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Old 05-04-2010, 07:57 AM   #1
Propeller Head
bigfeller's Avatar
Join Date: Jan 2008
Location: Toronto
Posts: 283
CD43- Amp Audio Problem

I've been running a 'BMW Business CD43' paired with an old 4 channel Blaupunkt amp, powering a set of components up front, and an 8" sub for the past year.

I had been using a Scosche SLC-4 line output converter (LOC) to accept the 10V balanced differential inputs from the CD43 head unit, and feed front and rear 'low level' stereo RCAs to the amp. Over bumps recently, the system has started cutting out/ making a static noise. In an effort to track down the problem, I removed the Scosche LOC and spliced a set of RCA ends onto the twisted pair balanced differential outputs of the deck (in the trunk). Great sound! More amplitude, cleaner etc.

Sounds great in the driveway, but when I turn the motor on, however, the deck cuts out after about 5 seconds. This would indicate a short somewhere (thus triggering it's protection circuitry), but all of the connections look good, and when the motor is off, it's fine. What I don't understand is what is changing from a load or circuitry perspective, when the motor's running.

If memory serves, someone put it on a scope once and it was outputing around 10 V peak to peak. And don't forget, those voltage outputs are the deck's maximum output- I'm not even getting close to that with my low volume settings. 10V is a lot, to be sure, but I can't see how this would change when the motor's running. That said, if the voltage was too high, there should be some telltale distortion, crackling, noise, etc, but there isn't. It sounds great. Then I turn the engine on....

My initial thought was possibly a bad ground, but I've checked the amp's ground, and it's good, and the system runs just fine when the motor's not running. I'm stumped.
'97 M3

SOLD- Now Enjoying Honda Civic

Last edited by bigfeller; 05-04-2010 at 08:01 AM.
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