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Schematics and PCB layout for a voice card with multimode filter for a polyphonic synthesizer

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polykit/polykit-x-voice-card

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Polykit-X Voice Card

This repository contains schematics and Kicad PCB layout for a voice card with pole-mixing multimode filter. Multiple cards like this are typically used in a polyphonic synthesizer. It can also be used in a monophonic setup. There is only minimal additional circuitry like some pots, input jacks and voltage regulators needed.

Voice card specs:

  • Single oscillator with saw, triangle and pulse output
  • Multiple VCAs to blend between different input signals, a main VCA for the envelope and a panning VCA for stereo output
  • An additional external input that can be mixed in, eg. a noise signal
  • One envelope generator for the VCA. It can modulate filter frequency and resonance, also with an inverted signal
  • A pole-mixing multimode filter with 8 filter responses:
    • 4P LP
    • 2P LP
    • 4P HP
    • 2P HP
    • 4P BP
    • 2P BP
    • 3P AP + 1P LP
    • 4P N
  • There are multiple options two switch and blend between different signals and filter responses
  • PWM, FM and synchronization inputs

Video playlist on its functionality and design process: https://www.youtube.com/watch?v=2grRbG7IwDw&list=PLUKLEU8jmlbHH4_13YZM0MI6ez4gYQPfy

Polykit-X Voice Card Rendering

Printed Circuit Boards

Geber files for PCB production can be downloaded here. There is a small batch of PCBs for sale on Tindie. If you like to support me you can get them from https://www.tindie.com/products/27330/

Pinout

Pin # Description Short Voltage Range Type Direction Note
1 Ground GND
2 +12V +12V
3 -12V -12V
4 +5V +5V
5 -5V -5V
6 Pitch control PITCH ±5V CV 1
7 PWM PWM 0V to 5V CV
8 FM FM Signal
9 Soft sync SSYNC Signal
10 Hard sync HSYNC Signal
11 Envelope Attack ADSR_A 0V to -5V CV
12 Envelope Decay ADSR_D 0V to -5V CV
13 Envelope Sustain ADSR_S 0V to 5V CV
14 Envelope Release ADSR_R 0V to -5V CV
15 Envelope Trigger TRIG 0V/5V Trig
16 Envelope Gate GATE 0V/5V Gate
17 Envelope Output ADSR_OUT 0V to 5V CV
18 Enable saw signal SAW_ON 0V/5V Switch
19 Enable triangle sig. TRIANGLE_ON 0V/5V Switch
20 Enable ext. signal EXTERNAL_ON 0V/5V Switch
21 Neg. freq. by ADSR FREQ_ADSR_NEQ 0V/5V Switch
22 Freq. by ADSR FREQ_ADSR 0V/5V Switch
23 Res. by ADSR RES_ADSR 0V/5V Switch
24 Neg. res. by ADSR REF_ADSR_NEQ 0V/5V Switch
25 Input level 1 LVL1 0V to 5V CV
26 Input level 2 LVL2 0V to 5V CV
27 VCA VCA_CV 0V to 5V CV
28 Output panning PAN_CV -2V to 2V CV
29 External input EXTERNAL_IN Signal
30 Freq. control FREQ_CV 0V to 5V CV
31 Res. control RES_CV 0V to 5V CV
32 Filter mode A FILTER_MODE_A 0V/5V Switch
33 Filter mode B FILTER_MODE_B 0V/5V Switch
34 Filter mode C FILTER_MODE_C 0V/5V Switch
35 Output left OUT_L Signal 2
36 Output right OUT_R Signal 2

Note 1: A 100k ohm resistor on pitch CV input is needed. Multiple CV sources can be added by using multiple resistors in parallel

Note 2: Max. output volume can be adjusted by changing R32/R33

Filter modes

# C B A Filter response
0 0 0 0 4P LP
1 0 0 1 2P LP
2 0 1 0 4P HP
3 0 1 1 2P HP
4 1 0 0 4P BP
5 1 0 1 2P BP
6 1 1 0 3P AP + 1P LP
7 1 1 1 4P N

Select filter mode on pins 32-34

0=GND, 1=5V

Tuning Procedure

VCO

You need a control voltage 1V/octave source like a keyboard or two fixed, precise voltages in order to tune the VCO. Also a tuning device or a mobile tuning app is needed.

RV3 (TUNE) is for trimming the overall tune, RV5 (SCALE) is for adjusting the distance between two notes.

Take two notes that are one octave apart, eg. C3 and C4. play the first note and trim RV3 to that note.

