Integrated Amplifiers

This new amp benefits from the 'Direct Digital' technology we first saw in the NAD M2 Master series [HFN Jun '10]. Historically, the Direct Digital Feedback Amplifier (DDFA), a proprietary and highly scalable digital Class D audio amplifier technology, was originally developed by Zetex Semiconductors back in 2006. Also called 'Class Z' at the time, the know-how was latterly acquired by Cambridge Silicon Radio Ltd in May 2013, which was mopped up by mobile tech giant Qualcomm in August 2015.

In the PMA-150H, the DDFA's (PWM) modulator is synchronised directly to the incoming sample rate (96kHz for all analogue sources) and supplies a discrete range of pulse 'widths' to the high-speed FETs at the output. The high frequency analogue stream is filtered to reveal the audio signal but, like other Class D amps of this ilk, the final frequency response is influenced by the impedance trend of the attached loudspeaker. In practice, speaker impedance trends vary with frequency but into a 'flat' 8ohm load the PMA-150H's response rises from +1dB/20kHz to +2.2dB/38kHz [black trace, inset Graph] but into 4ohm falls from –0.4dB/20kHz to –1.8dB/38kHz [red trace]. The treble is more severely rolled-off into tougher 2ohm [blue] and 1ohm [green] loads. PM