Question about Audio Players & Recorders

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Jamo D6 Subwoofer osillating sound through speaker.Phase cutoff and vol control do not change it.With speaker leads o/c no oscillation. With dynamic feedback sensor out of circuit no oscillation. Has anyone had this fault previously please. Best regards........ mbeech@paradise.net.nz

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  • Mike Beech
    Mike Beech Jul 26, 2008

    Jamo D6 subwoofer. This unit is self oscillating giving a low level approx 1.5kc's sound through the speaker in addition to the correct signal.Also with no signal input.Appears to be caused through the servo feedback input which if disconnected the problem goes away.

  • Mike Beech
    Mike Beech Jul 26, 2008

    I have this unit apart the oscillation is visible on a scope as a multi frequency centered around 1.5 Kc's triangle waveform.Disabling the servo feedback circuit stops the fault. Amplitude at speaker leads is about ten volts peak to peak.

  • Mike Beech
    Mike Beech Jul 26, 2008

    s4u311 no i cannot explain it i was hoping someone on fixya would have the answer. Too much loop gain?

  • Mike Beech
    Mike Beech Jul 26, 2008

    Robotek
    Thanks for your advice re input earthing.Will let you know if it helps later. Can i contact you via email or telephone i would like to remain in touch.

  • Mike Beech
    Mike Beech Jul 27, 2008

    Hi to all you folks who answered. First time i have used this service, very pleased with responses. How do i enter "award" points where appicable? Fault turned out to be the feedback transducer mounted on the back of the cone, a piezo disc plus op-amp inside a small plastic box hotglued in place. Now where to find a replacement!?
    Thanks again all.......
    mbeech@paradise.net.nz



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6 Answers

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Shielding is one of the main factor, Make sure cables are shielded well at the source. A tiny cut or crack on the unamplified cable lines will give chance to feedback and oscillation.

Posted on Jul 26, 2008

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  • Master
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No oscillation cause by Loose ,crack or cold solder joints parts..
Cold solder issues - each component is soldered into small holes on the printed circuit board which in turn connects said component to the other components. Due to time, heat, use, some solder joints would be cracked or loose. It is possible that the cold solder is still minimally connected (hanging by a thread). When the receiver turns on heat would be generated which would be sufficient to further loosen up the solder joint to the point it no longer provides electrical contact. Jarring the unit creates movement inside enough to re establish the contact, sometime good enough sometimes not enough.

Again, since the unit still operates (sometimes) then it is highly probable that there are no defective components. As you have surmised, it is possibly a loose connection, solder joints are "connections" except that molten soldering lead is used to attach/connect the component to the board, "loose connection" in this sense is equal to "cold solder".

Again on the assumption of no defective parts, then seeking a more experienced buddy for the soldering might be to your advantage. This is of course in addition to the possible electrical hazards when repairing....If i have helped,please click to rate.It is the only way i get paid for my work...

Good luck:

Regards,
VOTIT

Posted on Jul 26, 2008

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I would be looking for leaky capacitors in the sub woofers output stages. Some times the easiest way is tpo replace all the caps in the device that are in the signal section, and see if that stabilises the problem. Also check for low value resistors that may be used as a fusible connection to your input earth connections. It may have gone open circuit producing an earth loop from the rest of the system. You need to check that the sub is operating on the same phase as the equipment driving it or you will get earthloop problems from this also.

I am more than happy to answer any other questions you may have about it, please get back to me here, or use the ASK function to go straight to me :) If you are still unsure, have problems, or we need to look at the situation more deeply, then before you give a rating for this advice, hit the ASK button If you consider the advice I have given you as sound, please accept my solution and we can work through your problem.
Thanks for using FixYa

regards
robotek



Posted on Jul 26, 2008

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  • Graeme Ross
    Graeme Ross Jul 26, 2008

    oh, by the same phase I am referring to the AC mains power supply phase. if they are on GPO,s on either side of a wall, they may be fed from different phases. Just pull the circuit breaker for plugs in the room, if the power is gone from all the GPO's then they will be on the same phase.

  • Graeme Ross
    Graeme Ross Jul 26, 2008

    no worries, just accept my solution and we can go from there. :)

  • Graeme Ross
    Graeme Ross Jul 27, 2008

    Good to here you found the problem. I have a pair of old Philips active HiFi speakers that have a peizo transducer like that. You can get parts from hereservice@qualifi.com.au Its in Australia, but still be quick for you. Cheers mate

  • Graeme Ross
    Graeme Ross Aug 07, 2008

    How did you get on mate? Any uck sourcing the par to fix the problem?

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Can u explain it

Posted on Jul 26, 2008

  • sandip panchal
    sandip panchal Jul 26, 2008

    connecting a 0.1uF/100V capacitor across the speaker terminal



    humming will be off

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Hi,

On a similar concern, troubles are normally internal to the amplifier of the subwoofer. If it is a 60cycle hum, then it may very well be insufficient/defective filter of the power supply; if on the other hand, it the sound is rumbling, then it is amp noise and if it feedback sort, then it is a shielding problem.

Correct procedure would require opening the unit for a look and see, familiarity with electronic circuitry and components, use of a DVM and a soldering iron.

Alternately, possible temporary workaround may include:

  • shortening/replacing/relocating the speaker cable and the input cable;
  • connecting a 0.1uF/100V capacitor across the speaker terminal;
  • decreasing the input impedance of the amp (preloading); and/or
  • using a filtered power strip for the AC mains connection.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards.

Thank you for using FixYa.

Posted on Jul 26, 2008

  • Louie  Role
    Louie Role Jul 26, 2008

    Hi again,

    Appreciate the additional information. What is the output/amp IC? Any op-amp or pre-amp IC too?


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Hi

what do you mean oscillating sound?

Thanks!

Posted on Jul 26, 2008

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84c9065c-b7c6-4417-be21-0be7a6e74b41.jpg


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7a25e260-d39b-4992-8bce-8c2d2c9b07e5.jpg

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