Simple pure sine-wave test source...

[Nick]

How good are modern audio DACs? This seems pretty good: http://www.ti.com/product/DAC1280

Ummm. The subject line contains the word "simple"

There are obviously ways to get a purer sine wave - the DAC1280 is a 24-bit device with THD+N down at -120dBc, which is stupidly low. It also costs USD 25 just for the chip, let alone all the stuff round it.

But you need astonishingly good design & layout to achieve those figures, plus a CPU and a bunch of other stuff, plus software to synthesise the sine wave too. All do-able, but none of it... simple... or cheap.