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Old 15th November 2012, 05:39 PM   #1
Mitchel is offline Mitchel  Brazil
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Default Power Transformer Noise (not Hum)

Hello Everyone,

I just finished a Leach amplifier (I drew a new PCB), as you can see in this picture, which was taken prior to the wiring.

Click the image to open in full size.

The amp sounds amazing, but after testing it on a scope with a resistive dummy load, I found that the power transformer (EI) is making a weird noise, that strangely changes according to the sine wave injected on the signal input.

This is what I did: I connected the 8 ohms dummy load to the amplifier output. With a function generator, I injected a sine wave to the amp input. The transformer started to produce a noise, in the exact frequency as I injected. I tried to vary the freqeuncy, and the noise changed accordingly. Also, the noise increases with the amplitude of the sine wave. It's not really loud, but it's weird.

Anu clues? Thanks!
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Old 15th November 2012, 05:45 PM   #2
Mooly is offline Mooly  United Kingdom
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Are you sure its the transformer ?

Its actually quite common to "hear" TO3 package output transistors "sing" in the way you describe. They can be quite loud.
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Old 15th November 2012, 09:02 PM   #3
Mitchel is offline Mitchel  Brazil
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I'm gonna test it again, but I was pretty sure it's the transformer! I'll wrote again later! Thanks
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Old 15th November 2012, 09:16 PM   #4
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Have the bolts through the transformer been tightened down properly ?
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Old 15th November 2012, 09:54 PM   #5
mlloyd1 is offline mlloyd1  United States
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btw, that's a nice looking build
:-)

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Old 16th November 2012, 12:50 AM   #6
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I had this happen recently with some chip amps that had an on-board output inductor. It was the inductor that was making sound, especially above 1k Hz. It's like a little voice coil!

-Charlie
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Old 16th November 2012, 02:45 PM   #7
Mitchel is offline Mitchel  Brazil
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Hello guys,

You are right, it's not the transformer. It's hard to point out exactly the source of the noise, but it's definitely from the amplifier board - the TO-3 output devices or the output inductor.

Is this normal? What should I do?
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Old 16th November 2012, 02:47 PM   #8
Mitchel is offline Mitchel  Brazil
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Now that I know it's not the power supply, I will post this thread on the right forum.
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