Solid State Power Amp Reviews

I didn't really expect any surprises in our measurements of the Threshold SA/12e, and none were found. It would, in fact, be hard to imagine better objective performance at the current state of the art. Frequency response was a total non-event—that is, flat within 0.1dB from 20Hz–20kHz (fig.1). Fig.2 shows the Threshold's THD+noise. The three lowest curves indicate (respectively) the low power figures of 1W into 8 ohms, 2W into 4 ohms, and 4W into 2 ohms. The third curve from the top is the 250W into 8 ohm measurement. It edges just barely over the 0.1% specification above about 18kHz, but by a trivial amount which could be accounted for by slight differences in line voltage and grounding. Note that at 850W output into 2 ohms, the distortion remains well below 1%. Although not shown here, levels of intermodulation distortion were equally low.

Fig.1 Threshold SA/12e, frequency response at 1W into 8 ohms (0.5dB/vertical div.)

Fig.2 Threshold SA/12e, THD+noise (%) at (from bottom to top): 1W/8 ohms, 2W/4 ohms, 4W/2 ohms, 250W/8 ohms, 500W/4 ohms, 850W/2 ohms.

Clipping of the SA/12e (for 1% THD) was measured at 310.9W (24.9dBW) into 8 ohms (line voltage 115.5V), 542W (24.3dBW) into 4 ohms (line voltage 114V), and 898.9W (23.5dBW) into 2 ohms (line voltage 111.5V). Gain measured 27.5dB unbalanced and 26.7dB balanced (source impedance 50 ohms), both measured with the amplifier loaded with 8 ohms, implying a balanced sensitivity for full output into 8 ohms of 2.3V.

Output impedance from 20Hz–1kHz remained below 0.032 ohms for any load from 2 to 8 ohms; it rose slightly at 20kHz, but still below 0.04 ohms with the same loads. This verifies Threshold's claim of a high damping factor across the entire audible bandwidth, and indicates that the SA/12e response should be a constant with virtually any real-world loudspeaker/loudspeaker cable load. (Remember, when a loudspeaker "looks back" at the amplifier, it sees not only the amplifier's output impedance but the impedance of the cable as well—including the loudspeaker's internal wiring. That will determine the ultimate damping at the loudspeaker, and may also have an effect on the latter's actual response, though rarely a significant one.)—Thomas J. Norton