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Synth

These are my custom modules for a from-scratch Eurorack analog synthesizer.

https://rabid.audio/projects/synth

Modules

  1. Amplifier
  • LM13700
  • matched-pair PNP exponential converter
  • CV of 10V -> Gain=1
  • CV of <=0V -> Gain=~0, 6 decades below Gain=1
  • allow overdrive to Gain=2
    • ideally this would begin to clip
  • mixdown
    • mono - full mix -> left and right
    • stereo split - A+C->left B+D->right
  • Direct stereo input jack
    • mix arbitrary signal with mixdown
    • e.g. for accepting signals from other synths
  1. Oscillator
  • 0-10V, 1V/octave, A0 to A10
  • Square/Triangle core via LM13700
  • Waveshapper for triangle -> sine
  • PWM control
    • triangle wave applied to comparator with CV controlling reference. Triangle wave means CV is linear
  • abs(triangle) can be used for 2x square
    • adds a lot of parts for little benefit
  • blending shapes isn't useful here, since we could use other oscillators for that
  • Option to sync with another osc?
    • in theory, grounding osc at a regular interval would reset the osc over and over, effectively changing it's frequency
  • CV control for shape?
  • Room for more shapes
    • white noise?
    • PDS
      • https://en.wikipedia.org/wiki/Phase_distortion_synthesis
      • a bit of wild harmonics
      • multiplication techniques
        • log/antilog
        • https://en.wikipedia.org/wiki/Gilbert_cell
        • AD633 ($11-$13 /unit on DigiKey!?)
        • EL4450C ($4/unit on Digikey but out of stock). It's a little more awkward to use since it's max differerential input voltage for linearity is 2V
      • the hard part will be the "slightly higher frequency" bit
        • perhaps instead you have another osc (555?) running at a fixed frequency and clipping the sine
        • square PWM control could also adjust this clip frequency
      • In Max you do PDS by kink~-ing a phasor~ and using that to drive the phase of cycle~. Kink is somewhat straightforward to do - use a comparator to adjust the charge current after a threshold. But how you drive a sine wave phase with that I'm not sure
    • down ramp wave with just a JFET
      • up ramp wave probably needs another JFET and an inverter
    • sub octave
    • octave
      • full wave rectifier
    • multiplying the triangle with a square wave with different phases and pulse-widths gives some neat sounds
    • polynomial
      • The EL4450C datasheet describes using a multiplier ic to create a polynomial of the form (k1 v^2 + v) / k2
      • it sounds like crap on a triangle wave and does nothing for a square wave, but it adds a nice second harmonic to a sine wave
      • from experiments in Max, good values are k1=1.5-2.5, k2=2
      • but at that point, should you just use 2 oscillators?
  1. A/D/S/R envelope
  • 4 envelopes required for 4-voice polyphony
  • wired to VCA by default
  • save surface area with 1 set of ADSR controls for a pair of envelopes
  • fully modular means you could borrow one for VCF instead, or send to both VCA and VCF
  1. Filter
  • 4P LP(/HP?)
  • Cutoff frequency control has same range as oscillator
    • 0-10V, 1V/octave, A0 to A10
  • filter we designed for class
    • standard vactrols are available on Thonk
  • CV control of cutoff + resonance
  • Default to track cutoff with VCO-CV
  • individual filter knobs, plus a global knob for filter sweeps
  1. Utilities
  • Buffer, Sum, Attenuate, Invert, Gate
  • Could be all-in one
    • A input (default ground)
    • B input (default ground)
    • level knob attenuator
    • gate digital CV in, default on
    • (A+B)*level*gate output
    • -(A+B)*level*gate output
    • Constant CV out if A and B disconnected

Bitcrusher? 4 filters or one? Noise? Clock? Headphone output no longer required 2D mixer?

TODO

  • utility module
    • Verify on breadboard
    • design PCB
    • order parts
    • come up with a cute name
    • design front panel
    • assemble Rev. A PCB
      • bend the regulators first
      • plenty of 4066s
      • flip orientation of power connector
      • pot orientation - both channels:
        • right side (facing) to inside of board (from top)
        • left side (facing) to outside of board (from top)
    • test with Neutron
      • constant output is only 9 to -7.8
      • high noise when used as a buffer
        • perhaps input signal is clipping?
        • white noise when the gain is turned down
    • adjust design for Rev. B
    • assemble Rev. B proto board
    • fix prototype issues
      • sum pot is backwards
      • a bit of noise on the constant output near the center
      • verify output levels
      • hard click on gate
        • needs a LPF?
    • assemble Rev. B PCB
    • manufacture front panel
    • assemble module
  • VCA
    • Confirm amp on breadboard, finalize CV levels
    • figure out where tuning trimmers are needed
    • build prototype
    • fix prototype issues
      • output is too quiet for headphones, need about 2x gain
      • CV range is too wide, adjust scaling for faster change in db/V
    • finish PCB schematic for complete amplifier
    • Create PCB
    • design front panel
    • manufacture front panel
    • assemble module
  • VCO
    • design core
      • core osc - square/triangle
      • wave shaper (sine)
      • pwm control
      • additional shape
      • shape selection
    • Confirm amp on breadboard, finalize CV levels
    • fix breadboard test issues
      • pitch is very sensitive to power supply noise
      • tuning is rather difficult
    • Build prototype
    • fix prototype issues
      • TBD
    • design PCB
    • Create PCB
    • design front panel
    • manufacture front panel
    • assemble module
  • Envelope
    • design in SPICE
    • verify on breadboard
    • build prototype
    • fix prototype issues
      • TBD
    • design PCB
    • Create PCB
    • design front panel
    • manufacture front panel
    • assemble module
  • VCF
    • TBD
  • build tools
    • tiny Breakout pcbs for headphone jacks
    • transistor matcher circuit
      • bought matched pair BJTs instead
    • quantized selector with hysteresis

License

Schematics, board layouts, documentation, etc. is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Creative Commons License

If you're interested in commercial use, please ask first.

Any source code is licensed under MIT.