Solid State Power Amp Reviews

A more limited spectrum of tests was run on the Sumo than is usually the case because its floating ground requirement was incompatible with some of our test equipment. As can be seen from fig.1, the Andromeda II has only a very slight rolloff at the frequency extremes. There was a slight mismatch in gain between the two channels: an insignificant (in practice) 0.3dB. The 1kHz input impedance measured 42k ohms. The output impedance was below 0.06 ohms up to 1kHz, rising gradually to a maximum of 0.09 at 20kHz.

Fig.1 Sumo Andromeda II, frequency response at 1W into 8 ohms (right channel dashed, 0.5dB/vertical div.).

The gain of the amplifier was 27dB. Unweighted noise was 75dB (left channel) below 1W into 8 ohms. Channel separation (fig.2) was less impressive than I have measured with a number of other amplifiers, but was still greater than 60dB (except for a small blip at about 130Hz)—a more than adequate figure.

Fig.2 Sumo Andromeda II, L–R crosstalk (10dB/vertical div.).

Distortion of the Andromeda II at low power is shown in fig.3, which plots the THD+noise percentage against frequency at 2.83V into 8, 4, and 2 ohms. Distortion does increase at higher frequencies, due to the TL circuit not being able to fully linearize the output in this region. The distortion on the right channel was also noticeably higher than that on the left (the curves for the right channel at 8 and 4 ohms are shown dotted in fig.3).

Fig.3 Sumo Andromeda II, left-channel THD+noise % vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms.

Fig.4 indicates the THD vs power for 8 and 4 ohms at 1kHz, one channel driven (bottom and top curves respectively; line voltage of 116V). The maximum output power at 1% THD (one channel driven) was 356W (25.5dBW) into 8 ohms and 514.5W (24.1dBW) into 4 ohms, well above specification. With both channels driven, the Andromeda II's output power was 304W (24.8dBW, left channel) and 300.5W (24.8dBW, right channel) into 8 ohms. Measurements into lower impedances were limited by fuse failure. The "knees" of the distortion curves (fig.4) lie at 260W (8 ohms) and 360W (4 ohms)—again, in my judgment, the amplifier's maximum truly useful power output.

Fig.4 Sumo Andromeda II, distortion (%) vs output power into (from bottom to top at 100W): 8 ohms, 4 ohms.

Finally, DC offset in the Andromeda II was 2.2mV in the left channel, 20mV in the right. The Sumo operated very hot—its heatsinks became far too hot to touch—during its 1-hour, 1/3-power preconditioning into 8 ohms, but without any problem.

The Sumo's measurements were good for an amplifier in its price class. It appeared to be less robust than the Adcom in driving loads below 4 ohms, blowing fuses quite readily if driven for more than a second or two into such impedances.—Thomas J. Norton