Re: <E30> Amplifier wiring

[Bob Hazelwood <woodyh@xxxxxxxxxxxxx>]

I can't give you specifics like color codes and such, since I sold my
'87 325 about a year and a half ago and the ETM and Bentley went with
it. However, I can tell you this more general info:
You can use the wiring up to the factory amp location as the input
wiring to your new amp. It is common ground at about a 4V maximum level.
If you choose an amp with enough input sensitivity adjustment range, you
should not need any additional adapters. Amp inputs _like_ common
grounds, it's amp outputs that don't.  However, your fader will not
fade. This is because the factory amp presents a low impedance to the
signal from the fader, and most aftermarket amps present a high
impedance because it is better for more universal compatibility. There
are two ways to deal with this issue. You could place a 10-Ohm resistor
across each amplifier channel input, which will fool the fader into
"thinking" its connected to the factory amp, or you could ignore the
problem. I recommend ignoring the problem because the additional load
represented by the 10-Ohm resistor will make the head unit sound as bad
as it did from the factory. By running the radio into the higher
impedance of an aftermarket amp, distortion is reduced dramatically and
the head unit can begin to actually sound OK. (Not great in this BMW
generation but still much better than you'd expect) The next step is to
simply bypass the fader. Of course you lose the fader operation, but if
you choose a multichannel amp then you can set up the system balance
with the amp's input level controls. A word of warning if you decide to
run new signal leads from the radio to the amp. The 6 cyl E30's are very
noise prone due to the rear mounted battery. Run any signal leads either
up the center under the console or along the driver's side. Stay away
from the passenger side sill, which is where the alternator to battery
feed wire runs.
The BMW output wiring is common between the individual speakers on each
channel, but is independent for each channel, so it should also present
no problem for an aftermarket amp. It is however pretty thin, so you may
want to run new wires if you are a member of the "fat wires sound
better" cult. (If so: welcome brother!)
Come to think of it there is no excuse to not run new wires to the rear
speakers since they are right there by the amp. The Bentley manual shows
the wiring and color codes pretty well, and I strongly recommend that
you have it in front of you as you attempt anything in this department
using the factory wiring!

Best regards,
Bob Hazelwood
VP Product Management, a/d/s/
'93 525i 5-sp
BMW CCA (Boston)

Murf asked:
Hello BMW Experts!  I'm trying to upgrade my stereo, but the dealer
>I just spoke with said that I really should get a new head unit before
a
>new amp, which is what I want first.  I thought I could make a 4 or 5
>channel amp work if I messed with the wiring, but the salesman said I
need
>a $50. thing to sort out the ground wires.  Seems the 'common ground'
and
>the front-rear fader switch in the dash are incompatible with
after-market
>amps.
>     Can anyone help me with this one?  I have the four speaker,
tweeter
>in door, little amp in trunk system.
>     I put in DEI 5.25 components in the front last weekend, and along
>with the Pioneer 5x7 coaxes in the rear, music is getting much better!
>               Murf '87 325(8)is   BMWCCA 79550

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