Pass Aleph 1.2 monoblock power amplifier Measurements

Sidebar 3: Measurements

A full set of measurements were run on the Pass Aleph 1.2 in unbalanced mode, with selected measurements repeated in balanced mode. Unless otherwise noted, the following measurements reflect unbalanced operation.

The 60-minute, 1/3-power preconditioning test left the Aleph 1.2's heatsinks very warm for several seconds, but not uncomfortable to touch. The amplifier's input impedance measured 10.3k ohms unbalanced, 20.5k ohms balanced. Its voltage gain into 8 ohms is 23.1dB (23.3dB balanced), a little lower than most amplifiers. I wouldn't recommend the Aleph 1.2 for use with "passive" preamps in most systems, unless your room is small, your playback-level tastes modest, and/or your loudspeakers very sensitive.

The amplifier is noninverting (pin 2 positive in balanced mode). Its S/N ratio measured 83.6dB from 22Hz to 22kHz, 80.9dB from 10Hz to 500kHz, and 93.7dB A-weighted. (The balanced S/N measured about 6dB worse unweighted, and about 8dB worse A-weighted.) The output impedance remained at or below a low 0.085 ohms at 1kHz and 20Hz, increasing to 0.09 ohms into 8 ohms at 20kHz. DC offset at the outputs measured 16.2mV.

Fig.1 shows the Pass Aleph 1.2's frequency response. The unbalanced response is slightly flatter than the balanced, and the effect of our simulated real load is essentially negligible. The 10kHz squarewave response in fig.2 has a very fast risetime and only a small rounding of the leading edge. The 1kHz squarewave response (not shown) is close to ideal.

Fig.1 Pass Labs Aleph 1.2, frequency response at (from top to bottom at 10kHz): 1W into 8 ohms, unbalanced; 2.83V into simulated speaker load; and 1W into 8 ohms, balanced (0.5dB/vertical div.).

Fig.2 Pass Labs Aleph 1.2, small-signal 10kHz squarewave into 8 ohms.

The distortion curves in fig.3 show quite low THD across much of the band, rising only at higher frequencies as the design runs out of open-loop bandwidth. There is nothing at these low power levels that gives away the Pass's single-ended nature. The waveform in fig.4 indicates a heavy second-harmonic content—which was also true at 8 and 2 ohms (not shown).

Fig.3 Pass Labs Aleph 1.2, THD+noise vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms, unbalanced; 2W into 4 ohms, balanced; 1W into 8 ohms, balanced; 2W into 4 ohms, unbalanced; 1W into 8 ohms, unbalanced; 2.83V into simulated speaker load, unbalanced.

Fig.4 Pass Labs Aleph 1.2, 1kHz waveform at 2W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

The 1kHz, THD+noise vs output power sweeps are shown in fig.5. Note the increase in distortion at lower impedances in the mid-power range; the "double knee" character here is not common with conventional solid-state power amplifiers. The discrete clipping levels (for 1% THD+noise) at this frequency were inconsequentially below spec. at 197.1W into 8 ohms (22.9dBW), 263.9W into 4 ohms (21.2dBW), and 132W into 2 ohms (15.2dBW). The voltage of the power line was 113–114V during these measurements. At 117V, the amplifier will probably meet its power specification.

Fig.5 Pass Labs Aleph 1.2, distortion (%) vs output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.

A plot showing the output spectrum resulting from a 50Hz input (201W into 4 ohms) is shown in fig.6. The second harmonic is at –50dB (0.3%), the third at –52dB(0.25%). Because of the Aleph 1.2's high power output and the limitations of our simulated test load, I didn't run a 50Hz simulated-test-load spectrum.

Fig.6 Pass Labs Aleph 1.2, spectrum of 50Hz sinewave, DC–1kHz, at 201W into 4 ohms (linear frequency scale).

Fig.7 shows a similar spectral plot, here the result of a combined 19+20kHz signal. (Note that "OdB" in this graph is actually –6dB ref. the waveform peak.) The residual indicates the sum and difference tones created by the amplifier with these frequencies at the input—in other words, the intermodulation between these two tones. The output of the amplifier in fig.7 was 39.5W into 8 ohms—the highest output possible with this input signal before clipping was evident on a 'scope trace. The IM is –47dB (about 0.45%)at 18kHz, –55dB (about 0.17%) at 17kHz, and –45dB (about 0.6%) at 1kHz. At 25.7W into 4 ohms (again, just prior to clipping) the graph (not shown) looks similar, though the corresponding distortion is somewhat higher: –44dB (about 0.6%) at 18kHz, –50dB (0.3%) at 17kHz, and –41dB (0.9%) at 1kHz.

Fig.7 Pass Labs Aleph 1.2, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 39.5W into 8 ohms (linear frequency scale, 0dB = –6dBr).

All of these distortion-spectrum results—50Hz or 19+20kHz—are higher than we normally see on the test bench from the best-measuring push-pull solid-state amplifiers. Nevertheless, the other results here are very good, and far outstrip those of any single-ended tube amplifier I have yet measured.—Thomas J. Norton

Pass Labs
P.O. Box 219, 24449 Foresthill Road
Foresthill, CA 95631
(916) 367-3690

kiosa's picture

He managed to explain pretty well the setup and the whole system of a Pass Aleph 1.2 monoblock power amplifier, but his analog source was a VPI TNT III. If he had chose a RF combiner for example, because it manages to combine RF from a number of different sources, he might have been much more to the point. Anyway, interesting presentation, he totally worked for it.