From digest.v6.n526 Mon Apr 14 13:45:12 1997
From: KTM300_at_aol.com
Date: Mon, 14 Apr 1997 11:52:33 -0400 (EDT)
Subject: Re:it hums when it plays..

>I was in the passenger seat of my friend's '95 325i when I noticed a

>hum (actually a bit higher frequency than a hum but its difficult to

>discribe) that appears to be originating from the back. The noise is

>proportional to the RPM. At idle, the noise is barely audible.
However,

>when the tach shows 2k-3k, the noise surfaces and gets louder as the
RPM

>rises.

> If the <underline>frequency</underline> of the noise rises and falls
with the rpm's of the engine, it's most likely that you are getting D.C. leaking back to your stereo. I've had this problem in a few of my cars. You can get your local car stereo guys to fix this. Or even go to Radio Shack, there is a 'isolation transformer' type of thing that prevents this. I'll be seeing one of my electronic genious type friends this week, I'll ask him the details. But this otta get you started down that road of investigation.

My answer: Trevor is correct, it is DC leaking back into the system , but it is coming from the alternator. This is what we used to call an "audio tachometer". It comes from a poor chioce of the power feed for the amplifier for the subwoofers. I'd bet it is hooked up directly to the battery. If not on the battery posts, then it is awful close. At least, it is in between the battery and alternator. The factory amp gets its power way downstream from the alternator and battery where the power is nice and clean and filtered, ie. no "audio tach". The ONLY fix for this is change your power location for the subwoofer amp, I'd use the factory amp power, or get a "choke filter" that Trevor talks about from Radio Shack. Crutchfeild used to sell them, but its not a big problem with expensive amps anymore because the filter is built in.

Good Luck,
Dave Schultz
'The "pseudo" hack mechanic'
BMW CCA / Pittsburgh