Isolating Transformer

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Cressy Snr
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#16 Re: Isolating Transformer

Post by Cressy Snr »

This isolating transformer for hi-fi use is not a new thing. Paul B was talking about it years ago but I don't know whether he ever got round to doing one. I think he was on about the theory of winding an isolating TX with a gapped core to handle DC on the mains and also designing in deliberately poor HF coupling, in an attempt to keep HF hash out of the isolated side.

The big medical one I'm using has an earthed screen between the primary and secondary, to ground out the hash from the supply side. One useful side effect of using isolating transformers is that there can be no DC on the primaries of the mains transformers in the equipment on the isolated side. The slight transformer buzz I had on the amps has been eliminated and there are no earth loops possible.

The drop in the system noise floor, as I said earlier is palpable; the silence utter and the music seems to have had a wash and brush-up. Now the single ended vs push-pull debate will move to single ended isolator or push-pull BMU. I'll get a comparison set up next week, with our Ant's BMU.
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RhythMick
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#17 Re: Isolating Transformer

Post by RhythMick »

Ok thanks. Will pop over sometime.
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Nick
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#18 Re: Isolating Transformer

Post by Nick »

One useful side effect of using isolating transformers is that there can be no DC on the primaries of the mains transformers in the equipment on the isolated side.
Not sure that’s entirely true. DC is the result of the +ve going signal not being equal to the -ve going one, so you can still have a net DC imbalance after the transformer. It doesn't get turned magically into a perfectly equal sine. It does however mean that the neutral is at the "center" position of the sine, which may not be the case on the primary side.

The above is not so much me disagreeing, just thinking out loud,
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#19 Re: Isolating Transformer

Post by IslandPink »

I'm just trying to understand how it's safe - if you touch something that's HT in the amp, does it just go to 0V and hence the isolator secondary just flaps up and down in potential at 50Hz ?
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Cressy Snr
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#20 Re: Isolating Transformer Dont do it!

Post by Cressy Snr »

(fig 4) Using PME (unbalanced) power with an isolating transformer:
Image
Bridging between either terminal and ground is safe because there is no circuit through which the current can flow.

It doesn't absolve you from taking proper precautions with the high voltage DC in the amp itself and I never suggested for one second that it did.

Also if our friend in the pic were to grab both ends of the transformer secondary, he would be just as dead as he would be if he had grabbed the live and neutral on the primary side.

This from Sound on Sound magazine.

"A much safer, much more familiar, and more accessible alternative to a balanced mains supply — but one that provides exactly the same benefits of reduced ground noise — is to use a mains-isolating transformer on standard unbalanced (PME) mains for each audio device that is susceptible to earth line noise (see figure 4). The working principle is that the output side of the mains transformer 'floats' — there is no earth connection at all — and you cannot, therefore, get a lethal shock off the equipment powered from it under any circumstances. Equally, there can be no leakage currents to or from the earth if there is no connection

EDIT just found this in the SOS article. I've now abandoned the project and I suggest nobody else tries it.
Continued from above
" However, it is critically important that you connect only one piece of equipment to an isolation transformer. Connecting multiple devices completely negates the safety aspect, and a fault in one item will render all connected equipment live!"

That was close. Only the old wireless is connected to it now.
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Cressy Snr
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#21 Don't do it FFS !

Post by Cressy Snr »

Glad you brought that up Mark!
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Nick
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#22 Re: Isolating Transformer

Post by Nick »

However, it is critically important that you connect only one piece of equipment to an isolation transformer. Connecting multiple devices completely negates the safety aspect, and a fault in one item will render all connected equipment live!
I think that needs a bit more reading. What isn’t drawn in the diagram is where the PE of the equipment is connected. As long as the protective earth is connected to household earth (or they are class II devices), then the isolating transformer doesn’t make them any more prone to danger (it would seem to me).

BTW, whats also missing from your diagram is the inter winding capacitance of the transformer. Unless it was specifically would as a safety isolating transformer with dual bobbins, then that capacitance would make me not want to test the level of the isolation.
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#23 Re: Isolating Transformer

Post by IslandPink »

I was just trying to work out where anything would be earthed , it seems the casework would have to be connected to normal mains earth otherwise there'd always be a possibility the case could go live. The speakers outputs are isolated again by the output transformer, so that's more or less safe. Hence if you have a PE to the casework as Nick says then it's not necessary to abandon this.
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Cressy Snr
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#24 Re: Isolating Transformer

Post by Cressy Snr »

Thanks chaps. I'll just connect the earth through to the mains side to give continuity all the way.

I'm absolutely paranoid about safety and thus as soon as I saw that problem, I went to the "if in doubt dont!" option instantly until the thing could be clarified one way or the other.
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#25 Re: Isolating Transformer

Post by Nick »

it seems the casework would have to be connected to normal mains earth otherwise there'd always be a possibility the case could go live
Well, assuming the casework was not earthed, then with a single device (and ignoring leakage current) there is no way the case could go "live" as there is no current path from the secondary live or neutral to ground. Its why a site isolation transformer is used. You should be able to jump into a swimming pool holding a power tool and the transformer will keep you safe.
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Cressy Snr
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#26 Re: Isolating Transformer

Post by Cressy Snr »

We're back up and running again, with the earth pins of all output sockets connected through to the mains earth.
Music once again beautifully clean and equipment transformers nice and quiet.
Last edited by Cressy Snr on Sat Aug 04, 2018 10:26 pm, edited 1 time in total.
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#27 Re: Isolating Transformer

Post by pre65 »

Still sound better ?
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#28 Re: Isolating Transformer

Post by Cressy Snr »

pre65 wrote: Sat Aug 04, 2018 10:24 pm Still sound better ?
Yes it does. The improvement is not reduced in magnitude, but there was no way I was going to continue with this project without having cleared up the "multiple device" issue no matter how good the system may have sounded.
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pre65
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#29 Re: Isolating Transformer

Post by pre65 »

I'll try modifying my lash up BMU and see if it makes any difference.

As a BMU it did not seem to make any difference here, but then my mains is quite good. Nothing in the locality to pollute the supply. :)
The only thing necessary for the triumph of evil is for good men to do nothing.

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Cressy Snr
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#30 Re: Isolating Transformer

Post by Cressy Snr »

pre65 wrote: Sat Aug 04, 2018 10:32 pm I'll try modifying my lash up BMU and see if it makes any difference.

As a BMU it did not seem to make any difference here, but then my mains is quite good. Nothing in the locality to pollute the supply. :)
I'm not so lucky, my mains is crap! The shop on the corner of our street has two aircon units and every time they start up, the blasted transformers in my system devices start buzzing. In this weather their aircon units are running non stop.
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