analog-synth
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SPI clock noise
The filter overlays the filtered signal with a nasty 1kHz signal. The SPI dacs update with 1kHz update rate. Changing this changes the hum.
Since SPICLK is running at 5MHz, and since touching the SPICLK pin with a large metal mass (such as a wirecutter) leads to amore pronounced hum.
This also happens with a proper 7805-7905-battery-based power supply, so likely not a virtual GND problem.
wrapping the filter in (grounded) aluminum foil strongly reduces the hum.
Seems like we have built a radio station :(
to do:
- [x] is the buzz still there without the actual transistor ladder connected? (i.e. only U1D with R36,37 dangling). (try this on the breadboard) -> no; the transistor ladder needs to be connected at least to (-) of the OpAmp for buzzing
- [x] does the buzz disappear when changing R36,37 and R42,43 to 100 / 10k? -> it is reduced a bit, but still not great
- [ ] consider increasing the capacitors and the current passed through the ladder
- [ ] increase Vcc/Vee to ±12V
- [x] place capacitor parallel to the opamp's feedback resistor
- [ ] connect the GND planes. or completely disconnect them and separately route them to dgnd
- [x] does the issue still occur when
- [x] connecting the blue pill to power only?
- [x] completely powering the blue pill externally?
- [ ] increase source termination resistor (100-1k)
Disconnect at C2: -> buzz is visible on the oscilloscope at R21/R22. -> buzz is also visible across C3 and C6 -> buzz is also visible at output
Disconnect R36,37 -> buzz is visible everywhere except the output
=> OpAmp isn't the issue. Transistor ladder and prescaling are both affected. wtf.