Counterpoint Natural Progression NPS-400 power amplifier Measurements

Sidebar 2: Measurements

A full set of measurements of the Counterpoint NPS-400A was made in its unbalanced mode, with selected measurements repeated for the balanced configuration. Unless otherwise noted, the results presented are for the unbalanced configuration.

Following the 1/3-power, one-hour preconditioning test, the NPS-400A's heatsinks were very warm, but not unusually so. The NPS-400A is non-inverting in the unbalanced mode; in the balanced, pin 2 is configured as the positive leg, pin 3 the negative.

The NPS-400A's input impedance measured 96.5k ohms at 1kHz (unbalanced) and 205k ohms (balanced). Its output impedance was between 0.14 and 0.15 ohms, depending on frequency and the load impedance used to take the measurement (footnote 1). Voltage gain into 8 ohms measured 29.2dB unbalanced, and virtually the same balanced. In the bridged mode, the gain was 34.6dB. DC offset, though fluctuating with time, peaked at just above 27mV in the left channel, 25mV in the right. Signal/noise ratio (unweighted, 22Hz–22kHz ref. 2.83V output, 1W into 8 ohms) measured 82dB.

Fig.1 shows the frequency response of the NPS-400A driven from its balanced inputs at 1W into 8 ohms (the response with a 4 ohm load was virtually identical). The response in the bridged mode into 8 ohms is also shown (bottom trace). Both show some rolloff in the top octave; only the rolloff in the bridged mode, –1.25dB, is likely to be audible under some conditions. (With only one sample of the amplifier on hand, I didn't listen to it in the bridged mode.) The amplifier's response to a 10kHz squarewave input is shown in fig.2. The rounding on the leading edge is a result of the HF rolloff visible in fig.1. The 1kHz squarewave response was virtually perfect but for the smallest rounding on the leading corners, and is not shown.


Fig.1 Counterpoint NPS-400A, frequency response into 8 ohms in normal mode (top) and in bridged mode (bottom) (0.5dB/vertical div., right channel dashed).


Fig.2 Counterpoint NPS-400A, 10kHz squarewave.

The channel separation shown in fig.3 indicates a good performance. While noticeably better in one channel, this difference will doubtfully have any audible consequences when the worst crosstalk is still more than 70dB down (at 20kHz).


Fig.3 Counterpoint NPS-400A, crosstalk: L–R (top), R–L (bottom) (10dB/vertical div.).

The THD+noise vs frequency curves are plotted in fig.4. The distortion increases at higher frequencies—more so at lower load impedances. The 1kHz distortion waveform (fig.5) shows primarily a second-harmonic component. Into a lower load impedance (fig.6), however, higher-order components enter the picture and are clearly visible in the spikier nature of the spuriae curve (bottom trace).


Fig.4 Counterpoint NPS-400A, THD+noise (%) vs frequency at (from bottom to top): 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms.


Fig.5 Counterpoint NPS-400A, 1kHz waveform at 1W into 8 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.6 Counterpoint NPS-400A, 1kHz waveform at 4W into 2 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

The spectrum of the NPS-400A's output reproducing a 50Hz input signal into 4 ohms at a 268W level is shown in fig.7. Only the second harmonic, at –52.4dB (about 0.25%), is of any consequence—a fine result overall, though the persistence of the higher odd-order distortion harmonics should be noted. Fig.8 shows the output spectrum of a combined 19+20kHz signal—the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies—at 268W into 4 ohms. The largest artifacts here are at 18kHz and 21kHz (about –44dB, or about 0.6%), with the next largest at 17kHz (–59dB, or about 0.11%). The 1kHz difference tone lies just below 0.1%, –60dB. The 19+20kHz artifacts into 8 ohms (at 134W) are at even lower levels, and are not shown.


Fig.7 Counterpoint NPS-400A, spectrum of 50Hz sinewave, DC–1kHz, at 268W into 4 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level at –52.4dB (0.25%).


Fig.8 Counterpoint NPS-400A, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 268W into 4 ohms (linear frequency scale).

The 1kHz, THD+N vs output power curves (at 1kHz) are shown in fig.9, the same for the bridged mode in fig.10. The NPS-400A is not short of power—particularly bridged, when it will deliver more than 1kW into 4 ohms! The NPS-400A's discrete clipping levels (at 1% THD+N) are shown in Table 1, indicating that the amplifier comfortably exceeds its rated power specification at the usual 1% THD+N limit. At no time during the tests did the Counterpoint blow any fuses—a notable achievement for an amplifier this powerful.


Fig.9 Counterpoint NPS-400A, distortion (%) vs output power into (from bottom to top at 100W): 8 ohms, 4 ohms, and 2 ohms.


Fig.10 Counterpoint NPS-400A in bridged mode, distortion (%) vs output power into (from bottom to top at 100W): 8 ohms, 4 ohms, and 2 ohms.

The NPS-400A meets its power specs handily, its clipping power levels well in excess of its rating. In fact, with the exception of high-frequency linearity, all of its bench-test results are excellent.

Measurements addition: All of the measurements presented above, unless otherwise noted, were made on the initial sample.

I did measure the frequency response of the newer sample of the NPS-400A. The curve for this isn't shown, but it did show a steeper rolloff in the high-frequency response—just under –0.45dB at 10kHz, –1dB at 16kHz, and –1.6dB at 20kHz. Whether or not the added rolloff is responsible for the sweeter sound of the new sample, it is consistent with it. But the difference between the two samples is about 0.3dB at 10kHz, 0.7db at 16kHz—arguably an audible, if subtle, change. There was also a channel-balance mismatch in the new sample of about 0.35dB—which I had measured prior to my auditions and accounted for in my listening comparisons.—Thomas J. Norton

Footnote 1: We assess output impedance by comparing the measured output voltage at frequencies of 20Hz, 1kHz, and 20kHz into a very high resistive impedance (100k ohms), with the voltage into 8 or 4 ohms.—John Atkinson
Counterpoint Electronic Systems
Company no longer in existence (2021)

Glotz's picture

More hybrid AB amps need to come out!

Or impress us with more tube hybrid Class D... Rogue and AVA come to mind along with AR.

And thank you for the PS Audio '1200 review.