Wavelength Audio Gemini monoblock power amplifier Measurements part 2

Though its minimum midband distortion into a matched load was quite low, at around 0.25% (2A3) or 0.16% (45), the Gemini is not a low-distortion amplifier. This can be seen in figs.6 and 7, which show the small-signal THD+noise percentage from the amplifier's 8 ohm tap plotted against frequency into resistive loads ranging from 16 down to 2 ohms and into our simulated speaker. The distortion rises significantly into all loads in the bass, which suggests RD was wise to use a powered subwoofer. But it also rises into loads both higher and lower than the nominal transformer tap value, with the 2A3 slightly worse than the 45 tube. The picture was similar from the 4 ohm tap (not shown); again, the lowest midband distortion obtained was into a matched load. The variation with frequency into the simulated loudspeaker, however, was reduced from the 4 ohm tap, and the 2 ohm distortion remained below 0.9% across the audioband with either tube.

Fig.6 Wavelength Gemini, 45 tube, 8 ohm tap, THD+noise (%) vs frequency at 1V into (from top to bottom at 100Hz): 2 ohms, 4 ohms, 16 ohms, 8 ohms, and simulated loudspeaker load.

Fig.7 Wavelength Gemini, 2A3 tube, 8 ohm tap, THD+noise (%) vs frequency at 1.4V into (from top to bottom at 2kHz): 2 ohms, 16 ohms, 4 ohms, 8 ohms, and simulated loudspeaker load.

Figs.8 and 9 reveal that the distortion harmonics with the 45 tube with a midrange signal (fig.8) are more pure second-harmonic than with the 2A3 (fig.9), which has more higher-order harmonics present in its output. This may well correlate with RD's preference for the 45. At low frequencies, however, the 45 (fig.10) has more third harmonic than the 2A3 (fig.11). But with both tubes the harmonic series descends with increasing order, which, all else being equal, always tends to sound musically consonant. The harmonic content was not significantly different from either transformer tap, as long as the tap was matched to the load. Note that the 60Hz hum component lies below -70dB in these two graphs. It is the second harmonic at 120Hz that seems to dominate the hum content of the amplifier's output.

Fig.8 Wavelength Gemini, 45 tube, 8 ohm tap, 1kHz waveform at 1V into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.9 Wavelength Gemini, 2A3 tube, 8 ohm tap, 1kHz waveform at 1V into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.10 Wavelength Gemini, 45 tube, 8 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 1V into 8 ohms (linear frequency scale).

Fig.11 Wavelength Gemini, 2A3 tube, 8 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 1.4V into 8 ohms (linear frequency scale).

Company Info
Wavelength Audio
3703 Petoskey Avenue
Cincinnati, OH 45227
(513) 271-4186
Article Contents
Share | |
Site Map / Direct Links