README.md
Synth


  
These are my custom modules for a from-scratch <![CDATA[]]>Eurorack<![CDATA[]]> analog synthesizer.


Format


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
minimum 100K input impedance
maximum 1K output impedance
connecting rail voltages to signal inputs should not fry any components (although it doesn't have to perform correctly)


<![CDATA[]]>Pinout<![CDATA[]]>:



Design goals


"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
Overview:


4-channel polyphonic
that means dedicated VCO/VCF/VCA streams
"fully-modular with defaults"
semi-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/timbres
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

Direct output jack?


mix arbitrary signal with mixdown
e.g. for accepting signals from other synths



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


<![CDATA[]]>https://en.wikipedia.org/wiki/Phase_distortion_synthesis<![CDATA[]]>
a bit of wild harmonics
multiplication can be done with log/antilog or <![CDATA[]]>https://en.wikipedia.org/wiki/Gilbert_cell<![CDATA[]]>
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

down ramp wave with just a JFET


up ramp wave probably needs another JFET and an inverter




MIDI-CV converter

digital MIDI input to OSC CV
DAC resolution:


for 1 step = 1 cent on a 10V space with 1V/octave:


10V / 2^x = (1V/12/100) => log2(10/(1/12/100)) 13.55 bit minimum
16-bit DAC would be incredibly accurate, 0.18% resolution


up to 4 voices
round-robin outputs on each key press


use open output if available else replace oldest key pressed

Modes:


off
1-voice: A
2-voice: A + C
3-voice: A + B + C
4-voice: A + B + C +D
4-voice-split: A + B, C + D


like standard 4-voice mode, but round robin is in 2 pairs of slots instead of 4 slots
allows for 2-voice bass and 2-voice melody
on mode selection, waits for a keypress to set the note to split at


capable to expand to 8 voices?


Filter

4P LP(/HP?)
filter we designed for class


standard vactrols are available on Thonk

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
could 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


LFO

2x is plenty I think?
OSC circuit, but adjusted to lower frequencies
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)


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/B disconnected



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

7-segment display of BPM for source clock?
sweep control option


LFO or voltages to trigger and CV external LFO for 4-bar sweep
e.g. automatic filter sweeps

random note drop feature
Could be externally controlled, e.g. iPad


save space and iterate on features
allow it to be used as a drum machine too


White noise generator?
Sample+Hold?


combined with white noise creates RNG
alternatively sequencer could do RNG digitally
random is a lot more interesting if it can quantize
S+H has little use to me outside of random

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


For multiple options (like VCO shape), use a CV that is binned to options
Easiest solution is probably an ATTiny driving an analog switch
uC seems like overkill, but it's got an ADC and lots of digital pins
Lots of specialty components would be required for a native IC solution
Firmware code can easily provide hysteresis and arbitrary numbers of divisions, unlike CDXXXX series CMOS chips




TODO


utility module


[x] Verify on breadboard
[x] design PCB
[x] order parts
[x] come up with a cute name
[x] design front panel
[x] 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)

[x] 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

[x] adjust design for Rev. B
[x] assemble Rev. B proto board
[x] test with Neutron
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


[x] Confirm amp on breadboard, finalize CV levels
[ ] build PoC amp and test with real synth
[ ] figure out where tuning trimmers are needed
[ ] finish PCB schematic for complete amplifier
[ ] Create PCB
[ ] design front panel
[ ] test with Neutron
[ ] manufacture front panel
[ ] assemble module

VCO


[ ] design core
[x] core osc - square/triangle
[ ] wave shaper (sine)
[ ] shape selection
[ ] Confirm amp on breadboard, finalize CV levels
[ ] Build PoC osc
[ ] PCB for oscillator

Envelope


[x] design in SPICE
[ ] verify on breadboard
[ ] assemble proto board
[ ] test with neutron
[ ] design PCB
[ ] manufacture front panel
[ ] assemble module

MIDI


TBD

VCF


TBD

LFO


TBD

Sequencer


TBD

build tools


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



License

Schematics, board layouts, documentation, etc. is licensed under <![CDATA[]]>Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License<![CDATA[]]>.



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

Any source code is licensed under MIT.

Reference


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