The ART Head Amp provides simple headphone monitoring for several performers. It's built into a small folded-steel box and powered by an included mains adaptor which is thin enough to fit alongside a normal 13A UK mains plug. There is a TRS stereo jack input alongside four TRS stereo jack headphone outputs, each output having its own volume control — everyone gets the same headphone mix.
The output impedance is 51Ω, which means that headphones of most common impedances can be used, and the input is designed to accept the line-level output of a mixer send, but you can also feed it directly from another headphone socket. There's no mono switch, so you'll need to make up a special cable if you want to feed a mono source into the unit and hear it coming from both phones. The maximum input level is +14dBV, and the signal-to-noise ratio is a perfectly adequate 90dB, with distortion below 0.01 percent.
I checked out the Head Amp using my own selection of Beyer, AKG, and Audio-Technica studio headphones, and found there to be plenty of volume in all cases. In fact, with my Beyer DT250s, the level was as loud as I could comfortably tolerate at around halfway up! The sound quality was subjectively crisp and clear — more than adequate for performance monitoring — and the outputs showed no sign of running out of steam when cranked up. While there's nothing at all fancy about this product, it does do exactly what is asked of it with no fuss, and it does so at a very modest UK price.