I came up with an idea to convert my PC speakers into Bluetooth speakers. It's not really a serious project, rather a proof of concept, but I want to get it to work as well as possible.

Here's my idea: I wanted to use a Bluetooth audio receiver, connect its output to speaker's input and power it from speaker's power adapter.

So I got a cheap Bluetooth audio receiver. It's powered by an internal Li-Ion battery that you can charge via its microUSB socket. It obviously has a minijack audio output too.

Speakers are powered by a 10 V power adapter, so a tad too much for the Bluetooth module. I got a step-down adapter to get it down to 5 V. Here's how it's all connected:

schematic

simulate this circuit – Schematic created using CircuitLab

It more-or-less works, but I can hear a clearly audible high-pitched noise from the right speaker. I must note that all original circuits of the speakers are in the housing of right speaker and my additions are packed in there too. The left speaker is connected with a 1.5 m cable and it doesn't emit any high-pitched noise.

I tried to pinpoint the problem by powering the module from speakers' power adapter, but plugging in a set of headphones. No noise in that case, neither from speaker nor headphones.

Here's a screenshot of a sound analyzer app when the noise is audible:

As soon as I disable the BT module (just turn it off, without unplugging) the speakers work correctly again.

Is there anything I can do to filter out the noise?

  • Are you running your power cabling next to/close to the right speaker cabling? – Nick B Jun 22 '17 at 11:53
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    Describe more please. High pitched noise as in high pass filtered white noise? High pitched tone as in approximatly what frequency? – winny Jun 22 '17 at 12:20
  • @NickB everything's packed in the right speaker's housing and these are small PC speakers, so yes, everything in the right speaker is very close to each other. – gronostaj Jun 23 '17 at 9:09
  • @winny I've added a frequency graph. – gronostaj Jun 23 '17 at 9:12
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    Probably coming out of the 5V regulator (the part you have labeled "converter.") Many regulators using switching converters to step down the voltage. They can make noise like you describe. Try delivering a clean 5V to the BT Module. – JRE Jun 23 '17 at 10:16

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