Measuring Class-D amplifiers is complicated by the amount of ultra-sonic noise that rides on the top of any low-frequency audio signal they are amplifying which, in the case of the M1200, was around 400mV.
You can’t hear this noise of course, it’s too high in frequency, and it’s also filtered out by the loudspeakers themselves, but it interferes with the circuitry of the test equipment. The limited bandwidth of Class-D amplifiers (typically 50kHz) also makes it impossible to establish typical marker points of performance such as the onset of clipping at high frequencies.
All of which makes it necessary to connect an external low-pass filter to the speaker terminals to remove that noise to allow accurate measurements. Newport Test Labs used both an external passive Prism-Sound dS-LPF passive low-pass filter and an active AES-17 filter for the great majority of the measurements.
Power output was measured only at 1kHz and measured it as coming in exactly on specification, at
