B&K ST-140 power amplifier 1989 Measurements

Sidebar 2: 1989 Measurements

The B&K ST-140 exhibited some strange behavior on the test bench. It had no problem driving an 8 ohm load, delivering 115.3W (20.6dBW). Driving 4 ohms, however, proved to be too much of a challenge. When the amplifier was driven hard into 4 ohms, its distortion increased due to the waveform becoming flat on the negative voltage swing. It finally went into a runaway condition, its distortion increasing and finally going into LF oscillation (fig.1). This condition occurred well below its clipping point. It was thus impossible to measure its continuous maximum output level at 4 ohms, much less 2 ohms.

Fig.1 B&K ST-140, 1/3-octave spectral analysis of 1kHz tone at 90W into 4 ohms (onset of LF instability) (10dB/vertical div.)

The 4 amp output fuses blew during the bench testing into a 4 ohm load. By driving the 4 ohm load for less than a second, I found that the ST-140 would deliver 157W (19dBW). This suggests that the ST-140 would most definitely not be a good choice to use with loudspeakers possessing impedances dropping much below 8 ohms—remember that Guy did report a sense of strain with this amplifier.

The ST-140 also showed a marked rise in distortion at high frequencies, which together with its problems into low-impedance loads, may correlate with the amp's slightly fizzy tonal balance. Fig.2 shows the increase in THD with frequency, reaching 0.7% at 20kHz with 5W output into an 8 ohm resistive load. Output impedance was 0.076 ohm at 20Hz, increasing to 0.21 ohm at 20kHz. Frequency response (fig.3) showed about a 0.4dB rolloff at 20kHz.—Robert Harley

Fig.2 B&K ST-140, THD+noise (%) vs frequency at 5W into 8 ohms.

Fig.3 B&K ST-140, frequency response at 1W into 8 ohms (0.2dB/vertical div.)