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In Reply to: Hmmm... so you are saying... posted by Chris Sawwa on November 01, 1999 at 13:43:17:
: that basically I should be able to tap into the wires going from the radio to the amp and get a decent 4V pre-amp like signal?
Yep. Most modern radios use a "BTL" output stage for high power. The BMW radio uses a single-ended output, with common ground, which is basically the same as a typical preamp output. The difference is that the BMW radio has a high current output which could drive speakers in a pinch. However, when you connect this to a power amp input, the high current stage effectively turns itself "off". The result is that the onew amp is driven with the output of the prevuious stage, which is a class-A voltage amplifier stage exactly the same thing as a "preamp" output. except at a higher than usual voltage level. Now, this higher level is a good thing, as long as the amplifier is a high performance amplifier with enough input level adjustment range to handle this high voltage properly. Presently, the vast majority of high end amps will have no problem with this requirement. The only amps which may be confused by this are the low and mid priced stuff in the department stores.
: I believe my installer has done that, at least that's what he told me ;), and according to him, the signal is there but in the bacground there is some kind of digital switching noise (which I hear from the Navi challenged folks, is not present in their systems). It sounds kind of like a fax machine. I personally have not heard it so I can only rely what I was told. But... I am thinking that maybe this was some external noise... ?
I suspect some sort of ground fault specific to your system. If the noise was truly in the audio signal from the radio, the factory amp would also amplify it and send the same noise to the speakers. One mistake often made by installers is to connect to the four (+) wires from the radio, and take a chassis ground for the (-) connection. This will not work in the Bimmer. There is s finite resistance in the vehicle chassis, and a current resulting from all the various signals travelling around the vehicle results in a voltage which modulates the ground. This in turn can be picked up and amplified by the amplifier and BINGO! You have noise. The ONLY place to get a ground reference for the audio signal is from the radio chassis, and the easiest way to get it is from the BMW harness itself. I know many people who have done this (including myself) without noise problems. NAVI may be the device which is creating the noise, but it MUST be a ground-related mechanism which is causing it to be audible. I know of a few people with alternator whine problems, but this is specific to a few systems and is not universal, since the vast majority of people I have communicated with have quiet systems. Since your experience is unusual, I suspect that either your installer made a mistake (even good ones do now and then) or there is an unusual fault in your particular wiring harness.
: Because, logically, if it's an analog signal, then if there was a digital component to it, how could the amp (or whatever it is) possibly extract it from the analog signal and leave just the analog sound signal intact!? It makes no sense to me, but then again I have not mastered the electronics to the point that I could definitely say that it's impossible.
You've got it right. If the noise was there, the BMW amp would reproduce it too. It MUST be some sort of ground fault or connector problem.
Also, the installer is a very good and experienced guy, so I really don't have any reson to distrust him.
I'm sure he's not being dishonest. He is probably just confused by something which didn't do what he expected. I know of a local dealer who has done a lot of E39's here in the Boston area who uses this connection scheme regularly with good results. Maybe he can help your installer figure it out. He is Rich Inferrera at Rich's Car Tunes www.cartunes.com
Best regards,
Bob
: Hmmm...
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