I just installed a handsfree bluetooth kit, the HF850 from Motorola. It works great! I had no problems at all. However I do need some help.

About the HF850:
It has a dedicated 5W speaker.
It has a mute wire (active low, open collector).
It has no line-level outputs.

About my car:
I've got a PIE aux-in unit for my sat-radio. It works great. I almost never use the AM/FM/CD.
I've built an audio switch, driven by the mute wire from the HF850.
I don't want to pay $100 for the qubit (or whatever it's called).

The idea is to wire the HF850 to the switch, so it automatically changes to the phone when a call comes (someone else did this here, and it worked great for them).

My calculations:
Since the speaker is 5W and 8ohms, that's approx 6V at max power. I'll use a voltage divider to get it to 1V max. That's line level, right?

Speaker:
I know some speakers are driven with differential signals (one wire swings positive while the other swings negative). Others (like headphones or my sat-radio) have a common ground, and are driven single-ended (one wire swings in voltage).

Problem:
I can't determine if the HF850 has differential or single-ended speaker amp.

Neither of the leads is connected to chassis GND, I've checked with a meter. Does this mean they are differential? I think so.

If so, how do I wire them to the line-level PIE input?

Am I being an idiot? Are line-level inputs compatible with differential signals? Should I just connect it and go?

Thanks for any help.

 

OK, after some more research and some scribbling on paper, I think I found a reasonable solution.

The real issue is if the negative speaker wire is driven actively (differential signal), tying it to ground would be a big no-no. The op-amp would be toast.

Simple solution, use a ground loop isolator. The transformer guarantees that no matter what is in the PIE, it won't electrically connected to the HF850.

So I'll use a voltage divider to bring the signal down to line level, and a ground loop isolator to protect the circuits.