Bmw Reverse AUX
I have found a perfect solution for an AUX IN on the BMW reverse Radio. At frist the Tape has to be taken out. Then the two pins of the TEA0676T 1+16 should be removed. I just lift them up from the motherboard. This is important otherwise the music will be very bad because the noise from this IC will be in the way. After that on the back side of the motherboard are also two points we’re the pins 1+16 are connected to. On this points you need 1x 10umF capacitor on yeach pin. That’s because the audio IC is putting out 4.2V DC offset voltage and the AUX cannot be connected to the MP3player. The AUX cable can now be connected on the other side of the capacitor. The last important thing is to connect the pins from the last picture together so the logic in the radio thinks that some casset is playing and don’t connect the cabel with many colors again. Now everything can be put together again and the AUX cabel cann be put somewhere in the car. I did it in the middle console for an easy access. When the AUX is on the radio will show on the display tape 2.
Comments
Nice workaround, some clarifications are needed though.
10 µF capacitors (micro-Farad. not micro-mili-Farad) are there to filter out the lower frequency part of the signal from the IC, which poses a problem as it may provide high noise levels. This two capacitors are mentioned in application notes in the datasheet of TEA0676T.
However, these caps can prove to be a filter for lower frequencies for other sound sources, like the one in your modification. Impedance of the capacitor will be 160 Ohm at 100 Hz. 10 µF capacitors should already be on the board, you could have used them.
If your new music/sound source has usual output impedance of approx 50 Ohms (or somewhere in that ballpark, number is good for smartphones and such), that means your 160 Ohms impedance from caps will greatly lower your signal.
I believe you would get much better sound (not only in the low-end frequency range) if you removed this capacitors or replaced them with 100µF ones. You cannot destroy your DSP chip if you lower the volume of your sound device before first try.
If you take the cassette module out of the radio, you might found DSP chip (usually a larger one, in my CD radio I have Motorola XC390239FU and audio pre-processor (should go by the name of TDA7348 or similar.
I have done cheap and dirty trick directly connecting 3.5mm jack to Motorola chip - this overrides any other sound source, meaning, you can play radio or Cd or cassette and it will play your audio source, as long as there is signal coming out of it. The sound quality of such solution is very good (the best one can get basically), but you cannot control the volume over the usual radio controls. However, the full mute function works.(when you turn volume knob complete to the left).
If I were to do this modification again, I would trace the audio signal lines from my CD player and see where they connect to. I have a hunch there is a better solution to this.
As it works without impacts on sound quality, however, I have no need to mess with it.
If you try without caps or with higher capacitance, report back.
Hey, nice idea and thanks for replaying! I already tried something similar with this radio and it doesn’t work that great. The problem with this radio is that the audi chip tda 7342 is providing 3.5V on every output and input pin. That’s some sort of digital voltage which cannot be used as normal because it would drop down very fast. I think this voltage is working like an offset and reference voltage so the amplifiers have the same voltage an can amplifie this, the voltage is only there if the soft mute isn’t enabled . From the aux and the other sorcec the voltage amplitud is very low and maybe the amplifiers and the chips would not work properly.
Yes you are right I have 10uF that’s is just an mistake because of too fast writing 😅.
I use them as a blocking capacitor otherwise on the AUX cable the audi chip will put out 4.2V and the Smartphones cannt connect it. Either the old mp3 players. I also tried to connect the AUX direc to the other two 10uF capacitors which you mention, but the results from this wasn’t very good at all.
The audio is honestly perfect with this solution there ist absolutely no background noise or something like that. The low, middle and high frequenzes are absolutely clear and for now everything is working perfect. The volum control works also and the radio function too.
Your solution and idea is interesting an if I have some time I will test it and report back.
Seems they have multiple amp stages for sound source before reaching the last stage, would bet the problem is there, not on signal levels.
Since you are providin analog INPUT and not output, there is no way it cannot work, it can just have too low or too high volume
Hm I thought the same as you but somehow if there are 4.2 V on the aux cabel the smartphone doesn’t recognize it and plays the music with the internal speaker. In order to make it play the music I had to connect my phone first then play the music and turn the radio on. Wit the blocking capacitor this problem is solved. I will studi the voltage that is coming out with an oscilloscope and report back👍