Now go back and forth between both notes and adjust RV5 so that the correct distance between them gets closer. The first note will drift away while doing this so go back to it by adjusting RV3 again from time to time. Repeat this until both notes are perfectly in tune.

RV4 (HFT) is for trimming the higher frequencies if neccessary.

BOM

Reference Qnty Value Description
C1, C2, C3, C4, C5, C6, C17, C18, C19, C20, C22, C27, C28, C33, C34, C35, C36, C37, C38, C39, C40 21 100n Unpolarized capacitor
C7, C9, C13, C14, C15, C16 6 2,2uF Polarized capacitor
C8, 1 330p Unpolarized capacitor, film or mylar
C10, C11, C12 3 33n Unpolarized capacitor, film or mylar
C21, C23 2 1n Unpolarized capacitor
C24, C25, C29, C30 4 10n Unpolarized capacitor
C26 1 1n* Unpolarized capacitor, *polypropylene or similar
C31 1 10n* Unpolarized capacitor, *polypropylene or similar
C32 1 33n* Unpolarized capacitor, *polypropylene or similar
D1 1 1N4148 100V 0.15A standard switching diode, DO-35
D2 1 LED 3mm
J1 1 Conn_01x36 Pin header 2.54mm 1x36pin 90 degree angle
Q1 1 2N3904 NPN Transistor, TO-92
R1, R3, R4, R43, R44 5 1M Resistor
R2, R28, R29, R38, R42 4 470k Resistor
R5, R6, R7, R21, R22, R24, R25, R54, R55, R56, R60, R63, R68, R73, R74, R75, R76, R93, R101, R102, R103 21 10k Resistor
R8, R9, R10, R11, R12, R13, R14, R15, R18, R19, R20, R69, R70, R71 14 100k Resistor
R16, R17, R23 3 20k Resistor
R26, R27, R41, R72 4 68k Resistor
R30, R31, R34, R35, R37, R64, R65, R66, R67 9 1k Resistor
R32, R33, R36, R62 4 33k Resistor
R39, R40 2 22k Resistor
R45 1 330k Resistor
R46 1 1M5 Resistor
R47, R48, R57, R58, R59 5 470 Resistor
R49 1 24k Resistor
R50 1 5k6 Resistor
R51 1 1k8 Resistor
R52 1 10M Resistor
R53 1 47k Resistor
R61 1 680 Resistor
R77, R79, R80, R81, R82, R86, R87, R89, R90, R91, R92, R96 12 30k Resistor
R78, R88, R97, R98, R99, R100 6 15k Resistor
R83, R85, R95 3 7k5 Resistor
R84, R94 2 4k99 Resistor
R104 1 4k7 Resistor
RV1 1 10k NEG_RES_OFFSET Trim-potentiometer
RV2 1 10k NEG_FREQ_OFFSET Trim-potentiometer
RV3 1 10k TUNE Trim-potentiometer
RV4 1 10k HFT Trim-potentiometer
RV5 1 10k SCALE Trim-potentiometer
U1, U3 2 4016 Quad Analog Switches
U2 1 TL072 Dual Low-Noise JFET-Input Operational Amplifiers, DIP-8/SOIC-8
U4, U8, U9 3 TL074 Quad Low-Noise JFET-Input Operational Amplifiers, DIP-14/SOIC-14
U5 1 AS3372E 4-pole VCF, dual channel input mixer, VCA, panning VCA
U6 1 AS3340 Voltage Controlled Oscillator (VCO), DIP-16/SOIC-16
U7 1 AS3310 ADSR Voltage Controlled Envelope Generator, DIP-16/SOIC-16
U10 1 CD4051B CMOS single 8-channel analog multiplexer demultiplexer, TSSOP-16/DIP-16/SOIC-16

References

Article on Pole Mixing Filters: https://electricdruid.net/multimode-filters-part-2-pole-mixing-filters

Oberheim Matrix-12 Service Manual: https://archive.org/details/oberheim_MATRIX-12_SERVICE_MANUAL

Oberheim Xpander Service Manual: https://archive.org/details/oberheim_XPANDER_SERVICE_MANUAL

Sequential Prophet T8 Service Manual: https://archive.org/details/sm_SCI_T8_Service_Manual_Complete_sm

AS3372E Datasheet: https://www.ericasynths.lv/media/AS3372E.pdf

AS3310 Datasheet: https://www.alfarzpp.lv/eng/sc/AS3310.pdf

AS3340 Datasheet: https://www.alfarzpp.lv/eng/sc/AS3340.pdf

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Schematics and PCB layout for a voice card with multimode filter for a polyphonic synthesizer

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