First All-Digital Amplifier?
"The TACT Millennium employs a PWM (pulse-width modulation) amplification stage to amplify a digital signal and couple it directly to the loudspeaker without converting it to the analog domain." Among the claimed benefits of the design are high efficiency---close to 90%---and "constant dynamic range regardless of volume setting." A pre-production sample is on display in room 1654 at the Alexis Park.
It is 20 years since Sony first brought a PWM amplifier to market; Infinity introduced an amplifier with a PWM output stage in the late '70s, designed by John Ulrich, and Ulrich's Spectron company currently markets a range of amplifiers with PWM output stages. (It was announced at CES that semiconductor manufacturer Harris Technologies is to licence Spectron's switching circuit for mass production in chip form---the target market is energy-efficient boomboxes and the like.) These designs, however, accept a conventional analog input, while the Millennium is based on EQUIBIT technology developed jointly by NAD London and Denmark's Toccata Technology. EQUIBIT is a PCM (pulse-code modulation)-to-PWM technique that applies the digital signal directly to the amplifier's output stage, in effect using the amplifier as a digital/analog converter.
NAD claims that "the audio performance of this amplifier challenges any combination of conventional DAC and amplifier . . . regardless of price." The Millennium's all-digital design eliminates the need for negative feedback. It is spec'ed at 150Wpc and features four digital inputs: three S/PDIF (one RCA, two BNC) and one AES/EBU.
Stereophile's team of reviewers will comment on the Millennium's sound quality later this year.