Electronics > Projects, Designs, and Technical Stuff
Balanced audio output : opamp configuration
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Bassman59:

--- Quote from: Marco on May 20, 2019, 06:04:03 pm ---Does that ground compensated circuit actually work? When I simulate it, I actually get vastly increased coupled signals ... just leaving the ground feedback out (by deleting R2) improves matters immensely assuming a differential input on the other end (ie. measuring differential voltage across R10).

Am I missing something?
--- End quote ---

Yes, it does. It certainly worked on the older Soundcraft and other British consoles from the early 80s.


--- Quote ---On another note, what's even the point of a true differential output?
--- End quote ---

Double the output level (6 dB gain) for free. Well, at the cost of a second driver.


--- Quote --- These lines are not properly terminated
--- End quote ---

Audio lines want to see a bridging load impedance, which is why the source driver impedance is ~50 ohms and the load side is ~10 k ohms. You're not dealing with RF so power transfer is not interesting and therefore you don't match impedances. (There's no such thing as a 600-ohm line in modern professional audio.) The point of the bridging impedance is that it minimizes the voltage-divider effect.  So there's no "termination" as one might think about in the RF or digital-signal world.


--- Quote ---  Keeping impedances symmetrical is all that's necessary AFAICS, so a common mode signal impressed on the signal pair stays common mode (a ground loop current is also a common mode signal). Tie one end to ground with a given resistance, the other end to a single ended amplifier with the same resistance, done ... why not?

--- End quote ---

What you describe is what is done on pretty much every mixing console and piece of analog gear for the last, oh, 25 years, when they want to save a few pennies. It's got the unfortunate (and redundant) name "impedance balanced." To separate the lower-tier products from the higher tier, only the latter will have fully-differential outputs, everything else is impedance balanced. As a marketing bullet point, the manufacturer may want to advertise that the output can swing +26 dBu, which you can really only get with differential drive. Not that it matters a whole lot when everything is calibrated for +4 dBu "nominal," but headroom is nice and so is the ability to drive stupidly-long lines from FOH to the amp racks and still have some signal left.
Marco:
So what am I missing? A common mode signal impressed on the lines with identical impedance for the ground balanced circuit gets forced into a non common mode signal AFAICS.
David Hess:
The true differential output circuits like the two cross coupled examples I gave reject common mode changes even before the receiver gets involved.  If one of the outputs is shorted to ground for example, the differential output remains the same instead of being halved.  So asymmetrical effects on the differential pair are rejected by the transmitter.
Jan Audio:
Why would you need the inverted sound ?
@bson : he need to have more gain, use a non inverting amplifier and some amplification for the inverting one.
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