shielding an output transformer from ac hum. .

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arkestra
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shielding an output transformer from ac hum. .

Post by arkestra » Mon Oct 06, 2008 9:14 pm

Hey I have some quad eight cards im racking up and the output tranny is huge and seems to be picking up hum from the power transformer. The level of hum changes as I move the transformers closer together so I am guessing its induced hum into the primary? or something? how have people avoided this in the past? this is bumming me out because I dont want to move the power supply out of the chassis for portability sake, but assume the card was never designed to be right next to the power transformer. Ideas? can you shield it like a guitar pickup?

runrunrun
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Post by runrunrun » Mon Oct 06, 2008 9:29 pm

you want the psu transformer as far away from the circuitry as you can get it. are you using a torroid or a "box style" transformer?

arkestra
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Post by arkestra » Mon Oct 06, 2008 9:36 pm

its a power one box type transformer. my next step is to move them further apart in the chassis but things are tight in there especially with the stupid +48 supply but that is another story. too bad you need to use a switching circuit to double the voltage. the supply is + and - 24v. got the big dumb linear 48v supply taking up valuable real estate.

The Scum
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Post by The Scum » Mon Oct 06, 2008 10:16 pm

The first thing to do is to verify that the hum is really being transferred from transformer to transformer. Output transformers are less susceptible to hum than mic input tranformers...do these cards have input transformers?

Doublecheck grounds, and verify that the power supplies are giving you clean DC.

If you can, just for testing's sake, move the supplies far away and retest.

Power one supplies are great in many situations, but if you really have a transformer-to-transformer induction issue, you might want to look into something using a toroidal transformer.

arkestra
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Post by arkestra » Tue Oct 07, 2008 6:48 am

I put input transformers on them. I know this is a general test but if I move the input trans closer to the power trans there is no change in sound. the further the output trans is from the power the quieter it is. Ill double check the power with a scope. im going to move them around today to get them at opposite ends of the chassis. Right now everything is "star" grounded. lifting grounds does not seem to make a difference. I have the grounds going to the chassis too. maybe that is an issue?

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mixedupsteve
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Post by mixedupsteve » Tue Oct 07, 2008 6:49 am

sometimes a sheetmetal or copper wall can be placed between Xformers will pick up and drain some of the hum. Must be grounded of course.
Good luck!

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emrr
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Post by emrr » Thu Oct 09, 2008 9:02 am

check rotational orientation too. Usually the power one supplies are pretty well shielded. If you have two power ones, it could be either causing the problem.
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Post by planresonate » Fri Oct 10, 2008 1:38 am

Sheet copper can be wrapped around the outer layer of coil insulation to create a flux carrying path that will redirect magnetic energy. This can be a helpful EMF mitigation technique. The copper might be available at a local scrap lot.

EI lam power transformers may already have steel "bell caps" around the coil. These don't necessarily need to be a precise fit to be helpful. The curvature of the bell is more effective than a shielding steel box.

The mixer I'm rebuilding uses bell caps on the power transformer and also two of the techniques mentioned above. Sheet steel barriers are used to separate the power transformer and output transformers from the surrounding circuitry. Also the six output transformers are mounted in a pattern of alternating 90 degree rotational orientation. This layout will reduce the concentration of fields that would otherwise reinforce each other in one plane or the other.

arkestra
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Post by arkestra » Fri Oct 10, 2008 6:57 pm

cool, thanks for the input. I found a compromise of sorts by taking out the 48 supply, putting the power and output xformers 90 from eachother and putting them at opposite ends of the chasis. There is still a hum but it is about the same level as the noise floor. you only notice it when its MEGA cranked, as in, can hear yourself breathing from 4 feet away. these quad eights sound sweeeeeet! will have to use external 48 for now. oh well.

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