Wharfedale 8.1 Diamond Pro Active Hum

Discussion in 'Mastering' started by Tommohawk, Jun 7, 2011.

  1. Tommohawk

    Tommohawk Active Member

    Hi all. I hope someone can offer some help.

    I got a pr of Diamond 8.1 pro-active speakers just 1 week ago second hand. Initially they worked just great but now I have a loud buzzing - sounds like mains hum – from both speakers. I’ve checked the input cable and its good. The problems occurs on the unbalanced inputs – I’ve never used the balanced. There is no hum when the input cables are unplugged.

    If I unplug either input – the hum stops from both speakers, and the other unit plays perfectly. If I switch off the mains to one unit, the hum stops from both, but not if I switch off the other.

    If I swap the input cables round, I get the same result ie the same unit when switched off prevents the hum from both.

    They haven’t been played loud – only about 1/3 volume – and have just been sat on my desk ie haven’t been dropped.

    I would really appreciate any ideas you can offer.

    Thanks

    Tom
     
  2. jimmys69

    jimmys69 Active Member

    Hey Tom. Could be a bunch of things. Are the cables crossing a power cord? Is there a reason for not using balanced cables? Are the monitors plugged into the same outlet as 'all' of your gear?

    Jimmy
     
  3. Tommohawk

    Tommohawk Active Member

    Hi Jimmy

    thanks for your prompt response. The signal input cable is nowhere near power cables. I'm not using balanced input just because I dont know how to hook this up to my laptop. The monitors are both plugged into the same power socket set - but I tried changing them and it makes no difference.
    The thing is - it was just fine when I used it yesterday. I'ts strange that killing the power to one monitor kills the hum - but not killing the power to the other. I wonder if one monitor has lost the earth somehow - or similar.

    I have to sign off now - midnight here - but will get back tomorrow ASAP.

    Appreciate any other thoughts you might have.

    regards
    Tom
     
  4. jimmys69

    jimmys69 Active Member

    How exactly are you connecting your monitors now? Headphone out of laptop to RCA? Interface? Does the noise stop when you unplug the laptop power supply and use battery?
     
  5. Boswell

    Boswell Moderator Distinguished Member

    You describe an observable difference between the two active loudspeakers: switching off one of them stops the hum in the other, but not the other way round. This indicates a problem in the power input circuitry of that one loudspeaker that is coupling mains current into the ground circuit. I don't know how much experience you have in this sort of fault-finding, but if you can get the back off this speaker cabinet, I would do some checks.

    Since the hum disappears when you switch off the problem speaker rather than just unplugging it, the fault must be after the mains switch, although it is worth inspecting the EMC suppression components, particularly the Y-capacitors on the mains input filter.

    I don't know the innards of those monitors, but I do know that they use separate power amps for the bass and treble drivers, so it could be one of the amps has a ground leakage fault at its input. If I were trying to diagnose this, I would probably try removing the mains from each amplifier in turn to see whether that also removed the hum, and, if not, swap amplifiers one at a time between the two cabinets to see if the problem transferred to the other unit.

    Don't try this sort of thing if you don't feel confident about doing it.
     
  6. Tommohawk

    Tommohawk Active Member

    Thanks for comments.

    Jimmy - yes I'm connecting phones out from laptop to phono (RCA) input on monitors. Laptop on battery makes no difference. Its odd though that if I connect it all up without powering up the laptop there is no hum - I only get the hum once the laptop powers up - even if on battery only with mains disconnected.

    Boswell - sadly I think you are near the mark. This evening I tried again swapping the monitors but not the mains and input cables ie to eliminate cable faults. And the same result ie hum stops when one monitor switched off but not the other. Definitely suggests a prob with the monitor itself. But i cant understand why either one works just fine on its own - I only get the hum with both inputs plugged in.

    Just to be clear - the hum stops if I disconnect either audio input. It also stops if I switch off mains or unplug mains to one unit, but not the other.

    I am reasonably confident with this sort of thing - did quite a bit of elementary audio stuff years ago. I had the same idea of checking component sections form one monitor against the other. I took the back off and had a preliminary look at the guts of it - however the amp output cables to the speakers run through small hole in the false cabinet back and there isnt much slack to withdraw the amps and power supply. One group of wires has a connector block, but there are some wired soldered straight to the board - great!

    It may be that removing the speakers from the front would generate some slack.

    Any other ideas gratefully accepted. Do you think it may be a problem only with the unbalanced inputs? Maybe it would be OK with the balanced IP? Wishful thinking??

    Regards
    Tom
     
  7. hueseph

    hueseph Well-Known Member

    One more thing before you start gutting. Have you tried a different cable from your laptop? Buy a new Y cable. They're cheap. It could be that the cable is wonky. There may be a short in the stereo mini plug end that only manifests itself when you have both ends plugged in. Have you also tried swapping the left and right and then powering off one of the monitors to see if the results change? I kind of get the feeling too that it could be the output on the laptop or even that you may not have the plug in the jack all the way. I'm just stabbing in the dark here since you said it worked fine a day before. It seems odd that the problem would crop up like that all of a sudden.
     
  8. Tommohawk

    Tommohawk Active Member

    Hi Hueseph.

    I wondered about the cable - but it tests OK with a meter - though I know sometimes you get small unmeasurable leaks/losses. More significantly I get the same problems with different cables using different audio sources.

    The weird thing is I can use any combination of IP / mains cables and its always the same monitor which when depowered fixes the hum.

    BTW since last post - I tried removing the earth to the suply of the suspect monitor - problem sorted - no hum. So it definitely seems like some kind of earth related issue. BUT is was definitely OK before.

