Uniti Atom Headphone Edition features a 1.5 Watt per channel headphone amplifier measured into 16Ω. A larger amplifier would naturally require bigger power transistors, which by their nature get slower as they get larger and need more feedback compensation. The more feedback compensation an amplifier has typically the more flat it sounds. We wanted an articulate sounding amplifier for all moving coil headphones, balanced with the ability to run lower efficiency planar headphones as well.
Damping factor is a big subject. It can tailor an amplifier sound for sure. Naim power amplifiers have an 0.22R output impedance so the damping factor is 8/0.22 = 40. Headphones have a more resistive impedance compared to speakers and so damping factor is considered to be less important. In reality it has the effect of a level control more than damping. Increasing the damping factor allows the amplifiers feedback to breath, resulting in better sound. Damping is also controlled mechanically in the headphones. Some design philosophies favour very high damping factors, whilst some favour very low. During development a wide range of damping factors were tried, and 4.7Ω was chosen purely on sound quality.
How loud will they go?
Headphone impedance, sensitivity and amplifier power output are intrinsically linked to maximum volume level. This is especially important with headphones as hearing damage is certainly possible with prolonged exposure to high sound pressure levels. For instance, the United States NIOSH (National Institute of Occupational Safety and Health) standard classifies the maximum exposure level for an 8 hour day at 85dB SPL (A-weighted), and cuts the exposure time in half for every 3dB increase;
88dB = 4 hours
91dB = 2 hours
94dB = 1 hour
97dB = 30 mins
100dB = 15 mins
103dB = 7.5 mins
etc
Take the example of a headphone that has a sensitivity of 83dB and an impedance of 60Ω. Uniti Atom Headphone Edition will be able to provide a maximum output of 105.5dB, which equates to 7 minutes per day of listening at this level to prevent hearing damage.
A headphone with a sensitivity of 103.5dB and an impedance of 60Ω would result in a 119dB maximum output, safe for less than one minute per day to prevent hearing damage.
Naim recommend a sensible maximum volume level is set to the users preference on Uniti Atom Headphone Edition
What about electrostatic headphones?
The Uniti Atom Headphone Edition is designed to provide exceptional performance for the majority of moving coil and planar headphones. Electrostatic headphones generally are provided with their own energisers in place of a headphone amplifier. A Uniti Atom Headphone Edition can be used as a dedicated pre-amp in this scenario, still giving the benefits of a single, app-controlled system.