Classé Omega power amplifier:
Measurements 3
The Classé had a tendency to blow its internal protection fuses if even slightly mistreated on the test bench. This first occurred when I inadvertently unplugged an unbalanced input connector with the amplifier powered up. The second and third times it happened was when the Omega was driven into clipping with the 19+20kHz IM test signal. This should not be a concern in normal operation. And, of course, if you value your loudspeakers you should never plug and unplug input connections—particularly unbalanced ones—with this powerful amplifier turned on and the speakers hooked up. The internal fuses are tiny metric designs not readily available in every RadioShack. In any event, Classé cautions strongly against users rummaging around inside the amplifier.

I also ran several tests on the Classé Omega in its mono, bridged configuration. The frequency response was within 0.1dB of the results in fig.1. It popped internal fuses (yet again) when I attempted to perform a balanced frequency-response measurement in the mono mode. In bridged mode, the Omega's voltage gain into 8 ohms was 35.3dB, its input impedance 3.8k ohms (again, fairly low), and its output impedance 0.037 ohms at 1kHz and a maximum of 0.068 ohms at 20kHz into a 4 ohm load. Because the low-power THD+noise readings had been previously determined to be strongly dominated by noise, I did not run this test in mono mode.

Because of the Classé Omega's extremely high power rating and the limitations of our test-bench load, I did not attempt to investigate the amp's clipping performance. However, its proclivity to pop fuses at clipping with that 19+20kHz signal suggests that it might do the same in normal clipping tests. However, John Atkinson investigated the amplifier's maximum power using low-duty-cycle 1kHz tonebursts generated by the Miller Audio Research Amplifier Profiler.

The Classé Omega proved a powerhouse on these tests (fig.8), generating 506W into 8 ohms (27.05dBW) for 1% THD+N, 985.4W into 4 ohms (26.9dBW), 1886W into 2 ohms (26.7dBW), and an astonishing 3425W into 1 ohm (26.3dBW)! As well as comfortably exceeding its specified power, the Omega acts close to a true voltage source, as can be seen in fig.9, which plots output voltage (divided by root 2, because JA had to use a 3dB pad to avoid overloading the input of his National Instruments DSP card) against THD+Noise, this time in dB. Only into 1 ohm is there any significant reduction in maximum output voltage or increase in distortion. Note the slight rise in THD, however, around the indicated 20V level, equivalent to a real 28.3V RMS or 100W into 8 ohms. Perhaps this indicates the changeover from class-A to class-AB output stage operation. Whatever, the distortion level is still very low, at below -70dB (0.03%).

Fig.8 Classé Omega, distortion (%) vs burst output power into: 8 ohms (black trace), 4 ohms (red), 2 ohms (blue) and 1 ohm (green). Note that the actual power is twice that indicated in this graph.

Fig.9 Classé Omega, distortion (dB) vs burst output voltage into (from bottom to top): 8 ohms (black trace), 4 ohms (red), 2 ohms (blue) and 1 ohm (green). Note that the actual voltage is 1.414x that indicated in this graph.

This is a solid set of measurements. While there is nothing remarkable here, neither is there anything serious to criticize. I do recommend that the Classé be used with a preamplifier with a low output impedance, particularly in mono mode and in its balanced configuration.—Thomas J. Norton