Quite a lot is left, regarding application circuits and deeper view. Below a comparison of THD vs. output voltage of op-amp swapping in the Topping D10s DAC.
@amirm could repeat the test, if he had an op-amp stock available.
View attachment 441695
There will be many basic circuit where op-amp swapping would make almost no difference, and there will be many circuits where it would make a big measurable difference. Depends if your analysis is deep or shallow.
The distortion amounts for the op amps
are different - the spec sheets even state it outright:
Device | THD + N at 1 kHz (typ) (%): |
OPA2134 | 0.00008 |
NE5532 | 0.002 |
LME4562 | 0.00003 |
I'm personally a bit more interested int he 1 kHz frequency range than 10 kHz, but it's a continuum regardless.
The big key, though: Was the
rest of the amp engineered to take advantage of the op amp's sweet spot, or are you just going for name recognition and pushing it past reasonable bounds to get that last little bit of gain?
With apologies to those who practice amp design more regularly than I: It's been a while since I've looked at power amp designs, but I seem to recall the whole point of the op amp in this case is to let you control the volume?
ie. the oversimplified version: The volume knob is connected to a potentiometer, which is one of the resistors that sets the op amp's (ie. preamp) gain. And that is what lets us increase or decrease strength of signal going to the main power amp stage?
If that's the case, then... yeah. You
absolutely would want to optimize your circuit for less noise, full stop. Moreover, I'd pretty much
not care about the THD when the gain is set so low the output voltage is near zero, because the amount of actual signal to distort is also near zero.
For all of the above (@ 10 kHz) , I'd probably design the preamp circuit to not exceed 0.1 V. Yes, it leaves gain on the table, but it also means owners can change the op amps at will and
nothing would happen. Which, by all appearances, is exactly what Fosi has done.