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Balanced audio output : opamp configuration
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ratatax:
Hello

For the sound generator I design I have a PCM5102A DAC from Texas Instruments which outputs the sound, however it's a bit weak for line-level and i'd like to have the output as a balanced TRS jack (complementary/reversed signals) for use in studio conditions.

So we decided to use opamps to do the trick. I had someone giving me this schematic but he hadn't explained much about it, so as a developer (NOT an electronical engineer!) I tried to understand it and while I think it would work, I don't get the point of its relative complexity compared to the goal. I don't get the point of the first inverting amp, and why the final signal inversion is done with opamps in series instead of parallel.

If I follow the schematic it seems to do the job : the Tip of the jack will receive the original (inverted two times) amplified signal, while the Ring will get the reversed signal (inverted three times).

I feel it could be simplified with only two opamps instead of four : inject the DAC signal in two parallel opamps which both provide a gain, one in non-inverting mode the other in inverting mode.
However maybe I'm in a noob trap here, -- I struggle a bit understanding impedance-related things, maybe i'm missing something.

What do you guys think about it ?


darrellg:

--- Quote from: ratatax on May 16, 2019, 09:25:25 pm ---For the sound generator I design I have a PCM5102A DAC from Texas Instruments which outputs the sound, however it's a bit weak for line-level and i'd like to have the output as a balanced TRS jack (complementary/reversed signals) for use in studio conditions.

--- End quote ---

That's not what balanced means. That's the definition for differential signaling. A balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. You can have a balanced line without a differential signal.
DaJMasta:
Two parallel amps may be fine, but unless you're using matched resistors or very tight tolerance ones, their gains will be slightly different basically always, so keeping it single ended then converting to differential minimizes the independent drift/variation of two channels somewhat.


If it were me, I'd lay it out differently, there seems to be a fair bit of extra parts in this design.  First stage would be a unity gain inverting amp (like the output stages right now) but with the potentiometer in series with the feedback resistor, then a small capacitor in parallel with the pair of feedback resistors to take out high frequency noise above the audio band.  That would give you plenty of gain, and depending on the input impedance of your target device, you could either have a single inverting unity gain amp to give you the inverse signal and just the output of the gain stage to drive the negative differential output directly (half the number of opamps, many fewer parts), or, if you need to be able to source some current, you could have an inverting unity gain amp and a follower amp after the gain stage (3 opamps, but now each differential output stage can drive the full current rated for the opamp).  If running a single supply instead of a split supply, then you'd also want a cap on the output to remove the DC bias, and the resistor on the output probably doesn't need to be so high unless you want full protection from a constant short on the output.
oPossum:
http://www.thatcorp.com/1600-series_Balanced_Line_Driver_ICs.shtml
Someone:
There is a fairly comprehensive coverage of the topic here:
http://sound.whsites.net/articles/balanced-io.htm
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