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schematic

simulate this circuit – Schematic created using CircuitLab

I want to power a chromecast and an amplifier from the same power supply.

When the dc/dc converter is powered by a separate power supply, there is no noise coming from the amplifier.

But when the dc/dc convertor is powered from the same power supply as the amplifier there is noise coming from the amplifier.

The DC/DC converter is isolated, so this shouldn't be a ground loop.

What could be causing the noise? How can I fix it?

Isolated DC/DC converter

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  • \$\begingroup\$ Welcome to EE.SE! Please describe the noise. Is it white noise, 50/60 Hz humming? \$\endgroup\$
    – winny
    Apr 26, 2019 at 7:30
  • \$\begingroup\$ The noise sounded like digital data on an audio signal, if that makes any sense. Arhythmic bursts of hum or varying intensity. \$\endgroup\$ Apr 26, 2019 at 20:26

1 Answer 1

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If it is a ground loop, it's an AC ground loop....

Then, it's called "lack of common-mode rejection" and occurs because the isolating DC converter will have high frequency AC switching artifacts (noise) on both DC lines that power the chromecast. That in itself may not bother the chromecast but, given that the audio output from the chromecast will have a fraction of this noise superimposed on its audio output and that will cause the amplifier a problem.

That problem will be significantly worse when there is power supply sharing of the DC converter and amplifier.

If it isn't an AC ground loop...

Then it's likely that the DC converter is putting ripple and noise on the power line shared by the amplifier and then this is called "lack of power-supply rejection".

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  • \$\begingroup\$ What would be a good method of determining whether it's an AC ground loop? What makes lack of common-mode rejection worse when sharing a power supply? With separate power supplies the amplifier makes no noticeable noise, so is this the less likely option? Would adding filtering input capacitors fix a lack of power supply rejection? \$\endgroup\$ Apr 25, 2019 at 13:40
  • \$\begingroup\$ Try 100 nF capacitors on each power rail from the dc converter to amplifier 0 volts. Report what difference it makes. \$\endgroup\$
    – Andy aka
    Apr 25, 2019 at 16:22
  • \$\begingroup\$ Adding 100 nF ceramic capacitors, connecting to the power supply ground, to positive and negative outputs of the dc/dc converter fixed all noise issues \$\endgroup\$ Apr 26, 2019 at 20:22
  • \$\begingroup\$ I'm guessing this means the issue was: "DC converter is putting ripple and noise on the power line shared by the amplifier and then this is called "lack of power-supply rejection"." \$\endgroup\$ Apr 26, 2019 at 20:23
  • \$\begingroup\$ Probably more likely to be common mode rejection. \$\endgroup\$
    – Andy aka
    Apr 27, 2019 at 20:00

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