Pass Labs X1000 monoblock power amplifier Measurements

Sidebar 4: Measurements

Because of the Pass X1000's power output and the restrictions of our test load, I couldn't perform our usual 1/3-power preconditioning test. Instead, the amplifier was warmed up for one hour at 50W into 8 ohms before any measurements were taken.

The X1000's input impedance measured 23.7k ohms. The output impedance measured between 0.18 and 0.20 ohms (the higher values were at 20kHz)—slightly higher than normally experienced with solid-state amplifiers, but not enough to be a serious concern. The voltage gain was 30.14dB, and pin 2 of the (balanced) input is positive. The signal/noise ratio (ref. 1W into 8 ohms) measured 78dB from 22Hz to 22kHz, 77.9dB from 10Hz to 500kHz, and 82.5dB A-weighted. DC offset at the output was 40.5mV.

Fig.1 shows the Pass X1000's frequency response into 8 ohms. (The 4 ohm response is essentially identical and is not shown.) The deviation due to the amplifier's output impedance with a typical real-world load is clearly visible in the simulated loudspeaker plot, but is relatively benign. The 10kHz squarewave response in fig.2 has a very fast risetime and only a small rounding of the leading edge. The 1kHz squarewave, not shown, is virtually perfect.

Fig.1 Pass Labs X1000, frequency response at (from top to bottom at 6kHz): 1W into 8 ohms, and 2.828V into simulated loudspeaker load (0.5dB/vertical div.).

Fig.2 Pass Labs X1000, small-signal 10kHz squarewave into 8 ohms.

The distortion curves in fig.3 show quite low levels of THD+noise across much of the band, increasing only at higher frequencies and into lower impedances. The waveform of the distortion at 2W into 4 ohms in fig.4 has a heavy second-order content, a hint of higher even-order harmonics, and some noise. The distortion waveform into 8 ohms is similar, and the waveform into 2 ohms shows a trace of odd-order harmonics. Neither is shown here.

Fig.3 Pass Labs X1000, THD+noise vs frequency at (from top to bottom at 10kHz): 4W into 2 ohms, 2W into 4 ohms, 2.83V into simulated loudspeaker load, and 1W into 8 ohms.

Fig.4 Pass Labs X1000, 1kHz waveform at 2W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Again, because of the limitations of our test load, I took continuous power-output measurements only into an 8 ohm load. Actually, we used a load slightly more rugged than usual—8.4 ohms—if not durable enough for full-power testing into 4 or 2 ohms. This slightly higher value, together with the fact that the X1000 will cause the line voltage to sag when driven to high power, explains the discrete-clipping power figure I obtained: 889W (29.5dBW) into 8.4 ohms (113V line voltage). The corresponding THD+noise vs output curve is shown in fig.5. Because of the X1000's balanced output topology, it wasn't possible to investigate its burst output power using the Miller Audio Research Amplifier Profiler, which grounds the test amplifier's negative terminal.

Fig.5 Pass Labs X1000, distortion (%) vs output power into: 8 ohms.

A plot showing the output spectrum resulting from a 50Hz input (600W into 8.4 ohms) is shown in fig.6. The second harmonic here is at -72dB (0.025%). Fig.7 shows a similar spectral plot, here the result of a combined 19+20kHz signal. 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. In fig.7, the amplifier's output was 455W into 8.4 ohms—the highest output possible with a 19+20kHz input signal before clipping was evident on a 'scope trace. (If anyone knows of a real-world tweeter that can take this much power, let us know.) The IM distortion here is -67dB at 1kHz (about 0.045%) and -32.5dB at 18kHz (about 2.5%).

Fig.6 Pass Labs X1000, spectrum of 50Hz sinewave, DC-1kHz, at 600W into 8.4 ohms (linear frequency scale).

Fig.7 Pass Labs X1000, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 455W into 8.4 ohms (linear frequency scale).

There is nothing to complain about in the Pass X1000's test-bench performance.—Thomas J. Norton

Pass Labs
24449 Foresthill Road
Foresthill, CA 95631
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