DC receiver - AF chain

The application is DC receivers, the target load is headphones with L/R connected in parallel.
It uses 2N3904/3906 only, can work from 7..12V (with the regulator installed) or 5V (regulator bypassed), or even from a single Li-Ion cell (with reduced max output and sound quality). If not the onboard regulator or a battery, the supply must be well filtered. Straight from USB is likely to be noisy.
The total gain is about 40..50dB, similar to LM386. But there is no intrinsic hiss, and the LPF for the DC receiver is built in (no bulky inductors required).
The idea of Sallen-Key filter using directly coupled BJTs is from Ashhar VU2ESE.
The input impedance is around 1k, not optimal for loading a diode ring mixer.