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
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/
| 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
| # | 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
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.
| 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 |
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