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:
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?