test flight of the spacebird

Continuing with the late-70s Japanese synth recreations that I’ll probably never finish – that thing I was working on back in January has grown into this:

my four-voice "compuphonic" synthesiser

Four voices, “compuphonic”. You know. My version has a sawn-off version of the originals’ motherboard in the vague hope that I might be able to fit it in a 4U (or maybe 3U?) box. Maybe in some alternate universe this actually happened and was released with a Roland model number, maybe MKS-4?

So many problems, some of which have been fixed. The chorus sounds a bit too swooshy. The envelopes leak clock noise into the output. The LFO leaks into the audio path, although that one is at least a known problem with the original.

The oscillators required some resistor-twiddling to get the pulse-width wave square. I fret I might have altered the level of the VCO into the filter slightly, I could do with checking further.

I mean, it can sound quite good. With all four voices in unison and chorus on it’s proper shouty raaaargh full-on “I am a synthesiser”.

We’ll get to the chorus later on but here’s a bit of pulse-width single voice then unison:

Also the LFO randomly goes quite fast on that, quite nice for making a horrible racket.

Pile of (not quite finished) boards!

pile of circuit boards

Initially I made voice boards with single row connectors but they were really wobbly. And the voice board was the wrong way round.

I changed the spacing on the card connectors so they’re on a 0.1″ grid, with the initial thought that I would build the motherboard on stripboard, but eventually decided I was done with such masochism for this one.

Predictably then the module controller board and the voice board don’t quite line up in the same way as the original does. I’d re-do the motherboard and the module controller but… it’s a lot of work. Otherwise the voice board and the module controller should be the same as the original, the track layout should be fairly close.

As IR3109s aren’t falling from the sky (give 2020 a chance, though) I went with the rev D voice board, meaning that I had to make loads of BA662 clones. Praise be to openmusiclabs.

Here’s one of the billions of 662 clones/clowns, missing the two transistors and resistors for the buffer ‘cos they weren’t needed in this case:

seemingly one of six-hundred-and-sixty-two BA662s

Maybe if I get round to doing a Promars I’ll try building it with LM13700s in adaptors, using homemade 662s is an exercise in masochism.

The chorus works and sounds ace but for the none-too subtle swooshing from the LFOs. I bought two sets of MN3007 from eBay – inevitably the cheapest pair made my power supply panic, I guess they were MN3207 or something, re-marked.

Here’s a bit of that – listen out for the (stereo) chorus kicking in second time round the bass loop. Later on (at about 39 seconds) when the filter closes you should be able to hear the zapping on the chorus, and some clock noise from the envelopes.

Poking at the output of the BBD with an oscilloscope suggested that the two outputs are at slightly different levels, so maybe I have to balance them out to cancel the swooshiness. Originally the JP-4 didn’t have anything here for trimming the individual outputs from the BBD, but the System-100M delay/chorus module does.

So much dust, it’s been here for a while:

chorus board

The original used a pair of MN3005s but I adapted the layout for 3007s using the tactics published by fitzgreyve.

The brain of this mess is a dusty Teensy 3.6, which is massive overkill for such a simple synth, but the inbuilt microSD card is handy for saving and loading patches. And the plentiful I/O will be handy when I come to making a front panel for it (In about five years, at this rate).

Teensy 3.6 on a breadboard, running the rest of it

The DAC is a DAC8562, which has two 16-bit channels. It’s comically tiny, especially next to the through-hole parts on the weird circuit board I had made.

The output from the DAC for the compuphonic settings and the MIDI-to-CV is 5V, giving five octaves of pitch, which matches the four octave keyboard (plus one octave down via the transpose switch) range on the original, but I’m not sure if it’s a bit limiting when it comes to the keyspan.

It does a few different voice modes as per the original which may or may not work reliably: different flavours of unison, round robin, voice per note held down, and some bonus single voice modes to help me tune the individual voices.

This demo uses the unison mode for the bass drone and the round-robin for the wobbly lead that comes in at about 21 seconds, so the notes (with a long release) smear over each other.

