Oh Nick...
Now I'm going to throw a curve-ball your way.
When I say "once upon a time" this was probably 8-10 years ago, when I
was first getting in to this, um, sport (?). With the installation
I'm talking about, I was running 4 speakers right off the HU, and one
sub off of a class D amp.
Now, this doesn't change your overall theory, I suppose, just the
specifics.
First, I'll be honest, I don't remember if the whine was coming out of
the interior speakers - I DO remember the whine coming out of the
sub. 0_o The deck I was using didn't have a built in crossover, and
the amp said it was automatically crossed over at 300Hz (so I didn't
worry about it). But there was high pitched AND a low-pitched
undertone whine coming from the sub.
Weird, eh?
I always figured it was a difference in current between the power wire
and a long/poor ground (both were 4 gauge)...
~Mister.Lull
On May 29, 5:47*pm, "MOSFET" wrote:
Once upon a time, I attempted to ground a single Class D Subwoofer amp
with a two foot length of 4 Gauge wire and the whine was unbelievably
loud and obnoxious. *Simply by shortening that same wire (ground to
the same point on the chassis) by a foot (making it only 1 foot long),
the whine disappeared completely.
You know Lull, I thought about this.
Remember, I'm still recovering from my heart operation so I have lots of
time to think about things 
.
And I can't think of a single time I ever had a ground loop issue that dealt
with the subwoofer amp, PROVIDED you, and me in past installs, were feeding
the subwoofer amp it's own output from the HU. *The RCA outputs of a HU are
isolated in such a way that although you certainly COULD be experiencing a
ground loop via your subwoofer RCA's, I can't imagine you would HEAR such a
high-pitch whine over your subs?
So I'm assuming the ground loop noise was coming over your other speakers..
And again, I THOUGHT that those outputs were isolated in such a way that you
wouldn't hear such a ground loop unless your midrange amps were also
experiencing a ground loop problem.
Now maybe I'm way off on this issue, but let's suppose the curent draw of
your Class D amp was so great that although the loop was only through your
subwoofer RCA's, the difference in ground potential between the two
componets (HU and Class D amp) was sooooo great it effected ALL the RCA's as
the HU desperately tried to null out the difference. *You see, the other
thing that intrigues me about your story is that at two feet you have this
huge ground loop problem, yet at one foot it apparently completely goes
away. *Logically, that's a little hard to reconcile as you would EXPECT (if
your problem was caused by internal resistance due to this two foot length
of four guage) that by cutting the length in half, your problem should
logically be still there, but half as bad (as your ground differences should
now be half as severe as they were before). * If the problem before was
HUGE, by cutting the voltage differential in half would it COMPLETLY go
away? *Hmmmmm.....
Let me postulate something: I can't help but wonder if at two feet, your
ground loop was so severe, you crossed some threshold on your HU (sucking
current over your subwoofer RCA's) that made it impossible for it to isolate
the ground differences amongst the other RCA outputs. *But at one foot,
whatever ground-loop that might still be present would be small enough to be
restricted to your subwoofer outputs only, and as ground-loops are typically
high-pitched, they would be inaudible on a subwoofer system, and presto,
your problem would APPEAR to completely go away. *Simply stated, at two feet
your HU WAS OVERWHELMED, effecting all RCA's as it desperately tried to null
out what to it was an enormous difference. *At one foot, the problem was
resricted to your subwoofer output only, as it should be, and therefore,
inauduble.
I know, I know, I don't have enough to do. *That's what you're thinking
right now. *I know it.
It was just intriguing that at two feet the problem was huge, and at one
foot it seemed to completely go away. *That just seemed to not entirely
track for me and I was thinking of a reason why this might be so.
Again, I'm NOT challanging what you are saying, just approaching this as an
intellectual excercise. *Feel free to elaborate if there are more details to
this story.
Take care Lull,
Nick