Hi All,

just recently joined this forum, I just completed the upgrade of my '08 TL sound system...(replaced all front/door speakers with Infinity series, and rear 6.5s with BA Pro60 SE, 8" sub in separate box in trunk, 2 Blaupunkt amps in the trunk, one for sub, one for BAs Pro60s)

I tapped into the low-level signals going into the stock AMP, soldered on RCA connectors, and ran standard shielded RCA lines into the truck, for the Rear L/R signals, and the SW signal.

I'm picking up quite significant alternator and electrical noise on ALL of these signals (Rear L, R, and SW), same noise level on all.

I'm an EE, so I'm quite familiar with circuit design, etc. I'm fairly certain now these low-level signals between the audio unit and the stock amp are differential signals. Since they used standard separate wire for the + and - lines from the back of the audio unit, all the way over to the stock amp in the passenger kick, they will pick up so much noise since they are not shielded.

If they were truly single-ended signals, I would certainly think they would have run shielded lines for the audio-signals, but more expensive to run it versus just designing the audio unit and stock amp to use differential signaling.

But I've reading through various discussions on this board, and I see many have fought with using the low-level signals, and fighting these noise issues...

They would have to be doing some serious filtering/etc in that stock amp to take out that noise, if these were just single-ended signals, plus you'd notice some serious impact on the output sound. The stock amp puts out very clean sound, certainly no sign of any serious noise coming out to the speakers......

it seems BMW uses differential signaling as well out of their head units....

Just wanted to throw this out there in case no one was quite sure yet, I'm fairly certain that is what is going on.

I'm going to throw together a quick little circuit with op-amps, in the simple differential config (may even give them a 2:1 or 3:1 gain as well, give me a little more leverage on the amp's gain)....to verify this theory..

if these signals are truly differential, the outputs from the differential amps will be nice and clean, all engine noise should be completely gone.

I'll post back once I test this out...

 

you need a processor of some sort (3sixty.2, bit one, mls 8, etc...)

you should then take the high level signal from the amp, not the HU. the signal is a full range signal as well. You can tap RCA's on the end of speaker wire that you add coming from the amp (this is what I did, works great).

dumb question.... what side of the car do you have your speakerwire/rca's, and what side is your power wire on? make sure you are running them seperate from each other, as well as away from any other electrical wiring, motors, parts, etc... in the car. they will pics up noise easily.

 

Hi,

yeah, my RCAs are run down the right side (passenger), and the power wire I ran separately down the left side....

I would be a nice plus to add the fosgate 3-sixty in, already has built-in differential input support, etc, plus I can really tweak my rear sound....I may consider doing this..

But, just for anyone reading, it just really make sense that is what is really going on with the low-level signals from the audio unit to the stock amp, they are differential/balanced signals...that is why the voltage/sound levels seemed so low for others that tried to use the line level outs with no processor...

the differential/balanced signals are normally about 1/2 of the usual RCA/line level voltage on each signal of the pair......so when this balanced signal is converted back to a single-ended/unbalanced signal, you will see the expected 4-5v signal we are used to..

I will prob. breadboard up a quick differential opamp circuit, and throw it in my trunk for now, just to make sure it cleans up the signal as I'm expecting...

...then probably order the 3sixty for the permanent solution, plus tweak the sound spectrum better...

 

Hi Everyone,

just an update, these pre-out signals (low-level signals from audio unit to stock amp) are indeed DIFFERENTIAL/BALANCED signals.

I hooked up the simple audiocontrol matrix, which of course can take in balanced inputs, and gives you single-ended/buffered outs.

The large amounts of noise I was hearing (on all 3 channels I ran via RCA to the trunk - Rear L/R, and SW) is completely gone. Absolutely zero noise present when properly utilizing the differential signals.

Only caveat is that something is odd in the way the audiocontrol matrix inputs were setup, I could not feed the L and R signals for the rear into the same input block on the matrix, seems it was most likely grounding out the (-) of the R signal, therefore differential signaling was hosed, so I'd get the huge noise mess on the Right. So I had to run the Right signal through input block #2 by itself, and it was fine.

That's the only problem with using differential signals, BOTH (+) and (-) connections for each input need to be ISOLATED, ie NOT in any way tied to Ground of the car, or even Ground inside any signal processor.

Also, the signal strength was much improved as I stated when the differential signals are decoded back into single-ended, I didn't even have to use any gain on the matrix.


So just for anyone in the future that may want to use the true low-level outs at the stock AMP (versus down-converting the speaker-outs), you just need to:

1) Use the audiocontrol matrix (or similiar device that can convert differential signals to single-ended) IF you are using older amps that expect single-ended inputs

OR:

2) Purchase a newer amp that CAN handle differential/balanced low-level inputs... (ie JL Audio, etc)


I picked up a nice 4 channel JL Audio amp for my trunk, which I'll be installing next week to replace my two older 2 channel blaupunkt amps....

This way I can have a nice simple setup, no audiocontrol matrix needed, and only a single amp to drive the rear 6.5s and my 8" sub.

 

Great info, thank you. My recently installed system has been plagued with noise too. I've got a Rockford 360.2 feeding to my older ADS and Soundstream amps. THe installer said the noise was due to the older amps but I was having a hard time understanding that. I have been considering an amp upgrade, so this might just seal the deal. Now I just need to know how to break the news to my fiance. She'll understand, right?