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I have a circuit made up of:

  • A 12V power supply
  • A "TracoPower TSR 1-2450 Single Output DC-DC Converter 1W" which converts this 12V supply to 5V.
  • A pre-assembled audio amplifier
  • A Raspberry Pi

They are connected together as shown below (along with a bunch of other buttons and lights and things which I think are not interesting for this question):

enter image description here

When I play audio from the Raspberry Pi there is an incredibly loud hiss/hum sound.

Some observations:

  • If I plug headphones direct into the Pi it sounds normal.
  • If I put the audio amplifier onto a separate 12v supply the hum is diminished though not completely gone.

Could anyone help me with what I can do to diagnose / mitigate this hiss/hum? I'd like to understand the problem as well as solve it, and I don't really want to use two different power supplies. I'm almost a complete novice at electronics.

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    \$\begingroup\$ Which is it, a hiss or a hum? If hum, is it 50/60 Hz (you can find 50/60 Hz tone clips online to compare)? \$\endgroup\$
    – TypeIA
    Commented Jul 4, 2022 at 13:01
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    \$\begingroup\$ 1 watt is nowhere near enough for a Raspberry Pi. It might not be the cause of your hiss/hum, but you are overloading the DC/DC. \$\endgroup\$
    – TypeIA
    Commented Jul 4, 2022 at 13:04
  • \$\begingroup\$ It appears to be a 1 A DC/DC (so 5 W). This is still marginal for a Pi 4 but might be okay for an older Pi (you should check this out). \$\endgroup\$
    – TypeIA
    Commented Jul 4, 2022 at 16:05
  • \$\begingroup\$ @TypeIA it is a model 3 B. I thought it was probably OK because: 1. According to my 12v supply it's drawing ~.2A at 12V with just the pi plugged in (probably low because I'm not using any USB devices or screen or anything like that, just a couple things on the GPIO pins). 2. The rpi itself doesn't complain about being throttled via vcgencmd get_throttled. \$\endgroup\$
    – Simon StJohn-Green
    Commented Jul 4, 2022 at 17:47
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    \$\begingroup\$ 0.2 A at 12 V is 6 watts. Assuming (ballpark) 85% efficiency, your 5 V output current is 1.02 amps... which exceeds your DC/DC rating. (Still, may well be unrelated to your audio issue.) \$\endgroup\$
    – TypeIA
    Commented Jul 4, 2022 at 17:52

1 Answer 1

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You most likely have switching noise on your ground lines. Using thick ground wires will help. But, the easiest thing to try is to make the changes shown. You also want the 12V to the audio amp to come straight from the power source, and the 12V to the 5V converter to do the same, don't daisy chain the power. Posting a picture of your setup may help. enter image description here

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  • \$\begingroup\$ Thanks @Mattman944, I rearranged it like that and it has reduced quite a bit of the noise. A bit still remains but it's a level I can deal with. \$\endgroup\$
    – Simon StJohn-Green
    Commented Jul 4, 2022 at 20:08

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