YBA 1 Alpha HC power amplifier Measurements

Sidebar 3: Measurements

Following the 1/3-power, one-hour preconditioning test, the heatsinks of the YBA 1 Alpha HC were very warm, but not uncomfortably hot to the touch.

The input impedance of the YBA 1 measured 28.9k ohms. The output impedance was 0.04 ohms up to 1kHz and under 0.06 ohms at 20kHz. Voltage gain into 8 ohms measured 28.9dB. DC offset measured 14.8mV in the left channel and 51mV in the right. The latter figure is slightly higher than we normally see, but not enough to be a concern. Unweighted signal/noise (ref. 1W into 8 ohms) measured 87.7dB over a 22Hz-22kHz bandwidth, and 74.8dB over a 10Hz-500kHz bandwidth (93.6dB, A-weighted). The YBA is noninverting.

Fig.1 shows the frequency response of the YBA 1. The effect of a simulated real load is insignificant, as we might have predicted from the YBA's very low output impedance. The 10kHz squarewave response in fig.2 is excellent, with only a slight rounding at the transitions and a short risetime. The 1kHz squarewave (not shown) is virtually perfect. The channel separation shown in fig.3 indicates very good performance, with only the normally expected increase in crosstalk at high frequencies apparent, due to capacitive coupling between channels.

Fig.1 YBA 1 Alpha HC, frequency response at 1W into 8 ohms (top at 5kHz) and into simulated speaker load (right channel dashed, 1dB/vertical div.).

Fig.2 YBA 1 Alpha HC, small-signal 10kHz squarewave into 8 ohms.

Fig.3 YBA 1 Alpha HC, crosstalk (from top to bottom at 1kHz): L-R, R-L (10dB/vertical div.).

The THD+noise vs frequency result (fig.4) is also good, though with the commonly encountered increase at higher frequencies. The YBA's distortion waveforms (fig.5) indicate a predominantly second-order component, plus some higher-order harmonics at a much lower level. The results for 2, 4, and 8 ohm loads were practically the same; only the 4 ohm result is shown.

Fig.4 YBA 1 Alpha HC, THD+noise vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms, 2W into 4 ohms, 2.83V into simulated speaker load, and 1W into 8 ohms (right channel dashed).

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

The YBA 1's output spectrum reproducing a 50Hz input at 114W output into 4 ohms is shown in fig.6. This is a very good result; all of the artifacts are below -80dB (0.01%) save the second and third harmonics, at -63.8dB (about 0.065%) and -66dB (0.05%). The result into 8 ohms at 56W was even better (not shown), and the distortion into a simulated real load was little different from the 4 ohm result except for slightly higher fourth and fifth harmonic components—still under -80dB (0.01%) (not shown).

Fig.6 YBA 1 Alpha HC, spectrum of 50Hz sinewave, DC-1kHz, at 114W into 4 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level, at -63.8dB (about 0.065%).

Fig.7 shows the response to a 19+20kHz signal—the intermodulation spectrum resulting from an input signal consisting of an equal combination of these two frequencies—at 111.5W into 4 ohms. The largest artifacts here are at 1kHz (-55.7dB, or about 0.17%) and at 18kHz and 21kHz (-46dB and -45dB, respectively, or about 0.5%). The artifacts from the same test signal at 57W into 8 ohms (not shown) were even lower than this.

Fig.7 YBA 1 Alpha HC, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 111.5W into 4 ohms (linear frequency scale).

The 1kHz, THD+noise vs output power curves for the YBA 1 are shown in fig.8. The discrete clipping levels (at 1% THD+noise) are listed in Table 1.

Fig.8 YBA 1 Alpha HC, distortion (%) vs output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.

Table 1

YBA 1 Alpha HC Clipping (1% THD+noise at 1kHz)
 Both Channels DrivenOne Channel Driven
8119.9 (20.8)120.7 (20.8)127 (21)
4207.6 (20.2)209.8 (20.2)230 (20.6)
2  378.4 (19.8)

The YBA more than meets its power ratings [although we assume the 1.8kW specified into 0.7 ohms is a short-term burst result, not steady-state—Ed.] and appears remarkably robust for such a physically small amplifier. Its test-bench results were all very good to excellent. The YBA was a breeze to measure compared with so many other amplifiers seen here lately—mainly of the thermionic persuasion—for which many measurements had to be repeated to confirm that they really were as mediocre as they appeared on the first pass. With the YBA, no such repetition was needed.—Thomas J. Norton

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