I got mine from pepboys. Rebuilt valeo alt. Swich over your VR. Paid like 140 for it. 3 year warranty. Been running well for over a year and a half now. No more battery light when i hit 7 k rpms

 

Its more likely your charging system is failing than the stereo is overloading the 120A alternator. You could check this by measuring the current your stereo draws. Mine draws about 14A without the lights and stereo. That leaves a few amps for the stereo.

 

How did you check your amperage draw?

 

Correct and thats at idle RPM too. I'd expect there would be a slight increase as revs go up too. Must try that some time.
The P.S. shows Amps and Volts.

 

Thanks I give that a try

 

I was just looking up the specs on my amps and my 4 channel draws 60amps max @13.8 volts and suggest a 75 amp alternator and my 2 channel draws 80amps max @13.8 volts and also suggest a 75 amp alternator (which doesn't make sense)
So by the spec sheet that would be 140 amps max but I run my amps turned down about 75% and that's just on paper not actual gauge readings

 

Those current draws will be at continuous full power (volume) from all channels. As the volume comes off max or the signal changes from continuous tones the current draw will reduce. With music there will be high peak currents that may reach those quoted figures but the average current which is what matters to your charging system. How they suggest alternator sizes is quite subjective as the inconsistent recommendations show.
I ran a system (4*150W & 1*1000W RMS) without any trouble although it's not a good idea to use it for extended periods without the motor running.

For now I'd just make sure your charging system is doing its job before upgrading the alternator.

 

I have 3 amps.in my car. The factory, runs just the rear mids and tweeters. I have a rockford fosgate prime r150X2 amp tjat runs my door speakers and a power acoustics amp that runs my kicker 40cwd104 free air subwoofer. Never a problem with my battery or alternator

 

alternator is a weak point for the e46 mine died at 30k looking to see if i want to rebuild might check for sale section