guypettigrew

Balanced mains transformer

56 posts in this topic

A couple or so weeks ago I bought a 3kVA balanced mains transformer to power all my Hi Fi.

It has significantly improved the sound, but this is the technical forum, so let's press on !!

Two queries, please.

Firstly. The transformer has a slight hum. I'm assuming it's the primary, as it hums even when there is nothing on the secondary. The transformer is a (huge !) toroidal. The hum goes up and down in volume at different times of the day, although there is no pattern.

What causes the hum, and why does it vary?

Secondly. The transformer puts out a balanced supply. 120 volts on both the live and the neutral. Do the transformers in the various bits of the Hi Fi then also put out a balanced supply? Or do they unbalance it?

Thanks.

Guy

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I don't quite get this balanced supply business with mains supplies

Voltage is potential difference so -120v to +120v is a potential difference of 240v which is all the power supply is looking for so no difference from 0v to 240v

I understand transformers on building sites are 110v centre tapped to earth for safety reasons i.e -55v to +55v but they still supply 110v.

Am I missing something?

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I am also struggling with what exactly the OP means by "balanced transformer". Further detail would be helpful.

As regards the hum, this is most likely caused by magnetostriction and will be at 100Hz. (See: http://en.wikipedia.org/wiki/Magnetostriction)

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If it helps, it's one of these. http://www.airlinktransformers.com/transformer/bp3120-bp3120-balanced-power-supply.asp

Thanks for the suggestion the hum could be 100Hz. I looked up some test tones but, on my very low level computer sound system, it's impossible to hear them properly.

Oddly, the transformer is almost silent this morning. It seems to be getting quieter every day. Long may it continue!

Guy

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OK. This says that it is centre tap to earth, meaning that it is like a 110V tool transformer but with higher voltages. What possible advantage can it provide? All I can see is a safety issue because there is no fusing in the (formerly) neutral line of any connected equipment. Any fault causing excessive current draw on this line will not be protected by a suitable fuse, meaning that the cable could catch fire.

However, simply fusing the neutral line is itself not without risks: With two fuses, one could go leading to the impression that the equipment is dead when there are in fact hazardous voltages there. Note also that an isolation transformer of this type also negates the protection of any RCD in your consumer unit, again exposing you to a potential electric shock risk.

Overall, my advice would be that the use of such a device in a domestic environment is highly inadvisable.

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Hum may be magnetostriction, but as it's a toroidal, it could also be DC on the mains. High power toroidals have very low DC resistance primaries,and so the core can saturate with very low voltages of DC, less than a volt will do it. This then will cause mechanical noise as the AC superimposed on the small amount of DC takes the core in and out of saturation.

As to the sonic benefit of balanced mains, I also can't see why. Balanced mains has safety benefits, but considering that all connected equipment will be wired for unbalanced mains, the mains balance will be gone as soon as anything is connected. Not that it would make any difference, as far as I can tell.

S.

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Hum may be magnetostriction, but as it's a toroidal, it could also be DC on the mains. High power toroidals have very low DC resistance primaries,and so the core can saturate with very low voltages of DC, less than a volt will do it. This then will cause mechanical noise as the AC superimposed on the small amount of DC takes the core in and out of saturation.

At this rating, it is unlikely to be a toridal. The illustration also strongly indicates a conventional 'E' 'I' laminated core device.

As to the sonic benefit of balanced mains, I also can't see why. Balanced mains has safety benefits, but considering that all connected equipment will be wired for unbalanced mains, the mains balance will be gone as soon as anything is connected. Not that it would make any difference, as far as I can tell.

Agreed. As per my previous post, it can also be dangerous. My advice to the OP would be to STOP USING IT IMMEDIATELY!!!!!!

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Hi Chris217

It is a toroidal, and a flippin' huge one !!

Of course it can be dangerous. So can any electrical supply if you stick your fingers in while it's live. As to whether a huge current draw on the neutral line would occur, I'm unsure. From your comments I'm assuming you're a qualified electrician, otherwise you wouldn't be able to offer such definite advice.

Let's assume the neutral, at 120v after the balanced transformer has done its work, shorted to earth inside one of the Hi Fi components. A worst case scenario. Would that cause the cable to burn up, or would some other mechanism shut the whole thing down? Would the fuse on the primary supply side pop? I don't know, but perhaps someone else here can tell us.

What I do know is these are sold commercially, must meet safety standards and should, surely, be safe. If they aren't, then how can they be sold? Perhaps a neutral short to earth would overheat the secondary, which would then trigger the circuit breaker.

Thanks for the thoughts about DC on the mains, Serge. Perhaps that's why it's quieter at different times of the day?

As to why it makes the system sound better, no idea, but this isn't the forum to open that can of worms !!

Guy

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Been thinking about this !!

If the neutral from the secondary, at 120v, shorts to earth, then we'll have 120v on the earth at the consumer unit. This will, presumably, trip the RCD instantly. Then all is well and safe.

