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

2. 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
    • theory and example circuit: http://www.valvewizard.co.uk/uboat.html
  • 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?

3. 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

4. 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

5. 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.

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

Any source code is licensed under MIT.