Conrad-Johnson Premier Twelve monoblock amplifier Measurements

Sidebar 3: Measurements

Following its preconditioning test, the Conrad-Johnson Premier Twelve was hot—typical for a tube amplifier. The bias was found to be slightly off during the distortion measurements, was recalibrated, and the prior measurements were rechecked to ensure that the results had not changed significantly. (Except for the distortion, they had not.)

The amplifier's voltage gain into 8 ohms measured 28.5dB. The input impedance measured 98k ohms. The DC offset was a very low 1.1mV. The Premier Twelve is noninverting—a positive-going input emerges positive at the output. The S/N ratio (ref. 1W at 8 ohms) was 86.8dB over a 22Hz-22kHz bandwidth, 84.1dB over a 10Hz-500kHz bandwidth (both unweighted), and 91.7dB A-weighted. The Premier Twelve's output impedance measured 0.87 ohms at 20Hz and 1kHz, increasing to 1.09 ohms at 20kHz (into a 4 ohm load)—high enough to moderately affect the frequency response of the amplifier with a typical loudspeaker.

Fig.1 shows that frequency response at the conditions indicated. Note that with our simulated real load, the deviations due to the amplifier's output impedance are clearly visible, though they remain within approximately +0.2 and -1.2dB. Caught up in the moment, I also measured the frequency response into a 2 ohm load—a test we do not normally perform. The result is interesting, as it shows clearly the change in the amplifier's response above 10kHz as the load varies. The amplifier's top-octave balance will be dependent on the speaker it is used to drive.

Fig.1 Conrad-Johnson Premier Twelve, frequency response at (from top to bottom at 20kHz): 1W into 8 ohms, 2W into 4 ohms, 2.828V into simulated loudspeaker load, and 4W into 2 ohms (0.5dB/vertical div.).

The output waveform of the Premier Twelve in response to a 10kHz squarewave is shown in fig.2. There is a small overshoot together with a single cycle of ultrasonic ringing, this quickly damped. This overshoot is also visible in the 1kHz squarewave (not shown).

Fig.2 Conrad-Johnson Premier Twelve, small-signal 10kHz squarewave into 8 ohms.

The THD+noise result is shown in fig.3. This is not an unusual result for a tube amplifier. The distortion does increase noticeably at the frequency extremes, though it remains moderate at the low power used for this test. The 1kHz THD+noise waveform at 2W out into 4 ohms, shown in fig.4, is heavily second-harmonic. The same is true at other loads (not shown).

Fig.3 Conrad-Johnson Premier Twelve, THD+noise vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated loudspeaker load.

Fig.4 Conrad-Johnson Premier Twelve, 1kHz waveform at 2W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

The distortion spectrum resulting from a 50Hz input at 93W into 4 ohms (2/3 rated power) is shown in fig.5. The largest artifact is the second harmonic, at -38dB (about 1.2%). The 19+20kHz IM spectrum at 67W into 4 ohms—the highest power available with this signal before visible signs of clipping appear in the waveform—is plotted in fig.6. The 1kHz intermodulation artifact lies at -42dB, or about 0.8%; the distortion at 18kHz is -49dB (about 0.35%). The measured artifacts were slightly lower at 48W into 8 ohms (not shown): 0.4% at 1kHz, 0.2% at 18kHz, and correspondingly less pronounced elsewhere.

Fig.5 Conrad-Johnson Premier Twelve, spectrum of 50Hz sinewave, DC-1kHz, at 93W into 4 ohms (linear frequency scale).

Fig.6 Conrad-Johnson Premier Twelve, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 67W into 4 ohms (linear frequency scale).

The Premier Twelve's THD+noise vs continuous output-power curves are shown in fig.7. Its discrete clipping is shown in Table 1. The Premier Twelve does not make its specified power rating at 8 ohms, though it gets within a watt of the 4 ohm spec (1kHz).

Fig.7 Conrad-Johnson Premier Twelve, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

Table 1 Conrad-Johnson Premier Twelve Clipping
(1% THD+noise at 1kHz)
Load (ohms)W (dBW)
898.5 (19.9)
4139.2 (18.4)
2113.1 (14.5)

JA used the Miller Audio Research Amplifier Profiler to examine the Premier Twelve's power delivery on a toneburst waveform (10 cycles of 1kHz followed by 40 cycles of silence) more typical of a music signal. As can be seen from fig.8, the C-J sample as reviewed is happiest working into 4 ohms, where it does meet its specification on this toneburst signal: 142.5W was available into 4 ohms at the normal 1% THD+N limit (red trace, horizontal pink line), with a maximum current delivery of 6A RMS. (152.6W was available at the relaxed 3% limit we also use for tube amplifiers.)

Fig.8 Conrad-Johnson Premier Twelve, distortion (%) vs burst output power into: 8 ohms (black trace), 4 ohms (red), 2 ohms (blue) and 1 ohm (green).

The power delivery fell into both higher and lower loads, but the amp could still deliver 133W (3% THD) into 2 ohms (blue trace). The distortion remained acceptably low below 10W into both 4 and 8 ohms, which is where the amplifier's output will remain most of the time on typical music signals, as pointed out by Michael Fremer.

The overall measurement results are not atypical for a tube amplifier: acceptable if not particularly impressive.—Thomas J. Norton

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