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DIY subwoofer repair?

krabapple

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I have two identical powered 12" subs, the quite budget friendly Chinese-made Monoprice 9723, operating in a 5.2 system via dual sub outputs of my Denon AVR.

This model gained some notoriety for having intrinsic hum issues out of box...on some units. Wirecutter tested two and found no hummers. In my case, one hums, the other doesn't. It's not the cables or the AVR or the position of the sub. I even packed up and returned the humming unit under warranty and was sent...another humming unit. At that point I chuckled and called it a day... it's very low level hum anyway and these subs will surely be swapped out for others evernutally. Until then, I'm curious to know --

-- what tends to be the cause of an intrinsic hum in a sub? (I presume it's 60Hz but haven't actually measured) . Grounding? Amplifier?
-- is it something a very modestly experienced soldering-iron/multimeter wielder like me could diagnose and fix without electrocuting himself? Accessing and removing the power amp plate is not a problem.
 

norcalscott

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I’ve had hum issues with a couple of Polk cheap SW10 powered subs and fixed each of them in two different ways. One worked fine in a previous home, but in a new home, it was exhibiting hum quite badly when connected with the same Onkyo AVR via new unbalanced interconnect I built and ran about 20 feet. I thought that unbalanced input was the source, so I tried an isolation transformer and it still hummed. While doing that however, I noticed if I moved the power cable one specific direction the hum went away, so I permanently ran the power cable that way and it now doesn’t hum unless it is on and the AVR is not.

For the other one, I ended up replacing the amp plate, which died, with a Nobsound amp (using an old Dell laptop power supply) and it now works better than ever. That little amp packs quite a punch with a good power supply, and no hum whatsoever. The downside is that it lacks auto power on, but I just leave it on all the time.
 
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krabapple

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Interesting option. How did you connect the Nobsound to the AVR and the existing subwoofer after removing the plate amp?
 

antennaguru

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Ground Loop is usually the hum problem with powered subs. Try running an extension cord from the outlet strip where your other electronics are plugged in over to where the sub is, and then plug its AC cable into the extension cord (rather than into the nearby wall outlet).
 

norcalscott

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Interesting option. How did you connect the Nobsound to the AVR and the existing subwoofer after removing the plate amp?
I removed the fuse from the plate amp and cut the wires going from that output to the speaker then connected the speaker wires to two of the spring connector terminals on the inside of the plate amp. I then ran the output of the Nobsound to those spring connectors on the outside of the plate amp. The Nobsound is sitting in my audio rack for ease of access. I feed the Nobsound with the sub out on my amp (RCA to 3.5mm cable). I taped up the electrical prongs on the plate amp for safety.

Kind of down and dirty but it works!
 

MattJ

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A lot of times the hum isn't coming from your sub, it's coming from the cable box. I had a hum in my sub and got a cable ground loop isolator. Worked perfectly, no hum now. Dead quiet even with my ear right next to the driver. Inexpensive, too. Mine was about $25 online. I put it on the wall outlet, attached the cable and POOF no more hum.
 
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