NAD C 298 Stereo Amplifier Measurements

Link: reviewed by Evan McCosham on SoundStage! Hi-Fi on March 15, 2021

General information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The C 298 was conditioned for 30 minutes at 10W (8 ohms) before any measurements were performed. All measurements were taken with both channels driven, unless otherwise stated, with the C 298 connected to a dedicated 120V/20A circuit.

The C 298 offers one set each of balanced (XLR) and unbalanced (RCA) inputs, as well as a set of fixed line-level unbalanced (RCA) outputs, for daisy-chaining another amplifier or amplifiers. Unless otherwise specified, the balanced input connections were used for all measurements. No differences were seen between unbalanced and balanced input connections in terms of THD+N. There is a toggle switch on the back panel to choose between fixed gain or variable gain, for which a small potentiometer offers adjustments from 8.5 to 28.5dB (fixed gain is 28.7dB). There was a difference between the fixed- and variable-gain modes in terms of THD+N. In fixed mode, THD+N (A-weighted) at 10W into 8 ohms measured approximately 0.0004%, while variable mode yielded 0.0006%. All measurements, with the exception of gain-control accuracy, were performed in fixed-gain mode.

Variable-gain-control accuracy (measured at speaker outputs): left-right channel tracking

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the C 298 compared directly against our own measurements. The published specifications are sourced from NAD’s website, either directly printed on the site or from the manual available for download. With the exception of frequency response, where the Audio Precision bandwidth is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

* the power level at the “knee” from our measured THD ratio vs output power graphs below

** the NAD THD specification on their website is published as “20Hz-20kHz,” however, after discussions with NAD, it was discovered that the company actually measures THD from 20Hz to 6.5kHz, with a 20Hz to 20kHz input bandwidth filter.

Our primary measurements revealed the following using the balanced line-level inputs (unless specified, assume a 1kHz sinewave, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the C 298 was not able to sustain 528W (4 ohms) using an 80Hz tone burst (500ms) without causing the protection circuit to almost immediately shut down the amp. But we were able to achieve 517W into 4 ohms for 500ms, alternating with the same signal at -10dB of the peak (51.7W) for five seconds, for five continuous minutes without inducing a fault or the initiation of a protective circuit. During the test, the top of the C 298 was slightly warm to the touch. This test is meant to simulate sporadic dynamic bass peaks in music and movies.

Frequency response (8-ohm loading)

In our frequency-response chart above, the C 298 is essentially flat within the audioband (20Hz to 20kHz), with the exception of an insignificant 0.02dB rise between 10 and 20kHz, and a 0.02dB dip at 20Hz. NAD’s claim of +/-0.2dB from 20Hz to 20kHz is corroborated by our measurement, as is the -3dB point at 60kHz. The C 298 cannot be considered a high-bandwidth audio device however, due to a steep rolloff starting at about 30kHz. In the graph above and most of the graphs below, only a single trace may be easily visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the balanced line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink is an actual speaker (Focal Chora 806, measurements can found here), and the cyan is no load connected. The graph above shows that all four plots are indistinguishable from one another. This is an indication of a high damping factor, or low output impedance.

The graph above shows the same data as the graph above it, but with the vertical axis expanded to show differences. Here we find that there’s a total deviation of only 0.02dB in the flat portion of the curve between 4 ohms and no load. At 20kHz, the spread is a little larger, perhaps 0.03dB. The maximum variation in RMS level when a real speaker was used as a load is extremely small, deviating by less than 0.01dB within the flat portion of the curve. There is a rise around 10kHz, which is due to the C 298’s inherent frequency response and not the output impedance of the C 298 interacting with a load that varies with frequency.

Phase response

Above is the phase-response plot for the C 298 from 20Hz to 20kHz. The C 298 does not invert polarity, and there is virtually no phase shift throughout the audioband, with a worst case of under +10 degrees at 20Hz.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the C 298’s output into 8 ohms as a function of frequency (20Hz to 20kHz) for a sinewave stimulus at the balanced line-level inputs. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the rated 185Wpc. There is about a 10dB improvement in THD ratios at 10W versus 1W. At 10W, the THD values are extremely low from 20Hz to 2kHz, hovering above and below 0.0001%. Due to these extremely low values (as low as 0.00007%), it’s important to note that we are approaching the limits of the APx555 analyzer, which inherently measures at approximately 0.00001-0.00002% THD when fed its own signal at these output levels. The rise in THD above 2kHz is steeper at 10W compared to 1W, measuring about 0.01% at 20kHz against the 0.003% value at 20kHz for the 1W data. The general trend at 185W is more of a linear rise in THD as a function of frequency. For the 185W data, at 20Hz, we measured just below 0.0001%, 0.0005% at 1kHz, and rising all the way up to 0.03% at 20kHz, which is still low.

THD ratio (unweighted) vs. output power at 1kHz into 4/8 ohms

The chart above shows THD ratios measured at the output of the C 298 as a function of output power for the balanced line-level inputs, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). At 50mW, THD values are near 0.001% for both 8- and 4-ohm data. At 10W, the 8-ohm THD value is at 0.0001%, and just above 0.0002% at 4 ohms. At the “knees” in the curves, the 8-ohm THD value is at around 0.0004%, with power nearing 200W, while the 4-ohm THD value is at around 0.002%, with power at just over 400W.

