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Balanced -> Unbalanced and CMRR

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Peter Kowalczyk:
I have a situation where I need to take an unbalanced -10dBu signal and send it across a building to another unbalanced input.  250' or less, probably more like 150'. 

I had planned to use a passive Summing / Balancing box at the source end, and another Balanced -> Unbalanced device at the receiver end.  The Audio Control BLX-10 transciever looks like it would do the job, but it doesn't appear to be available anywhere. 

It occurred to me that I could just take one leg of the balanced signal at the receiver, but then I realized that this would negate any possible CMRR benefit from using a balanced line. 

Then I started wondering if ANY *passive* balanced -> unbalanced receiver would give a CMRR benefit.  My instinct says that it wouldn't; you'd need an active receiver for that. 

What would you use?

Mike Caldwell:
Look at Radio Design Labs aka RDL.
They have a quite a few unbalanced to balanced interfaces/converters both passive and active and
balanced to unbalanced interfaces.

I would look at using an active unit at the sending end and a passive unit at the receiving end.

Keith Broughton:
I have done something similar with a Rapco Isobox at each end.
It's a transformer so you maintain CMMR over the long distance run and just wire the in and out sides for an unbalanced line. (short pin 3 to pin 1 and use 1 and 2)

John Roberts {JR}:
+1 transformers

JR

Ivan Beaver:
One thing you need to be a little concerned with is the level.  Small transformers will saturate with higher levels.

Look for a +4 dBu transformer and you should be fine.

I used to have a pile of old Altec octal plug in transfomers that I have used in all kinds of situations regarding unbalanced to balanced and vice versa configurations.  They always did the job.

Peavey used to make them also for the balanced inputs into the older amplifiers.

Just don't use a mic level transformer.

Transformers are also good, because the provide physical isolation.  A VERY GOOD thing.

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