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Speaker hum.

Been out of the hi-end scene for a while, but finally did what I've wanted to do for some time-get back some into equipment I regret selling years ago. Anyway, recently picked up Krell models KRC-3 preamp and FPB-200 amp. Yes, they are old, but after gyrating for years with other models and brands, I found that I preferred the design of these. Anyway, set them up shortly ago and have a very subtle hum coming from the speakers (an even more elderly pair of Mirage M-1s). At first, I first had a conspicuous hum coming from the preamp, but that turned out to be caused by some incandescent lamps/lights using the same outlet. I solved that with some LED bulbs, but I then noted the hum from the drivers. I expect the hum is form an AC power ground loop, but beyond the equipment sharing the same outlet, I am wondering how to best solve this. It is hardly noticeable, but you know how that is. Thanks for the help.

isgau8
ghdprentice
7,464 posts
 

OP,

 

Sorry you cannot put in direct lines… two is standard. One for components and one for amp. It would improve sound quality and take care of ground loops.

 

Before I got my second direct line, I just had to plug everything into a single outlet.

 

There are some fixes that may help. You are using two different circuits for your system… if you replace all the duplex receptacles on each line this may be what is causing the differential resistance between the two. You’ll have to do a bit of exploring to find them all. 
 

I am not sure this really does you any good. But make a heavy duty extension cord… I used 10 gauge and plug the second half into a nearby outlet that is on the same circuit as the rest of you system. That is basically does the same thing as plugging in to the same outlet. 

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mijostyn's avatar
mijostyn
8,570 posts
 

Put a ground cheater on the amp to "float" the ground and the hum will go away. You can also break the ground lug off of a spare power cord.

mlsstl
1,452 posts
 

Have you ever had the units serviced? The models you have could be 20 or more years old. It is very common that the power supply capacitors need to be replaced at that age. 

Power supply capacitors, especially the big ones on a power amp, are like tires and brakes on a car. At some point they wear out and need to be replaced. If the capacitors are shot, nothing other than replacement will fix the problem.

isgau8 OP
7 posts
 

Thanks for all the suggestions, everyone, I’ll give them a try. Yes, the units are indeed old and servicing history is unknown-a common risk when buying such equipment. I would guess these are north of 20 years in age given their introduction year. Anyway, I only noticed the noise when I leaned over to pick up some debris off the floor while just inches from the speaker. Otherwise, I’m not certain I would have noticed it at all. Thanks again.

 

 

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