Early on I hadn’t realised how much current this thing was drawing – the wires I was using were too thin and causing the pitch to wander about, so tuning was a regular thing. Beefing up the cables has mostly sorted the drifting pitch, but I still think about incorporating some sort of autotune.

One downside with autotune is that I’d lose a bit of pitch range, so I’d definitely need to amplify the voltage from the DAC. Maybe that’s against the spirit of the wobbly original though.

Portamento is done in software as part of MIDI to CV – the original used a 662 for each voice but I’d had enough of making them by this point.

All the compuphonic settings from the original are MIDI-controllable. How will I get the MC-4 to talk to it? I guess I could do with some sort of assignable CV/gate input thing on the Teensy.

Another sort of demo – I was enjoying playing in Ableton for a change and actually being able to play chords. Loads of reverb on the pad, maybe a bit of chorus on the unison bass bit and a single voice for the squeaky acid bit at the end.

There’s something pleasing about the stepped voltages from the DAC, although it was a struggle to get them right.

Oscilloscope DAC output (before the sample and hold for each channel)

It didn’t help that my 90s-era Philips oscilloscope died quite early on into this project, and it turns out now that the same model is a hundred quid or so more expensive on eBay. This picture is from my replacement scope, which is a Tektronix 2445 that I’m not so keen on.

I found I had be quite careful in the sequencing and timing of the DAC and S&H multiplexer to get reliable behaviour, and even then I’m not 100% sure I’ve sorted it. The DAC8562 is posh compared to my usual MCP4922, but it still needs at least 10 microseconds to settle down to a consistent voltage. Boo physics.

I’ve got a screen! It’s in colour! Although I’m not sure that’s a good idea, because I’m having to shift more bits out through the SPI interface than I would otherwise with a monochrome display. Here’s my cheesy splash screen.

MKS-4 splash screen

There’s nothing much actually happening on the screen yet, but here’s a video to show off the patch loading and how the parameter changes are represented on the screen. When it comes to patch 5 I’m changing multiple parameters through MIDI, so predictably enough the screen goes a bit apeshit. (And the camera is a bit out of focus on the last part, soz about that)

The envelope noise is really obvious on patch one. The voice mode display currently doesn’t get updated when the patch changes so that’s all lies. At least the voice allocation numbering is correct.

The random noise burst when it turns on is alarming, but I quite like it. It reminds me of Robotron starting up, or a robot angrily being woken from its slumber. It does need sorting out though, it’s on the list.

I spent way too long on implementing the arpeggiator and got tied up in how it should respond to MIDI clock, so that’s in a half-broken state.

On the arp front I saw a rumour (…in YouTube comments) that the original didn’t do a real random, but that it was actually based on a bunch of numbers stored in ROM. So if you hold the right keys down, eventually you get the “Rio” arp line, god help you.

Presumably that would be in the key assigner board mask ROM, which unfortunately hasn’t been dumped out as far as I can tell. Not that I’m sure I’d be able to do anything with it if I had it.

Next move is to experiment with temporarily taking the voice outputs through a different route to see if we can make the induced clock noise from the envelopes disappear. I suspect it’ll mean cutting tracks on the motherboard – there’s already a suggested fix for the LFO bleed on the originals mentioned elsewhere that I’ll look at as well.

I’d already tried to balance out the chorus swooshiness with a pot and promptly fried one of the BA662s on the output, so that’s next after the envelope noise. Still a long way to go.


made a cake

Something like a Yorkshire tea loaf – about half a kilogram of fruit soaked over-night in tea, one egg, brown sugar, some bicarb, a bit of butter, plain flour, and some brandy that I randomly threw in ‘cos it’s Christmas.

And it turned out alright! Usually everything I make turns out flat, but this one wasn’t too bad. I suspect because it has a vast amount of fruit propping it up.

Apart from that, mostly I’ve been frenziedly drawing circuit boards to order from China to try and beat the UK customs changes come January 1st.

To explain a bit – at the moment there’s an arrangement called Low Value Consignment Relief, which means that any goods imported under the value of £15 don’t attract any VAT charges, which most of my PCB orders squeeze into.

