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"Shurthi-1 Digital/Analog hybrid synthesizer (new)
Fully assembled and calibrated
The Shruthi-1 is a hybrid digital/analog monosynth. Its hardware design is deceptively simple, but the sonic range is wide: sometimes grungily digital like a PPG-Wave, fat and funky like a SH-101, videogame-y like a Commodore 64, weird and warm like an ESQ-1 ; but more often than not, truly original.
The Digital Side
An 8-bit microcontroller plays the role of the two main oscillators + sub-oscillator. This allows the generation of (aliasing-free) classic analog waveforms, but also wavetable/transwave synthesis and a trove of weird digital algorithms (sinewave bitcrushing, formant synthesis, phase distortion, bit toggling, XOR modulation).
Shruthi-1 firmware has plenty of great features: a complete MIDI implementation (large CC coverage, NRPNs, SysEx data dump including wavetable transfer), a 12-slots modulation matrix, a 16-steps notes/CC sequencer and advanced arpeggiator with some unusual time warping modes, and even a polychaining feature to turn several Shruthi-1s into a monster polysynth!
The Analog Side
The filter board is based on the classic SSM2044 filter chip. Providing a VC 4-pole LPF (with VC resonance), the SSM2044 was used in the PPG Wave, the Korg PolySix and MonoPoly, but also in the SP1200. Its sound is very warm and “juicy”. This board is no longer in production and becoming rare
To complete this filter, a linear VCA based on the high-quality SSM2164 quad-VCA chip is added. Don’t be fooled by the magical SSM letters: this one is a modern chip. The VCA cells on the SSM2164 have an exponential response curve and need to be linearized. One such cell is used in a feedback configuration to convert the linear CV into a logarithmic CV (the little man in the op-amp can find the inverse of functions!), and a second cell is used for the actual amplification, using the inverse CV.
It would have been very unfortunate to leave the 2 remaining VCA cells on the SSM2164 unused, so we decided to wire them into a 2-pole low-pass, the cutoff frequency of which is controlled by CV1. Yet another low-pass? No, because the SSM2044 has two inputs: an inverting one and a non-inverting one. The original signal is sent to the inverting input, and the low-passed signal is sent to the non-inverting input. The SSM2044 will thus process the difference of both signals, which will sound like a high-pass filtered version of the original signal, with cutoff controlled by CV1. Why didn’t we go with a high-pass? The circuit was a bit-simpler this way, and it sounded like an interesting use of the inverting and non-inverting inputs of the SSM2044.
External signals can be processed through the VCF and VCA easily, the envelope can be triggered via midi to "play" the filter or can be left open for "dumb" processing. The digital oscillators can be turned off and the filter can be run in self oscillation for a monster analog sine as well.
Case
The case is my personal design and is a black, one sided textured acrylic which resists fingerprints and scratches and includes all red LEDs. The LCD is protected by a clear window.
I warranty the device for 1 year."
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