DACs and the Differential Dilemma: a resolution

Which way?

This is a bit of an overdue followup to my last post, DACs and the Differential Dilemma. I describe here one solution to the standoff between single-ended and differential primacy discussed there.

One way to resolve this issue is to take a single-ended version of the circuit and place another output stage in parallel with the fist but connected with reverse polarity:

Solution to the differential dilemma: Two differential filters in anti-phase.

In this setup, differential outputs are differential, single ended outputs are single-ended, and no path goes through more than one active stage. Simple, and in many ways so obvious!

However, there is a drawback: The differential output will be twice the level of the single-ended version (i.e., 6dB higher). Whether this is a significant issue is up to you, but for most consumer applications, it isn’t. Factor of two differences in RCA and XLR outputs of consumer gear is pretty much the standard these days.

In designing an output stage along these lines, rhere are some potential issues that you’ll want to be careful about — for example balancing the load seen by the DAC IC against the thermal noise added from the resistors. But careful design can resolve these. I’m using this approach in a soon-to-be released commercial product and can confirm that the approach works very well.

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