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Date: Fri, 12 Jan 2001 22:42:33 -0500
From: "Thomas E. Tice" <>
Subject: [E36M3] Factory Stereo Design, was Stereo Specialist Needed....


Historically, BMW has used a somewhat modular head unit/amplifier stereo system. What I mean here is that they designed a system that can mix and match, add and delete components. On low end models the head unit would drive the speakers directly (with low power output - approx. 3 watts per channel). The next step up would be to use the same low end head unit and insert an external amplifier into the path to increase the power. Most 80s model US BMWs with the premium sound had this configuration (perhaps with a slightly better head unit). To simplify things they simply used the high level (more accurately termed speaker level) outputs from the head unit. BMW head units of that era had no Preamp level outputs at all. The next upgrade would simply add a head unit with more features and/or external amplifiers with more power. In Europe you may have been able to select the head unit with the features you desired independant of the size of the external power amp or even choose not to have an external amp (if you were happy with 3 watts).

So while I haven't dug into my M3 audio system, I believe the E36 cars use the same system so the most compatible signal from an aftermarket head unit would be the speaker level outputs. I have installed several aftermarket head units in E28 and E30 cars using the factory amps (it was cheaper to get a head unit and CD changer than buy a BMW changer) and had no problem with a connection of this type. If you use the preamp outputs of the head unit you likely will not get satisfactory levels from the BMW amp. You would also be less likely to damage the head unit driving from the speaker level outputs as these outputs are designed to drive very low impedances (8 or 4 ohms). Most preamp outputs are designed to drive something in the 10K ohm range. For those electrically challenged, low impedances are much harder to drive (requires more current).

Most preamp outputs should be current limited (ie protected) but if not, this is the mode more likely to damage the head unit if the amp is not intended to operate this way. I don't think it will cause damage either way as I believe the BMW amps have high input impedance (relative to a speaker) even though they receive a high voltage signal. Note that an additional complication is that most high power head units have differential speaker outputs. I won't attempt to explain that here but you need to be sure to only use the positive speaker output to connect to the amplifier. The negative speaker output should be left floating.

So if anyone still reading this is curious why the aftermarket uses the preamp outputs to drive amplifiers, its to avoid distortion. The power amp stage that is designed to drive the tough load (the speaker) usually is the dominate distortion generator in the signal chain (especially the wimply ones in a head unit). In BMWs system, you are adding the distortion of an unnecessary high power amplifier stage.

Sorry for the long winded response to your question. I believe the original installation was correct.

Tom Tice
'98 M3/4
'99 540/6 with Audio system so integrated into the car electronics I wouldn't dare attempt an upgrade.

"Ben C. Tickner" wrote:

> Date: Fri, 12 Jan 2001 13:08:49 -0800
> From: "Ben C. Tickner" <>
> Subject: Stero Specialist Needed....
> Hello all,
> My alpine head unit stopped sending a good volume signal to the amplifier. I
> placed the stock unit back in for now and everything works ok. The audio
> store sent my alpine cd unit back for repairs, stating that the amp (in the
> head unit) went bad.
> Here is my problem. The install they did originally had the high outputs
> from the radio go straight to the amp. Is this correct. I thought that only
> lows (RCA connections) should go to the amp. By the way I am using the
> factory amp. I am recieving my alpine head unit back this weekend and I am
> debating installing it myself, and correctly. What would be the correct
> procedure to install an aftermarket head unit on an otherwise stock 95 M3.
> Thanks in advance...
> ben
> 95 m3 -Cd player

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