Beolab 12 on pre-out > humm.

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L1NO Posted: Fri, Jun 20 2014 9:51 AM

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Hi guys, 

I've installed my new beolab 12 in the bedroom and they sound splendid. I have used cat7 to connect (daisychain) the speakers to my yamaha receiver.

I am using an B&O original line to rj45 adapter connected to a B&O splitter which connects to the speaker wiriring. (Splitter also accepts a second rj45 that suplies the 5v trigger from my custom beo4 to ir converter)

so far soo good! But i'm detecting a low humm that is noticable from 40cm or less. Both speakers have the humm and the humm is abcence when the receiver is turned of or muted. The humm is not volume controlled. 

Is this a ground loop problem? I've connected the shieldimg of the cat7 to phono ground on the receiver. (Turntable connection?) Removed most of the connections but the humm stays.

Any suggestions or experiences?

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w5bno123 replied on Fri, Jun 20 2014 7:18 PM

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StUrrock:

is the hum just coming out of the acoustic lens?

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L1NO replied on Fri, Jun 20 2014 10:05 PM

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Oops! I've ment the beolab 12! 

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StUrrock replied on Fri, Jun 20 2014 10:07 PM

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w5bno123:

I've never seen an acoustic lens on a BeoLab 11

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Steve at Sounds Heavenly replied on Sat, Jun 21 2014 9:44 AM

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Hi,

Is your Cat 7 cable fully shielded? (Most Cat 7 cables are, you can usually tell by checking that both RJ45 plugs have a metal surround just behind the row of 8 pins).  Beolab 12s use the plug case to ground the digital amplifiers, so if this connection is missing, it can cause the humming you described.

Technically, either Cat5e, Cat6 or Cat 7 cables can be used for audio, as long as they are correctly shielded (normal Ethernet data cables are not suitable).  You can see details of suitable cables here: http://www.soundsheavenly.co.uk/bando-cat5e.htm

Hope this helps, please let us know how you get on.

Kind regards, Steve.

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L1NO replied on Tue, Jun 24 2014 7:18 PM

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During installation i discovered that the rj45 connectors needed the metal shield to cancel any interference.

later testing revealed that the receiver grounds itself using the coax radio cable. Which would need a complete remoddeling to ground the coax cabeling. Without the coax the receiver uses any connected device, hdmi for example. But the tv does not ground well either..

i'm taking the small humm for granted. Need to piut my ear on the speaker to hear it anyways.