I've built a little soundmachine using a Raspberry Pi 3, the Adafruit MAX 9744 Amp and two recycled Car-Stereo-Speakers. Since this installation is ment to be used on a boat, I have an incoming 12v current. To reduce the voltage down to 5v for the Pi3 I use a DCDC stepper. The wiring can be seen in the poor diagram attached ;)

enter image description here

The Amp runs on 12 volts, so it is directly connected to the power source. For if it matters, here at home I'm using an AC/DC stepper (to simulate the boat battery) which turns the local 230 volts AC into 12 volts DC.

On to the problem. When I connect this setup I hear a very audible humming in the speakers. When I disconnect the audio jack from the PI and I use my phone as an audio source, the humming disappears.

I should mention that the picture doesn't show the entire setup. The 12 volt power source feeds another DC/DC stepper which in turn sends 7.5 volts into an arduino. Also a I have used the same power curcuit to feed 12 volts via Power over Ethernet into a device. Since I believe (or rather hope) that these do not have anything to do with the problem at hand, I didn't include them.

I'm a beginner when it comes to electronics, please take this into account when providing a solution ;)

  • \$\begingroup\$ You wont likely get AC hum on a boat, but floating DC may have hum as a large common mode signal. Any earth ground avail?, or use your grounded finger on 0V to see it if gets worse or better. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 4 '18 at 15:50
  • \$\begingroup\$ @TonyStewart.EEsince'75 Thanks for your reply but I don't understand what you mean... I have put my finger on the 0v line (don't know if my finger is grounded) but it didn't change anything anyways. \$\endgroup\$ – Mats Mar 5 '18 at 17:15
  • \$\begingroup\$ earth ground one hand and other to test \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 5 '18 at 17:28

You didn't show where your PI 3 made its signal ground connection to the main ground rail so I added one in blue.

enter image description here

I'm not saying this is where your humming/pick-up is coming from but I can tell the way you presented your question that you didn't understand about attaching a 0 volt signal return into a power rail that may take amps of current.

The PI 3 signal return should be tied right into the closest 0 volt connection on the MAX9744 as you can i.e. it should share as little of the power rail as possible.

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  • \$\begingroup\$ Thanks for your reply! The PI doesn't have a connection to the main ground. The PIs minus (ground) is connected to the DCDCs minus (on the output 5V side). And the DCDCs 12 volt minus is then connected to the main ground. Should I put the DCDCs minus closer to the MAX minus? And do I assume correctly that "minus", "ground" and "0 volts connection" are interchangable terms? \$\endgroup\$ – Mats Mar 5 '18 at 17:12
  • \$\begingroup\$ Gnd is defined as local 0V , earth ground is earth 0V \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 5 '18 at 17:29
  • \$\begingroup\$ Your PS is presumed to be floating and not earth grounded. Magnetic induced hum can be shielded or proximity isolated or if capacitively coupled, diverted to earth ground as mentioned by testing. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 5 '18 at 17:38
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    \$\begingroup\$ @Mats if the output from the DC/DC converter is isolated and there is no connection to 0 volts then you need to make one close to the MAX chip and from the 0 volts on the PI. -12V is wrong because that implies a negative supply below 0 volts. It's 0 volts or ground or GND. \$\endgroup\$ – Andy aka Mar 5 '18 at 17:47

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