# AC hum on Subwoofer



## Kaisergrendel (Jul 19, 2011)

Hi all, I'm having problems with a kit subwoofer. I was able to play it last year after some initial problems with humming (which were never properly diagnosed but somehow went away after enough fiddling), but took it apart to apply veneer. Now I've reassembled it but as soon as I turned it it starting producing a 50hz hum. 

Since it's a powered sub with no ground prong at the back, I'm inclined to believe it's not a ground loop issue.

Details:

- The S250 emits a 50hz hum when it's turned on, greatly interfering with the signal from the receiver.
- The hum is present with or without RCA cables attached, but become louder if they are attached. 
- For some reason, the hum gets softer if I place a hand anywhere on the SW amp radiator.
- I've tried connecting the sub to 4 different power outlets (although they might have been on the same house circuit) with no change to the hum.
- I removed the SW amp for testing. Tried tying a wire to the ground prong on the power lead and touching it to the amp. Hum still present.
- After pulling and reconnecting cables many times, I turned on the SW amp again and no hum was heard. I was able to play a clean signal. At this stage the amp was not screwed in as I was testing it, and I placed the amp back onto the SW cabinet, placed a few screws in, and tested it again. No problem. I put in the rest of the screws and turned it back on, and the hum was back.
- I removed the SW amp yet again to see if the attaching process had affected the circuitry. No visible sign of short circuits or damage. Hum still present.
- I had actually used this SW last year where I had some hum problems at the beginning to which disappeared all of a sudden and I was able to use it for a few months without further issue. I am not sure if that was related to my current problem or if it was a wiring fault.
- The SW amp doesn't come with a ground prong.

What gives? I've only used the subwoofer intermittently for a few months, and I was careful in disassembly/reassembly not to damage anything. Is my amp simply dying?


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## Kaisergrendel (Jul 19, 2011)

Another strange twist to this story.

Because the amp is not grounded, I attached a wire to the power cable (which has 3 prongs) and attached it to exposed metal in the amp chassis. The hum disappeared.

Not wanting to celebrate so quickly, I unplugged the RCA cables and tested it again. This time the ground wire had no effect.

Plugged the RCA cables back in with the ground wire - no hum again.

So, basically, I have RCA cables installed with a grounded amp in order to eliminate hum - which I hear is most common cause of ground loop hum.

What is going on? :dontknow:


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## Mauritzvw (Sep 19, 2010)

Is this the s250 from lsk?


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## Kaisergrendel (Jul 19, 2011)

Hi! Yes that's right.


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## Mauritzvw (Sep 19, 2010)

Have you tried contacting them? I have been keeping an eye on them for a while but cannot find enough reviews.


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## Kaisergrendel (Jul 19, 2011)

I got in touch with Scott from Essential Audio (Victoria LSK reseller) but since I'd found a workaround we didn't really talk about it much. I might call him again soon and ask about it if you want. The sub itself is superb, strong and tight, detailed.


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## Mauritzvw (Sep 19, 2010)

No need to call him on my account mate, cheers tho. Just thought you could possibly get the amp looked at or replaced.


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## Kaisergrendel (Jul 19, 2011)

Mauritzvw said:


> Just thought you could possibly get the amp looked at or replaced.


I'd like that, but I don't think the MDF cabinet can stand another ripping out and I'm quite happy with the way it works now. 

I'd still like some ideas on what's going on though.:sneeky:


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## Mauritzvw (Sep 19, 2010)

Is the cabinet flat where the amp mounts? If it is not it could be that the amp is being distorted when you tighten it down, possibly causing a bad track or connection.


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## Kaisergrendel (Jul 19, 2011)

Mauritzvw said:


> Is the cabinet flat where the amp mounts? If it is not it could be that the amp is being distorted when you tighten it down, possibly causing a bad track or connection.


That's pretty insightful, and I did consider this, but the back is nearly mirror-flat(I polished it), and the chassis is made of a thick slab of aluminum, and I made sure to apply even pressure when inserting screws, so I doubt this is the case.


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## Mauritzvw (Sep 19, 2010)

I just read your post again. You do not mention trying different RCA cables, might be worth a try if you have not done so already.


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## Kaisergrendel (Jul 19, 2011)

Mauritzvw said:


> I just read your post again. You do not mention trying different RCA cables, might be worth a try if you have not done so already.


Considering the hum is there whether or not RCA cables are attached, do you think this is worth pursuing?


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## Mauritzvw (Sep 19, 2010)

IMHO yes because it could be that the RCA inputs is picking up the hum and the RCA cables may have a break in the ground side that's possibly why the hum goes away sometimes when it happens to make contact. Worth a shot I reckon.


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## Kaisergrendel (Jul 19, 2011)

Mauritzvw said:


> IMHO yes because it could be that the RCA inputs is picking up the hum and the RCA cables may have a break in the ground side that's possibly why the hum goes away sometimes when it happens to make contact. Worth a shot I reckon.


Hmm, good point I might try that when I can afford another set of cables


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## Mauritzvw (Sep 19, 2010)

No worries. See how you go. If possible borrow some off a mate just for testing it out. Hope it works out mate.


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## Kaisergrendel (Jul 19, 2011)

Cheers


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## Rodstintshop (Apr 26, 2011)

Kaisergrendel said:


> Cheers


Diff need a sub cable I know people say you can't tell a dif but a good sub cable will make it were you will have to turn your sub down to be set right from rca


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