Solid State Preamp Reviews

The THD+noise percentage vs frequency curves are plotted in fig.8. The levels here—quite typical for a good solid-state power amplifier—rise slightly (but not alarmingly) as the load impedance is reduced and the frequency increased. The waveform of the distortion at 2W into 4 ohms is shown in fig.9. It is heavily second harmonic, with indications of higher-order components that become a bit more obvious into a 2 ohm load (not shown), plus the inevitable noise.

Fig.8 Moon W-5, THD+noise vs frequency at (from top to bottom at 10kHz): 4W into 2 ohms, balanced; 4W into 2 ohms, unbalanced; 2W into 4 ohms, balanced; 2W into 4 ohms, unbalanced; 1W into 8 ohms, balanced; 2.83V into simulated loudspeaker load; and 1W into 8 ohms, unbalanced.

Fig.9 Moon W-5, 1kHz waveform at 2W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

The spectrum of the W-5's output while it reproduces a 50Hz input at 200W into a 4 ohm load is shown in fig.10. The distortion products are all extremely low—the maximum value is -72.9dB, or just under 0.025%, with now the third harmonic a little higher in level than the second.

Fig.10 Moon W-5, spectrum of 50Hz sinewave, DC-1kHz, at 200W into 4 ohms (linear frequency scale).