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Old 4th February 2007, 08:02 AM   #1
CBRworm is offline CBRworm  United States
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Default squeeling toroids -- help

I have 3 almost identical Mmats class A/B car amplifiers. They are great amps, but are getting old. Not that it has any bearing on my issues, but they use MJ15003/15004 TO-3 outputs and sound great. They are Mmats LM2125's and one SQ2150.

My problem is that all 3 of them now make a squealing noise from the area of the torroid. I called Mmats - they said that it is a common problem that happens to all of them. They said they would replace the caps, bench test, etc for $175 per amp. There is no trace of the noise on the output, it is just a noise that eminates from the amp itself. Not a problem if they are mounted in the trunk, but not ideal.

You can see the transformer in question in this picture: http://ampguts.realmofexcursion.com...125/inside1.jpg

The impression I got was that replacing the caps would cure the problem. I opened up two of the amps planning on getting a parts list of caps to order. I expected that it would be power supply caps around the switching devices. What I realized with the amps open is that if I move the windings on the torroidial transformer the noise goes away or changes volume/frequency. I measured the frequency at the coil, it measures exactly 40khz. Obviously I am not hearing 40K, but it sounds like it could be 10.

So my questions are:

1. Could this be caused by bad caps? All the caps on the board are Nichicons rated at 105C. none of them show any visual signs of being too hot. They all look perfect - not that looks mean much.

2. I assume that what I am hearing is the coils in the transformer resonating or vibrating. Is there a thin lacquer or something that I can pour into the torroid that would stop the noise? If I put my fingernail between a couple of windings the noise goes away?


ps now one of my xxk2500's is doing the same thing. also silent if i put pressure on the toroid.
Suggestions?

Thanks!

I have tried to post this a bunchof times not seeing the searched checkbox. DUH
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Old 6th February 2007, 05:52 AM   #2
Jeb-D. is offline Jeb-D.  United States
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SMPS can be prone to oscillation. Heres how to check.
Since you have a scope, check the signal coming from the PWM to the switching FET's when it's making the noise. See if the pulse-width signal on the gate of the switching devices is freaking out. Frequency will remain fixed, but look for pulse width deviation. If this is the case, adding like a .01-.1uF cap from the "Comp" pin to ground, on the PWM IC will usually cure it.

If the pulses look ok, then the toroid winding may be too loose. If that is the case, a small amount of high temp RTV sillicone should be fine if you don't mind it looking a little ghetto. Otherwise just bite the bullet and send them in for repair.
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Old 6th February 2007, 09:34 AM   #3
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I have two 20 year old Adcom GFA-565 monobloc amps and when I turn them on, there is an slight audible hum emanating from the massive toroidal transformer. I'm not sure if its because the transformer is old or because of the old filter caps, but I too would like to know what causes this.
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Old 7th February 2007, 03:53 AM   #4
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replace the smaller electrolytics in the power supply circuits of the car amps. both toroid questions are also examples of magnetic "slap". as the current through the wire changes, the magnetic field around the wire changes, and causes the wire to move, since the magnetic fields around the wire and the core are interacting. one way to cure it is to use enamel or some filler compound to keep the wire from moving. another cure (more likely the best in the car amp case) is to reduce any current spiking in the circuit by replacing the caps in the PWM circuitry. it's possible the adcom amp needs caps replaced as well, but the more likely cause of that noise is the windings in the transformer have got a little loose.
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Old 7th February 2007, 06:44 AM   #5
CBRworm is offline CBRworm  United States
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Great! Thanks guys. As soon as I get back to Florida I'll scope it, then replace or add caps as needed. If everythig looks good but they still squeal, i'll try something on the coils themselves
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Old 7th February 2007, 08:31 AM   #6
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Spray the toroid windings with 'conformal coating', a sort of super laquer for electronics.
Spray them good the first time as that way the laquer will penetrate the windings, otherwise you'll just be sealing the top windings together, which may not be the squealing ones..
Caps are worth changing anyway as some have a finite life (mainly can types - check for bulging and venting).
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