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hummmmming toroidal transformers

F

firestorm

pfm Member
My Aleph J amplifier use a pair of 250VA 24-0-24v toroids. They're in a 5U modushop aluminium dissipante chassis.

It suffers from 100Hz hum, that I can hear from 3m away and is quite intrusive. I've bought two sets of 92mm neoprene washers to increase the mechanical isolation from the chassis, as it's amplifying the noise. This has helped reduce the hum, however it's still there.

Wanting to get rid of the remaining humming I looked further.

Searching the Internet has unearthed the possibility of DC on my AC mains. There's a great article by Rod Elliot, https://sound-au.com/articles/xfmr-dc.htm
It shows a simple circuit.

xfmr-dc-f3.gif


I found a ready made one by ATL Hifi in Bulgaria.

20190712_164855-1.jpg


This one has a DC mains filter as well as DC blocking function.

Wired it in-line this evening, but it still hums, it hasn't made any discernible difference.

Before I go replacing the existing standard toroids, is there anything else I can consider?

Or do I need to go to Canterbury Windings or perhaps toroidy.pl for "audiograde" toroids?
 
If the hum is not caused by DC in your mains then you could consider immersing your toroids into electrical varnish. Some people even use normal wood varnish because electrical varnish is costly. The procedure could be quite challenging if you want to do it right (better results by using vacuum chamber). It's reported to have better results than potting the center or even the entire transformer.
Anyway, the difficulty involved and probably the cost is a thing to consider and as always nothing is guaranteed. Probably worth as a last chance solution.
 
You seem to have been a little unlucky here

You have done all the right things...but still have the hummmm

I'm surprised the DC blocker made no difference too... again..it usually helps

Couple of thoughts..it could be something else in your house making the pollution and being amplified..try going round switching off everything you can and see if that helps identify a noisy appliance

Also..are the transformers saturating due to too high a voltage...ie amp rated at (eg) 220v mains at 245v
 
ramona said:
If the hum is not caused by DC in your mains then you could consider immersing your toroids into electrical varnish. Some people even use normal wood varnish because electrical varnish is costly. The procedure could be quite challenging if you want to do it right (better results by using vacuum chamber). It's reported to have better results than potting the center or even the entire transformer.
Anyway, the difficulty involved and probably the cost is a thing to consider and as always nothing is guaranteed. Probably worth as a last chance solution.
Click to expand...

Varnish? Is that to minimise movement and therefore the humming? I assume the plastic covering will have to be stripped first?
Then dip in varnish and wait to dry. Reapply plastic.
Sounds messy.

Mynamemynaim said:
You seem to have been a little unlucky here

You have done all the right things...but still have the hummmm

I'm surprised the DC blocker made no difference too... again..it usually helps

Couple of thoughts..it could be something else in your house making the pollution and being amplified..try going round switching off everything you can and see if that helps identify a noisy appliance

Also..are the transformers saturating due to too high a voltage...ie amp rated at (eg) 220v mains at 245v
Click to expand...

My mains voltage is quite high, between 243-250V depending on the time of day.
The toroids are rated for 230V (ISTR)

I could use a variac to see if it makes any difference and if it does, a buck transformer may do the trick. Thanks for reminding me about saturation.
 
Mynamemynaim said:
Couple of thoughts..it could be something else in your house making the pollution and being amplified..try going round switching off everything you can and see if that helps identify a noisy appliance
Click to expand...

I doubt that'll cause the 100Hz vibration, it may conceivably cause higher frequency interference, like a poorly designed SMPS, but that's usually RFI.
 
This might sound counter-intuitive, but have tried to loosen the transformer mounting bolts a little?
 
Yes, use a variac if you have one. As you say if that cures it you rig up a bucking transformer on the primary to drop the mains to 230ish. Low VA is all that is needed as you will have circa 0.5A max in primary, so even allowing for 1A current, 20VA at 20V should do nicely. Can you scope it so you get the phase the right way around? Don't want 270V by accident!
 
advice i was given was to loosen mounting bolt and try rotating the transformer to reduce hum.

It does work to a point but hard if wires all dressed I was advised to to try before final install.

failing that have a chat with Tiger Toroids I found them to be very helpful and no was no hard sell to change to their transformers but happy to make to any spec required.
 
Just to clarify - I'm talking about hum from the transformers, not through the audio chain.

1) I'll have a go at loosening the nut on top of the toroids to see if that makes any difference
2) Reduce the incoming voltage (I do have a variac)
3) I've got a feeling that I may have to go for new toroids or to SMPS (hiss, boo..!)
 
Just to rule out a couple of things.
Unlikely, but they are not mounted in such a way to create a 'shorted turn' ?
Do they still hum if you disconnect the secondaries, so there is no load on them?
 
How close are both transformers from each other, what is the outside diameter and height ?
Also, can you see or feel some slightly loose wires on the outer portion of the tranfos ?
A picture would be helpful.
 
Bigger transformers >500VA are subjects to hum due to DC in the mains. Also another electrical device in your house you could cause this kind of problem. My transformer shakes like old motor when I switch on my oven.
If no other transformer is affected it could be that it not good quality tranformer. In this case I don't know if trying remedies like the one i mentioned it would be worth it.
In order for the varnish to enter in the tinniest gaps shaking it and higher temperature it is advised. Some let the transformer soak in varnish in the sun and others, more daring, while the transformer is plugged in (so that it is shaking).

If you have the time, better have a step by step inspection to find out what's the cause.
With or without load. Bolded or in the air (holding it).
 
Right.
I've unbolted it and disconnected the secondaries.
They are rated for 230V. 250VA each at 18-0-18V.

Incoming mains is 247-249V AC this afternoon at 50Hz.
I took the video in landscape mode, but YT has made it portrait :confused:

Here is a photo:
 
It hums even when I pick the thing up with no load on the secondary and the primary at mains voltage.
I'll dig out my VARIAC to see if it stops humming with 230V AC.
 
Mynamemynaim said:
Couple of thoughts..it could be something else in your house making the pollution and being amplified..try going round switching off everything you can and see if that helps identify a noisy appliance
Click to expand...
Data Point: When my SO uses her hairdryer, I can hear my Sugden's toroid LOUDLY across the room.

BugBear
 
firestorm said:
Right.
I've unbolted it and disconnected the secondaries.
They are rated for 230V. 250VA each at 18-0-18V.

Incoming mains is 247-249V AC this afternoon at 50Hz.
I took the video in landscape mode, but YT has made it portrait :confused:

Here is a photo:
Click to expand...
You have a large black wire going from between the two transformers, wrapping around the left transformer and going to the back of the amplifier, can you try move it away from the transformer ? It may or may not work but worth a try.
 
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