Basic transformer debalancing question

[ej_whyte]

Just a quick question to clear this up in my head, I don't have anything to test here with me. When de-balancing an input with a 1:1 transformer, is there still a 6dB loss due to single-ended vs. differential?

A RaneNote on unity gain/balancing I found says the following about output transformers, but nothing about input, however I would assume it is the same?

"...because the output signal develops across the secondary winding, it does not matter whether one side is grounded or not. Grounding one side gives the same 2 volts output. Only this time it references to ground instead of floating. There is no gain change between balanced and unbalanced operation of output transformers."

Cheers

 

[bruce0]

Not sure exactly what you are asking, but here is what I understood about the 50% loss (6db loss) when "debalancing".

If you have an electronically balanced output and you ground one side, then one amplifier is basically going to ground the other is still generating signal, so in that case you have half the output, thus the 6db loss.  (electronically balanced outputs usually are two opamps generating the same signal 180 degrees out of phase)

If you have a transformer there on output, assuming no change on the primary side, and you ground one side of the output, the ungrounded end of the winding is still generating signal, and the signal induced into the winding by the primary side is unchanged, so the signal will be unchanged (although instead of a potentially symmetrical signal that might have existed, all the waveform (but not all the noise...doing this will probably make the signal more susceptible to noise... see the jensen site) will now be on one conductor.  But that signal will have twice the amplitude (so there is no 6db loss).

You are referring to an input situation, so :

assuming that you have an input transformer fed by an output transformer... you should get no 6db loss.

if you are feeding the input transformer from an electronically balanced output, and you ground one side of the primary on the input transformer, that is similar to grounding one side of the output and you will get the 6dB loss

I am not sure of this, but that's the way I think it works.

Sketch out a schematic and your question will be easier for smarter guys than I to answer.  I was wondering when you said debalancing if you were taking a balanced signal and putting it in an unbalanced input with a transformer... which would not have the 6 db loss.

 

[MagnetoSound]

It is the same. 1:1 gives the same voltage into the amp. The loss would only occur if you were to ground one side of an active differential input.

 

[ej_whyte]

Bruce, By input I was meaning balanced signal to both sides of the primary, ground one side of the secondary, typical 312-style configuration.
Thanks for the quick response Magneto.

Cheers

 

[abbey road d enfer]

It is the same. 1:1 gives the same voltage into the amp. The loss would only occur if you were to ground one side of an active differential input.

No. You may loose 6dB (or less, or more...) if you forget to connect the - leg of an electronically-balanced input to the -leg of the source that drives it. If the source is unbalanced, the - leg of the input must be connected to ground.
You can ground either leg in a fully-balanced configuration without loss.

 

[MagnetoSound]

You can ground either leg in a fully-balanced configuration without loss.

Oops, yes, you are right ... still, there is no loss in grounding after the transformer.