I was asked for a recommendation for a headphone amplifier but I don't have much experience of these.
I took a quick look on fleabay and this came up as the cheapest on there, so I though I would give it a punt and see what's what.
https://www.ebay.co.uk/itm/Mini-HIFI...72.m2749.l2649
This was from a UK stock, from China you can get it 35p cheaper
The amplifier uses an LP-2020A integrated circuit, this is a class D chip feeding the rear speaker outputs but interestingly there are separate analogue outputs to the front headphone socket.
It's a tiny little thing
As usual the power LED is rather overdriven, given that modern LEDs are super efficient, the current was about 8mA providing a blinding light.
With a quick swap out of the LED series resistor from 1.5K to 56K the light level was substantially reduced.
Given the small size you only get a 3.5mm input socket and a block connector for the speakers.
The class D power amp switches at 480KHz, the standing supply current is 48mA (8mA to the LED) rising to less than 200mA for a very good domestic volume. The instruction specify a 2A supply but imo that isn't needed.
When the headphones are plugged in the switching power amp is cut off, which is a good thing and the standing current drops to 15mA (7mA with LED mod) so very suitable for battery operation as a headphone amplifier.
The headphone output certainly goes loud and stays clean. All my headphones are 64 ohms impedance so I can't comment on how well it would drive high impedance phones.
Cheap as chips and excellent performance, apart from the blinding LED
Postscript. Just tried it at 9V supply and it works no problem. It doesn't work at 5V supply.