Conrad-Johnson Premier Eleven power amplifier:
Measurements part 2
In the Conrad-Johnson's THD+noise vs frequency plot (fig.5), the distortion increases more than we normally see at the frequency extremes. Though such a characteristic is not uncommon in tube amplifiers, it's somewhat more pronounced in the Premier Eleven than in most others. The rise in the bottom end may result in a softened bass performance in some circumstances; the top-end rise will produce distortion components well above the audible range.

Fig.5 Conrad-Johnson Premier Eleven, THD+noise (%) vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms (right channel dashed).

(All of the measurements presented here, except for the THD+N vs level and clipping, were made at a time when our line frequency was at approximately 110V (footnote 1). When I discovered that the Premier Eleven didn't quite meet its power specs at 110V, I made later measurements of the maximum power output when the power line had increased to 121V. While it was impractical to then repeat all of the other measurements, I rechecked the THD+N vs frequency results at 8 ohms. They were not significantly different, but the maximum THD+N did stay below 1% at the higher line voltage.)

The waveform of the distortion associated with a 1kHz input signal at a moderate power output (10W into 4 ohms) is shown in fig.6. Though the content here is primarily third-harmonic, the spikes occurring twice per cycle are most likely crossover artifacts, the audibility of which is open to question. Looking at the distortion content another way, but with a lower signal frequency, fig.7 shows a spectral analysis of the Premier Eleven's output reproducing 50Hz at 47W into a 4 ohm load. The only artifacts lying above 0.1% are the second harmonic (-49dB, or about 0.35%), the third harmonic (-57B, or about 0.15%), and the fourth harmonic (-59.5dB, or about 0.11%). Note that the relative level of the harmonics decreases with increasing order—a musically natural result. Note also the absence of high harmonics that would otherwise be associated with the crossover spikes in fig.6.

Fig.6 Conrad-Johnson Premier Eleven, 1kHz waveform at 10W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.7 Conrad-Johnson Premier Eleven, spectrum of 50Hz sinewave, DC-1kHz, at 47W into 4 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level at -49dB (0.35%).

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Footnote 1: Unlike some testers, we don't attempt to control the power-line voltage with a Variac when making our measurements. We prefer to measure a product as an owner would most likely use it—right off the line.—Thomas J. Norton