MEASUREMENTS

I measured NAD’s Masters Series M33 using my Audio Precision SYS2722 system (see the January 2008 “As We See It”1). Usually, before I test an amplifier, I precondition it with both channels driving a 1kHz tone at one-third power into 8 ohms for an hour. This thermally stresses an amplifier with a conventional class-AB output stage, but as the NAD is a class-D design, this wasn’t necessary. However, before doing any testing I ran the M33 for an hour to ensure that it was fully warmed up. At the end of that time, the top panel was slightly warm, at 100.7°F/38.2°C. Because class-D amplifiers emit relatively high levels of ultrasonic noise that would drive my analyzer’s input into slew-rate limiting, all measurements other than the frequency response and squarewave tests were taken with Audio Precision’s auxiliary AUX-0025 passive low-pass filter, which eliminates the ultrasonic noise.

Looking first at the analog inputs: With the NAD’s volume control set to its maximum of “0.0dB,” the voltage gain