On the Bench
I tested the M2.2 using its XLR balanced input. I also
set the M2.2 to its upper toggle position, meaning 1 to 6 ohms, even though
I used 8 ohm loads as well as 4 ohm loads. This was because I did not want
to add a second variable, namely, the toggle position. The toggle basically
changes the bias and limits the voltage output. However, the 5 volt output
is within the Class A output for both toggle settings, and 20 volts output
is out of the range of the Class A output for both toggle settings.
At 1 kHz and 5 volts output into 8 ohms, THD+N was
0.024%. Signal-to-Noise (S/N) ratio was 84 dB (unweighted).
At 20 volts into 8 ohms (50 watts RMS), THD+N went up
to 0.06%. S/N was 94 dB (unweighted). I could not hear any hum or hiss at
all with the CD player on pause.
Moving to a 4 ohm load, THD+N was 0.02% at 5 volts
output.
At 20 volts output, and a 4 ohm load, THD+N was
slightly more than 0.1%.
In the following graphs, I used a standardized SMPTE/DIN
Intermodulation Distortion test, which has input sine waves of 2 kHz and 60
Hz. IMD readings represent the peaks immediately surrounding the 2 kHz peak
(i.e., ± 200 Hz). In this case, IMD was 0.060% at 5 volts output into 8 ohms.
At 20 volts output into 8 ohms, IMD was 0.247%.
For the sake of comparison, here is a graph of the IMD
for 20 volts output into 8 ohms, using the lower toggle setting (8 - 16
ohms). IMD was 0.228%, not significantly different than with the upper
toggle setting (1 - 6 ohms).
Click Here to Go to Part V.