The main board of the synth: It accepts a STM32F103 Blue Pill board and offers two 10-channel DACs. Additionally, it contains a voltage converter, a virtual ground circuit and pin headers for power distribution.
Adjust to R2 to 540 Ohms to get a supply voltage of 10V.
U3 is optional. If U3 is equipped, do not close JP1. (C1, C2, R4 and R5 can be left out in this case.) Otherwise, do close JP1.
JP2 controls whether the digital ground is connected to the analog ground, or whether DGND is a separate buffered voltage source.
Similarly, JP3 allows to switch between the analog supply voltage and the blue pill board's 5V supply voltage.
Use Vdd = Vcc (JP3). Taking Vdd from the blue pill board creates weird "digital singing" on the audio signal.
Use a separate dgnd (JP2). Joining agnd and dgnd together causes strong humming (approx. 100Hz; I measured 116Hz?!) and buzzing (from the SPI bus; completely goes away as long the BU2505FV DAC isn't used (i.e., during startup/init), reduced when the MCP4822 isn't connected to the SPI bus, very present when it is). Using a separate GND completely cancels the humming, and reduces the buzzing to ~-70-80dB when the VCA is closed, but still audible if the VCA is open).
A TLE2426 ground splitter (U3) seems to be unneccessary.
- rev01: Has no marking and no wind fish on the back side.
- rev02: Small overlapping silk screen fixes. Has a wind fish and "rev02" on the back side. Was never fabricated.
rev01 has been used successfully with the following configuration:
- U2 was changed to TL072.
- JP1 closed, U3 not equipped.
- JP2 (dgnd select) in sep configuration.
- JP3 (Vdd select) in blue pill configuration.
- right half of the DAC section (i.e. J8, J10 and associated resistors and capacitors) not populated.
The gerber outputs used for fabrication of rev01 with JLCPCB are
located under fab/.
The fabrication layers indicating which component values to put where are here.