Pass Labs Aleph 3 power amplifier Measurements

Sidebar 5: Measurements

Following its 1/3-power, one-hour preconditioning test the Aleph 3's heatsinks were hot, though not unexpectedly so for a class-A design.

The input impedance of the Aleph 3 measured 22.3k ohms. The output impedance was at or under 0.1 ohms up to 1kHz, and just slightly higher (0.12 ohms) at 20kHz. The amplifier's voltage gain measured 20.3dB—low, as amps go, but close to specification. DC offset measured 48mV in the left channel, 46mV in the right, both fairly high figures but not high enough to cause any real problems. Signal/noise (ref. 1W into 8 ohms) measured 83.2dB over a 22Hz-22kHz bandwidth, unweighted, and 81.7dB over a 10Hz-500kHz bandwidth, unweighted (88.7dB, A-weighted). The Aleph 3 is noninverting.

Fig.1 shows the Aleph 3's frequency response. It is nearly ideal—even with our simulated real load, the deviation from flat is inconsequential. The 10kHz squarewave response in fig.2 is excellent, with a fast risetime, sharp corners, and only the slightest trace of an overshoot, but no ringing. (The 1kHz squarewave response, not shown, is virtually textbook.) The crosstalk shown in fig.3 is hard to comment on, with only the usual increase at high frequencies due to capacitive coupling between channels. The two channels are virtually identical, and the overall separation is very high.

Fig.1 Pass Aleph 3, frequency response at (from top to bottom): 1W into 8 ohms, 2W into 4 ohms, and 2.83V into simulated speaker load (right channel dashed, 0.5dB/vertical div.).

Fig.2 Pass Aleph 3, small-signal 10kHz squarewave into 8 ohms.

Fig.3 Pass Aleph 3, crosstalk (from top to bottom at 20kHz): L-R, R-L (10dB/vertical div.).

The THD+noise vs frequency result (fig.4) is good. The rise at high frequencies is not unexpected given the single-ended design, but in general this is far better performance than you'll see with any single-ended tube amplifier of similar power output (at least judging from the ones we've tested). The Pass's distortion waveform (fig.5) indicates a predominant second-order component at low power into 4 ohms. (The results for 8 and 2 ohms were virtually the same and are not shown.)

Fig.4 Pass Aleph 3, THD+noise vs frequency at (from top to bottom at 100Hz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated speaker load (right channel dashed).

Fig.5 Pass Aleph 3, 1kHz waveform at 2W into 4 ohms, (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 shows the Aleph 3's output spectrum, reproducing 50Hz at 37.5W output into 4 ohms. This is only a fair result for a solid-state amplifier, though not at all unusual for a single-ended design. The largest artifacts are at 100Hz (-47.3dB, or about 0.4%) and 150Hz (-50.8dB, or just under 0.3%). Fig.7 shows the spectrum resulting from a combined 19+20kHz signal—the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies—at 26.7W into 4 ohms (the highest output power available with this signal without visible signs of clipping). The low-frequency IM products are the most likely to be audible, though they are not disturbingly high: -58.7dB (about 0.12%) at 1kHz, and -54.7dB (about 0.2%) at 2kHz. The results with 15.4W output into 8 ohms (not shown) indicate lower artifacts than the 4 ohm result at most frequencies, though at 1kHz in particular the 1kHz/8 ohm result is notably higher (-47dB, or about 0.45%).

Fig.6 Pass Aleph 3, spectrum of 50Hz sinewave, DC-1kHz, at 37.5W into 4 ohms (linear frequency scale).

Fig.7 Pass Aleph 3, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 26.7W into 4 ohms (linear frequency scale).

The 1kHz THD+noise vs output power curves for the Aleph 3 are shown in fig.8. The discrete clipping levels (at 1% THD+noise) are in Table 1. Note that the distortion into both 4 ohms and 2 ohms rises rapidly at the "knees" of the curves, but slows again prior to its final increase to clipping and beyond.

Fig.8 Pass Aleph 3, distortion (%) vs output power into (from bottom to top at 40W): 8 ohms, 4 ohms, and 2 ohms.

Table 1
Pass Aleph 3 Clipping (1% THD+noise at 1kHz)
Both Channels DrivenOne Channel Driven
Load W (dBW)W (dBW)
ohms(L)(R)(L)
8 32.8 (15.2)32.6 (15.1)32.7 (15.1)
(line)117V116V117V
455 (14.4)55.8 (14.5)55.8 (14.5)
(line)117V117V117V
239.1 (9.9)
(line)117V

The Pass Aleph 3's measurements, while not striking in any particular respect, are nevertheless good, and are very respectable for a single-ended design. Used within its power limits, it is reasonable to conclude that any particularly appealing sonic signature of the Aleph 3 is probably not the result of a euphonic—but inaccurate—distortion or frequency-response deviation. I wish I could say the same for the single-ended tube designs that have crossed our test bench.

I do, however, advise caution concerning your overall system gain with the Aleph 3. Its 20dB voltage gain is unlikely to be adequate when used with a "passive" or very-low-gain preamp and loudspeakers of typical sensitivity. (The gain of different amplifiers varies, but most of those we have measured have gains 6-9dB higher than the Pass.) But much will depend on your system, listening preferences, and room. I recommend confirming the Aleph 3's suitability for your system before committing to a purchase.—Thomas J. Norton

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