EMI Interference In Rod Elliott's DOZ Amplifier - diyAudio
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Old 7th January 2005, 04:05 AM   #1
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Default EMI Interference In Rod Elliott's DOZ Amplifier

This project has taken bloody forever (and cost over twice my original estimate), but finally got it powered up last night for the first time. Sounds quite good, and so far I'm impressed, but I have two issues to solve... (#1 is easy, #2 I need help with, so for those who are easily bored with rambling, skip to #2 )

1) Apparently my heatsinks, which I thought were pretty large, are not large enough. Output devices get too hot to put your finger on after its been on for an hour or so, and bias keeps slowly rising. No big deal...I'm going to get a pair of 120mm 12V Vantec Stealth fans and power them from a wall-wart, and bolt them close to the heatsinks, blowing air across them. In the meantime, I have bias reduced to about 1.2A to at least listen for a while.

2) This is the issue I need help with. I hear a faint (but loud enough to be annoying) 'buzz' through the output, especially with headphones (the amp has a couple of headphone jacks). I poked and prodded all over the amp, and finally I unbolted the transformer (an AVEL toroidal from Parts Express) and swung it away from the driver boards. The buzz immediately went away. The chassis is not large enough for me to move the transformer anywhere else...

How do I tell if this is electrical noise, or EMI? I suspect that it is EMI. If so, and the solution is to use either mu-metal or perhaps the TI metal from Michael Percy, how do I go about using the stuff? Does the entire transformer need to be sealed in the stuff? Or just wrap the outer circumference?

Or could you take strips of mu-metal, and wrap them through the toroid, working your way around the thing? And does this new shield need an electrical connection to chassis ground?(the amp is powered by only hot and neutral...no earth ground).

I lean towards trying the TI metal from Percy, since the claim is that it works for both electrical and EMI shielding, but it is awfully expensive...

What works, what doesn't? Your thoughts are greatly appreciated...
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Old 7th January 2005, 06:53 PM   #2
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You can probably reduce the noise by placing a high-
magnetic steel or mu-metal band around the outside
circumference of the transformer.

Often too, rotating the transformer while watching
the noise will yield much quieter results.
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Old 7th January 2005, 06:58 PM   #3
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Default Re: EMI Interference In Rod Elliott's DOZ Amplifier

Quote:
Originally posted by EchoWars

Or could you take strips of mu-metal, and wrap them through the toroid, working your way around the thing?
Don't do this! You will be creating a shorted secondary that will get very hot and possibly destroy your transformer!
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Old 7th January 2005, 08:41 PM   #4
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This seems to be such a common problem that I don't understand why no one makes toroid shields. Simply a round steel sheet metal case that the torroid sits in and is grounded to the chassis. Seems pretty straight forward. Hmm, maybe there;s a market there and I should just shut up and make them...

Hmm.

Like an upside-down metal bowl, replaces the upper washer, bolt goes through it, throught the torroid and to the chassis. Any takers?

-Brian
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Old 7th January 2005, 08:46 PM   #5
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Quote:
bolt goes through it
NO! You have made a short circuit.

Try Nelson's suggestion.......or move it a few inches away.

BTW.....if you can hear it....I can assure you that your amp will sound better when the noise goes away.

Jocko
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Old 7th January 2005, 08:51 PM   #6
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The bolt would go through the center of the donut, just like toroids are mounted now, just replaced the big washer with something that covers the sides as well... I don;t think that would short anything. Could be wrong...

-Brian
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Old 7th January 2005, 09:01 PM   #7
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Thumbs down Trafokiller



You are making a VERY lo impedance shortcut secondary
winding with that cup and the bolt.

This is NOT a good idea

Uli

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Old 7th January 2005, 09:10 PM   #8
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Okay, how about without the bolt. More like a potted design, just without all the messy potting. Case surrounds the outside only.

-b
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Old 7th January 2005, 09:21 PM   #9
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Bingo

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Old 8th January 2005, 12:43 AM   #10
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Thanks guys!!

I can't rotate the transformer...it sits on top of a Hammond enclosure, and most of the electronics (including the bridge) are down below, and I have a circular cutout with a rubber grommet feeding the wires through to the inside of the chassis. I can rotate it a cm or two...not enough to matter.

I wish someone DID make enclosures for toroids. I'd be in line to buy one right about now.

I believe that I'm going to buy a sheet of the TI shield from Percy. Supposedly you don't have to worry about deformation of the metal reducing its effectiveness.

If I do this (and I believe I have a method that may work to enclose it) does the shield need to be run to chassis ground?
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