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60 Hz Hum from Powered Sub

Hi All-=

I have an older (late 90's?) Tannoy PS110 powered subwoofer. It has a 3-prong (grounded) permanently connected power cable. Over the last month it has developed a 60 Hz hum. I've plugged it in different locations in the house to rule out ground loop. The hum is present whether there is an audio source connected or not. Is it time to just move on or is this something worth repairing? Could this be something simple, and if so, how can I diagnose it? Any suggestions are appreciated!

Thanks, 

G
128x128spacecadet65
noromance's avatar
noromance
9,766 posts
 
nob and tube
Knob and tube. Cost me a fortune to replace after some flood damage.
spacecadet65's avatar
spacecadet65 OP
58 posts
 
OK. So I was wrong about one thing. The permanently attached power cable is a 2-prong, polarized version, where you can't invert it in the wall socket. So there was no way to use a cheater plug to get out of this. 

@jea48: It is definitely the middle YouTube video hum. Great videos, BTW.

This is a pretty old unit. $400 gets me a basic 12" Emotiva sub that will run rings around it. I may open it up and take a look at the caps. If it looks like there's something obviously amiss, maybe I'll try to do a repair. It's not worth paying someone to work on it given what it is...

I really appreciate all of the advice!
erik_squires
16,047 posts
 
Hey,

If you get the hum with NO audio connections and just plugged in you need to have it serviced. It’s probably a power cap that’s going. Get them all replaced before it blows.  If you can afford it and they are available, get higher temp caps than you originally had.  They'll have a much longer lifespan.

Best,

E
fiesta75
1,378 posts
 
If you're going to open it, be careful to discharge all large electrolytic capacitors through a resistor. These caps can store some high and potentially lethal voltages for a long time. All plate amps I've worked on use 105C capacitors.
jea48
4,423 posts
 
I assume the DC power supply uses full wave rectification.

: a rectifier that converts alternating current into continuous current and that utilizes both halves of each cycle of the alternating current.
120 ripples per second.
I would think if it is a bad electrolytic cap in the DC power supply the hum buzz would sound like this:

120Hz Hum
https://www.youtube.com/watch?v=pC4Uzt0qm2E

or this,

Like a 120Hz buzz
https://www.youtube.com/watch?v=pf-vULfjYK8
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