After purchasing orthodynamic headphones with a resistance of 40 ohms, it turned out to be a problem that not every audio output can normally swing these headphones. Amplifier 47 Headphone amplifier was found for sale.

When buying, the seller suggested installing high-quality OPA2134 chips from Burr-Brown Corporation. As a result, I liked the sound of the amplifier, although there are strange jambs in the original circuit. For example, the input signal connector allows the input to be connected to ground with the plug not inserted, but this has not been done and the amplifier, in the unconnected state, catches pickups. I looked at the board layout, a lot of unused space, plus all this can be transferred to small-sized SMD components and decided to make this amplifier in a smaller version.

Full project review - https://ashiba-projects.blogspot.com/2019/09/smd-47-headphone-amplifier-40.html (Russian)