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Tardcore

12v trigger causes ground loop type hum

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Probably not the best place to get real assistance with this issue but I may as well throw it out here in case somebody has some troubleshooting advice....

 

I recently replaced my AV receiver with a new Marantz SR7012 and am having an issue with a humming noise that goes away when I disconnect the 12v trigger cable I use to activate my subwoofer (Monitor Audio RXW-12).

 

Troubleshooting:

 

Tried a different 12v trigger cable.  No change.

 

My power situation is slightly janky.  I primarily use a Furman Elite DMi power conditioner but I have to plug it into a power strip (Tripplite Isobar) basically because the power cable isn't quite long enough.  I swapped some things around there.  No change.

 

I plugged the sub into my desktop setup (Emotiva XDA-2 digital preamp into a Emotiva fusionflex amp) using the 12v trigger and no hum.  everything works fine.

 

I tried plugging in another 12v trigger capable device to the receiver.  No hum.

 

The subwoofer itself has 3 power settings:   On,  Auto and Off.   By design, whenever the 12v trigger cable is connected, the sub simply powers on/off based on the trigger signal and ignores the power settings.   I found that when I unplug the 12v trigger cable the On/Auto power settings no longer do anything.  I have always had the 12v trigger cable connected so I really don't know how long it's been like this.


My thought is that the best case scenario is there's something wrong like a bad solder joint in the sub's power mode selector switch.  I ran the issue by Monitor Audio and they said the entire plate amp unit needs to be replaced and quoted me a part price which wasn't much less than I paid for the subwoofer new.

 

Unless somebody can offer up something else I can try, I think my next step is to pull apart the sub to see if I can find anything obviously wrong with the power mode selector switch.  I do have a a multi meter but I'm not really knowledgeable enough to do any advanced troubleshooting. 

 

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1 hour ago, kannibal said:

Why are you pulling apart the sub when it works fine in another setup? You replaced the amp and the problem started. Sounds like it’s either the amp or the interaction between the amp and sub. 

 

Does the amp have any menu options or other adjustments relating to the 12v trigger? 

That's a good point.  The fact that the power mode selection switch on the sub is not working correctly (which directly interacts with the 12v trigger port) and that other triggered devices connected to the receiver do not cause any issues lead me to think the sub is the culprit.

 

I checked and the only options available related  to the 12v triggers control are to control when a connected device is On/Off depending on channel (example sub ON for DVD channel but OFF for CD).  In my case, they are all switched to ON.

 

Additionally, I didn't mention in my OP that there are 2 trigger out ports and I tried both of them with the same result.  Also the hum is present in all speakers including my L/R fronts which are driven by an external amp (PM8004).

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Of course I forgot about the malfunctioning switch. 

 

There were a lot of info when I searched on the topic. Most figured out that a ground loop was being created with the interconnect to the sub and the trigger cable. There were a few ways to deal with it, but not sure any were true fixes at the source. More like work arounds, like a couple of people cut the sleeve of the trigger cable. There’s probably a better way to do it with a grounding of some kind, but I wouldn’t know where to start.

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Emotiva makes an active box that takes a 12v trigger input and splits it into up to 3x 12v trigger outputs.   I ordered one to see if putting it in between the receiver and the sub somehow solves the issue.  Even if it doesn't it was only $40 and is sort of a useful thing to have regardless.

 

If (when) that doesn't pan out....I'm kind of digging having the sub in my desktop setup at the moment.  I may just say fuck it and order an SVS sub for my AV setup and keep the Monitor Audio sub with my PC rig.

 

EDIT:   You mentioned the sub interconnect cable and I realize that I hadn't tried to disconnect the sub interconnect.   I hooked everything back up and unplugged the sub interconnect cable rather than the 12v and confirmed that the hum went away.  I tried a different sub interconnect.  No change.

 

I also tried connecting my desktop amp (Emotiva fusion flex) to the same sub output and 12v trigger out on the receiver and confirmed no hum.

 

It does seem to confirm the issue lies between the sub interconnect cable and the 12v trigger.  It's not completely clear whether the problem is with the receiver or the sub but I still believe that the non-functioning power mode selector may be responsible.

Edited by Tardcore

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UPDATE:

 

I remembered that my Furman line conditioner has a trigger in/output.  I ran the trigger line from my receiver into the Furman and on into the sub and  fired everything up.  The sub correctly powered up with my receiver and the hum is gone.

 

So...I guess problem is solved?  I'm still concerned that there is some kind of grounding issue in my subwoofer, but I guess if my apartment burns down, it's Furman's fault.

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