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Plumbing solved, onto electricity! Posted by Justin VanAbrahams [Email]  (#32) [Profile/Gallery]
(more from Justin VanAbrahams) on Tue, 29 Oct 2013 16:46:14
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I'm decent with electrical stuff, but it appears I have run out of talent. I'm hoping someone here can 'splain something to me...
A few months ago I bought a 2000 Celica GT-S so I could teach my girl how to drive a manual. I probably could have taught her in something I already owned, but everything I own is some mix of old, quirky, and more likely than not high strung. The Celica is a fantastic, swift, and totally stock car. :) Well, it was stock. I installed a stereo. Mostly.
The problem is a hiss from the speakers. I know, the immediate thought is "ground loop" but I am 99.9% sure that is not the case. The hiss is apparently a common Toyota thing, a byproduct of the factory amp not getting along with aftermarket head units. I have read that the problem is an "impedance mismatch," a low impedance aftermarket head unit outputting to a high impedance factory amp. I've read the problem described as "the mismatch turns the entire wiring harness into a big antenna, which the amplifier then amplifies" as well as "the mismatch causes the amp to dramatically increase the gain." The end result is that the amp is making noise. Here are some things:
1. The system has been VERY well grounded. It's grounded via the factory harness, but also grounded to the dash crossmember. I've futzed with grounds directly linking the amp and the head unit as well as linking the the individual RCA cable grounds. I also went so far as to ground the shield that surrounds the signal wires. No improvement.
2. I've installed and removed ground loop isolators, just for grins. Like I said, It's not a ground loop.
3. The head unit has a 5v pre-out - it should not be very susceptible to noise.
4. There are two noises from the speakers - a hiss and a whine. The hiss is always there, the whine is from the alternator and RPM dependent. You can really only hear either of them when there is no signal, or very low signal. Once the source is at a normal listening volume, you can't hear it. The fact that it's there between tracks is annoying enough for me.
5. Finally, most importantly, the hiss and whine are present when the stereo is *physically disconnected* from the car. If the amp is powered on with no signal and the car off, there's a hiss. That certainly supports the "impedance mismatch results in amp gain" scenario.
Years ago, a dude on the Celica forum posted a message about the issue, and said his solution was to install 60 ohm resistors between the signal wires and ground. Here's his Tripod (*giggle*) page:
http://pacificbluegts.tripod.com/id1.html
The thing I fundamentally don't understand is he says "the termination resistors should match the output impedance of your preamp outs." I have no idea how to determine that. Frankly, I don't even understand exactly what it is this modification actually does, either. Can anyone help me? This is way beyond my pay grade. :)
posted by 12.195.130...
, Tue, 29 Oct 2013 16:46:14 <-- Viewing This Message
, Wed, 30 Oct 2013 05:17:15, Wed, 30 Oct 2013 09:58:36, Wed, 30 Oct 2013 12:05:13, Wed, 30 Oct 2013 12:44:53
, Wed, 30 Oct 2013 12:48:41, Wed, 30 Oct 2013 11:12:52, Wed, 30 Oct 2013 12:42:03, Wed, 30 Oct 2013 12:50:54
, Wed, 30 Oct 2013 01:46:55, Wed, 30 Oct 2013 10:05:34, Thu, 31 Oct 2013 01:36:50, Wed, 30 Oct 2013 12:44:29
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