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Beolab Penta mains hum / buzz noise

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Gavin
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Gavin posted on Tue, Mar 10 2015 11:57 PM

Hi all,

Further to my project of upgrading my Beolab Penta IIs, I have completed all the cross-overs, and re-foams. The results are great. I used to think the Penta design had limitations in the tweeter with poor high frequency response but now with the mods I am very happy. 

One issue remains which was present before the upgrades, which is a mains hum coming through one of the speakers. You can hear it from about 1 meter away but not when music is playing. 

I was hoping that replacing all the caps might have fixed it but no. When I replaced the caps, op amp, and trimmers I set up the voltages very accurately.

I have a circuit diagram but rather than aimlessly measuring voltages, can someone point me in a more specific direction. If all else fails, I'll remove the amp and take it into the B&O shop but thought someone here may have had a similar experience.

 

cheers

 

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Gavin
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Answered (Verified) Gavin replied on Tue, May 19 2015 11:38 PM
Verified by tournedos

For the record, and if anyone else stumbles across this through Google I thought I'd update what I found.

I swapped the L & R RCA cables but the problem remained.

I finally moved the speakers and used some other cables and the problem stopped. Upon closer examination of the cables, the ones that cause the problem had been wired with the core and shield (2 wires) for each speaker. The cable which had no noise had only the core (1 wire) per speaker.

The speakers now 'go to sleep' as per normal. Noise is introduced by wiring up the shield side of the RCA connectors.

GC

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tournedos
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Perhaps it is just a cabling/mains problem - have you tried swapping the speakers around to make sure the hum follows with that particular amp?

The mains transformer itself may hum, but that would be mechanical noise and not coming through the drivers.

--mika

Gavin
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Gavin replied on Wed, Mar 11 2015 8:54 PM

Yes, swapped the RCA cables and it's the speaker.

The noise is coming through the cones and is amplified. Having swapped most of the caps I am now wondering if it's a component or something else.

Gavin
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Answered (Verified) Gavin replied on Tue, May 19 2015 11:38 PM
Verified by tournedos

For the record, and if anyone else stumbles across this through Google I thought I'd update what I found.

I swapped the L & R RCA cables but the problem remained.

I finally moved the speakers and used some other cables and the problem stopped. Upon closer examination of the cables, the ones that cause the problem had been wired with the core and shield (2 wires) for each speaker. The cable which had no noise had only the core (1 wire) per speaker.

The speakers now 'go to sleep' as per normal. Noise is introduced by wiring up the shield side of the RCA connectors.

GC

tournedos
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Glad to hear you fixed it (and posted how)!

--mika

Rauk
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Rauk replied on Thu, Oct 22 2015 4:40 PM

I have a similar problem with Penta MK2 . Would be nice to get some suggestion of what I can do.

The Pentas are connected to a Sonos system through the Line in .

You can hear the noise when you have low volume or when music is paused. Noise is 100Hz.

First I thought it was the main power caps (10000uF) and shifted them out. No change.

It does not matter if Line in cables are connected or not.

I also tried to replace Line in cable with a resistor between signal in and GND. No change.

 

Gavin
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Gavin replied on Sat, Oct 24 2015 11:35 PM

Hi, I also use Sonos and since my upgrade I have had perfect sound. 

Just to understand your issue - the speakers are powered (plugged in to the mains) but no line input connected to the 'audio in' of the speaker?

Do both speakers suffer the same problem?

Do any of the switches change this buzzing like the input level switch, or the audio video switch underneath?

Have you tried setting the speakers up using the trimmer pots as per the voltage specs in the manual?  If you can't align to these voltages then this may indicate another problem.

 

Let us know

Filip
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Filip replied on Sun, Oct 25 2015 12:39 PM

Have you performed the no-load current adjustment? Connecting DC millivoltmeter between R103 and R104 should give 30mV. Otherwise adjust R95 accordingly. Speakers have to be disconnected from amplifier which needs to be cold. Be careful not to short anything. 

Misalignment will cause the hum, but this is rather from amplifier itself.

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