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I did this on a Sony boombox but used the CD audio lines for the input. The tape input has an EQ curve based on tape type standards and won't sound right. There is 5V to power a Bluetooth USB stick. Connect the audio to the junctions of R101,102 and R201,202 through a 10k resistor. You may have to play with the value or add a DC blocking capacitor. The service manual is online. On the Sony, i had to disable CD muting but you shouldn't have to on this one.
If anyone else has made one, please post your opinions. I'm surprised no one else has. I do believe there are too many choices in the instructions so I'll try to streamline things a bit. If there are questions, ask away.
I have made some changes since I built this and will be posting them soon
DIY Bluetooth Speaker
Let me know if you got it working. I just discovered I have one like that and Might experiment with it.
I'm not sure about that module. You could use a 5V step-up converter. On my next project, I was going to try separate step-up converters for the BT module and amplifier. This might help with the noise. 5V to the amp will make a huge difference in power output too.
The easy way is to use a 5V step up converter.
You could try using a power bank type portable phone charger. The fun of building your own isn't necessarily saving money. Rather it is having something that is exactly the way you want it.
Can you take a pic of the other side?
DIY Supercharged Bluetooth Speaker (v2.0)
Powerful Portable Bluetooth Speaker 2x5W
Modify The HC-05 Bluetooth Module Defaults Using AT Commands
Cheap 2-Way Bluetooth Connection Between Arduino and PC
Portable Bluetooth Speaker 2x3W Finished !