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| Balanced audio output : opamp configuration |
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| ratatax:
Thanks all for your answers ! I understand it a bit more now... maybe i'll keep the four opamp stages (just tweaking the values if needed after testing) since while sourcing the components I've noticed that that the quad version isn't much more expensive compared to the dual (like $ 0.2 vs 0.25) so we won't gain much by optimizing it except if we remove half of the opamps, which doesn't seems to be possible without affecting precision and balance if I understand correctly. |
| chris_leyson:
LTC1992 is another option if you want to run everything from a single 5V supply. Rail to rail input and output with fixed gain of 1, 2, 5 and 10 or adjustable gain. |
| Marco:
--- Quote from: Someone on May 17, 2019, 12:39:27 am ---There is a fairly comprehensive coverage of the topic here: http://sound.whsites.net/articles/balanced-io.htm --- End quote --- I think it's misleading that he says the ground compensated circuit has no common mode rejection, it's more accurate to say it introduces next to no common mode error to begin with. It will blow everything else out of the water in that regard. |
| David Hess:
--- Quote from: bson on May 17, 2019, 04:17:36 am ---Don't cascade the bridge halves, just pair an inverting and non-inverting amplifier. --- End quote --- The reason two cascaded inverters are actually not that bad is that an inverter and a follower have different noise gain and inverters have less distortion than followers which suffer from common mode effects. An added difficulty is that your inverter and follower example presents different impedances to the operational amplifiers further compromising differential performance. Walter Jung gives the higher performance example shown first below which cancels out the differences and the common mode distortion. Note however that this is not a true differential output although it is suitable in most cases. The second example is cross coupled for a true differential output and what THAT does. Another example is shown here: https://www.rane.com/note124.html |
| Marco:
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? On another note, what's even the point of a true differential output? These lines are not properly terminated and even if they were the wavelengths at audio frequencies make keeping currents anti-symmetrical irrelevant, you don't have a fully differential signal chain which you want to keep fully differential either ... so what's the point? 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? |
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