Advice on quieting transformer hum?

[Scott75s1974]

This is an e-core transformer on a plate amp in an AR subwoofer. Definitely physical transformer hum, not electrical. The firm attachment to the plate gives it resonance with the box.

I don't see any way to tighten the laminations (I assume that was a much older era of construction) and it's not worth the trouble to shellac it, I do have a larger quiet transformer of similar voltage, but there's no easy room for that. (If I wanted to go crazy I could mount it in the speaker enclosure...not sure what that loss of volume would do.)

My plan is to try some vibration isolation.

It' would be easy enough to loosen it and put some rubber padding underneath. Maybe a cut of a sorbothane insole. But the screws are still going to be a direct connect. Ideally they'd be inside of isolating rubber washers. Not sure there's room for that. Perhaps I could drill out the flanges.

Advice? Thanks!

 

[Lavane]

Put rubber washers between the transformer mounting holes and the chassis. That may help some. I've dome it before and it helped. I put a washer, then a rubber washer on the mounting screws. Then put another rubber washer between the mounting tabs and chassis.

 

[Lavane]

Also maybe cover the metal housing of the transformer with Dynamat? Maybe put some on any exposed areas of the chassis or amp plate around the transformer.
I've used Dynamat on the spokes of speaker baskets and to quieten up the cab of a pickup. I added it to the inside back of the cab. That stuff really works. You can buy a smallish sheet of the stuff on Amazon or Ebay.
That's where I got some when I covered the speaker baskets.

 

[Scott75s1974]

That outer steel shell is held on just with fold over crimp tabs on bottom. Nothing providing compression. When I squeeze it very hard, it quiets. (alas the circuit board contacts make putting a clamp on it a no-go)

Any thoughts on dribbling some thin cyanoacrylate in there as easy-potting material? (Edit: well I'm trying it. Will report back.)

 

[Lavane]

You could try and squeeze some thin foam insulation between the metal cover and the winding. Anything soft like thin rubber, etc. I've had iron core transformers that would buzz. I would take the end bells off and put folded card stock, cardboard, etc between the bell and the winding. It usually made it quieter, or dropped the noise all together.

 

[Scott75s1974]

Thanks for all the help.

The cyanoacrylate seemed to help a bit. I dripped it in around the steel jacketand clamped. While clamped it remained loud, but once dry and unclamped it was significantly less loud. The problem may be in the plastic bobbin on which the windings sit vibrating in the core. It's still vibrates to touch, but with less noise.

I also undid the tabs on the bottom of the jacket and installed a strip of butyl rubber (something a bit like dynamat.. left over from roofing). When I reclosed it all up it was quieter still.

Then I mounted it on some rubber washers I made from salvaged seals on carbon water filters and used old faucet seals for top rubber washers. Had to lengthen the screws.

All in all, I'd say 75-90% reduction. If you go listening for it, it's still there. But no longer obvious from across the room.

 

[Lavane]

That's a pretty good drop in noise though. It could be the winding vibrating on the bobbin. Not much you can do about that.

 

[Watthour]

Depending upon where there is access, it may be possible to pack more fish paper between the bobbin and core to restrain its movement. I've had success on some that way.

 

[elvi]

Silicone.
Run beads where the txfmr sits, and allow to harden.
Then, place the txfmr in place. Compressing the silicon will keep the fastener tight. ( or a touch of nail polish on the threads.. use red.. looks better )