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Old 13th August 2005, 04:04 PM   #1
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Default SMPS buzzing?????

I have four 14v SMPS's that I put in series to make a 28-0-28 supply for some 41hz.com AMP-1B's. The problem is when I connect my bulk capacitors to the supplies there is a strange buzzing that comes from them that doesnt stop until there is a load of a couple amps applied.

The setup works perfectly fine with and without the caps attached, but the supplies only buzz when they are connected . Voltages between the supplies vary about 0.2v, is this a problem?

Oh, and they will also make the noise if I connect just one cap to one supply.
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Old 13th August 2005, 04:56 PM   #2
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Ok I fixed the buzzing but in a very strange way, and I have no clue what it did to actually fix the problem.

I just put a very low resistance in series with the power wire to the cap.

I dont have any low value high wattage resistors so I used a piece of NiCrome wire. The piece of wire is 2" and has a resistance of about 0.5 ohms.

If no one sees a problem with the way I'm doing this I guess this will be my solution. The NiCrome wire can handle hundreds of watts of heat so I dont have to worry about it burning up like a resistor would.
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Old 13th August 2005, 08:10 PM   #3
Eva is offline Eva  Spain
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This buzzing is probably due to control loop instability or discontinuous mode operation. Sometimes some SMPS have a hard time working with very light loads, so adding dummy load resistors in paralell with the output of each SMPS will probably be a better solution.
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Old 13th August 2005, 08:49 PM   #4
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Thank you for the reply Eva. A dummy load across the output requires quite a bit of current before the buzzing stops.

Will having 0.5 ohms of resistance in series before the bulk caps have any negative effect on the amplifiers?
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Old 13th August 2005, 10:19 PM   #5
Eva is offline Eva  Spain
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Nothing bad.
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Old 14th August 2005, 12:33 AM   #6
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Thanks Eva, a simple statement from you like "nothing bad" is extremely comforting to me.

The DIY resistors seem to be doing the trick. After playing a 20hz sine wave near clipping the makeshift resistor gets pretty warm, but it doesn't glow red so thats good. I still don't know why such a small amount of resistance in series with a cap fixed whatever was wrong, but I guess it doesn't really matter why as long as it works.

Here's a pic of my DIY resistor and the complete test setup....... on the carpet no less. don't tell Mrs. Anonymous, shes away for the weekend.

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