Crystal Log Synth
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As part of our workshops at Ale and Cakes mine, we used a prototype 'crystal log synth' that kids could plug their crystals into to make techno music.
The ideas was to building up the circuits created from the Cardboard Crystal synth we used in previous workshop into things that can be played/listened to a little more easily in a hardware synth form. It was also used as a testing method so prototype ideas for sound generation that use natural semiconductors alongside 'off the shelf' synth hardware. We used it for taking to mines in the organised atoms workshop (where it has to have a kind of presence and playability) as well as recording tracks.
Using a volca modular as a temporary(?) solution that provides modules and element we need without having to build them. Voltages seem compatible with what we have been building so far (6V).
The synth as a whole is wired together via springs like the cardboard boxes, but is built out of wood.
- Circuits
- Square wave fm voice
This is directly based on the workshop circuits - osc1 modulates the frequency of osc2 via a transistor, this feeds into osc3's frequency via the crystal used as a transistor. This is fed into a low pass filter to allow us to remove (or enhance) some of the nastier frequencies the natural crystal provides.
[ osc1 ] -> [ osc2 ] -> crystal cat's whisker -> [ osc 3 ] -> [ resonant low pass filter ]
The inputs can be controlled via any of the sequencer vactrol resistances (see below)
Inputs:
- Osc1 frequency
- Osc2 frequency
- Filter cutoff
- Filter resonance
Outputs:
- Signal
- Sawtooth fm voice
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Using a common 'saw core' circuit, this is a new design where osc1 modulates the frequency of osc2, where the crystal is working in diode configuration, controlling the shape of osc2 as well as it's frequency. The resulting output is filtered by a resonant state variable filter that allow you (via a jumper) to pick between high, band or low pass outputs.
[ osc1 ] -> [ osc2 ] -> [ svf ]
|
[ crystal as diode ]
Inputs:
- Osc1 frequency
- Osc2 frequency
- Filter cutoff
- Filter resonance
Outputs:
- Signal
- Crystal noise generator
This circuit passes a controllable DC current through a crystal and a 1K resistor. The DC is removed by a RC high pass filter and any resulting fluctuations are amplified (X ~100) and passed through a resonant state variable filter. This provides a noise source that is different for different mineral specimens. The voltage passed across the crystal can be controlled, as the noise seems to change based on this and the point contact location.
[ noise ] -> [ svf ]
Inputs:
- DC offset
- Filter cutoff
Outputs:
- Signal
- Shift register/XOR sequencer with 2 vactrols
Takes a clock signal from volca (or an internal oscillator) and provides resistance changes that can be used in any other circuit as modulation/automation.
An XOR gate feeds to the shift register input and can be plugged into any of the 8 register bits to provide semi-random patterns where you can control the length.
Has a momentary button to insert 'data' which shifts the patterns, and a 8xtoggle switch which mixes outputs to provide different levels, basically a digital to analogue converter.
The output CV can be smoothed via a RC lowpass filter. It can be wired to one of the on board vactrol (LED/LDR combination) CV's - these are exposed as inputs so we can also wire them from the volca modular. The reason to put them here rather than on e.g. voice PCBs is partly due to their size, and the ease of can just hooking up connections to any pots we want, and expose them to the spring connectors.
The second CV is provided via 2 hardwired XOR gates on randomly picked outputs.
clock -> [ shift register ] -> [ mixer ] -> [ low pass filter ] ->
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\--> [ XOR mixer ] ->
CV1 -> [ vactrol1 ] -> resistance CV2 -> [ vactrol2 ] -> resistance
The vactrols are not completely covered, meaning light falling on the circuit is also a factor - and can be controlled expressively.
Inputs:
- Clock
- CV1
- CV2
Outputs:
- Analogue voltage
- Resistance 1
- Resistance 2
- Double shift register/XOR/vactrol
Unfinished but will provide 2x8 bit or 16 bit sequencing with XOR.
Inputs:
- Clock
- 2 vactrol CVs
Outputs:
- Analogue voltage
- 2 vactrol resistances
- Triple distortion envelope/gate/sequencer
A hacky replacement for the volca modular's low pass gates (which I made heavy use of previously). This circuit does several things in one:
- Takes clock input and generates pulse trains from two 4 bit binary counters. These counters can have their starting point set (using the 8 dip switches), which get automatically read at the end of each cycle. This can be used to create polyrhythms that can be changed live.
- Converts the pulse trains from the counters into envelopes with a decay control.
- Uses the resulting CV to attenuate a signal. This only uses a single transistor, so as the decay lowers it clips and adds distortion to the signal.
clock -> [ 4bit counter x2 ] -> [ env generator x3 ] -> [ gate x3 ] ->
^
[ audio x3 ] --'
Inputs:
- Clock
- 3 audio signals
Outputs:
- 3 gated/attenuated audio signals
- Mixer
Todo