De-mystifying, slaughtering and modding a Roland CMU 800

This project did not come out that perfect. I bought this unit (for to much money) to have some drumsource for testing a step-programming-drumsequencer-module. But i decided to give the cmu a supersimple midi to trigger-interface instead, mainly to gather experience 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. 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 …?

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 606-like and the schematics in general is also very similar.

Ringing-Oscillator for Hihat and Cymbal

Normally the Hihats and Cymbal get a mix of detuned squareoszillators. I added another noisesource in form of a tiny23 Microcrontroller with the following features:

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

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 couldn’t resist. The following was improved:

Snare Tune -> replacing R72 (560k) with 220K and a 1M-Pot in serial (this mod was removed after a while. the long cable leading to this sensitive section caught to much noise)

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 ringing-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 (this mod was also removed later due to too much noise)

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 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.

Demo of the modded CMU-800

