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The sound coming from the speaker cuts out and just hums loudly whenever I touch or displace the unplugged RCA cable.

How do I get rid of this noise? The whole should be somewhat robust. The speaker and amplifier will be moved around constantly so I expect the cables to move frequently.

The unplugged RCA is part of a stereo pair. Since the speaker and configuration of the amplifier is meant for mono I only use one of the RCA cables. So one of them just dangles unconnected. On the other end, the RCA pair is connected to a Bluetooth-module via a 2x RCA to 3.5 mm jack (TRS) connector.

I am using a

  • STEVAL-CCA044V1: class-D amplifier board
  • VHM-314: Bluetooth-module
  • Qoms2: Monopole omni-directional speaker

Before the Bluetooth-module and amplifier used to be powered with the same battery supply but the output kept varying in volume over a set of time. So after reading about the potential problems of a ground loop problem or bad decoupling of the power supply, I tried using an external supply for the Bluetooth-module which seemingly helped with the varying output.

Bluetooth-module (left) Amplifier (center) Battery supply (right)

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  • \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$
    – Community Bot
    Commented Aug 14 at 14:23
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    \$\begingroup\$ "How do I get rid of this noise?" Stop touching the exposed plug? Though as it's a stereo Bluetooth module you will only be getting half the musical detail, so you might be better combining the left and right channels with resistors. \$\endgroup\$
    – Finbarr
    Commented Aug 14 at 14:54
  • \$\begingroup\$ I mean the idea is that needs to be at least somewhat robust against physical manipulation. \$\endgroup\$
    – blaporte
    Commented Aug 14 at 15:18
  • \$\begingroup\$ Make your own 3.5mm TRS to single phono plug cable? But bear in mind these devices should really be mounted in an enclosure for protection anyway. \$\endgroup\$
    – Finbarr
    Commented Aug 14 at 15:23
  • \$\begingroup\$ How do you power the thing? The problem might not be hum because you touch the plug, but the amplifier floating around and touching you or anything that is nearly grounded will make the amplifier hum. \$\endgroup\$
    – Justme
    Commented Aug 14 at 16:17

1 Answer 1

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The dangling cable should be the output of one channel of the Bluetooth module, normally you wouldn't get hum from touching an output. If you were touching a cable plugged into the input of the amplifier you might expect hum.

So how is touching the output getting back to the input? Well, both channels out of the BT share a ground, and depending on how the amplifier is configured this could be connected to the input of the amplifier. The amplifier can be setup for balanced inputs, this connects the outer conductor on the RCA jack to an amplifier input instead of ground, if it's set up this way the noise your body picks up could travel up the outer conductor on the unused red plug, down the outer conductor on the used white plug, and into the amplifier.

I would check the amplifier setup, I believe it should be setup for mono, single-ended as shown in the datasheet. Also it looks like you are using the right input but for mono you need to use the left, unless the jacks are the other way around from the way they would usually be. Found a picture of the board, it looks like the jack positions are indeed reversed from what I would expect, so you should be okay on that.

If it's configured correctly and still has the hum you could do several things, terminate the unused plug with a suitable resistor, or as mentioned in the comments combine the two stereo BT outputs using resistors.

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  • \$\begingroup\$ Thanks! I ended up connecting it in stereo configuration as a sort of "termination" of the unused plug. So far the hum is gone even after giving all the cables a proper shake! \$\endgroup\$
    – blaporte
    Commented Aug 14 at 16:46

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