Saturday, December 20, 2008

modified korg poly-800 synth / processor

The Poly-800 mkII includes a wonderful, warm (stereo) digital delay (with modulation!), a great-sounding VCF, three envelopes (one for each of the two oscillators and one for the noise source/VCF), and an LFO to modulate filter cutoff or pitch.

I've also added several well-designed modifications on this unit. First, it now has a VCF CV Input, which allows the filter cutoff to be modulated by a modular synth, CV pedal, or other signal generator. The input is designed to receive 0-5V. Clamping diodes and a current-limiting resistor ensure that voltages outside of that range will not harm the Poly-800's circuitry.

Second, I added an External Audio Input to the Poly-800's VCF and digital delay. The modulation delay provides great analog-type echoes, flanges, and choruses. Modulated filtering can also yield some very interesting effects.

This input is designed to work perfectly with guitars or other low-level signals. An added circuit provides the right impedance and just enough gain to boost such a signal, and a pot allows for attenuation before the gain stage.

The Noise / External Audio Switch disables the white noise source, so that the external signal can run through the Poly-800 alone. The DPDT switch also holds the VCA level at maximum (regardless of the settings of parameters 61-65).

The Filter FM control pot adjusts the depth of Filter Cutoff Modulation from DCO1's audio. This allows for an extreme range of sounds- from growling synth bass to ring modulator-type noises. It should typically be used with Chord Memory in 'mono' mode.

And VCF Cutoff and Resonance pots allow the cutoff frequency and the amount of resonance to be tuned in real-time.

The program parameters (31 and 32) now set the maximum levels. The filter will self-oscillate, and the resonance pot offers its full useful range when parameter 32's value is set to 12.

Here are a few examples of this Poly-800's more unique capabilities:
- FMsyn.mp3 (FM, cutoff, and resonance settings are being tweaked.)
- SHgtr.mp3 (Guitar is being processed with a random S/H input to the VCF CV Input.)
- spacegtr.mp3 (The delay sounds beautiful as a guitar effect.)
- SHsyn.mp3 (Again, the filter is being modulated by the EML101's random S/H output.)
- warblegtr.mp3 (The filter is being modulated by the Poly-800's internal modulation generator.)

E-mail me if you'd like to have your Poly-800 modified similarly!

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.)