3b hum

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stevo_man

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3b hum
« on: 5 Dec 2003, 06:13 am »
Recently received a used 3b amp. I am now using it to drive my sub in a mono setup. Everytime I turn on the amp with my digital processor on, I get this rather annoying hum coming from the sub. I have tried the amp in both configurations for the ground position - to no avail. Should I just ground the chassis to the house ground? Lots of work, but if it gets rid of the hum.....
Thanks for the input.

James Tanner

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3b hum
« Reply #1 on: 5 Dec 2003, 01:14 pm »
Hi stevo,

If you unplug the input from the amp - turn it on and connect it to the speaker does it still hum? If not the groung loop is being fed to the amp from the processor.
Try leaving the 3rd prong off the amp (cheater plug) and see if that gets rid of it.

james

Espo

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3b hum
« Reply #2 on: 6 Dec 2003, 11:41 pm »
Many people recommend using a cheater plug for ground loop problems; isn't that dangerous?  Usually it's the TV cable ground that's causing the ground loop. If so, isn't better to use TV ground isolator?

James Tanner

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3b hum
« Reply #3 on: 7 Dec 2003, 12:59 pm »
Yes - cheater plugs are illegal - I just wanted you to see if the loop was prevented using the cheater. If it was then we can look further down the line.
Cable systems are notorious for ground issues so that is certainly the first place I would look.

james

stevo_man

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3b hum
« Reply #4 on: 8 Dec 2003, 05:23 am »
Thanks for the info. Will try out your suggestions and reply back again.
Cheers!

nicolasb

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3b hum
« Reply #5 on: 8 Dec 2003, 03:09 pm »
Another possiblity might be to use the amplifier's XLR input. (I'm assuming the sub has a phono - RCA - output).

Make up an RCA-to-XLR cable that is wired RCA-signal to XLR-phase, RCA-earth to XLR-antiphase, and XLR-earth to the cable shielding. That way the cable screening remains connected to earth (ground) at the amplifier end, but there's no possiblity of a ground loop.

Disadvantages:

1) You'd probably have to get a cable made specially, as phono-to-XLR cables and adapters aren't usually wired this way.

2) Your signal will be reduced by 6dB.