Palmer PHDA02 - a reference-class headphone amplifier.

The device offers three operating modes: Stereo, Mono, and Dual Mono. In Stereo mode, the input signal is distributed left/right to the corresponding ear cups. The volume is controlled by the left potentiometer labeled Stereo/Left. Two headphones can be connected in parallel to the jacks on the front panel simultaneously. In Mono mode, the left/right channels are summed to a mono signal and routed identically to both headphone jacks. The volume is again controlled by the stereo knob. In Dual Mono mode, the left and right channels can be used independently and the volume can be adjusted separately for each headphone.

All commercially available headphones with load impedances between 8 and 200 Ohms can be connected. The adjustment is made by switching a specially designed matching transformer for this purpose. This somewhat unusual and elaborate solution ensures equally good dynamics and performance across all load impedances. Another advantage is an extremely low noise level.

The inputs of the PHDA02 are combo jacks that can be used both symmetrically with XLR and unsymmetrically with a jack plug. Parallel XLR jacks for signal looping are available, allowing multiple headphone amplifiers to be cascaded, for example. The PHDA02 features a built-in wide-voltage power supply for mains voltages between 100 VAC and 240 VAC.

The device is built with low-noise operational amplifiers, and the output amplifiers are equipped with discrete power transistors mounted on heat sinks. The housing dimensions correspond to half of a 1 HE 19" rack case.