[W126 Coupe] Becker radio iPod hook-up

[Anuj Varshney]

I've tried it temporarily before, and it is fairly easy.  The gist of it is,
you simply feed the L/R audio channels from the Ipod directly into the
preinput for the factory amps. The factory amps are located behind each rear
passenger seat, but the wires are accessable either at the Becker radio or
fader switch. At the radio, this wire is thick with three pins,
red/blue/yellow I think.  The power on/off button of the radio will turn the
amplifier off or on.

You will need a DPDT relay to switch between either your radio or the Ipod
as a source for the amp. The relay can be controlled from something like
your antenna switch. So if you lock the antenna in the down position,
presumably you aren't listening to the radio, so the relay feeds the Ipod to
the amplifier. Put the antenna back up, and you're listening to the radio.

There are a few problems with this setup. For some reason I never completed
this. From what I remember, with just my test wires and aligator clips,
everything sounded great with the engine off. Clean, smooth, dynamic sound
from the Ipod. Then with the engine on I was getting a ground loop and faint
engine noise. Furthermore, the radio should be OFF if you are using the
relay to switch between radio/aux input, to prevent potentially damaging the
amplifier or speakers with a loud signal pop when the contacts bounce.

I wanted to come up with a softer switch instead of a relay, to switch input
when it sensed voltage from the ipod, but didn't have the time to do it.
Also, at one point, I was trying to get all the right connectors to make
this "plug and play" without cutting any wires.

Here's my post from April 2002. It has a lot of extraneous information that
I left as a mental note for me in case I resurrected the project in the
future:

***Date:* Sat Apr 20 2002 - 15:07:14 EDT

"...I have an Apple Ipod that I carry around with me, and found a good way
to
use it in the car. With a few simple cables, you can actually feed its
output into the factory amplifiers. You can access all of the wires either
behind the radio, or in the preferred location at the center fader switch.
Basically, the signal from the radio goes into the fader switch, where it
goes equally into front and rear speakers when the fader knob is at the
center position. (Incidentally, if you turn the fader all the way to the
front, the rear signal gets dropped over 22.4kohm of resistance, and
vise-versa.) The trick is to cut the signal wires feeding the fader switch,
and us a DPDT switch to select between either your radio, or auxiliary
source. Just remember to keep a common ground. It's preferable to access
the wires at the fader switch, since there seems to be a lot of shielding
on the connector at the radio.

You can route your aux wires to a headphone or RCA jack, and place that
virtually anywhere. I think mine will end up in either the center glove
box, or coin tray, or ashtray.

One problem with this setup is that the volume now depends on your source,
and not the radio knob. The factory amplifiers have a fixed gain setting,
so the volume output is limited by the source. On my Ipod (which uses a
standard headphone out), I was able to obtain enough volume. The sound was
loud enough that your mother would complain if she was riding with you. But
I guess that depends on your mother. I measured the maximum volume on my
Ipod as the equivalent of 26 clicks of volume from zero, with antenna all
the way up on a fairly strong station.

If you're still interested you can read on for more details....

Data:

Station: KIIS 102.7 from So. Cal, antenna fully extended

Radio Output Voltages:

OFF: While the radio's off, the signal has a DC bias of -2.0V to -1.40V,
presumably to prevent a harsh click/pop on startup. My theory is that the
radio ignores any signals with a DC offset below 0V. When you turn the
radio on, the factory amp turns on, and you see the DC offset slowly rising
up to around 3V-4V.

ON: As you turn the radio on, there's a slow drift of the DC bias to 3.15V
and upwards, presumably to give noise immunity and plenty of range to
prevent clipping at high volumes. This way even if you crank the radio up
all the way, the negative peaks of the waveform will not fall far below
ground (The radio can only generate up to 2V below ground). The interesting
thing is that there is a lot of drift in the DC offset. Hence, the radio
does not mind even if you use NO offset. This is what makes using an aux
input possible, without additional circuitry.

Ignoring DC bias, here are the AC signal values that feed into the amp:

Vsig = 0.200Vpeak-to-peak at "normal" loudness
Vsig = 0.500Vpp when loud enough for my tastes

Vsig = 1.0V and beyond when radio cranked up, it probably goes 2x as high
as this (not tested)

Apple Ipod Output Voltages:

DC Bias = 0.00V

Vsig = 1Vpp at maximum volume - equivalent of 26 clicks on radio volume
knob on a well tuned radio station.

Most other audio sources should also be able to generate 1Vpp at the
headphone jack. Check the source you are using before you start.

Of course it would be a simple matter to make a "preamp" for any source you
are using. You may even want to add a bit of DC offset, if you are
listening LOUD.
 -Anuj

On 3/25/06, Paul Samarin <pauls at sbcglobal.net> wrote:
>
> Couldn't find this info in the archives; maybe I've got the wrong list.
>   Don't we have a guy from Becker who knows how to convert an old Becker
> to accept an iPod input via RCA jacks?
>
> Any help appreciated.
>
> Paul
>
> The MB Coupes Website!
> W126 SEC Mailing List
> Postings remain property of MB Coupes, L.L.C.
>
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