Plan.md
Good Intro:


]]>https://aisynthesis.com/how-to-get-started-in-synth-diy/]]>
]]>https://www.schmitzbits.de/index.html]]>
]]>http://www5b.biglobe.ne.jp/~houshu/synth/]]>


Format

Eurorack is super popular

]]>https://en.wikipedia.org/wiki/Eurorack]]>


3.5" mono patch cables
3U, 1HP
10 or 16-pin ribbon power - +/-12V, sometimes 5V digital
Audio signals are typically a maximum of 10V peak-to-peak (i.e. between -5V and +5V)
Control voltages can either be unipolar or bipolar. Bipolar control voltages are typically 5V peak-to-peak (i.e. from -2.5V to +2.5V), unipolar voltages between 0V and 8V. The V/Octave scale is used for pitch information
Trigger, Gate or Clock signals are digital 0V-5V pulses typically used for timing and event signaling
100K input impedance ]]>typical]]>


]]>Pinout]]>:



Case:

TipTop is good starter product, has it has adapter for mounting modules in 19" rack, and included power supply

Power supply:


2000mA at +12V
500mA at -12V
170mA at +5V
]]>https://www.sweetwater.com/store/detail/HappyEndSV--tiptop-audio-happy-e...]]>
]]>https://www.amazon.com/Tiptop-Audio-Happy-Ending-Kit/dp/B074B29GPR]]>


Notes after playing with the Neutron


"Paraphonic" means multiple notes that share a filter stage, "Polyphonic" means a dedicated O->F->A for each note
Tracking the filter cutoff to the note is super important, otherwise the tone varies with the pitch
Neutron in paraphonic mode maps the note first note to both oscillators if only one is played. This
seems like a bad design choice to me
In addition to VCF cutoff, LFO can also reset with envelope, which is a very nice feature. Not sure how to make this Polyphonic without 4 LFOs...
Neutron has "osc sync" button. I don't have a good mental model of what it does, but it seems to link the oscillator frequencies together for creating harmonies
Choosing how you want to split the oscillators between polyphony, independent control, and sync/harmony is going to be a real control challenge, esp if keeping it modular is desired
Shared cascading controls with overrides is still a good idea IMO
Dedicating 1-2 oscillators to a rhythm with very good shared clocking seems important for making songs as opposed to just keyboard sounds. If not playing with others, need either that or a loop. Unfortunately with a loop you can't adjust the parameters later, you've probably reclaimed your oscillators for something else
Neutron sends envelope 1 to VCA and envelope 2 to the filter. You could accomplish a similar thing with the LFO in key sync mode, but you really only get attack


Design goals


Overview:


4-channel polyphonic
that means dedicated VCO/VCF/VCA streams
"fully-modular with defaults"
simi-modular-style sensible default paths connected under the hood
all defaults overridden with patch cables
"cascading controls"


unless overridden, CVs propagate from previous channel, allowing control of
all oscillators simultaneously, which is desired when using multiple voices

specifically targeting support for several modes:


4 totally separate, independent synths, each controlled with constant VCO or sequencer
1-voice, MIDI-,sequencer-, or constant-controlled synth with 4 oscillators, e.g. harmonizing intervals, mixing shapes/tambres
4-voice MIDI-controlled synth
2-voice MIDI-controlled synth, with a pair of oscillators for each voice
sub-combinations of the above, e.g.:
1-Voice MIDI-controlled synth with a pair of oscillators, 1 constant-source independent synth, 1 sequencer-controlled independent synth
1-Voice MIDI-controlled synth, 2-voice sequencer-controlled synths, with two oscillators for the  first voice

Ideally, one cascade switch between each signal path


need to find a way to do this while still respecting modularity
Perhaps defaults are controlled by a module which controls modes?

LED feedback is critical
Modules



Amplifier:



LM13700
matched-pair PNP exponential converter
CV of 5V -> Gain=1
CV of <=0V -> Gain=\~0, 6 decades below Gain=1
allow overdrive to 6V


ideally this would begin to clip

mixdown


mono - full mix -> left and right
stereo split - A+C->left B+D->right



Oscillator

0-10V, 1V/octave, A0 to A10
Square/Triangle core via LM13700
Waveshapper for triangle -> sine
abs(triangle) can be used for 2x square
blending shapes isn't useful here, since we could use other oscillators for that
square CV output for clocking?
Option to sync with another osc?
CV control for shape?


MIDI-CV converter

digital MIDI input to OSC CV
up to 4 voices


Filter:

4P LP(/HP?)
Moog Ladder?
CV control of cutoff, resonance
Default to track cutoff with VCO-CV


A/D/S/R envelope

4 envelopes required for 4-voice polyphony
wired to VCA by default
fully modular could mean you could borrow one for VCF instead


alternatively, perhaps simpler A/D envelope for VCF?



LFO

2x is plenty I think?
555 or relaxation
CV (+/- 5V) and digital out
ideally multi-shape: sine,tri,squ,ramp up,ramp down
CV control for shape
Trigger input (e.g. envelope or clock)
Future:
Sequencer


clock source for LFO? Or LFO input as clock?
divide down clocks?
likely digital


if MIDI output, can simplify duplicating VC outs for tuning,
can leverage MIDI converter for this
however, key-based offsets of patterns wouldn't work


White noise generator?
Sample+Hold?


combined with white noise creates RNG
alternatively sequencer could do RNG digitally
S+H has little use to me outside of random

Delay, overdrive, etc. are gimmicky and not useful, can be done via effects pedals



Amplifier

Calculations: ]]>https://docs.google.com/spreadsheets/d/1fOI5-cV5lOYskNise5XZh0knm-L3UJJy...]]>


]]>https://www.onsemi.com/pub/Collateral/2N5457-D.PDF]]>
]]>http://www.ti.com/lit/ds/symlink/lm386.pdf]]>
]]>https://www.schmitzbits.de/vca.html#]]>
]]>http://www.ecircuitcenter.com/Circuits/BJT_Diffamp1/BJT_Diffamp1.htm]]>


TODO


VCA


[ ] Confirm amp on breadboard, finalize CV levels
[ ] build PoC amp and test with real synth
[ ] finish PCB schematic for complete amplifier
[ ] Design housing
laser cut acrylic?
spacing?
mounting screws?
[ ] Create PCB

VCO


[ ] Confirm amp on breadboard, finalize CV levels
core osc (square/ramp or square/triangle)
wave shaper (sine)
sync? Zero-cross detector shorts fet to ground?
[ ] Build PoC osc
[ ] PCB for oscilator

MIDI/envelopes


TBD

VCF


TBD

LFO


TBD

Sequencer


TBD

Utils


[x] tiny Breakout pcbs for headphone jacks
~~transistor matcher circuit~~
bought 100s of matched pair BJTs instead