Jonkan

toroid shield question (yes its the tincan!)
« on: August 10, 2006, 06:02:30 AM »
I have some hum in my micpres, so im going to shield the toroid to try to lower it some.

I was wondering, will the shield (two to three layers of tin can) work better if it is on top of the toroid, or will it be enough to have a metal band around the toroid sides and leave the top open? Im not 100% shure how the radiation pattern looks for these things.

Im also worried i might be creating a closed turn if i put the toroid inside the cans with the top up.

/Jonas


Mendelt

Re: toroid shield question (yes its the tincan!)
« Reply #1 on: August 10, 2006, 08:13:33 AM »
Quote from: "Jonkan"

Im also worried i might be creating a closed turn if i put the toroid inside the cans with the top up.


You'll only get a shorted turn if you have a path through the center and around the side of the toroid. If your tin can touches the top of a metal screw through the center of the toroid you can expect fireworks.
I've seen people use plastic screws to fasten toroids. Seems like a good idea.

Jonkan

toroid shield question (yes its the tincan!)
« Reply #2 on: August 10, 2006, 08:41:17 AM »
good to know, safety first!!

How about the shielding then, will it be less effective if i lay the toroid inside the can with the opening up, so that im 100% shure to avoid creating a shorted turn..?

/J

gyraf

toroid shield question (yes its the tincan!)
« Reply #3 on: August 10, 2006, 09:11:11 AM »
Try turning the toroid - field-of-minimum-hum is in the opposite direction to the wire breakout point...

Jakob E.
..note to self: don't let Harman run your company..

Jonkan

toroid shield question (yes its the tincan!)
« Reply #4 on: August 11, 2006, 06:37:52 PM »
I have turned the toroid to every possible angle, and i still get some hum.

When i extended the cables from the toroid and moved it outside the box, i could get the preamp to be pretty quiet.

Could it be that im loading the +/- rails differently thats causing the problems?

btw, my box is pretty damn crammed....
/J

tommypiper

Re: toroid shield question (yes its the tincan!)
« Reply #5 on: August 12, 2006, 12:40:54 AM »
Quote from: "Mendelt"

You'll only get a shorted turn if you have a path through the center and around the side of the toroid. If your tin can touches the top of a metal screw through the center of the toroid you can expect fireworks.
I've seen people use plastic screws to fasten toroids. Seems like a good idea.


You mean if the metal can touches the screw AND gets through the insulation around the toroid, don't you?  I've done this by using the metal screw to hold sheet metal over and around the torroid side with no problem, and it did reduce hum.
Imagine a wet, slightly chilled from its gas release and decompression, with water droplets condensing, sucking surface tension, slowly sliding down the side, capped by a healthy virgin froth on top..

Kev

toroid shield question (yes its the tincan!)
« Reply #6 on: August 12, 2006, 04:02:02 AM »
put the toroid out of the box for a test
extend the leads if you have too

just make sure the hum is what you think it is

you may have a simple earth loop going on and no amount of shielding the trafo will change anything

it won't take long to try ... then you will know
Kev
DIY Factory

Flatpicker

toroid shield question (yes its the tincan!)
« Reply #7 on: August 12, 2006, 12:19:10 PM »
Quote from: "Jonkan"
Could it be that im loading the +/- rails differently thats causing the problems?

Yes, that is most likely the problem. I rarely see toroids get that noisy under normal circumstances. Measure AC (current) going to power supply and see if it is more than ~3/4 of the transformer rating.

Jonkan

toroid shield question (yes its the tincan!)
« Reply #8 on: August 14, 2006, 02:22:25 PM »
When i moved the toroid out of the box, it was almost totally quiet. But i had to get it atleast 30-40cm or so out before it was that way (10-12 inches aprx).

Im not 100% shure what you mean by this:
Quote
Measure AC (current) going to power supply and see if it is more than ~3/4 of the transformer rating.


Do you mean that i should measure the current from the transformer secondaries? (This has to be measured in series because its current, right?). Do i measure both secondaries or just one?

Or did you mean that i should measure the current before the AC reaches the PSU?

Sorry if this is a stupid question, but im not totally confident how to do this.  :green:

Thanks!

/Jonas

Flatpicker

toroid shield question (yes its the tincan!)
« Reply #9 on: August 14, 2006, 07:50:49 PM »
First, you have to sum the VA (volts x amps for a purely resistive load) of all the secondary windings. For example, using a transformer with a center-tapped secondary, measure the AC current and the AC voltage of one secondary side, using the center-tap for the ground reference. Multiply these together. Next, do the same with the other secondary side. Add the results and that should give you the total VA. Compare this with the transformer's power rating. (You should be feeding a sine wave through each channel with terminated outputs while taking measurements.) You can measure current by putting a meter (set to "amps") in-line with the secondary being measured. You'll get most accurate results using a "True RMS" meter.


 

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