hardware
First: a disclaimer: I drew the the pcb by hand (which is to say, in pcb view), so i don’t have a proper schematic. (Sorry, i know it’s helpful having one). it's simple though.
and here’s the basics, anyway.
-
as noted, pinout is identical with the pjrc "audio adapter"; except pin 15 (VOL), which isn't used here; pins 18, 19 (i2c) aren't needed, owing to the different codec used (pcm5102a). (to be be precise, those pins are used, but for the (i2c) display). MCLK (11) isn't used either, as pcm5102a has its own PLL. the audio API defaults to 44.1kHz.
-
being a module, this thing also has the following :
- 4x CV in < > pins A1,A2,A3,A6
- 2x clk in < > pins 5,8
- 3x switches < > pins 0,13,21
- 1x encoders < > pins 1,2
- OLED SH1106 < > pins 18,19
-
CV inputs: standard inverting op amp configuration / 100k input impedance / 0.33x gain; this is the MCP6004, running of the 3v3 rail:
-
digital inputs: wired as transistor switches, directly to the respective GPIO pins: collector tied to 3V3 via the internal pull up resistors; emitter to ground; the trigger signals go into the base via a 100k series resistor and 33k to ground. at the software end of things, these inputs can be/are treated as interrupts; they are normally high, and low if/when the transistor saturates. any signal > 3V-12V should do:
-
audio out: a simple RC filter (as per PCM5102a datasheet), followed by an output amp (standard non-inverting) and a 1k series output resistor. I’ve tried with both TL072s and fancier audio op amps, i’d say the difference is mainly psychological, though i tend to go for the latter (and YMMV in any case). either way, the DAC output is 2.1 V_rms (~6VPP); so 1.7x gain (V_out = V_in * (1 + R_fb/R_gr)) will get you into 10VPP territory. as per BOM, R_fb = 33k, R_gr = 22k, which is plenty; the outputs are DC coupled. (conveniently, the DAC puts out a ground-centred signal).
-
power: LM1117-5 goes into teensy VIN and supplies both the ADP150 (3v3_A) and LM1117-3 (3v3_D) regulators; the latter supplies the OLED and microSD (the onboard 3v3 regulator might have been used, but it's rated only for 100ma, which seemed a little non-conservative; peak-current draw with SD can be + 100ma, They say); the ADP150 supplies the codec (AVDD). the two diodes provide for basic / reverse polarity protection.