Kinergetics KBA-280 power amplifier Measurements
Following its 1/3-power, one-hour preconditioning test, the KBA-280's chassis was surprisingly cool, except for the fan-driven heat coming from its rear vent.
The input impedance of the KBA-280 measured 10.7k ohms unbalanced, 1062 ohms balanced. Its output impedance was a maximum of 0.027 ohms at 20Hz and 1kHz, and 0.034 ohms at 20kHz. Its voltage gain into 8 ohms measured 29dB unbalanced and 26.5dB balanced. DC offset was 2.8mV in the left channel, 5.8mV in the right. Signal-to-noise (at 1W into 8 ohms) measured 91dB from 22Hz to 22kHz, 88.4dB, 10Hz-500kHz, and 102.2dB A-weighted (unbalanced; the balanced S/N measured 1-2dB lower: ie, worse). The Kinergetics is noninverting at its unbalanced inputs, with pin 2 positive at its balanced input.
Fig.1 shows the KBA-280's frequency response overlaying its response for 2W into 4 ohms unbalanced, the same condition balanced, and 2W into a simulated real load, unbalanced. The responses are consistently good (and are the same for 1W into 8 ohms, not shown). The 10kHz squarewave shown in fig.2 is nearly ideal, with no ringing and a fast risetime. (The 1kHz squarewave, not shown, is similarly beyond criticism.) The crosstalk shown in fig.3 is fine, with only the normal increase at high frequencies due to capacitive coupling between channels.
Fig.1 Kinergetics KBA-280, frequency response at (from top to bottom at 10kHz): 1W into 8 ohms, 2W into 4 ohms (balanced), 1W into 8 ohms, 2W into 4 ohms (unbalanced), and 2.83V into simulated speaker load (0.5dB/vertical div.).
Fig.2 Kinergetics KBA-280, small-signal 10kHz squarewave into 8 ohms.
Fig.3 Kinergetics KBA-280, crosstalk (from top to bottom): R-L, balanced; R-L, unbalanced; L-R balanced; L-R unbalanced; (10dB/vertical div.)
The THD+noise vs frequency result for unbalanced operation (fig.4) is extremely good, though the 2 ohm curve indicates increasing distortion at that load—not an unusual occurrence. The THD+noise from the balanced inputs is not shown, but is marginally higher than the unbalanced result. The waveform of the harmonic distortion, shown for 5W into 2 ohms, has a discernible second-order component but significant higher-order harmonics (though these are at a very low level).
Fig.4 Kinergetics KBA-280 unbalanced), THD+noise vs frequency at (from top to bottom at 5kHz): 4W into 2 ohms; 2.83V into simulated speaker load; 2W into 4 ohms; and 1W into 8 ohms.
Fig.5 Kinergetics KBA-280, 1kHz waveform at 5W into 4 ohms, (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).