I’ve gotten busy again but had an hour to put together a proper audio amp circuit using the TL972IP, +12v on a single supply rail, and 5x gain.
Below is the result of the QA400 THD and THD+N test I ran.
In the previous post I noted that under full listening audio gain conditions, producing about 500mV output with a signal, the noise floor seemed to be in the range of about -140 dBFS. In this case you can see with a bit more gain, the noise floor raises to about -135 dBFS, but the THD at full gain is only 0.0011% and with noise contribution, around 0.0246%. Further, the dynamic range is about 99 dBFS which is quite good.
My final step will be to try this circuit in place of a LM386 to compare the relative quiet of the background noise using a TL971 single Op Amp or TL972 dual Op Amp circuit with earbuds as well as an external speaker.