I2C controllable CV output card

To clean up my SID organ I have made separate 4 channel CV output cards.

Features:

  • 4 channels +/-10V @16bit.
  • Buffered outputs with current limiting.
  • Addressable 0-3 so up to 4 card can stay on the same bus allowing up to 16 outputs.
  • Current controlled bi color led indication of output. Green for positive and red for negative.
  • Standard +/-12V eurorack power.
  • Output jacks are mounted so they fit under an eurorack panel and mounts with the jack nuts.

Click here for schematics.

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SID Organ

Behold the unholy marriage between an old wrecked reed organ and a SID chip.

Features:

  • 62 keys, monophonic CV/SID or polyphonic Midi. No touch sense.
  • Adjustable keyboard split high/low.
  • Individual envelope and arpeggio and glissando for each keyboard split.
  • 2 LFOs. Sine only so far.
  • 4 Control rods assignable to a lot of different functions and parameters.
  • Built in MOS6581 SID chip. You know. 3 channels triangle/saw/square/noise and all.
  • 4 0-10V CV out for Euro Rack module control.
  • 2 gate inn/out  for Euro Rack control and external arpeggio sync.
  • 10 save slots for settings (pre-named after C64 games from A to J)
  • some stuff i can’t recall…

I found the old reed organ in a really sorry state after at least a year outdoors in the Norwegian rain. It was already on its way to the trash yard, and the keyboard was so swelled it looked like a sine wave. I knocked it apart and kept only the keyboard section.

I have mounted pushbuttons and springs under each key. Instead of a keyboard grid I have used PCF8574 expansion boards (which I happened to have a pile of) making the whole keyboard I2C interfaced. An Arduino Mega takes it from there.

The sound is generated from a MOS6581 SID chip controlled straight from the Arduino Mega, but it can also 1V/octave control external oscillators trough one of the four CV outputs. I have made room and power for a few eurorack modules at the right side. At the moment I only use a reverb unit which I control with CV out patched to one of the Sound Rods, and patch the SID audio output trough this. Yes, and a home made eurorack master clock so I can sync arpeggio with my Ditto guitar looper.

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The old sound control rods are connected to potentiometers and sent to the AIs on the Mega. The transmission only moves the potentiometer a few degrees so the voltage difference is quite small. I have compensated somewhat for that by changing the analog reference voltage as low as possible so I don’t waste to much resolution.

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I have used one of the tiny 128×64 oled displays from ebay, together with an encoder, to allow serious menu diving. There is A LOT of settings available so far, so this unfortunately necessary. I would really want a bigger display, but that would require more memory for the display library and I’m already dangerously low on RAM.

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On the software side I had some challenges with the multi-level menu system and the lack of memory, forcing me to learn more about pointers. Which is a good thing I guess. I have made my own Arduino libraries both for the menu system and the SID, only the latter anywhere near clean enough to share with you guys.

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Problems so far:

  • The SID and CV-out circuits is a breadboarded rats nest and not trustworthy.
  • The Mega is almost out of memory. The oled library takes a lot. The menus take the rest.
  • The Mega is to slow to allow smooth LFO/Envelope/Appergio/Glissando processing together with menu operation. Glitching occurs and resources are maxed out. Okay, maybe with tons of optimization but ain’t Nobody Got Time for That!
  • The filter of the SID chip does not work. Yes they are bought on Ebay.
  • The +/-12V power supply made from scrap transformers gets dangerously hot. Also it is a bit hummy which you might notice on the video. I have used it for hours with no problem but I am afraid it will burn down the house if I forget to switch it off.

Future plans:

  • Replacing the Mega with a Teensy to satisfy my need for speed and memory. I am not planning on any audio processing in the teensy, only menus and control signals.
  • Getting a bigger display. Menu diving on the tiny oLed is mildly cumbersome.
  • Designing and printing some PCBs for the SID and the CV out,  trough Dirty PCBs. Both cards will be I2C controllable to save pins and to avoid level shifting a million pins from 3V3 to 5V if I move to teensy.
  • Building a new power supply. From new parts.

 

Gotten this far? Maybe add me on Instagram: https://www.instagram.com/tinkartank

I2C controllable MOS6581 card

To clean up my Sid Organ, I have designed a simple card for my SID chips, allowing I2C control. The card will be addressable from 0-3 allowing 4 SID cards on the same bus.

The card is supplied from +/- 12V to fit the eurorack standard but with the option of single power if wanted. There is also a jumper to choose between MOS6581 and MOS8580, the latter using 9V power supply. I think.

There are, maybe unnecessary, +/-9V regulators to clean up the power supply output buffer in case of noisy power rails. These can be bypassed.

Summary:

  • Selectable single or dual 12V supply.
  • Addressable up to 4 SIDs on same bus. Actually up to 8 if you use PCF8574B on half the cards.
  • Buffered Audio Output. Audio can be attenuated with resistors.
  • Optional 9V regulators for clean power to the output buffer.
  • Jumper for MOS6581 / MOS8580 selection.
  • I2C control. I will update my own Arduino SID library to fit this communication.
  • On board 1Mhz oscillator.

Click here for schematics.

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