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Discussion Starter · #1 · (Edited)
:thumbup: I got all ambitious and finally solved my Phatbox alternator whine problem (i.e. RPM dependent noise). The solution invovled building an adapter that doesn't require any destruction of the existing harnesses/cables.

It looks like this:



Some background... The OEM CD changer cable in the trunk seems to be constructed in a slightly dodgy fashion. According to wiring diagrams, the power, remote turn-on, digitial I/O, and analog audio signals are all supplied directly from connections at the back of the HU. But while the power line is supplied by the HU, the ground line in the trunk changer harness is supplied from a separate connection to
somplace on the chassis, below the center console. While this is supposedly the same place that the head-unit and Monsoon amp are ultimately grounded, in practice, most Phatbox installs exhibit a nasty amount of alternator whine. This suggests that either these grounds are not actually common (leading to a ground-loop problem), or that the CD changer harness is picking up alot of noise on it's way down the
driver's side of the car toward the trunk. This problem often gets worse when the Phatbox chassis is also grounded to yet another part of the car.

Additionally, the ground line in the CD changer harness is used to shield the data I/O lines between the Phatbox/CD-changer and head-unit. This might explain why some continuous (non-RPM related) clicking and popping can also be heard at high gain. Now, it isn't clear to me why Phatboxes seem to exhibit this problem much more often than CD changers, but I imagine it's because the Phatnoise design makes improper use of the the ground line in the audio circuits. It is also unclear why this noise/whine problem often gets worse when the HU is changed from the OEM Monsoon HU to the OEM MFD Navi HU.

Possible solutions may include running a better CD changer harness from the HU to the trunk, or perhaps by adjusting the ground point used by the existing harness. But, I didn't have the time or patience to try any of this. Instead, I just used a line filter to clean up the power supply, and a ground-loop isolator to break the electrical connection of the audio lines between the Phatbox and HU.

Parts required:

1x VW: 6Q0 972 736

12-pin connector housing.
Connects to trunk harness.


1x VW: 6Q0 972 726

12-pin connector housing.
Connects to Phatbox/CD changer.


5x VW: 000 979 131 A

Repair wire with two female connectors suitable for housing 6Q0 972 736.
The A suffix denotes gold contacts. For cheaper, non-gold contacts, drop the trailing A.


5x VW: 000 979 132 A

Repair wire with two male connectors suitable for housing 6Q0 972 726.
The A suffix denotes gold contacts. For non-gold contacts, drop the trailing A.
Note that the image above is for the non-gold version, 000 979 132.


1x Navone Engineering: N-555

Two channel ground-loop isolator.
This part is nothing more that a transformer that electrically isolates the input from the output. The audio signal is coupled from the input coil to the output coil using a magnetic field. You could substitute any other good quality ground-loop isolator. However, this model is advertised to introduce very minimal high/low-end attenuation, or any phase changes. (There is certainly no detrimental audible effects that I can discern, and I'm really, really picky...)


2x RadioShack: 274-850

Gold RCA Plug Pair.
Substitute whatever type/general of plugs that your gound-loop isolator requires.


1x RadioShack: 270-051B
See picture in this PDF: http://www.aeroelectric.com/articles/filter/RS_Noise_Filters.pdf
Line Noise Filter.
Apparently, this part is now out-of-production. However, any similar device can be substituted. Crutchfield carries several, rated for different current loads. As you can see from the PDF linked above, it's nothing more that a capacitor and inductor that filter out AC components from the (supposedly) DC input. The PHatbox uses very little current, so sustitute any line filter rated at a mere 1A or more.


Once you gather all these bits, assemble them as in the schematic below...





Should look something like this...



CD Changer Side:


Trunk Harness Side:


In practice, I've found that the ground-loop isolator, alone, is enough to remove all the noise in my system. I have yet to try the line-filter on its own, but it might also be sufficient on its own. I should mention that this will only solve your problem is the noise in your system is entering the system between the Phatbox and HU. In my car, the sound system was 99.9% noise-free when listening to AM/FM or Navi voice prompts (i.e. audio sources internal to the HU). It only exhibited noise when audio input was set to the Phatbox. Now, that noise is enitrely eliminated.

Good luck!
 

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Wow nice I've been looking for a solution like this...I will try with the ground-loop isolator alone first...
 

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he stated he didn't get a VW wiring harness - he bought VW connectors 6Q0 972 736 and 6Q0 972 726, as well as all the pins to populate the connectors and then custom built the wiring to include the ground loop isolator, etc.
his instructions are very complete, so if you have trouble understanding them, I don't know how to make it more plain.
 

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he stated he didn't get a VW wiring harness - he bought VW connectors 6Q0 972 736 and 6Q0 972 726, as well as all the pins to populate the connectors and then custom built the wiring to include the ground loop isolator, etc.
his instructions are very complete, so if you have trouble understanding them, I don't know how to make it more plain.
1x VW: 6Q0 972 736

Isn't that a wiring harness?

And can these parts be purchased from the dealer?
 

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Discussion Starter · #7 ·
1x VW: 6Q0 972 736

Isn't that a wiring harness?

And can these parts be purchased from the dealer?
Both 6Q0 972 736 and 6Q0 972 726 are only the plastic housings for the trunk CD harness. You have to buy the 'repair' wires that each have a pair of either male or female pins that snap into the housings. Those part number are also given in the original post.

If you go with that radio shack part, then you can either grab female RCA jacks to accept the male RCA jacks on that filter, or just lop off the existing connectors and splice the filter directly into the harness -- your call. The schematic I gave should explain everyhting you need to know.

The dealer can supply those parts, but 1stvwparts.com or worldimpex.com will have much better prices.
 

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I wonder why my phatbox never had the alternator whine issue. I wonder if there is somoething different in how the wiring loom is built from model year to model year.
 

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Both 6Q0 972 736 and 6Q0 972 726 are only the plastic housings for the trunk CD harness. You have to buy the 'repair' wires that each have a pair of either male or female pins that snap into the housings. Those part number are also given in the original post.

If you go with that radio shack part, then you can either grab female RCA jacks to accept the male RCA jacks on that filter, or just lop off the existing connectors and splice the filter directly into the harness -- your call. The schematic I gave should explain everyhting you need to know.

The dealer can supply those parts, but 1stvwparts.com or worldimpex.com will have much better prices.


Cool thanx. I'm really bad at electronics. Even in Advanced Placement Physics in high school, I got a 75 in the Electronics and Magnetism portion while I got a 100 in the Mechanics.

I also got a 3 on the Advanced Placement exam for E&M vs a 5 for the Mechanics.

I need as much help as I can get with electronic circuits.

Thanks for your help and I'll update this thread with my progress....
 
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