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Transformer epoxy or fill for diy potting
Transformer epoxy or fill for diy potting
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Old 11th January 2018, 04:39 PM   #1
Masonstorm is offline Masonstorm
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Default Transformer epoxy or fill for diy potting

So I built my first amplifier and decided to use a 1000va troidal transformer. It uses 6 lm3886s in a bridged tied load in an all aluminum case that I built with my Father. The problem that I have run into is that the no signal noise gets a lot louder when I put the lid on the case. I have a feeling that there is some type of magnetic interference that is coming from the transformer. The transformer is 165x65mm and it is hard to find a pot that would fit it. I ended up on stainless steel pot from good will that is about the right size. My question is what type of epoxy or fill should I use on this pot that does not cost a ton? Also how good does stainless steel work at blocking magnetic interference? Is this even worth pursing?
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Old 11th January 2018, 04:50 PM   #2
djoffe is offline djoffe  United States
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A Steel wall surrounding a toroid can be very effective in cutting down the stray magnetic fields. It can really make a difference if all the other hum sources are very well handled.

In the GT-102, I used a combination of spacing and a steel wall to get really low levels of hum in the output.

https://www.akitika.com/pictures/GT102AssembledC.jpg
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Old 11th January 2018, 05:59 PM   #3
MorbidFractal is offline MorbidFractal
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Quote:
Originally Posted by Masonstorm View Post
Also how good does stainless steel work at blocking magnetic interference? Is this even worth pursing?
YMMV...

Is stainless steel non-magnetic?

Try it before you commit yourself to potting it.
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Old 11th January 2018, 06:50 PM   #4
Masonstorm is offline Masonstorm
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Originally Posted by djoffe View Post
In the GT-102, I used a combination of spacing and a steel wall to get really low levels of hum in the output.
This looks like a great idea but I think Iíll have a hard time implementing it in my build. Also, the GT-102 looks epic.
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Old 11th January 2018, 07:20 PM   #5
djoffe is offline djoffe  United States
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There look to be many formulations for stainless steel, with wide range of relative permeability, and hence magnetic shield effectiveness...

Permeability (electromagnetism - Wikipedia)

I'm still looking for a good web page that goes through the relative permeabilities of various types of steel...this is about the best I've found so far:

Magnetic properties of materials 2.6.6

hopefully someone could find a better one...

From experience, galvanneal is plentiful, inexpensive, and probably pretty good, but I have yet to find the mu-relative data.
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Old 12th January 2018, 04:00 PM   #6
AndrewT is offline AndrewT  Scotland
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Originally Posted by Masonstorm View Post
......... stainless steel pot from good will that is about the right size. My question is what type of epoxy or fill should I use on this pot that does not cost a ton?
try glassfibre supply specialists. They usually stock epoxy and polyester compounds for GRP manufacture. Some are quite runny and may suit impregnating the outer layers as well as holding the transformer securely inside the iron ring. Might be best to ask about slow curing instead of standard or fast.
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Also how good does stainless steel work at blocking magnetic interference? Is this even worth pursing?
Many pipe fabricators have to use magnetic stainless to allow non destructive testing of their welds.
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Old 12th January 2018, 11:03 PM   #7
trobbins is offline trobbins  Australia
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Quote:
Originally Posted by Masonstorm View Post
... use a 1000va troidal transformer .... in an all aluminum case.... The problem that I have run into is that the no signal noise gets a lot louder when I put the lid on the case.?
Can you clarify if the 'noise' is just audible noise from the amplifier case region, and not a noise that gets in to the signal and becomes observable from the speakers?

Are you using a standard toroid mounting fixture (ie. a mounting scheme that doesn't introduce a shorting turn).

Is the transformer hard mounted to the case chassis, or have you included substantial mechanical compliance (eg. a neoprene foam that squishes to say 50% of unloaded height) with the mounting assembly such that the toroid is vibration isolated from the chassis and transformer mounting hardware?
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Old 16th January 2018, 12:46 AM   #8
Masonstorm is offline Masonstorm
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Can you clarify if the 'noise' is just audible noise from the amplifier case region, and not a noise that gets in to the signal and becomes observable from the speakers?
I also made speakers that are around very sensitive. Using pro drivers that are around 98 db at 1w. The noise sounds like it is around 60hz and is kind of annoying. I have tried moving things around and adding a star ground as well as the twisting of wires. But the thing that is interesting is that the hum is worse when the top of the case is on. I figure it is the aluminum reflecting the magnetic waves into the input section. Before I made the all aluminum case the transformer was further away and I did not have the noise problem. I also just ordered an cast iron pot that the transformer will fit in just a few days ago and I will try it and see what one works better be it the stainless steel or the cast iron. The weight that it will add would be great.

Quote:
Originally Posted by trobbins View Post

Are you using a standard toroid mounting fixture (ie. a mounting scheme that doesn't introduce a shorting turn). Is the transformer hard mounted to the case chassis, or have you included substantial mechanical compliance (eg. a neoprene foam that squishes to say 50% of unloaded height) with the mounting assembly such that the toroid is vibration isolated from the chassis and transformer mounting hardware?
Right now it is using a standard mount with a bolt and nut on the giant washer plate. There is rubber on both sides of the transformer. I do not think I have a short turn.

In the next few days I will try out the other mounting methods and take some pictures.

@AndrewT I will take your advice on the fiber glass specialist when I figure out what type of pot to use.

I also found another post where someone used beeswax with additives so that it was possable to take it apart in the future so I may go that route.

Many thanks from everyone so far. The ideas have been great.
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Old 16th January 2018, 08:15 AM   #9
AndrewT is offline AndrewT  Scotland
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....................But the thing that is interesting is that the hum is worse when the top of the case is on.
this points to shorting, see later
Quote:
I figure it is the aluminum reflecting the magnetic waves into the input section. Before I made the all aluminum case the transformer was further away and I did not have the noise problem.
Aluminium cases usually do not affect the magnetic fields around an amplifier. Steel panels definitely affect the magnetic fields.
Quote:
.............
Right now it is using a standard mount with a bolt and nut on the giant washer plate.
The field emitted by the transformer is usually not even.
Sometimes rotating the transformer can "point" a strong field lobe in a different direction and so affect nearby circuits in a different way. But also look at your LOOP AREAS. These are aerials that pick up interference. If you have a low level circuit that has a big loop area, then that could be injecting the hum frequency into the circuit.
Quote:
There is rubber on both sides of the transformer. I do not think I have a short turn. ............
The bolt securing the toroid transformer MUST ONLY be connected to the metal chassis at ONE END. The other end must be isolated from the metal chassis. A few layers of tough plastic over the bolt and washer can prevent accidental shorting.
There should be no electrical connection to the bolt, nor to the washer plate.
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Old 16th January 2018, 04:32 PM   #10
SemperFi is offline SemperFi  Wake Island
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Originally Posted by AndrewT View Post
this points to shorting...
The bolt securing the toroid transformer MUST ONLY be connected to the metal chassis at ONE END. The other end must be isolated from the metal chassis. A few layers of tough plastic over the bolt and washer can prevent accidental shorting.
There should be no electrical connection to the bolt, nor to the washer plate.
+1

A hard short of the toroid should blow the fuse, but perhaps the shorted turn is composed of material with high enough resistance so the tranny only hums from the load of a 'softly' shorted turn?
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