Shielded Power Transformers

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JamieOxford

Member
Joined
May 7, 2017
Messages
24
Hello!

Wondering if someone could chime in on whether shielding a power transformer is likely to reduce hum on nearby equipment?

I have a pair of EQs racked in a 1U case with a toroidal transformer in as well. There is a slight amount of hum from the transformer and I'm interested to know if this can be reduced by covering with a metal case? I can't feasibly mount it any further away from the EQ cards in the case.

I also have a Juno 6 which produces hum on the Rhodes that it sits on top of. I've tried some basic foil shielding under the power transformer with little effect. It looks as though the 100V transformers Roland used in their domestic Junos had full metal covers, where as the export 240v models have a plastic bobbin with only partial metal surround!

Same question for the Juno - could a replacement transformer, or suitable transformer cover reduce/remove the hum picked up on the Rhodes?

Thanks!

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JamieOxford said:
There is a slight amount of hum from the transformer and I'm interested to know if this can be reduced by covering with a metal case?
Not really. It may help a little if the metal has magnetic shielding properties like "mu-metal" or even something vaguely thick that has iron in it. But grounded metal shields are only really good at blocking electroSTATIC radiation. They are not so good at electroMAGNETIC shielding. Power transformers are pumping current which is going to generate magnetic fields and thus it is electromagnetic radiation and therefore a metal shield won't help so much. Certainly not copper tape.

There are other ways to reduce power supply hum that are more effective anyway. Toroidal transformers have a smaller magnetic field footprint and if you put them physically far away from any high impedance high gain circuitry, that can significantly reduce hum.

Using an external SMPS with chokes to filter out HF noise will probably be even better.

But at some point the hum will still remain because magnetic radiation from mains circuits it is all around us. Just putting a rack next to a wall that has a mains wire in can induce noise in gear that could be on a completely different mains circuit.

If you get mains hum down low enough and the gear has decent dynamic range (meaning not some -10dBu consumer thing) and you're actually using it (meaning put as much signal into it that it can handle before clipping), then you'll never hear the mains hum in which case it doesn't really matter.

Also, low-cut early and often. Low cut at the mic, the mic pre, ... That will not only reduce mains hum but also allow you to squeeze more signal into your downstream gear without clipping.
 
I had a choke in a power supply of mine for a while , its small but with a good quality mu metal sheild , no matter what I did it picked up hum from the nearby mains tx . In the end I moved it to the signal chassis ,now hum is non existant . I noticed in some old tape decks I dismantled that a special metal plate was placed under the transformer where it bolted down to chassis , I can only assume this was to prevent hum currents entering the metal work of the chassis ,cassette player transformers do tend to be much better screened than most transformers in domestic equipment . I suppose the benchmark could be the Telefunken V series , mains tx is very well screened and in close proximity to signal transformers , of course theres several thick metal sheilds between mains and signal transformers , shows its do-able ,but its a big engineering challenge . 

Just thought it would be worth adding , sometimes placing an output transformer in the shaddow of a choke can help with respect of mains hum coupling  , orientation of the core is  all important ,ideally somewhere about 90 degrees will give minimum ,two cores in the same plane and with the lams going the same direction will give a worst case scenario , monitoring the hum output of a circuit while physically moving things around will reveal a minimum hum position ,but remember its all relative to the position of everything else , a well sheilded smoothing choke on the signal chassis right next to the anode load is the best Ive been able to do , the inherent resistances noises in the circuit tottaly cover up hum , thats a simple single stage , once you have multiple gain stages ,you run into phasing of the hum signals and sum and difference , that can be used to good effect ,as it is in some older guitar amps ,thing is it tends to vary depending on the position of the controls , often the hum dinger is actually contributing to noise ,but if your carefull it can at least partially mask noise from another point in the circuit .
 
JamieOxford said:
Wondering if someone could chime in on whether shielding a power transformer is likely to reduce hum on nearby equipment?
Yes! However it takes a significant amount of metal to shield efficiently. I used to sell such shields, that were made of 2 plates of #16 steel (1.5mm) separated by 20mm of air.

I also have a Juno 6 which produces hum on the Rhodes that it sits on top of. I've tried some basic foil shielding under the power transformer with little effect.
Aluminium is utterly useless with magnetic interference. You may have better results with a heavy steel plate on the bottom of the Rhodes. Ideally two plates separated by an insulating material (wood, plastic, cardboard) is more efficient.

Same question for the Juno - could a replacement transformer, or suitable transformer cover reduce/remove the hum picked up on the Rhodes?
If you could find a toroidal transformer suitable for the job, that would be ideal, but I doubt you can find one off-the-shelf.
 
As other replays  have stated.

The Copper foil is not going to help as it will only help reduce the ELECTRIC FIELD and has no effect on the MAGNETIC FIELDS. The MAGNETIC FIELDS can be reduced by increasing the SPACING, full STEEL jacket or reducing the FLUX field by adding more turn on the transformer, lowering the AC input voltage, adding more core laminations or getting a NEW DESIGNED TRANSFORMER.

The 100 Volt transformers running on 120 volt line will saturate the core and make BIG MAGNETIC FIELDS.

Adding a steel plate between the products will help as many devices are Aluminum chassis and they stop ELECTRIC FIELDS and pass MAGNETIC FIELDS.
I hope this helps you.
Duke
 
If the hum is actually induced magnetically then it might be more productive to identify which component it is getting into and then shield that. Does you EQ have inductors in it? Do you have unshielded input or output transformers?#

Cheers

ian
 
boji said:
May I ask, what was the purpose of the air gap?
Flux lines are deflected when they meet a transitional barrier, which makes the assembly more efficient than a single plate of double thickness. It does not change the efficiency for flux lines that are normal to the plate, but it makes a difference for those that come at an angle.
 
the less flux, the less radiation, use a pwr xfmr with a big core and many primary turns,  flux goes down to 4 KG and things get quiet,

ever wonder why those Marantz power transformers were so big?

or HP voltmeters and signal generators?

look at this, pwr xfmr right next to RF front end, big core concentrates field inside xfmr>
 

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makes a difference for those that come at an angle.
Interesting. Hope it's ok I ask another question about it.  (terribly naive on the mech. of electromagnetism) Do the flux lines kinda get burdened with herding electron's in the metal plate across the parallel plane like a surfboard?

Edit: I guess I'm asking some fairly basic 'google it dummy' question about what magnetism does to electrons in a plate of metal that isn't grounded...

So cool! =
https://www.youtube.com/watch?v=pCON4zfMzjU
(Electromagnetic Levitation Quadcopter)
 
boji said:
Interesting. Hope it's ok I ask another question about it.  (terribly naive on the mech. of electromagnetism) Do the flux lines kinda get burdened with herding electron's in the metal plate across the parallel plane like a surfboard?

In a way yes. It is much the same effect as is taken advantage of in an induction hob (but that is not the mechanism that screens the magnetic filed)

Cheers

Ian
 

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