After installing an AUX input to the radio of my Mini a while back, I got sick of continuously having to plug my phone into the cable I wired up, and needed to go wireless.
Bluetooth A2DP receivers allow a phone to send music-quality audio to a receiver device, and this is the way I wanted to go. But for a while, they were just too expensive - around $50 for a receiver, so I put it off...
Until I discovered this cheap module on DealExtreme. At only $14, it's hard to go wrong!
After getting it and testing it out with both headphones and in the Mini, it turns out that the audio quality was pretty dismal. The main problem was very tinny sound - pityful bass and muddy treble. All the midrange was there, but that's about it.
So I opened up the module (just takes a fingernail to pry) and saw a bluetooth receiver module soldered to a PCB with an audio amp.
I looked around in the amp side to see if I cold figure out what was giving the crappy frequency recponse - nothing really struck me as wrong. But then I discovered a datasheet for the bluetooth chip. Turns out that it is designed to drive headphones directly!
It left me wondering just why there is a separate amp, given that the module has direct drive. I suspect something to do with isolation or perhaps different impedance headphones...
But I tried hooking headphones to the module output directly - and the audio was superb! Frequency response of the additional amp was awful and I had no need for it.
My end solution was easy - just disconnect the headphone socket from the amp, and wire directly to the module output, via 47uF electros.
There is a tiny bit of RF interference, but only on idle and not much of a concern in the car - but the music now has booming bass and crisp highs from the Mini - all by just bypassing half the components in the module.