Another Box o’ Techno

Another Box o' Techno

ABoT:  Another Box o’ Techno (2008)

A collection of experimental synth circuitry, linked together and mounted in a suitcase:

The ABot synchronizes with the BBoT.  Continuing with the BBoT ethos, there is no computer interface, storage, MIDI, etc.  Improvisation is the only approach.

The ABoT can be heard in the music of Operation Playmate… check out the bass drop at 0:40 in this clip.  The Bird Box generative sequencer is creating the tom-tom pattern.  Ragsdale is playing live keyboards (CS-15, MS20).  The Drone Commander appears at the end.

operation playmate - cryosleep

The bassline synth is a self-contained sequencer, melody generator, oscillator, LFO, and filter:

The designs for the SyncLFO, LFO-Sequencer, VCO, VCF, and Generative CV Sequencer were inspired by the wobbly-wub-wub bassline sounds used in dubstep music.  Although there are a variety of ways to make bass wobbles, I focused on this dubstep bass tutorial and translated the concepts to hardware.  Check out some recordings:

ABoT bassline + BBoT drums - 1

ABoT bassline + BBoT drums  - 2

ABoT bassline + BBoT drums - 3

 

wobble-bass machine

 left: SyncLFO, right: LFO-Sequencer

The SyncLFO is a recent design of mine.  It outputs sawtooth waveforms at LFO frequencies, locked to the tempo of the song.  A switch selects between UP and DOWN sawtooth variants.  Available rhythmic intervals are 1/2 note, 1/4 note, 1/4 triplets, 1/8 note, 1/8 triplets, 1/16 note, and 1/16 triplets.  There is also a “zero” interval which holds the LFO output at zero volts; this can be used as a mute or silent step.  It accepts a 32ppq +5V sync signal (similar to Roland DIN-Sync), optically isolated to eliminate ground loop problems.  It has a 3-bit parallel input to select the rhythm interval; this is connected to the LFO-Sequencer.  Finally, the SyncLFO has a glide control to smooth its output.  This is a really nice effect when controlling a VCF.  With glide off, it creates a sharp “wapp” sound, and when glide is dialed in nicely, it says “wub”.

The LFO-Sequencer is a minimal design implemented with 74HC- and CD4000- series discrete logic chips.  Its memory is 3-bits wide by 64-bits long (max), using MC14557 variable length shift registers.  The pattern is programmed in real time and repeats as a loop.  It can be tweaked step by step, morphing slowly, and you can change the loop length on-the-fly.  Its easy to double the length and punch in a different variation for the newly added “B-section” of the riff.  I also gave it an Auto Variation button that shuffles the bit pattern of the existing sequence in a mobius-strip sort of way… the pattern becomes six times as long and explores all of its rhythmic permutations.

VCO

 Dual VCO with sub-bass sinewave

The Dual VCO w/ sub-bass sinewave is a mutated variant of the CD4069 VCO.  I revised the 4069 VCO with several goals in mind - I wanted to build a dual version using only one 4069 chip; to do this required eliminating the pulsewidth modulator section.  I incorporated the NPN transistor sawtooth buffer that is recommended,  but I found that its bass output was rather weak; a little component tweaking was required to restore solid low frequency response.  And for the time being, I left out the tempco thermistors, using a zero-ohm jumper in their place… the pitch drifts slightly but I think of it as a virtue.  The sub-bass sine wave feature is an original design of mine.  It uses a PLL to track the output of VCO1, multiplying its frequency by a factor of 64 or 128 (-2 or -1 octave); this becomes the clock for an 8-bit digital lookup table, in which I have stored a representation of a sinewave, and also sine-squared.  Sine-squared is a very nice tone, when tuned to sub-bass frequencies, it offers those sick nasal harmonics that work well with an LPF to give the dubstep wobble-sound I was shooting for.  I’m using the TLC7524 8-bit multiplying DAC to convert the sinewave lookup table to analog.  Since it is a multiplying DAC, it was easy to add a cross-modulation option, which multiplies the synthesized waveform by VCO2; this gives a thinner timbre but can be manipulated for interesting warble effects if VCO2 is tuned several octaves higher…  And finally, this module has a crossfade control to smoothly blend between VCO1+VCO2 and the synthesized sine wave.  Effectively this is a tone control which brings varying amounts of sub-bass into the mix.

The 12dB/oct voltage-controlled LPF is based on the filter in the MFOS Sub-Commander guitar synthesizer.  I used an NJR13600 OTA instead of the LM13700 shown.  The filter sounds killer with no further tweaking.  (I think I used a 50K pot for the resonance control instead of 10K as shown.)

The Generative CV Sequencer is sort of a lackluster performer at this point; I’ll wait to discuss its design until I have a chance to revise it.