Amplifier toroid noise, proper way to fix?

Discussion in 'General Electronics Chat' started by MrSoftware, Nov 30, 2015.

  1. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    The toroid in my audio amplifier is making a horrible squealing noise and I'm looking for tips on the proper fix. I know it's mechanical noise, if I put something on the toroid to stop the vibrations it gets quiet, but surely this cannot be normal for an audio amplifier. So I guess my question is, did age just loosen up the toroid (the amp is probably 20 years old) and the proper fix is to try to tighten it up, or is this really an indication that I've got another bad component on the board somewhere that needs to be replaced?

    I bought it as damaged on ebay, replaced a bunch of bad caps and now it seems to be operating just fine except for this noise. I would like to resolve it properly so I can resell it (new hobby). Here's a video showing the noise, any tips would be great:

     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You can make some trials. 2) Reflow the solder to the six wires from the toroid.

    3) You can count the number of turns from each wire and measure wire size, then re-wrap the core - in case insulation is compromised and shorting to an adjacent wire on slight contact or to the core (unlikely)

    1) You can try to mechanically fix the wires in place with epoxy or laquer so they do not vibrate and cause mechanical noise.

    4) As you re-wind the core, make note of the core specirfications (diameter, inner-diameter and color code). Then order a new one. Sometimes, the iron powder in a Micrometals core oxidizes over time (expecially with heat and abuse - like your amp that had all caps swell). Since each particle of iron is insulated by a chemical treatement, then heating causes the insulation layer to break down and start conducting. Once you have conduction, you get further heating from eddie currents.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Sounds like oscillator whistle from the core of the transformer, bet its a switchmode psu....i used to coat them in hot melt or cover it wax in old tv sets, pesky line whistle!!!
     
    Last edited: Nov 30, 2015
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    The small gauge of the power supply wires may be the cause of the instability (the noise you are hearing). Try adding a very large external capacitor near the input terminals. This cap would be external to the amplifier and in parallel with your current setup.
     
  5. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    Thanks guys for the replies. More details; it is indeed a SMPS, input is 14v DC and draws ~1.5A at idle, the bench input wires are 16ga; the little tiny wire is the remote turn-on signal. The high side of the toroid is about 80vpp. It's an old Rockford Fosgate class A/B automobile amplifier with max claimed output of 500w RMS. The toroid noise goes away if I put some load on the amplifier, or after it gets warmed up a bit.

    I don't think there are any shorts in the toroid as I'm not seeing any arcing, the scope shows the waveforms are clean and the audio output is clean. If it's likely that age just loosened the toroid up or changed the core to cause more vibration (as opposed to a bad component causing new vibration) then I'm OK with damping the vibrations with some epoxy, maybe that's the way to go.

    @Lestraveled - Thanks for the suggestion, I will try a large cap on the inputs. If that makes a difference then I'll go ahead and replace those 4 additional caps inside the amplifier, I believe they're on the input side.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Or just slather it all over with some 2 part epoxy. Make sure it gets underneath to cement it to the PCB.
     
  7. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    I put 20,000 uF on the inputs and it made no difference in the noise, so I don't think it's an input line issue. That was worth a try though, thanks for the suggestion. So I studied the toriod a little closer and it looks like the lacquer might be getting old and deteriorating a little. So I'm thinking over time it must have gotten thinner, or even worn from vibrations over the years, and basically the coils have loosened up. I'm going to try mechanically sticking the coils in place to stop the vibrations.

    Taking aesthetics as well as functionality into account, would lacquer like this be the best way to go?

    http://www.all-spec.com/products/419C-340G.html?gclid=CL3MzoPRuckCFcpCXgodFMwAWg
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That is essentially the same as an aerosol clear version of gel fingernail polish. Either should work. The brush fingernail polish will be easier to keep from making a mess.
     
    MrSoftware likes this.
  9. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    I raided my wife's makeup bag and put some clear gel fingernail polish on the offending windings, problem solved! Thanks everyone!
     
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