Analog voltage comparator

Hi,

What I want to do is take the AC signal from the speaker wires of my car and then use this to turn on an aftermarket amp.

I am thinking about using a comparator circuit. I have two concerns with this though:

1. All the comparator circuits that I have looked at use one wire as input, whereas, it is two wires that come from the speaker. Do I simply ground the negative wire from the speaker and run the comparator with the positive wire?

2. Since the voltage on the speaker can be as low as 50 mV can an op-amp comparator circuit use this low of a reference voltage for comparing? (So that the amp will be on when 50 mV or more is present.)


I am open to any other suggestions for solving this problem.

Thanks for the help,
Michael

Usually one speaker wire is at the reference/ground level. Your easiest approach is to capacitively couple from the hot lead to an op amp gain stage and then go to your comparator. This way your comparator can use 500 mV for instance for comparison which is easier to do. You can protect the op amp with diodes across the feedback resistor.

Another method is to capacitively couple a step up transformer to the speaker leads. Then use your comparator.