Solid State Power Amp Reviews

The Classé's A-weighted noise floor was very low, at -94dB with respect to 1W into 8 ohms. Removing the weighting and extending the measurement bandwidth to more than 500kHz dropped the S/N ratio to 82.2dB, which is still very good. The CAM 350 will not compromise the dynamic range of the new hi-rez digital media.

Distortion levels were also very low, particularly with balanced drive. Fig.4 shows the THD+noise percentage plotted against frequency at a 3V output level. Yes, there is a rise in measured THD at higher frequencies and into 2 ohms, but the basic circuit appears to be extremely linear, with around 0.005% of THD and noise delivered over the entire audioband into 8 ohms. The unbalanced performance (not shown) was similar in character, but with the levels of distortion almost doubled.

Fig.4 Classé CAM 350 (balanced input), THD+noise (%) vs frequency at (from top to bottom at 4kHz): 4W into 2 ohms, 2.83V into simulated loudspeaker load, 2W into 4 ohms, and 1W into 8 ohms.

The actual distortion content, at least at fairly low output powers (fig.5), comprised the more sonically worrisome higher harmonics—worrisome, I should say, at much higher levels than the 0.005% featured in this graph. At higher powers, the subjectively inoffensive third harmonic dominated (fig.6), if "dominated" is the right word for something that lies 90dB down from the fundamental level! The levels of intermodulation distortion were also low, as can be seen in fig.7, which shows the spectrum of the CAM 350's output while driving a punishing combination of 19kHz and 20kHz tones at 320W into 4 ohms, just below visible clipping with this signal.

Fig.5 Classé CAM 350, 1kHz waveform at 22W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Classé CAM 350, spectrum of 50Hz sinewave, DC-1kHz, at 200W into 4 ohms (linear frequency scale).

Fig.7 Classé CAM 350, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 320W into 4 ohms (linear frequency scale).

On continuous sinewave drive (fig.8), the Classé more than met its specified power, delivering 450W into 8 ohms (26.5dBW) and 780W into 4 ohms (25.9dBW). Into a 2 ohm load, however, the protection circuitry cut in at 1000W, equivalent to an output current of 44.7A!

Fig.8 Classé CAM 350, distortion (%) vs continuous output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.

It wasn't possible to measure the CAM 350's output power using the Miller Audio Research Amplifier Profiler, due to this instrument's having a grounded rather than a floating input. But all things considered, I am not surprised BD liked the amplifier so much.—John Atkinson