De-mystifying, slaughtering and modding a Roland CMU 800

I bought this (for to much money) to have some drumsource for testing a step-programming-drumsequencer-module. I heard it should have a built-in 606. (Well, more about THAT later) But i think i will never make this step-programming-thing, will need much hardware. So i recently decided to give the cmu a supersimple midi to trigger-interface, mainly to gather experiance with the I2C-Bus.

Some general Info on the CMU 800

I happened to stumble somewhere over some fundamental misunderstanding about this thing. In contrast to the CMU 810 this one does NOT have analog synth functionality. It definately has NOT a SH101 or even a juno106 built it :) It was intended to be stuck together with a mac running some composition software to play some bip-bip-squarewaves polyphonically which come from some computerchips (which i dont know, was before my time). The bass-channel has a fixed lowpassfilter to sound different, There are also some rudimental decay-envelopes for the melodic voices. It had also the facility to output cv/gate. But who uses old macs today ... who uses macs at all? ;)

Much more interesting is the built in 7-voice analog drumsynth, It is not exactly the 606-scheme, e.g. the toms dont have a tune-envelope like the 606 has, the values for the parts are not identical. But it sounds very nice and 606like and the schematics in general is also very similar. More about how to mod it at bottom of page.

Supersimple Triggerinterface with 16 channels

• only 3 ICs: Optocoupler for Midi, µController as brain and a 6bit-8 channel DAC. Total cost with some resistors, quarz a.s.o: just 10 €.
• sorry i never draw schematics, i always make things on the fly with  ... ehmm ... the word for 'lochrasterplatine' ... is ... maybe.... 'holematrixboard'?
• Because the I2C-Bus is so simple (just 2 wires!) and the CMU already had 16 output-jacks i added another DAC to have 8 triggersignals more for external hardware.
• i learned a lot from the vacoloco opensource-midi-2-sds-triggerinterface. I had to change some things to work with the TDA8444. I couldnt get this max529-dac vacoloco uses. In addition its about 35times the price of the tda8444. The latter only has 6 bit in contrary to the 8bit of the max529 but i guess for drumtriggering its ok :)

click it for big size

Ringing-Oscillator for Hihat and Cymbal

• has 2 pots for TONE and TUNE
• Tone supports 10 different 4-Oscillator-Squarewave-relations and digital noise
• Tune can tune this Tone
• used as alternative soundsource for HiHat and Cymbal

... soundclip to follow

very simple, just a tiny26 (not attached yet as you see), 2 pots and a capacitor for decoupling. controller must be have been burnt on the developmentboard, no isp-jacks here.

just the idea, refer to datasheet for pinnumbers. capacitor mustn't be an elko

Modding

This CMU is quite a dream to mod. Everything open and a very simple schematic. Althoug i wanted to leave it as it is, i coudln't resist :) The following was improved:

• Snare Tune -> replacing R72 (560k) with 220K and a 1M-Pot in serial
• Snare Noise-Decay -> replace C22 (0,47µ) with 1µ to make decay longer (more than 1µ is not good, kills energy in circuit). Then add a 100k-pot towards mass at where R77 meets C22. This unloads capacitar faster or slower to set decay.
• CY-Decay ->1M-pot against mass at D16/R144
• OH-Decay -> replace R104 with 1M-pot
• Switch to change between original noisesource for HH/CY or my ringin-oscillator
• HiHat-filter -> the ringingoscillatortakes two ways. One to the long decay-noise of the cymbal, this one is lowpassed. the other path gets to the hihats and a short 'tsh' of the cymbal. This path is filtered at R102/C68/C69. Replace R102 with 10k-pot to alter the filter from its harsh, agressive exaggeration of high frequencies to a relaxed midrange tone, really nice.
• added the clave of the dr55 to have some nice additional, tunable percussion on drum-channel 8
• BD-Tuning for both filter-oscillators replacing R57 and R74 with 10k-pots against mass. Caution, put 330 ohm between pot and mass for R57 (lower BD) and 220 ohm between pot and mass for R74, otherwize they can be tuned so high that they will start to squear quite painfully
• BD-Tone, a simple lowpassfilter (see 808-schemes). Needed another free opamp to work. But an amplification was needed anyways for splitting the channels to the frontfaders.
• BD-decay with an additional opamp (for feedback, see 808-schemes) which was set free by killing the bip-bip-square-sound-unit, schematic to the left

For cabeling from the soundboard to the interfaceboard i used the already-there-multicable-jack. This way everything can be put apart without soldering. But it got a mess anyway.

I also cut the individual drumvoice outputs from their mixer and fed them into the mixerchannels for the not-neede-anymore melody-voicechannels. Has the advantage to have individual outputs for BD,SD,Tom/Clave and Hihat/Cymbal.

the main cpu was merciless cut away to reveil triggerpoints

backsideconnections of modpoints to the jack

the interfaceboard without the metalplate. i wanted to keep the original pots. the additional ones were screwed to the metalplate:

... and their cables were connected to the board with these ...

... cool ic-socket-connectors

BD-tone-scheme - click to enlarge

BD-decay scheme - click to enlarge

The 16 Jacks which formerly supplied gate/cv now spit the triggers. In the lower row the 8 trigger-signals of the internal drumvoices and another 8 triggers for external gear.

To the back is the midi-in-jack and a learn-puchbutton to change midichannel. Is stored in eprom of course.

To the right is a 8-pin-din-jack. Maybe i will need that in future for another drumsynth (cant get enought of them)

the whole thing finished. click for largescale

midi and trigger-out