Come January 1st, HMRC are binning this arrangement and have stated that the seller will be responsible for collecting the VAT, so that makes things less fun.

There were loads of things I was going to make (a Kobol clone with 3310s subbing in for the rare SSM2050s! an MS10! the DAC/gate output board for my imagined future MC-4 ripoff that I’ve been going on about for years) but mostly I’ve been churning out boring old clones of 100m boards. Yep, even the overly-complicated looking portamento module.

Other than that, the silliest is probably this board intended for my lovely Juno-6, which is basically a copy of the DCB interface in the Juno-60. Yeah! DCB!

At this point you’re understandably wondering why would anyone do this in this day and age, and I’d say, fair enough, it’s a bit daft.

Mostly it’s because I was too cheap to invest in a MIDI interface for this thing, but also because it’s more fun trying to work out how to get it going, even though it’ll be a bit more convoluted.

I’ve long missed the boat on buying an MSQ-700 for super-cheap, and OP-8/OP-8m are super-rare, so I guess I’ll end up writing a MIDI-to-DCB and CV/gate-to-DCB thing for an Arduino.

I’m sure it’d do-able to go from the Arduino to the Juno-6 direct, but I’m slightly nervy about knackering the MPU inside the Juno.

I opened the Juno up before I ordered the board just to see how much room I’ve got. Turns out I could easily fit some sandwiches and a banana in there as well as the board.

Loads of room. Probably not going to do the sandwich-box mod this time round.

Also there’s a suspiciously DCB-port-shaped hole covered by a plate at the back that will hopefully mean I can avoid any drilling, because you know that’s a bad idea. Potential Christmas fun to be had anyway.

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s950 screen replacement

Saturday afternoon, got a couple of hours, let’s change the screen in my S950.

Here’s the problem – backlight really dim in the late afternoon light. Even with the curtains drawn it’s hard to get a photo with the reflections. If anything this makes it look more readable than it was.

Dim looking screen on Akai S950

Let’s take it to bits.

Front panel removed from S950 to reveal buttons and original screen

I’m using the (slightly confusing) instructions from the Gearslutz Pimp my S950 thread. I bought a 40×2 display from buydisplay via Ebay – this is the one:

Stock photo of replacement display

The original screen has a 14-pin data/power connection on the left-hand side, and the backlight power connection on the right-hand side here:

Close-up of power connection on original screen

I was just interested to see what kind of voltage was appearing here, but it was hard to measure – didn’t seem to be getting anything much, maybe half-a-volt? Partly I suspected that screen is probably fine, but the inverter is stuffed – here it is, lurking in-place behind the front panel:

Power inverter for original screen - probably knackered

The connection on the left is the 5V and 0V connection from the panel – this is what we’ll use to provide the power for the backlight on the new display. The data and power connection for the display uses the bottom 14 pins on the connection, with the 15th and 16th pin taking 5V and ground respectively for the backlight.

As Don suggested in the thread, I connected a 1K pot (albeit linear, it’s all I had going spare) in series with the 5V connection to control the backlight – I nudged it down a bit from the maximum, and it’s probably a bit too bright still but it’s readable.

It took a bit of finagling to get the screen in place – it has to go behind the screw holes because the screen is thicker than the old one. The screws had to go in from the back, which was a bit of a faff.

New screen in place

Here it is with the front panel on:

S950 with front panel back on, screen glowing brightly

Looking loads better. Shame about the scratch through the screen, though not much I can do about that. It was only 75 quid when I bought it (including a massive box of discs) so I can’t complain really.

While I had it open I took some photos of the insides. Easy to see the eight voices on the top board.

View of inside S950 from the top

Getting a bit closer – loads of regulators attached to that chunky heatsink:

Top of power supply

Towards the back on the left we’ve got the power inlet, transformer and the MIDI board – PC900V opto represent.

Transformer and MIDI connection

And there’s this bit, caused me some trouble last time.

Jack board output and PCB

It’s all back together now and happily propping up a bunch of other relics.

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