Ferrari Mailing List: Re: More Stereo Installation & Wiring Expertise Needed

[Charles Perry (charles]

Robert -

I can't see in your F-List or F-Chat posts that you replaced the speaker
wiring at any time. While I don't think that the speaker wiring would be
bad, there were several wiring schemes on older cars that could cause
the problems you are speaking of. I have never worked on a GT4 so I
don't know if they apply.

Some older cars used a common negative for each pair of speakers,
meaning that they each had their own positive and then they shared a
negative back to the head unit. While you may see two separate negatives
at the head unit, there's no guarantee that they are independent all the
way back to the speakers.

In some really old cars, the car chassis was the negative for the
speaker wires.

This worked fine in older head units where there was a single sided
power amplifier in the head unit. However, newer units like your Kenwood
have two amplifiers for each speaker. They are wired out of phase and
operate in a push-pull configuration to send more power to the speaker
than you could otherwise get from just 12-volts with no switching power
supply. These amps are intolerant of common ground or chassis ground
configurations because you are essentially shorting two amplifiers to
each other or to ground under those conditions.

As to why it only happens with the car running, I have two guesses. One
is that one of the power leads you tapped into is hot under some
circumstances and ground (rather than open) under others. This can only
be verified with a decent multimeter. The other is that when you start
the car, you move from battery voltage (around 12.5 in a healthy
battery) to alternator voltage (around 14.4v in a healthy alternator).
This pumps up  the power output to the speakers by as much as 15% and
may be enough to put the amp into Protect if you have a bad situation
that is on the border of the protect trigger with the car off.

I would get your multimeter and set it on ohms first. Check each speaker
line (between pos & neg) and you should get roughly 4 ohms (+/- 20%
would be normal depending on brand of speaker). Next, take the meter and
check every individual speaker wire lead (positive or negative) against
the leads of all other speakers (pos & neg) one pair at a time. Under no
circumstances should you read anything but open circuit (infinite
resistance) between legs that aren't going to the same speaker. If you
do, that's a problem. Let me know which leads show resistance to which
other legs and we'll go from there. Next, check every speaker wire leg
(pos & neg) against the car chassis. You shouldn't read anything but
open circuit anywhere. If you do, that's a problem.

-- charles


 
_______________________________________________
 
Charles G Perry IV
Vice-President, Engineering
 
Carolina Sound Communications - MuzaK
7630 Southrail Road, BLDG B
N. Charleston, SC 29420 
(843) 571-4488         www.carolina-sound.com
_______________________________________________