technical diversions

a place to document my current project of connecting an iPod to a Saab 9-3 convertible with the factory audio system.

Monday, September 11, 2006

this may explain some strange results...

For some people the much more simple modification described by Seth works very good, but for some people it does not work, or stops working after some time. I have not researched on this and I now mostly enjoy my iPod but to give another example of the power of a network I will post an e-mail that I got from Arne that probably explains what is going on. Here is his e-mail:

Hello All,

Although I have not created an AUX input yet, I plan to and studied all the descriptions from all of you in advance.

Just studied my 9-3 2001 radio unit (newer type PU-2135C) a bit and have some remarks.

Did some PCB tracing and reverse engineering:
The TEL and CD inputs are DC separated by 10 uF/16V and buffered by an op-amp before it is fed to the pre-amp (the point on the PCB where the two TEL inputs are connected together)

Using a NJM2058 from op-amp from JRC they have created a differential TEL input, the CD inputs are just a 1x op-amp. There is also a 1K series+1K to ground attenuator at the input of TEL+

This does explain the lower volume on the unmodified TEL input.

And this could also explain why Seth's simple mod (soldering an input socket directly to the pre-amp) can damage your radio. Because you bypass the DC separation and buffering, a peak or static discharge could damage the pre-amp input.

I think that simply adding a electrolytic capacitor of 10 uF (minus towards signal source) in the L and R line and maybe a 100 Ohm series resistor should be added to make this mod less "dangerous".

Re-routing the CD inputs like German does is the safest way if you don't use the CD changer.

Currently I only added a 60 mm CPU fan to the back of the radio unit because the front and CD's became very hot after an hour !
When this mod proves successful, I plan to do some more testing on the TEL input.

The Netherlands

Arne, thank you very much for sharing this research!

-- German Alvarez


  • At 09 March, 2008 15:28, Blogger Bill said…

    Just finished this! I used two 10uF electrolytic caps, and wired it in to the phone input (splitting the traces for R & L, and cutting the trace with the old phone input).

    Previously, trying this without the caps was very unsatisfying. Audio quality was poor, and having that line going directly to the stereo board seemed dangerous.

    The caps must indeed be taking care of the DC coupling, leaving the op amp sufficient range to amplify cleanly. Volume level is just about the same as CD or FM (with a cheap sandisk MP3 player at near full volume).

    Thanks for the capacitor tip!

  • At 10 November, 2008 03:26, Blogger claude said…

    Hello German,

    just wanted to let you know that I've posted some of your pictures and "translated" the solution for adding an aux input to the italian saab forum

    hope it's not a problem to post the pictures.

    also thanks for finding a solution for that wonderful car.



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