jmp-1 transformer hum solved


New Member
May 28, 2022
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If you have a JMP-1 preamp and the transformer is humming and stops when you remove the top, this fix might work for you as well.

A lot of post on the web says it’s the transformer coming into contact with the cover. My experience was that mine was not. If I picked up the cover and slowly lowered it down you could hear the hum begin and get louder as you closed it. I put a straight edge across the case and the transformer was below it, so no contact with the cover.

From what I have read the plates in the transformer are not that good of quality and basically that causes the hum and the cover will develop a sympathetic vibration at that frequency therefore amplifying the sound. If I placed my hands on top of the cover and apply a little pressure the hum goes away.

On another forum I found someone that said using a thin rubber sheet with a peel off adhesive helped to muffle the hum. He listed the Amazon link, so for less the $20 usd I gave it a shot. He did not go into detail on how he applied his, but I did some experimenting and now mine is totally quiet.

this is my 1st post and it will not allow me to post a link.
get the rubber on amazon

"Black Heat Resistant Thin Silicone Rubber Gasket Sheet Adhesive Back,1/25 by 12 by 12 inch."

I cut a 4-5 inch wide piece and the length was about 6-7 inches. I cut mine about ½ inch short of the tube vent holes.

Place it on the underside of the cover and hold it about a ¼ inch or little more from the side, if not it will hold up the cover and the screw holes will not align.

Next, I cut 2 pieces to fit on top of the transformer to build it up so it does come in contact with the piece in the cover.

Now no hum what so ever, even if I put my ear almost on it.

I have no idea what the long term affect of the rubbers + heat will do

It might make it a little more difficult to remove the cover if the rubber pieces stick to each other. You could put a piece of paper between them if you think it might be an issue down the road..

Way cheaper that buying a new transformer and paying to have it installed.

Jon Snell

Active Member
Apr 12, 2020
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Jurassic Coast, England
Eddy currents emitted by the transformer cause the metalwork in close proximity to vibrate. A common phenomenum in a lot of equipment.
Rubber pads to pack the space does shut it up. There should not be enough heat to cause the pads to stick to the lid.
A new transformer of the same design will have no effect in reducing the vibration.

william vogel

Well-Known Member
Dec 18, 2016
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All transformers create a radiated magnet field around them when are operating. If you were to take a steel or iron object and get it close to the laminations of the transformer, it would probably pull it from your fingers forcefully and stick it to the laminations. Because the transformer is operating with AC the field is rising and falling at the frequency of the AC. The cover is not close enough to be pulled into contact and therefore vibrates at the AC frequency. You’ve figured this out but this is an explanation of why the vibration occurs. Good job.

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