SITUATION: Delphi XM unit connected to AUX1 on Alpine head model CVA-1005

SYMPTOM: Loud noise even when the Delphi is OFF but all the cables are
connected. Alternator whine added when engine running. Noise is NOT a hum
and NOT a hiss, but a shrieking mechanical sort of noise like an eggbeater
in a box of broken glass.

CAUSE: Noise is from DELPHI unit itself. They must expect you to use
headphones or other isolated output, so the noise reduction is limited to
common mode reduction on the output. The noise you hear is the digital
'hash' of the microprocessor and related digital stuff in the unit. The
noise is caused by the transistors that make up the ICs switching on and
off; causing the IC to consume different amounts of power. In order to
filter the Vcc rail (positive power), a 'bypass' capacitor is mounted next
to every IC. This filters the power to the IC making it work properly, but
puts this noise on the ground rail. (Of note, every 'stiffening' capacitor
sold is also a 'bypass' capacitor, adding positive rail filtering, but
adding noise to the ground rail.)

So, you have a device that had a ground rail noise. The positive rail noise
is OK, since you have a voltage regulator and heavy filtering to keep it's
noise from affecting anything. Your ground rail noise is now bonded to your
chassis ground, and the resistance of your power supply line changes the
ground rail current noise to ground voltage noise, since now the ground
voltage in your device is varying directly proportionally to the ground
current and supply resistance. This is what causes the 'ground loop' noise
in the output only when connected to another device sharing the common
ground.

A lot of people say the noise is from bad ground wires or careless wiring,
both of which may be true. Always follow the installation instructions
carefully and ground units exactly as indicated. Always route your RCA
analog cables away from all other wiring as much as possible. My experience
is that this noise is far less bothersome than the noise of a ground loop
which is the sonic equivalent of a digital chain saw. Horrible and loud.


REMEDY: One good solution is a ground loop isolator (www.radioshack.com
type in 270-054) which will degrade your audio quality somewhat, and reduce
your volume and dynamic range. It is only $15 so go get it. You'll
probably now have solved 90% of the noise. Whether you plan to connect up
an MP3 player, a laptop, or whatever, you are going to be glad you have this
part.

A better solution for the XM or any particular unit is an isolated switching
power supply. Normally called a DC/DC converter. These are available from
many sources. Sold as wither a discrete IC, or a prebuilt module. Newark,
Digi-Key, and Mouser all have a large selection. Be sure that the switching
frequency of the converter is above your listening level (15kHz).



Good luck and careful driving