I'd like to share a circuit which I use as a high-performance headphone-output buffer circuit.
It is intended to be used between a low-impedance circuit (like headphone output of a smartphone) to improve the audio quality. With an input source impedance of say 33 ohm, the output noise voltage in 20kHz is ~ 0.2 uVrms, or 2uVrms with the default input termination of 3.3kohm. The idea is of course not new. It just parallels 4 op-amp circuits (2 duals) for each stereo channel to boost output current drive, and spread the power dissipation over 4 dip packages. It can deliver about 120 mA output per stereo channel and can drive headphones down to even 10 ohm with very good performance. The circuit is very bare-bones ... no input protection, pop-filtering or frequency rolloff etc. Any comments welcome. The noise level is extremely low and the effective en is about 1.5nV/Sqrt(Hz) (proportional to Sqrt(Ncircuits).
Apologies for the use of an LM4562 op-amp but I had those handy.