer's metronome (the tempo track being controlled by the seq of course). The metronome one's audio out is on the seq right-hand side panel, where I connect my DIY monitor earplugs, which I made years ago by sticking earbuds into putty, then sticking it to my ears and waiting for it to cure :-)

The MIDI IO section is made up of one MBHP MIDI-IO (upstairs) plus a MIDI thrux5 PCB (downstairs), which just multiplies one of the outs from the seq. There's four MIDI ins (IN4 doesn't have a DIN5 socket) and fours MIDI outs (OUT3–4 don't have a DIN5 socket).


In addition to the DIY stuff, there's a DSI Tetra, a Waldorf Blofeld and an EHX V256 vocoder. The two former ones, the MBSID and one sample player (four stereo sources; while the sample player is mono, it's connected to both left and right channels) go into two Behringer MX400 mixers under the Blofeld. The MX400 mixes four mono ins into one mono out, so you need two if you want to mix stereo. This combination was the smallest simple stereo mixer I could find. I tried to make my own op-amp mixer first, but there was a lot of noise I couldn't get rid of, and given that the MX400 costs only 23€ a piece, it wasn't much of a threshold to go for that in the end.


All in all I'm happy with the box. It's got a mains in (with fuse) feeding the multi-socket inside the box (actually there's two, one upstairs and one downstairs in the drawer), and a single stereo out (for rehearsal room main mixer). The vocoder has its own in and out, however. But it's pretty easy to connect up at rehearsals, I'm done before anyone else (even the bass player!) after setting up this and the drums :-)



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