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Old 2nd Mar 2021, 3:35 pm   #1
Julesomega
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Default 600Ω isolation transformer

Why would you want an isolation transformer when a 600Ω line is or should be already balanced? Just to be sure? For particularly noisy and hummy environments? Maybe a Mobile Control Room would use 1 or 2 (or 21 )
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Old 2nd Mar 2021, 3:49 pm   #2
crackle
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Default Re: 600Ω isolation transformer

Electrical isolation of 2 circuits.
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Old 2nd Mar 2021, 4:32 pm   #3
barrymagrec
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Default Re: 600Ω isolation transformer

GPO used to be very keen on lots of isolation if you wanted to connect to their lines and would insist on approved equipment only.

This transformer could possibly have been used in such equipment.
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Old 2nd Mar 2021, 5:23 pm   #4
Julesomega
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Default Re: 600Ω isolation transformer

Thanks - mains isolation might be another requirement. For isolation from a GPO line I suppose the side without the centre tap would go to the line, while the CT could be used on the equipment side
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Old 2nd Mar 2021, 5:43 pm   #5
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Default Re: 600Ω isolation transformer

Avoidance of earth loops. Balanced and floating circuits are very common in professional studios. Otherwise, imagine the potential for hum pickup!

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Old 3rd Mar 2021, 10:22 am   #6
Valvepower
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Default Re: 600Ω isolation transformer

Hi,

I recently used a pair of 1:1 600Ohm isolating transformers to isolate digital and analogue environments.

Due to COVID restrictions a BBC local radio presenter needed to pre-record from home, and I helped - doing my bit for the pandemic - this presenter to setup a home studio. The problem was he wanted to play vinyl records as he does a “Vinyl” based show, so I ended up using an old school analogue desk to mix down the turntables, Mic and other sources as it was a case of doing it all with equipment, turntables and bits 'n' pieces which I had to hand and lying around the workshop. The problem was we were getting digital noise coming down from the playout laptop via the USB connected ADC.

I fitted an isolation transformer between the output of the desk + monitor switching and the analogue input to the ADC and this stopped the noise in its tracks

I did try a proprietary brand USB isolator gadget between the lap top and ADC, but a good ‘ole transformer did a far better job.

A pair of these transformers would have been useful

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Old 3rd Mar 2021, 10:58 am   #7
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Default Re: 600Ω isolation transformer

Quote:
Originally Posted by Julesomega View Post
For isolation from a GPO line I suppose the side without the centre tap would go to the line, while the CT could be used on the equipment side
Other way around, Jules. The GPO input side carrying programme balanced WRT earth to the transmitter (say) would go to the centre-tap side, then any magnetic interference on the line would send current the same way along each leg of the pair to earth via the centre-tap, and so cancel. The secondary side might or might not be tapped to earth on one leg.
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Old 3rd Mar 2021, 11:55 am   #8
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Default Re: 600Ω isolation transformer

Thanks for the updates. This transformer is now on its way to the BBC MCR21 outside broadcast van restoration project where they are looking for a few more isolation transformers
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Old 3rd Mar 2021, 10:19 pm   #9
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Default Re: 600Ω isolation transformer

HI all

The transformers are for a unit in MCR21 called the Talkback Junction Box, that we will have to recreate. It has 5 600:600 transformers with centre tapped secondariys or primarys depending on which way you are going.

The JB is not very clever just transformers and relays, a sort of "all-come-good" box. In the attached partial circuit diagram you can see the CT is used for DC signalling and also to phantom an extra audio circuit as the circuits are balanced the principle circuit does not suffer any interaction with the signals injected in to the balance points.

In this use the circuits go off to an additional external camera control engineer Via several XLR cables.

I am working towards scanning all the handbooks for the MCR, progress to date here :- http://mcr21.org.uk/technical-handbooks-slug/ much still to do.

I have now located the 5 transformers needed, but at least 3 are untested so a spare would be good.



Thanks Jules.
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