Ground loop whining sound with aftermarket head unit but stock M490 amp

[Vic S]

Hi guys,

 

Have a 2000 Carrera Cabrio.

 

When I got the car, it came with an aftermarket Alpine that didn't really work well, so never used it much, so not sure if this is an older issue or a new one. Replaced it last weekend with a Blaupunkt Liverpool series head unit - which is a budget unit I have to admit. It uses the stock M490 amplifier to power the speakers. 

 

The sound quality coming out of the speakers is pretty good, but there's definitely some noise (crackling / hum) from the amplifier when the unit powers on and whining sound as engine revs pick up, which is a grounding issue somewhere in the system as it's picking up the alternator. 

 

From my research, it seems like in a stock setup, the grounding for the original CD220 head unit and the amp are linked together - so the harness/CD220 has a grounding connection, not sure where. After all this splicing, etc, I'm not sure if that's been lost somehow. 

 

In my testing, even with volume set at zero, there is the noise and whine, making me assume it's the amp that needs grounding. Firstly, is this the correct assumption? The noise stays the same volume as far as I can tell when I crank up the volume on the head unit. 

 

If I do need to ground the amp and looking at the wiring diagram, I really can't figure out which wire from the blue plug (cause I have a cabrio) that goes into the amp that I should use to create a new grounding connection closer to the amp (I saw a grounding screw just behind the amp). 

[Vic S]

Here are the close ups of the plugs

[ProfJake]

Hello Vic.  Just saw this post of yours. Curious if you ever found the cause and a solution?  I have a similar situation as you (2002 C2 Cab. with the M490 audio package). Curious - is the whine ONLY coming from the rear seat speakers? Mine seems to be. I've been trying for a LONG time to fix this, but haven't sadly figured it out. Not sure what the stock radio/head unit was, but the previous owner pulled it, and put and installed a Blaupunkt Beverly Hills 71 unit.  I BELIEVE the stock CDR220 radio chassis is making contact to a metal "tab" on the left side of its dash mounting. directly next to THAT metal tab is the grounding wire of which you speak. SO it is POSSIBLE that your new head unit isn't making contact to that grounding tab. I didn't get a chance to test that yet on my end. 

 

In my case the "whine" is constant, unchanging with increase or decrease of radio volume, and has no correlation to engine rev. Matter of fact it exists even if only the ignition key is in the "ON" position.  

Edited yesterday at 12:42 PM by ProfJake

[Vic S]

It's solved now, this is what I did, and what I think helped. I am also using a Blaupunkt (110 Liverpool). Mine was 100% an alternator whine, so only when the engine was running. 

 

If yours is whining even without the engine running it's likely to be the grounding of the head unit. All the speakers I think use the chassis as the negative terminal (if I remember). So maybe just run a new grounding wire from the front bulkhead to the head unit, although if I remember the original unit used the original stereo bracket as the grounding point, so maybe there's somewhere to ground it near the head unit. 

 

For my car, the hypothesis I formed was that when the original head unit was installed, the ground for the amp was shared with the head unit. When that was replaced, I think the amp no longer shared the same ground / or didn't have a main ground.

 

1. So the first thing I did was to cut the ground wire on the amp and route it to a chassis bolt. That didn't help. 

2. I then wired a new ground wire to the head unit and shared that ground with the amp. This didn't help, so I would do these two first steps only if the next few steps don't work.

3. I bought some ground loop insulators for the amp from Aliexpress and tried a few, some reduced the whine but didn't eliminate the whine. But I'm not sure if it was the heat or the load on the capacitors, they cut out the power to the amp intermittently after about 20 minutes of using the stereo. I changed it one that ended up being more stable. 

4. I then bought another voltage regulator/insulator that connects to the head unit directly that I found recommended in an older forum post. I think that was the one that truly fixed it for me.

Edited 5 hours ago by Vic S