    Latest thoughts - what might have happened in the monitor to cause a problem?
    Cables shifted somehow? But all looks nicely tied down.
    Partial capacitor failure? But surely would cause hum in mono situation which wouldnt resolve with ground lift.
    Partial earth fault? But earths from plug to OP outer measure OK with (basic) meter.

    There are various inexpensive in-line hum eliminators out there - do you think that might help? If its really a problem with the monitor I'd rather sort it at source.

    Cheers, Tom
     
  9. Boswell

    Boswell Moderator Distinguished Member

    Be careful here. The audible hum is a manifestation of a fault that may have other implications, specifically those involving safety. Don't brush the problem under the carpet by experimenting with hum eliminators - that hum is telling you that something is wrong inside that loudspeaker cabinet at the mains level, and it needs fixing.
     
  10. Tommohawk

    Tommohawk Active Member

    Hi Boswell - well thats my thinking too -- but if a meter shows continuity between the earth and chassis --what next?

    Actually thinking about it I only checked continuity between earth at mains plug and IP outer - I guess chassis could still be floating. I'll try and measure this when I have a mo.

    The wiring looks good though. I wonder if theres a dodgy earth link from the amp section to the front section - should the speaker frame be earthed for example? Maybe use of the speakers with moderate/high volume has rattled something loose?

    Please hang on in there!

    Regards
    Tom
     
  11. Tommohawk

    Tommohawk Active Member

    OK well Ive had the back off both units -its difficult to access becasue the cable runs are so short and there are some cables - the LED feeds - which are directly soldered to the board.

    I checked earth to chassis / transformer body etc connections and generally all looked good. However I did note that the trasformer screen plate in one unit, which is made of coated ally was poorly earthed. No surprise becasue the connecting screws and the plate were not cleaned at the point of connection. Similarly there is a bracket which was not earthed at all in the other unit - again couplings not cleaned.

    Had to look again at the badge on the front - yes, definitly says Wharfedale. (Yes I know theyre made in China now)

    Once sorted I was hopeful one of these things things would get a result - nope - same level of hum exactly. And the only leads to the front are cone and LED so no issue of earth to speaker frame.

    So back to the drawing board.

    Help!

    Tom
     
  12. Boswell

    Boswell Moderator Distinguished Member

    Lots of points to you for perseverance, but I have a hunch this will turn out to be a component problem that is not identifiable from visual inspection.

    I think what you need to do is what I mentioned in an earlier post, that is, to try to see if it's possible to disconnect the live and neutral mains inlet leads from each of the two internal power amps in turn, powering up the speakers in between and listening for hum levels. Leave any earth leads in place. My guess is that the hum will disappear when one of the amps is disconnected. You will have to listen for the result in the other speaker, of course.

    Wharfedale sonics are still reasonably good, but it's not the quality manufacturer it once was.
     
  13. Tommohawk

    Tommohawk Active Member

    OK well I've had the back off both units.

    The transformer bodies and main chassis on both units appear well earthed. The tranformer screen plate on one was poorly earthed,, and a securing bracket on the other was not earthed at all - in both cases this resulted from the screws and plates not having the protective coat cleaned at the contact points (quick look at the badge on the front - yes it definitely says Wharfedale)

    So I was hopeful this would make a difference - but no - the same loud persistent unwavering hum.

    The only cables running to the forward section of the cabs are the 4 cone leads and 2 LED leads - so it look like there is no speaker frame earth to potentially come adrift.

    So now at my wits end - would be grateful for any ideas - time to try the hum eliminator?

    Or maybe I could try the balanced inputs? I'm getting mixed messages about the potential benefit of this.

    I read this: http://www.jensen-transformers.com/an/generic seminar.pdf esp page 29. This seems to suggest a potential gain in using unbalanced to balanced using a twin core + screen cable. (hi and low to pin 2 and 3 respectively and screen to 3) However, I guess this wont help if the imbalance is within the monitor?

    The most likely suspect component (disregarding the (significant) fact that one monitor behaves differently to the other) is the cable. So if I'm going to get a new cable it may as well be phono to XLR.

    These seem to be readily available - but I cant work out if they use the wiring method explained above, or simple single core per channel.

    Would really appreciate any other pearls of wisdom/wizardry/miracles.

    Regards
    Tom
     
  14. Tommohawk

    Tommohawk Active Member

    Well I've had the backs off both units.

    Chassis and transformers seem well earthed, but transformer screen plate on one unit and mounting bracket on the other poorly/not earthed due to failure to clean protective coating off screws and plates at contact points. Are these really Wharfedales?

    Hoped this would fix it - but no - same as before. Leads to front of cab are 4 drivers and 2 LEDS - so no earth to fall off.

    Am now getting desperate - how can each unit perform just fine on its own, but not work as a pair? If I was using them in a mono app I would be unaware of any problem!

    I wnodered about using connecting to the balanced IPs - but mixed messages here.

    REad a good paper here: jensen-transformers.com/an/generic seminar.pdf esp Page 29, which seem to suggest a possible gain using twin core + screen cable. But I~ guess this wont help if there is imbalance within the monitor itself?

    I see that phono to XLR cables are readily available but cant work out if they use the twin core wiring or simple single inner.

    The most likely component to have failed is the cable to I may as well get replacement I guess and see what happens.

    Any other ideas/thoughts please?

    regards Tom
     
  15. Tommohawk

    Tommohawk Active Member

    sorry about duplicate replies - thought theyd got lost - hadnt realised I'd gone to page 2!!
    Tom
     

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