THD+N ratio (unweighted) vs. output power at 1kHz into 4/8 ohms

The chart above shows THD+N ratios measured at the output of the C 298 as a function of output power for the balanced left and right line-level inputs, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). At 50mW, THD+N values are near 0.05% for both 8- and 4-ohm data, dipping down to 0.003% between 50 and 100W (8 ohms), and roughly 0.005% between 50 and 100W (4 ohms).

THD+N ratio (A-weighted) vs. output power at 1kHz into 4/8 ohms

The chart above shows THD+N ratios (A-weighted) measured at the output of the C 298 as a function of output power for the balanced line-level input, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). At 50mW, THD+N values are near 0.005% for both 8- and 4-ohm data, dipping down to 0.0003% between 20 and 100W (8 ohms), and roughly 0.0006% between 10 and 50W (4 ohms).

THD ratio (unweighted) vs. frequency at 8/4/2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the C 298 as a function of load (8/4/2 ohms) for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms and 80W into 2 ohms) for the balanced input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find increasing levels of THD from 8 to 4 to 2 ohms, with about a 5-10dB difference between 8 and 4 ohms from 20Hz to 3kHz, and a 10-15dB difference between 4 and 2 ohms from 20Hz to 5kHz. There’s a convergence of THD values between all loads at higher frequencies. Overall, even with a 2-ohm load at roughly 80W, THD values ranged from 0.001% at 20Hz to just above 0.01% at 20kHz.

FFT spectrum – 1kHz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W output. We see that the second harmonic, at 2kHz, is at a very low -130dBrA (relative to the reference 0dB signal), or 0.00003%, while the third harmonic, at 3kHz, is just below -120dBrA, or 0.0001%. The fourth and fifth harmonics are even lower, below -130dBrA. Below 1kHz, we see no noise artifacts, either at 60Hz or 120Hz, due to power-supply noise.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W output. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The second harmonic of the 50Hz signal (100Hz) can just be seen in the left channel at just over -130dBrA, or or 0.00003%, while the third harmonic at 150Hz is at -125 dBrA, or or 0.00006%, for both channels. Here again, no noise artifacts due to power-supply noise can be seen.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

Shown above is the FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone at the balanced input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at about -125dBrA for the right channel, -135dBrA for the left, while the third-order modulation products, at 17kHz and 20kHz, are just below -110dBrA. These are all extremely low values.

Square-wave response (10kHz)

The charts above and below are the 10kHz square-wave response of the C 298 into 8 ohms. The chart above is using the Audio Precision’s highest input bandwidth setting of 1MHz, while the chart below . . .

. . . is bandwidth limited to 250kHz. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, these graphs should not be used to infer or extrapolate the C 298’s slew-rate performance. Rather, they should be seen as qualitative representations of the C 298’s limited bandwidth (e.g., slow rise time and mild overshoot in the corners), but also as a representation of the noise artifacts present due to the class-D amp topology that the C 298 uses. An ideal square wave can be represented as the sum of a sine wave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot in the corners and softening of the edges, which are shown above. In the case of the C 298, however, what dominates the plateaus of the square waves in the first graph is a 500kHz sinewave, the frequency at which the switching oscillator in the class- D amp is operating (see FFT below). With the bandwidth limited to 250kHz (second graph), the 500kHz switching signal is no longer visible.

FFT spectrum of 500kHz switching frequency relative to a 1kHz tone

The C 298’s topology relies on a roughly 500kHz modulation frequency in the feedback network of the amplifier. The graph above plots an FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave, but with a wide 1MHz bandwidth. We can see that the peak at about 500kHz is quite evident, at -40dBrA or 1% relative to the 1kHz signal. There is also a peak at 1MHz, (the second harmonic of the 500kHz peak), which is -70dBrA relative to the 1kHz signal. Those two peaks—the fundamental and its second harmonic—are direct results of the design of the C 298 amp modules. The noise around those very-high-frequency signals is always present within the amplifier, but far above the audioband, and therefore inaudible. The noise is also so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway, since the output of most tweeters begins to fall off not far above 20kHz.

Damping factor vs. frequency (20Hz to 20kHz)

The chart above represents the damping factor (DF for an 8-ohm load) of the C 298 as a function of frequency. Damping factor is calculated by measuring the voltages across an 8-ohm load (V_L) and across no load (V_NL), which, in our tests, is the 200k ohms input impedance of the analyzer, and then applying the formula DF = V_L / (V_NL – V_L). For the C 298, both channels show a general trend of a higher damping factor at lower frequencies and lower damping factor at higher frequencies, varying by almost a factor of 2 between 20Hz and 20kHz. The right channel outperformed the left channel, with a peak value around 2700 between 25 and 60Hz, while the left channel peaked around 2100 at the same frequencies.

Diego Estan
Electronics Measurement Specialist