Counterpoint SA-100 power amplifier Measurements
All the measurements were made after each amplifier had been preconditioned by running it at one-third full power into 8 ohms for an hour, which thermally stresses the amp to the maximum. Though the Counterpoint didn't overheat, its heatsinks did get too hot to touch. The review logistics meant that all the measurements were performed on the first sample. I was a little surprised by the Counterpoint's small-signal frequency response; it showed a rising trend in the top audio octave (fig.1), which results in a degree of overshoot on the 10kHz squarewave shape (fig.2). Reaching +0.75dB at 20kHz, it actually peaks at +1.4dB at 70–80kHz. Note also from fig.1 that the two channels don't balance very well, the right (dotted trace) being 0.75dB less sensitive than the right. Overall voltage gain of the left channel was a high 31.9dB, suggesting a 1W/8 ohms sensitivity of 72mV, while the input impedance at 1kHz measured as 95k ohms. Channel separation was excellent, being greater than 75dB across the audio band, dropping to 95dB below 800Hz.
Fig.1 Counterpoint SA-100, frequency response at 2.83V into 8 ohms (0.5dB/vertical div., right channel dashed).
Fig.2 Counterpoint SA-100, small-signal 10kHz squarewave into 8 ohms.
Like the Muse 100, the SA-100 featured a high, tubelike output impedance, this implied by the specified damping factor of 8.9 at 1kHz. (Damping factor is the load impedance, nominally 8 ohms, divided by the amplifier output impedance.) This was a little better than spec, averaging 0.65 ohms at 20Hz and 1kHz, rising very slightly to 0.72 ohms at 20kHz. This will still result in some modification of a loudspeaker's response depending on its impedance curve—dips in impedance value will result in gentle suckouts in the sound at the relevant frequencies.
Fig.3 shows the Counterpoint's THD+noise plotted against frequency at 2.83V into 8, 4, and 2 ohms (bottom two, middle two, and top traces, respectively). While lower in level in the midband than the Muse, the distortion can be seen to rise significantly in the bass. Fig.4 shows the harmonic spectrum produced with the SA-100 playing a 50Hz tone at 50W into 4 ohms. A regular, decreasing series of harmonics can be seen, totaling –42.6dB or a shade over 0.75%, with the second harmonic the strongest. While the ear is relatively forgiving of low-frequency distortion, this would still be expected to blur the sound somewhat and might well correlate with CG's dislike of the amplifier's low-frequency quality.
Fig.3 Counterpoint SA-100, THD+N (%) vs frequency at 2.83V into (from bottom to top): 8, 4, 2 ohms (right channel dashed).
Fig.4 Counterpoint SA-100, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 4 ohms (linear frequency scale).
In the midband, the predominant distortion is also second harmonic, as can be seen from the waveforms in fig.5. Though both Corey and the listeners in Santa Fe felt the SA-100 to have a grainy quality to its sound, the distortion in itself will be relatively euphonic. High-frequency intermodulation (fig.6) was only a little better than the Muse, however, with the mixture of 19 and 20kHz tones throwing a number of sidebands and lower-frequency products.
Fig.5 Counterpoint SA-100, 1kHz waveform at 2W into 4 ohms (top), THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.6 Counterpoint SA-100, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 45V peak–peak into 4 ohms (linear frequency scale).
With a 116V AC line voltage, the SA-100's maximum output power (1% THD) with one channel driven was an inconsequential shade below specification, at 95W into 8 ohms (19.8dBW) and 151W into 4 ohms (18.8dBW). Testing into 2 ohms blew the series 6A fuse in the output at 125W, below what the SA-100 will probably deliver into this load under dynamic conditions. With both channels driven, the maximum output power drooped a little, to 86.4W (19.4dBW) and 127W (18dBW) into 8 and 4 ohms, respectively (this time at a 115V AC line voltage). Fig.7 shows the manner in which the distortion into 8 and 4 ohm loads rises with output power with one channel driven. Better than Counterpoint's spec, in that the overall THD levels are considerably lower, the somewhat gentle "knee" in the curves is reminiscent of tube amplifier overload (see fig.7 in the VTL measurements).
Fig.7 Counterpoint SA-100, distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 8 and 4 ohms.
Overall, I found the SA-100's measured performance to be a puzzle. With the exception of the bass frequencies, from its quite similar distortion footprint and output impedance it might be expected to sound somewhat similar to the Muse Model 100. Yet, from both Corey's auditioning and my own, it sounded significantly more grainy. Perhaps—and this is a monstrous "perhaps"—that ultrasonic peak in this sample of the SA-100's response frequency is indicative of a deeper-rooted problem that my simple measurements failed to reveal.—John Atkinson