I'm tinkering with various cheap Chinese Bluetooth modules these days. This one is the KCX BT003 - about $3.50 Canadian. I bought mine at icstation.com, but they're available all over the interweb. This module is Bluetooth 4.2. It does MP3, WMA, WAV, and FLAC, and claims to be lossless.
This board is pretty full featured. It has connections for a USB and TF card sockets. It has a LED indicator output and a microphone input so you could take cell calls through it if you wanted to. It also has an infrared input so you can add a remote control. The board requires 5V DC.
The board edges are castellated at 1.5mm pitch, so it doesn't match 0.1" breadboard. To make it breadboardable I soldered it on some (I think) 22 gauge copper standoffs on cheap perfboard and soldered in pin header for the breadboard. I realized halfway that I should have done it with the perfboard the other way around, because the pads are on one side only, but I made it work.
The chip on the board is branded JL. The number on it is AC1745AP16420-5A8. I googled for that but found absolutely nothing.
It uses one A/D input pin to do the five control buttons, as shown in the diagram below. The resistor values are pretty odd, but anything closish works fine.
I had only 4 breadboard friendly switches, so I had to make one up with perfboard and pin header.
I don't have a USB or TF card socket to test with yet, so I don't know how well that works.
I connected it to my little Lepy LP-2020A Class D amplifier and tested. Pairing is quick, and the recorded Chinese woman speaks decent English, unlike some other modules I've tinkered with.
This little 20W per channel amplifier is about $30 at amazon.ca and sounds surprisingly good. Makes a pretty good bench amp. Really good bang for the buck. It takes up very little space and makes almost no heat. It has both RCA inputs and a 3.5mm stereo input jack.
The Bluetooth audio is very good. FLAC sounds excellent. But there's too much high pitched Bluetooth RF noise getting picked up and amplified with the music. It's not quite so noticeable at high volume, but at anything mid volume or below it's really bad.
Tried a ferrite ring on the cable to the amp. No change. Tried various capacitors on the breadboard. No change. I suspect some of the noise may be coming from my power supply, and the breadboard is going to be picking up lots of that RF like an antenna. So, if you're going to be actually using it and not just experimenting, don't breadboard. Just do short point to point wiring.
That's it for now. I need to order USB and TF sockets. Then I'll ditch the breadboard and tighten up the wiring. Hopefully that will clean up the hideous noise in the audio. We shall see...
... Next day ...
I stripped the KCX off the breadboard adapter, built a control board and rebuilt with point to point wiring.
The super annoying Bluetooth noise is completely gone. I was correct about the breadboard being an antenna. Now the audio is clean and sounds perfect. Silence between tracks is dead silent. It's a good sounding module. Not bad for the price.