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I built a case that houses:

Everything is wonderful except for one problem:

There is an enormous amount of interference in the speakers. These noises correlate perfectly with the activity of the Raspberry Pi, and when I power the Pi with a separate 5v power supply all noise is gone.

So I built an RC filter for the 12v that goes to the 2 amps. I put a huge 4 Ohm resistor (https://www.amazon.com/gp/product/B008ML0LE4) with 6 capacitors of all sorts of values (3 chemical, 3 ceramic) and it is better but still unacceptably loud.

Is there a better way to clean up a DC power?

And if I feel lazy, is there a little device I can buy that does that extremely well so I don't need to build one more thing from scratch?

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    \$\begingroup\$ Welcome. Don't waste time with RC filters. They cannot block harsh noise from cheap switching power supplies, which can get into your audio circuits. \$\endgroup\$
    – user105652
    Aug 25 '20 at 22:08
  • \$\begingroup\$ "is there a little device I can buy that does that extremely well" - yes, a good quality separate power supply for the audio. \$\endgroup\$ Aug 30 '20 at 18:19
  • \$\begingroup\$ As I said it runs on batteries as a UPS... \$\endgroup\$
    – FlorianB
    Aug 31 '20 at 19:11
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Low Pass Filters need three structures

  • a series element (could be inductor, or FerriteBead, or Resistor); the resistor has DC_voltage drop

  • a shunt element (various capacitors, with ZERO ESR and ESL if possible; X2Y.com capacitors have FOUR terminals with internal charge_flow paths that minimize the "inductively" stored energy)

  • a well_controlled GROUND plane (copper sheet)

and need WELL SEPARATED return paths (ground connections) to input energy and output energy; you do not want to share VIAS between input and output Ground wires.

Be aware that PCB Vias have about 1 nanoHenry inductance.

At 100MHz, each via has 0.63 ohms reactance.

And why use a sheet of foil (ground plane) on which to build a successful Low Pass Filter?

Because WIRES are about 1 nanoHenry inductance per millimeter of length.

Use a plane.

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  • \$\begingroup\$ Ok, thanks a lot :) \$\endgroup\$
    – FlorianB
    Aug 26 '20 at 13:37
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The Pi series has little on-board filters in the form of SMD capacitors, so it cannot filter out noise from a power source. But inductors can. Use a 100 uH to 1 mH inductor rated at least 3 amps on the 5 volt input wire to the Pi. Normally this cable is power only, no data lines to muck up. Find the +5 volt wire close to the Pi and cut it and splice in the inductor.

Narrow body can capacitors can be soldered to the board behind the USB-C power port but not as effective as a series inductor on the 5 volt feed. That is why battery power creates no buzz or whining sounds-it is clean power.

Some car stereos have this problem, and it takes inductors on the head unit and external power amplifiers to make the "buzz" and alternator whine go away.

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  • \$\begingroup\$ Thanks! So I can just put one of these amazon.com/dp/B07FK8G5BS right before the 5v input of the RPI and it will prevent it from sending noise back to the battery pack and audio amp? \$\endgroup\$
    – FlorianB
    Aug 26 '20 at 1:33
  • \$\begingroup\$ Interesting for anyone like me who isn't very aware of inductors: tdk.com/news_center/publications/inductors_world/pdf/… \$\endgroup\$
    – FlorianB
    Aug 26 '20 at 1:44
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    \$\begingroup\$ Toroids make great inductors, even if a bit bulky. Due to high cost these 220 uH 5A inductors are a good buy. If you actually need to solder a can capacitor to the Pi board I will show you how. \$\endgroup\$
    – user105652
    Aug 26 '20 at 2:41
  • \$\begingroup\$ Thank you DAS :) \$\endgroup\$
    – FlorianB
    Aug 26 '20 at 3:51
  • \$\begingroup\$ I went overkill and put 4 inductors that I received from the link above: one on each wire (+/-) of power of the Pi and the Amp, and I kept the capacitors after also. Results: about 10 times less noise, very acceptable. I have to put my ear against the speaker to hear it. \$\endgroup\$
    – FlorianB
    Aug 28 '20 at 21:26
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Isolating the Pi isolated the noise.

Ergo the Pi is the noise source.

Filter the power INPUTS to the Pi and you will also be filtering the noise OUTPUTS to the PSU and thence to the audio. Although only the audio noise intrudes directly, RF may be present and may upset the PSU and/or audio circuitry causing secondary distortion, so use a low-pass filter such as an LC or LRC network.

Better still, leave the Pi's supply separate.

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