Kinergetics KBA-280 power amplifier:
Measurements part 2
Fig.6 shows the KBA-280's spectral response to a 50Hz input at 187.6W output into 4 ohms. This is a superb result; all of the artifacts are well below -80dB (0.01%). Fig.7 shows the output spectrum with a combined 19+20kHz signal—the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies—at 164W into 4 ohms. The only artifact here above -80dB is at 1kHz (-68.3dB, or about 0.04%), though there is a lot of power-supply-related noise floor "grass," implying that the amp is working hard with this high current output. The result into 8 ohms (89W) was similar and is not shown.

Fig.6 Kinergetics KBA-280, spectrum of 50Hz sinewave, DC-1kHz, at 187.6W into 4 ohms (linear frequency scale).

Fig.7 Kinergetics KBA-280, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 164W into 4 ohms (linear frequency scale).

The 1kHz, THD+noise percentage vs output power curves for the KBA-280 are shown in fig.8. The discrete clipping levels (at 1% THD+noise) are listed in Table 1 and comfortably exceed the amplifier's specified power. While the KBA put out significant power into low impedances, it tended to pop power-supply rail fuses when doing so on the test bench. We were nonetheless able to run a full set of measurements.

Fig.8 Kinergetics KBA-280, distortion (%) vs output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.

Table 1 Kinergetics KBA-280: Clipping
	(1% THD+noise at 1kHz)
	Both Channels Driven		One Channel Driven
Load	W (dBW)		W (dBW)
ohms	(L)	(R)	(L)
8	167.9 (22.3)	169.5 (22.3)	174.8 (22.4)
(line)	116V	115V	116V
4	301.9 (21.8)	302.1 (21.8)	323.2 (22.1)
(line)	115V	115V	115V
2			539.1 (21.3)
(line)			115V

The Kinergetics' test-bench results are fully consistent with the results of the listening tests: hard to criticize.—Thomas J. Norton

The curves in fig.8 were taken using continuous tones. However, Stereophile's acquisition of a new piece of measurement gear, the "Amplifier Profiler" from Miller Audio Research, allows us to look at an amplifier's behavior under conditions more closely resembling what happens with music. Fig.9 plots the KBA-280's output power against distortion using a 10-cycle 1kHz toneburst interspersed by 400ms of silence. Under these conditions, the Kinergetics clips at 197.7W into 8 ohms (black trace), suggesting a dynamic headroom of 0.3dB. Into 4 ohms the amp clipped at 360.8W (red), at 630W into 2 ohms (blue), and at 1015W into 1 ohm (green). The latter corresponds to an output current, maintained for the 10ms toneburst, of 31.85A!—John Atkinson

Fig.9 Kinergetics KBA-280, THD+Noise (dB) vs dynamic output power (W) into (from left to right): 8 ohms, 4 ohms, 2 ohms, and 1 ohm (2.5dB/vertical div.).