Mains transformer physical hum

[NOON]

I've got an otherwise lovely Murray Amplifiers power amp that physically hums like a *insert humming thing of choice*. It's enough to vibrate the rack it's in and set off other hums and resonances. I can feel a fair amount of physical vibration from all points of the transformer, including the mounting frame, when I touch it with a stick when it's powered up.

Any clues on the best way to dampen down this beastie?

It's a shame that it's unusable at the moment, circuit-wise it's a work of art. (Look up Murray Amplifiers if you haven't heard of them before. They've been used in the Sydney Opera House and all kinds of other highbrow places I'm not allowed into.)

 

[NOON]

Another angle.

 

[NOON]

And the other side

 

[Gus]

Is this amp new or used?

It looks like a toroidal like transformer in the pictures.

An example of a toroidal transformer issue I caused with capacitor changes in an amp. 

I changed all the power supply caps in a tube guitar amp from electros to polypropylene. The amp has solid state diode rectifiers and a toroidal power transformer.

The amp power transformer was fine before the cap change after the cap change the transformer made mechanical noise.

I needed to add a 1ohm power resistor before the first cap to limit the current surges to get rid of the transformer noise.

My best GUESS was it was due to the VERY low ESR of the first poly cap making the mismatch of the two Si diode rectifiers cause a DC imbalance in the core due to higher current turn on spikes. Who matches diodes for a power supply?

Toroidal transformers do not work well with unbalanced DC surges.

Can you measure the power supply caps to find out if one side is much different than the other. Maybe one power supply side has caps that aged differently. I am assuming it is a +- power supply

Have you powered up the transformer alone with the secondary disconnected to check if it has mechanical hum?

 

[RuudNL]

I had this problem years ago.
I have removed the transformer and immersed it in a hardening lacquer.
The lacquer must be able to penetrate between the windings.
In my case this solved the problem.

 

[JohnRoberts]

Transformers hum for different reasons (like eddy currents or even saturation)... I have a cheap transformer inside my microwave oven that hums along without the music depending upon mains power quality.

a) ask the manufacturer, if its a common problem they may have a solution.
b) consider physical isolation.. rubber feet (might help) but I've never seen/done this.
c) toroid transformers reportedly make less mechanical hum, (according to toroid manufacturers) because of the lack of an air gap

Good luck, reportedly load can affect mechanical noise.

JR

 

[Bo Deadly]

Could also be because too much current is being drawn from it such as because the filter caps are shot. Use a scope to check the filter caps for excessive ripple. If there are some smallish dropper resistors measure their value and then the voltage drop across them and compute current and see if it's vaguely correct. If not, it could be that something is half-shorting like a burned up power transistor.

 

[Gus]

I don't understand the posts after mine, There is an order to troubleshooting and if you can, split the circuit at a point to find what section is failing.

I gave a example of circuit change that caused transformer hum and at the end I posted what I think is the first test to do.
Actually a visual and smell test should be the first.
Disconnect the transformer secondary and power it up to check if it has mechanical hum.

If NOON does this test and it does not hum the next test(if you have a resistor load bank) would be to connect it to a load bank of resistors selected for the current draw and check for mechanical hum.

Then I would check the power supply and amp components and measure voltages and currents.
I would not keep it powered up because maybe something would fail and cause way more damage and if you every repaired power devices when something fails other other parts fail and it often turns into tested every part in the power section.

Maybe the amp is "trying to tell you it is sick from the transformer hum"

 

[e.oelberg]

A main transformer in my korg ms20 hummed. I anyway had to replace some caps in the psu, so I also pulled out the tx and poured some bulb colour into it. Now it is dead quiet, btw I do the same with zizzeling chokes on graphic cards with success.
https://www.amazon.de/Tauchlack-Lampenlack-30ml-orange-violett/dp/B01EWSOITY

 

[JohnRoberts]

Is this amp new or used?

It looks like a toroidal like transformer in the pictures.

Toroids are wound on a circular magnetic core.

JR

An example of a toroidal transformer issue I caused with capacitor changes in an amp. 

I changed all the power supply caps in a tube guitar amp from electros to polypropylene. The amp has solid state diode rectifiers and a toroidal power transformer.

The amp power transformer was fine before the cap change after the cap change the transformer made mechanical noise.

I needed to add a 1ohm power resistor before the first cap to limit the current surges to get rid of the transformer noise.

My best GUESS was it was due to the VERY low ESR of the first poly cap making the mismatch of the two Si diode rectifiers cause a DC imbalance in the core due to higher current turn on spikes. Who matches diodes for a power supply?

Toroidal transformers do not work well with unbalanced DC surges.

Can you measure the power supply caps to find out if one side is much different than the other. Maybe one power supply side has caps that aged differently. I am assuming it is a +- power supply

Have you powered up the transformer alone with the secondary disconnected to check if it has mechanical hum?

 

[Gus]

Toroids are wound on a circular magnetic core.

JR

I posted like

It looks like a C core, they work like a torrid core a little more loss due to the gaps

 

[Audio1Man]

Hi NOON
The power TX is “R CORE” and they use a split bobbin that snaps together and is wound on the core. The bobbin spins and is loose on the core. They typically need wedges to stop the vibration of the bobbin to the core. You may need to check the ripple voltage for symmetry (DC offset).
Duke

 

[Gus]

Audio1Man

Thanks for the R core information.
I assumed it was a C core.
It looks like a R core is more like a torrid then because of no gaps.

 

[Tubetec]

I was a bit late to be able to offer up anything new ,
but  it looks like  a c-core not an R or toroid core to me.
I second Ruud's suggestion of dipping ,but you could try applying a suitable laquer or transformer varnish with a brush and allow gravity to cause it to seep into the gaps in the windings and between the bobbins and core  , maybe as a precaution on the underside of the transformer place something to catch any extra that drips out rather than making a mess . Dipping would require desoldering and dismantling , application with a brush has done a satisfactory job for me on several ocassions. A quick check to make sure all the bolts are torqued down would also be worthwhile.

 

[NOON]

Thanks for the ideas. What comes out of the speakers sounds great and the voltage rails measure up as expected, so I don't think it's a transformer under strain unless there's some DC imbalance. I'll disconnect the secondaries and do some more investigation along those lines.

It's a definite deep hum rather than a buzz like you get from loose windings or brackets if that makes any difference. All mountings seem tight and secure.

Exactly what product would I be looking for as a suitable lacquer or varnish? It's not an area I've had anything to do with (except for hand winding an inverter transformer and dipping it into a vat of something as part of my renewable energy training many years ago!). I'm located in Melbourne, Australia, so will have limited sources compared to Europe and the US.