Monday, November 24, 2008

analog MIDI drum machine

This is a MIDI-controlled analog drum machine.

The enclosure came from one of our local thrift stores where I was hoping to find a cash box or similar metal enclosure. This recipe card holder happened to be exactly the right size, and its easy-opening lid was something I needed because I didn't want all of the controls to be mounted on the outside of the case.

The 8 knobs on the top of the case control Master Volume, Kick, Clave, Snare, Conga, Low Tom, High Tom, and Hi-Hat levels. The two LEDs indicate power and midi activity.

On the rear of the case I've added only the most basic connections: +15V DC input, audio output, MIDI input, and MIDI thru.

The drum sound circuitry was taken from a broken old Farfisa organ. I removed the entire rhythm accompaniment assembly and later built a power supply for it, turning it into a stand-alone drum machine.

The sounds were great, but the preset patterns and preset drum mix were limiting. I began to investigate the options for MIDI control.

The Farfisa circuit board is the larger one mounted on the box's top. I added the blue trimmers to the solder-side of the board to allow for easy adjustment of resonance on the kick, both toms, clave, and conga.

Originally, the drums' levels were simply mixed by resistors, but I added 10K volume pots for each sound, and replaced the resistors with new values for more matched levels.

I added MIDI control with a lot of help from John at who sold me a customized MSA MIDI decoder kit.

His chip turns MIDI data into 8 channels of +5V triggers. 7 of the channels were configured to trigger drum sounds, but the 8th momentarily switches on a 'long-decay' effect for the hi-hat.

By tracing the original Farfisa circuits, I realized variable attack and decay settings were set by logic to create a variety of cymbal and shaker sounds.

So I added the two controls inside the case to adjust hi-hat attack time, and hi-hat release time (which only has an effect when hi-hat 'long decay' is triggered by MIDI).

I originally meant to use the IEC mains jack and an internal power supply, but despite my experiments with filtering and shielding I could not eliminate the noise from the power transformer! Eventually, I gave in to a wall-wart power adapter.

The +15V from the adapter is filtered, converted by a regulator into +12V, which is filtered and powers the audio, mixing, and amplifier circuits. The MIDI board uses a 7805 to supply the +5V for trigger pulses.

I had to configure the MSA decoder's firmware by SysEx to respond to the right MIDI notes and with the right pulses and triggers.

The Kick and both Toms use an output mode which toggles and latches +5V and 0V alternately upon receipt of a "Note On" MIDI message. The Conga use an output mode which supplies +5V constantly, except when a "Note On" MIDI message triggers a short (.5ms) "off" pulse. The Snare, Hi-Hat and Hi-Hat 'Long Decay' Switch use an output mode which sends a +5V trigger for the duration of a MIDI note.

On these channels the drum sounds can have variable durations, which is very useful and allows for some interesting programming tricks.

I changed a few resistors to increase the gain of the original preamplifier and amplifier circuits, but the output level is still a little weak. Eventually, I'll probably replace them with some simple Op-Amp circuit.

Here are some sound samples (recorded without any effects or processing):
- drumtest1.mp3
- drumtest2.mp3
- drumtest3,mp3
- drumtest4.mp3

E-mail me if you'd like to have your analog drum machine modified similarly!

(photos by kate and robert.)


thegoodbutcher said...


so the Farfisa board runs on 5V logic?

that'd be handy, because all the old organ rhythm circuits I come across are running something like -26V. it's a pain to try and interface, since the PIC's upper supply is limited to 6V...

r scott said...

You could add some transistors to amplify the output of the PIC.. ?

Yeah, I was really glad that this board accepted such low-voltage triggers!


n.d said...

Hey, very nice.
Where did you get the farfisa schematics?, I have a broken VIP 61, and maybe I can do the same thing. Thanks.