So, for the sake of this discussion, let's say the board is not RCD protected. What happens if there's 120v on the earth? Does the sub station recognise it and do something?

Anybody any ideas?

Guy

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Been thinking about this !!

If the neutral from the secondary, at 120v, shorts to earth, then we'll have 120v on the earth at the consumer unit. This will, presumably, trip the RCD instantly. Then all is well and safe.

So, for the sake of this discussion, let's say the board is not RCD protected. What happens if there's 120v on the earth? Does the sub station recognise it and do something?

Anybody any ideas?

Guy

Death is always an option.

The point is, you clearly don't understand the implications and you are compromising safety. And that is dumb.

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Are you saying the supliers of the transformer are selling an inherently dangerous item, Mr Coco? That seems totally unlikely.

They sell a range of balanced power supplies, have a look here. http://www.airlinktransformers.com/balanced-power-supplies.asp

No way can these be unsafe.

Where is the safety compromise?

Guy

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Been thinking about this !!

If the neutral from the secondary, at 120v, shorts to earth, then we'll have 120v on the earth at the consumer unit. This will, presumably, trip the RCD instantly. Then all is well and safe.

So, for the sake of this discussion, let's say the board is not RCD protected. What happens if there's 120v on the earth? Does the sub station recognise it and do something?

Anybody any ideas?

Guy

I've just fixed a fault where a house had no mains... However, live was at 246V with respect to earth, neutral was at 112V to live and 57V to earth.

The whole installation was a 3 phase distribution protected by a 100mA RCD, which didn't trip. Local 3 phase mains transformer didn't care either way - the 100A incoming supply fuse would have had to blow before anything would even think of happening there.

Turned out the fault was a neutral cable in the main distribution board had been rubbing on an uncut live busbar. The insulation had been reduced enough to put a voltage on the neutral, but the resistance was still high enough not to allow enough current through to trip the main rcd. But would have still given someone a serious belt if they'd touched it!

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Are you saying the supliers of the transformer are selling an inherently dangerous item, Mr Coco? That seems totally unlikely.

They sell a range of balanced power supplies, have a look here. http://www.airlinktransformers.com/balanced-power-supplies.asp

No way can these be unsafe.

Where is the safety compromise?

Guy

If you have to ask, you probably shouldn't be using it.

Electrical safety is designed in layers, from the equipment fuse, through the plug top fuses to th MCB/RCD in the consumer unit. These measures are designed to remove power to the device in the event of a fault for two reasons - to prevent shock hazard and fire. I think you also misunderstand what an RCD does. An RCD is designed to trip if a (small) current imbalance is detected between line and neutral conductors, e.g. if there is a (low current) leakage to earth via a person. An MCB is for overload (high current) protection and more or less a replacement for a fuse, e.g. a short to ground causing a large fault current will trip the MCB. Modern domestic consumer units use RCBOs which is basically a combination of RCD and MCB.

These safety measures are all designed with the assumption of only one high voltage conductor (line) with the other at 0V (neutral). The basis for this being that it's easier to cut one conductor in the event of a fault, allowing for complete removal of high voltage to the device.

What balanced power does is introduce two high voltage conductors and this makes it far, far more difficult to completely remove power in the event of a fault. Simply adding a fuse in the neutral will not do, one fuse will always go before the other, leaving the possibility that even if one fuse blows there is still high voltage at the device, allowing for the possibility of electric shock or fire. This makes it inherently more hazardous. An RCD or MCB may or may not trip, depending on the nature of the fault, so you should never rely on only one safety mechanism - that's why there are several of them. Defeating some of them, even by accident seems foolish.

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If you have to ask, you probably shouldn't be using it.

I ask because I don't know the answer, Mr Coco. I also don't understand how a car engine works, or how my TV works, not in any detail. Doesn't mean I can't use them !!

The transformer is suppied as an in-line mains balanced transformer, from a reputable company. I have every reason, therefore, to assume it is safe for use.

So, are you saying balanced mains transformers are inherently unsafe and shouldn't be sold? Or are you saying there are additional safety precautions I can, and should, put in place? If it's the latter, then I'm very happy to add as many safety mechanisms are are neccessary.

Guy

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I ask because I don't know the answer, Mr Coco. I also don't understand how a car engine works, or how my TV works, not in any detail. Doesn't mean I can't use them !!The transformer is suppied as an in-line mains balanced transformer, from a reputable company. I have every reason, therefore, to assume it is safe for use.So, are you saying balanced mains transformers are inherently unsafe and shouldn't be sold? Or are you saying there are additional safety precautions I can, and should, put in place? If it's the latter, then I'm very happy to add as many safety mechanisms are are neccessary.Guy
What a completely spurious analogy. You have passed a test to drive a car, and your TV is unlikely to kill you unless to forcibly try to insert it into your anus.

I can buy tree felling equipment from several reputable companies, but I would be an idiot to try and use it unless I had a clue what I was doing.

Do you even know what the intended application is for this thing?

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