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RE: "differential bridged output " balanced or not?

Posted by Charles Hansen (M) on September 17, 2017 at 20:30:18

In Reply to: RE: "differential bridged output " balanced or not? posted by Ugly on September 14, 2017 at 21:26:19:

> > I had assumed the OP was using the term "balanced" in the more traditional sense. < <

I disagree with your assertion that a balanced impedance relative to ground was the "traditional sense". Balanced lines were first used soon after electrical recording was invented. Almost always there were transformers used at both ends of a long cable. The transformers were almost always center-tapped to create equal and opposite signals in each phase. The fact that they had the same impedance was simply it was less expensive and easier to use symmetrical cables than asymmetric ones.

Later on they figured out that instead of grounding the center-tap of a transformer that they could use it to transmit DC power from (say) a mixing console to (say) a microphone. This is called "phantom power", as it requires no special cables and allows the clever use of the third conductor. This line is at "ground" from an AC signal perspective, but not from a DC voltage perspective - which is true of all power supplies in all electronic equipment.

Eventually someone figured out that the source only needed to drive one of the lines - as long as there was a network to make the undriven line equal in impedance to the source impedance of the driven line. This saved the cost of a transformer (or balanced circuitry) but added the cost of an impedance matching network at the source. I have only seen this method of "balanced" connection describe in technical papers and never in an actual piece of audio equipment. Now that the cost of an op-amp has dropped to less than a dime, it seems silly to try and save a few cents by creating an impedance matching network. Perhaps when op-amps were $5 and transformers at least that much (up to 10x more, depending on the quality of the transformer), there may have been a few examples made. I've been in audio for decades and never seen one. That doesn't mean they don't exist - just that they are definitely not common.

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