Phono Preamplifiers

Headphone Hero?

The rotary that distinguishes the UltraPhono from the StudioPhono is not a volume pot but, instead, governs a Maxim MAX5440 attenuator chip which offers 31 log-spaced (2dB) steps and is designed to 'replace a mechanical potentiometer'. This drives a TPA6120A2 headphone amp IC from Texas Instruments, described by TI as 'a Class AB current-feedback amplifier with differential inputs and single-ended outputs'. In this application the IC delivers almost exactly 600mW/25ohm at <1% THD [see inset Graph] from a moderate 8ohm source impedance. The latter represents a power loss of 2.8dB into our 'average' 25ohm headphone load, and will influence the frequency response of low impedance 'phones, but has less impact into higher 600ohm headphone loads where 5050mV (43mW) is available.

These are not huge figures by the standards of the Mytek Brooklyn Bridge [2.7W/25ohm – HFN Dec '19] but certainly represent an uplift over most USB hub-powered DAC/headphone amps, including the AudioQuest Cobalt [53mW/25ohm – HFN Oct '19]. Distortion is low at ~0.001-0.008% (500Hz-10kHz) over the first 100mW of its range, increasing mainly at low frequencies under load (0.04% vs. 0.015% at 20Hz/10mW/25ohm). Again, the UltraPhono is not as quiet as many dedicated headphone amps, the 72dB A-wtd S/N ratio (re. 0dBV out) implying that lower sensitivity 'phones may prove the better partners. PM