This project attempts to build a low-cost analog synthesizer based on well-available components like LM324 or TL084 op-amps, which is easy to understand and learn.

The goal is a semimodular synthesizer which is compatible to existing other systems, where possible. The following features are planned:

• two 1-Volt-per-octave voltage controlled oscillators (VCO) with triangle, PWM-able square, and sawtooth waveforms
• the option to use them as digital controlled oscillators (DCO)
• a voltage controlled filter (VCF)
• a voltage controlled amplifier (VCA)
• digital or analog envelope generators
• analog waveshaping and distortion circuits
• USB or MIDI interface based on a STM32 microcontroller

For the sake of simplicity, the following trade-offs are being made:

• exact component selection is traded for automatic or semiautomatic tuning where possible:
• no exact 1-volt-per-octave frequency behaviour: The VCO's frequency response is not 100% perfect. While commercial synthesizers offer complex analog compensation circuitry, this project will measure the frequency deviation and compensate for it in software at the stage of the MIDI-to-Control-Voltage interface.
• no voltage controlled analog envelopes: These are complicated to build, and the difference to digital envelopes or LFOs is not noticeable.

So far, apart from standard resistors and capacitors, the synth uses LM324 op amps, and BC547 / 2N3904 bipolar-junction-transistors and 2N7000 MOSFETs.

The control logic and its firmware is currently based upon a STM32F103 based "blue pill" board, which can be sourced for less than 2 EUR from your favourite chinese seller. (Note: look for those boards with a rectangular white reset switch, not a flat round one.) Previous versions were developed for the STM32F411 discovery board, which should still work after adjusting the Makefile.

under simulation/, some circuitry was tested (using the KiCAD SPICE simulation).

Most importantly, simulation/vco shows a functional exponential VCO. There are detailed annotations on which part serves which purpose.

simulation/software/ contains tooling to do transient simulation on parts of the synth with different control voltages and to further analyze the results.

Under measure_vco/ you can find the tooling and test results for measuring the pitch deviation of the VCO.

hardware/ contains the actual hardware boards and drafts:

• The VCO development board mk I.
• A voice supplement board containing a VCA, VCF and some utility functions.
• A main control board with a STM32 blue pill board, utility DACs and the supply voltage generation.

To build libopencm3:

git submodule update --init
make -j8 -C libopencm3

If you're using a STM32F411 board, you must adjust measure_vco/firmware/Makefile and use the DEVICE=stm32f411re instead of the DEVICE=stm32f103c8t line.

To measure the vco frequency response:

cd measure_vco/firmware
make

then do whatever is neccessary to flash the program onto your board. For the blue pill board, putting it into bootloader mode by jumpering BOOT0 to 1, resetting, and calling stm32flash -w dac_test.bin /dev/ttyUSB0 does the job, while for my STM32F411-discovery, which has a non-standard blackmagic firmware on the STLINK chip, it is:

arm-none-eabi-gdb dac_test.elf # make sure it loads the .gdbinit file
load
run

Connect the pins correctly, then record the output with an UART adapter (or using the blackmagic uart device).

All software, firmware and schematics are released under the terms of the GPL3 license. Note that this does not include the simulation/**/*.lib files needed for simulation. Vendor licensing as noted in the respective files applies.