headphone to car amp which takes balanced differential signal, please review circuit - diyAudio
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Old 23rd August 2011, 05:19 PM   #1
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Default headphone to car amp which takes balanced differential signal, please review circuit

Hi everyone,

I'm attempting to stuff an android tablet in place of my factory head unit, and have basic stuff like regulated 5v, bezel, usb gps, software figured out, but I would appreciate some guidance on integrating the tablet's audio with the factory amp/speakers.

The car is a '02 330i and I've had it for 6-7 years (though it only has 40k miles...) and I don't mind ripping stuff out and frying parts of it, so I would like to use this opportunity to learn and experiment.

Being that it's an e46 (USA), it headunit and amp which communicate with balanced differential signals. This is the amp: BMW E46 10-Channel Amp

The tablet has headphone out, internal speakers, and hdmi out. After some research most forum members seem to recommend one of two possibilities:
1) line driver (e.g. phoenix gold tld22) to amplify and sven4 (discontinued but there's similar product from pac) to get balanced out from unbalanced...
2) replace factory amp with aftermarket amp which supports speaker level signals.

Now the reason I'm posting on this forum and not mp3car or e46 forums is the following:
I found one post on mp3car which uses a TDA7053A to get 2ch amplified BTL (from what I google, BTL seems synonymous with balanced diff signals??) BMW E46 AMP/No head unit

This seems like an elegant solution so I set out to build this circuit or something functionally equivalent. I see from nxp.com that TDA7053A is discontinued, with no replacement part# noted. After searching on this forum I gather DRV134 from TI and SSM2142 from analog device do similar things? Can anyone confirm that? The schematics in the application notes look similar but I'm not knowledgeable to determine whether they are functionally the same. Do I need some sort of an additional preamp if I do not use the TDA7053A? I read the factory amp doesn't take line level, but 11v peak to peak or something (does that even make sense?). I'm guessing headphone out on the tablet is nowhere near that, I will hook it up to my scope and see. There's a lot of conflicting information on various forums, so I'm still doing research.

Also would it make any difference if I tapped the tablet's speaker lines instead of using the headphone jack? I don't mind cracking open the tablet and soldering onto its pcb.

I'm a novice and although I've done all the maintenance/repairs on my car myself, i've never touched the audio/electronics stuff. I'm actually a software guy not hardware.... but I've messed around with microcontrollers in the past and have pretty decent equipment. I can put together circuits from schematics draw up by others but I don't really know what's going on , so please dumb down your answers for me. Thank you.


TLDR summary:
1) tablet headphone out = unbalanced
2) car amp takes balanced differential signal
3) found schematic for convertor/preamp but IC is discontinued, need functionally equivalent circuit or better solution.
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File Type: jpg bmw-e46-amp-no-head-unit-preampschematic1.jpg (55.4 KB, 27 views)
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Old 23rd August 2011, 05:32 PM   #2
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Join Date: Nov 2003
Location: Louisiana
A simple ground loop isolator will get you from unbalanced to balanced but there will be no gain (in the signal level).

How does the amp get it's turn-on signal?

Some look for 6v on the signal line. Others have a dedicated turn on signal.
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Old 23rd August 2011, 05:35 PM   #3
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i think the amp turn on signal has been figured out.
Is this picture the correct Pin Out Code of the E46 BMW Business CD53 Radios? - Bimmerforums - The Ultimate BMW Forum

i believe i just hook the +12 V ignition pin (pin 9) to pin 16 and it's supposed to turn on the amp on ignition.
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