Sidebar 3: Measurements
Before performing any measurements, I ran one Pass Labs XA200.8 (serial no. 29556) for an hour at one-third its measured maximum power into 8 ohms—thermally, the worst case for an amplifier with a class-AB output stage but not for a true class-A amplifier, which runs at its hottest with no signal. By the end of the hour, the top panel was warm, at 99.4°F (37.4°C), and the side-mounted heatsinks were hotter, at 118.1°F (47.8°C). Although Pass Labs specifies the XA200.8 operating in class-A, I suspect that even with its massive heatsinking, the amplifier doesn't have sufficient bias current to allow class-A operation up to its specified maximum power.
I performed a full set of measurements using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The voltage gain at 1kHz into 8 ohms was slightly lower than the specified 26dB, at 24.85dB for the balanced input and the expected 6dB lower for the unbalanced input (for unbalanced drive, sockets 1 and 3 of the XLR jack were connected with a jumper). The amplifier was non-inverting (ie, it preserved absolute polarity) with both inputs. Its balanced input impedance was the specified 100k ohms at 20Hz and 1kHz, dropping slightly to 93k ohms at 20kHz. The unbalanced input impedance is specified as 50k ohms; my estimate was 55k ohms at 20Hz and 1kHz, 38k ohms at 20kHz.
The XA200.8's output impedance was 0.11 ohm at 20Hz and 1kHz, rising to 0.15 ohm at 20kHz. The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, was just ±0.1dB (fig.1, gray trace). Into an 8 ohm resistive load (fig.1, blue trace), the XA200.8's response was flat up to 20kHz, then rolled off to reach –3dB around 120kHz. The response rolled off a little earlier into lower impedances, but was still just 0.5dB down at 20kHz into 2 ohms (red trace). With this wide a small-signal bandwidth, the amplifier's reproduction of a 10kHz squarewave into 8 ohms featured short risetimes and a well-squared shape (fig.2).
The XA200.8 is specified as delivering 200W into 8 ohms (23dBW). Using our definition of clipping—ie, when the output's percentage of THD+noise reaches 1%—the Pass Labs amplifier clipped at 258W into 8 ohms (24.1dBW, fig.4), 480W into 4 ohms (23.8dBW, fig.5), and 700W into 2 ohms (22.4dBW, fig.6). The THD begins to rise above the noise floor at powers greater than a couple of watts, but remains at or below 0.1% below 100W into any of these impedances. As with the XA60.8, the percentage of THD+N in the XA200.8's output slowly rises with increasing power, suggesting that the amplifier uses only a small amount of corrective feedback.
Fortunately, the XA200.8's distortion is predominantly the subjectively innocuous third harmonic (figs. 8 and 9). Still, at low frequencies into lower impedances, the second harmonic rises to equal the level of the third, and some higher-order harmonics appear (fig.9), though the latter all lie at or below –100dB (0.001%).
As with the other Pass Labs XA-series amplifiers we have reviewed, the XA200.8's top-octave decrease in linearity (fig.7) is associated with some higher-order intermodulation products that were fairly high in level when the amplifier was asked to drive an equal mix of 19 and 20kHz tones at high power into 4 ohms (fig.11). Even so, the second-order difference product at 1kHz, which will be more audible than the higher-order products, lies at a low –74dB (0.05%).
The measured performance of the Pass Laboratories XA200.8 reveals some well-balanced audio engineering.—John Atkinson
Fig.1 Pass Labs XA200.8, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (1dB/vertical div.).
Fig.2 Pass Labs XA200.8, small-signal 10kHz squarewave into 8 ohms.
The unweighted, wideband signal/noise ratio, ref. 1W into 8 ohms and taken with the input shorted to ground, was an excellent 83dB. This improved to 88.2dB when the measurement bandwidth was restricted to the audioband—and was even better, at 92.1dB, when the measurement was A-weighted. Like the other Pass Labs amplifiers we have reviewed, the XA200.8 is quiet. As with the XA60.8 monoblock that we reviewed in December 2017, both the odd and even harmonics of the 60Hz power-supply frequency were present in the amplifier's noise floor, though these all lay at or below –100dB ref. 1W into 8 ohms (fig.3).
Fig.3 Pass Labs XA200.8, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).
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Fig.4 Pass Labs XA200.8, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Pass Labs XA200.8, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.6 Pass Labs XA200.8, distortion (%) vs 1kHz continuous output power into 2 ohms.
I examined how the percentage of THD+N changed with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The THD+N was extremely low in the midrange into 8 and 4 ohms (fig.7, blue and magenta traces), and still below 0.1% into 2 ohms (red). It rose linearly as the frequency increased, the three traces converging in the high treble—which suggests that, again like the XA60.8, the circuit has limited open-loop bandwidth.
Fig.7 Pass Labs XA200.8, THD+N (%) vs frequency at 20V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).
Fig.8 Pass Labs XA200.8, 1kHz waveform at 50W into 8 ohms, 0.06% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.9 Pass Labs XA200.8, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (linear frequency scale).
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Fig.10 Pass Labs XA200.8, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 4 ohms (linear frequency scale).
Fig.11 Pass Labs XA200.8, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 4 ohms (linear frequency scale).
