NAD C 298 power amplifier Measurements

Sidebar 4: Measurements

The NAD C 298 can be operated as a conventional stereo amplifier or as a bridged-mono amplifier. (In mono mode, the signal is fed to the left input and the output taken from the left channel's positive binding post and the right channel's negative binding post.) As Kal Rubinson auditioned the NAD both in stereo mode and as a pair of monoblocks, I performed a complete set of measurements in both modes. The C 298 has an output stage operating in class-D, so I inserted an Audio Precision auxiliary AUX-0025 passive low-pass filter between the test load and my Audio Precision SYS2722 system (see the January 2008 "As We See It"). This filter eliminates RF noise that could drive the SYS2722's input circuitry into slew-rate limiting, and I used it for all the tests other than frequency response.

The C 298 has both balanced and single-ended inputs; I performed most of the measurements using the balanced inputs, repeating some tests with the unbalanced inputs. With the amplifier in stereo and fixed-gain modes, I measured a voltage gain of 28.6dB into 8 ohms with both types of inputs. With the amplifier switched to variable-gain mode, the voltage gain could be varied between 9.35dB and 28.5dB. In mono mode, the C 298's gain into 8 ohms was 25.3dB in fixed-gain mode but could be varied between 15.4dB and 34.55dB in the variable-gain mode. The amplifier preserved absolute polarity (ie, was noninverting) with both types of inputs in both stereo and mono modes, as well as from the preamplifier output. The XLR jacks are wired with pin 2 hot.

The input impedance is specified as a high 56k ohms with both types of input. My measurement was close to the specification for the single-ended inputs, at 49k ohms at 20Hz and 1kHz. This dropped inconsequentially to 35k ohms at the top of the audioband. The balanced input impedance was 100k ohms at 1kHz and a little lower at the frequency extremes. The gain at the preamplifier outputs was –0.1dB, sourced from an impedance of 655 ohms at 20Hz and 385 ohms at 1kHz and 20kHz.

With the NAD amplifier in stereo mode, I measured a very low output impedance of 0.06 ohms at 20Hz and 1kHz, rising slightly to 0.072 ohms at 20kHz. (These figures include the series impedance of a 6', spaced-pair speaker cable.) The output impedance was only slightly higher in mono mode, at 0.063 ohms at 20Hz and 1kHz, and 0.082 ohms at 20kHz. The modulation of the NAD's frequency response driving our standard simulated loudspeaker was therefore very low, at ±0.05dB (fig.1, gray trace). This graph was taken in stereo mode; the responses were identical in mono mode. The small-signal bandwidth was restricted by the low-pass filter between the amplifier's class-D stage and its output terminals. Into 8 ohms (fig.1, blue trace), the ultrasonic rolloff reached –3dB at 66kHz. This rolloff lengthened the risetimes of a 10kHz squarewave (fig.2). There is a critically damped overshoot on the tops and bottoms of the waveform, but there is no ringing.


Fig.1 NAD C 298, stereo mode, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (green) (0.5dB/vertical div.).


Fig.2 NAD C 298, stereo mode, small-signal, 10kHz squarewave into 8 ohms.

Channel separation in stereo mode was superb, at >110dB in both directions below 1kHz and still close to 80dB at 20kHz. Without the auxiliary low-pass filter, 180mV of ultrasonic noise was present at the C 298's output terminals. With the AP filter and the C 298 set to stereo mode and fixed gain, the unweighted, wideband signal/ noise ratio, taken with the single-ended inputs shorted to ground, was 78.5dB (average of both channel) ref. 1W into 8 ohms. It improved to 95.1dB when I restricted the measurement to the audioband and to 99dB with an A-weighting filter in circuit.

These ratios were affected only slightly by switching the amplifier in stereo mode to variable-gain mode and adjusting the gain from minimum to maximum. This can be seen in fig.3, which shows the spectrum of the low-frequency noise floor with the gain set to its minimum (green and gray traces) and to its maximum (blue and red traces). The only power supply–related component visible in this graph is at 60Hz, but this is negligible at almost –120dB ref. 1W into 8 ohms. With the C 298 set to bridged mono and with the variable gain set to its minimum, the noise floor was identical to what it had been in stereo mode other than no longer having a 60Hz component (fig.4, blue trace). However, increasing the gain to its maximum now raised the noise floor by around 10dB (red trace).


Fig.3 NAD C 298, stereo mode and variable gain, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with maximum gain (left channel blue, right red) and minimum gain (left green, right gray) (linear frequency scale).


Fig.4 NAD C 298, mono mode, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with maximum variable gain (red) and fixed gain (blue) (linear frequency scale).

The C 298's rated power in stereo mode is specified as 185Wpc into 8 ohms (22.67dBW) and 340W into 4 ohms (22.3dBW ref. 1W into 8 ohms). The NAD exceeded its specified power into both impedances. With both channels driven and clipping defined as when the THD+noise in the output reaches 1%, the C 298 clipped at 275W into 8 ohms (24.4dBW, fig.5) and at 510W into 4 ohms (fig.6, 24.1dBW). What is extraordinary about these two graphs is that the harmonic distortion reaches 0.0005% or lower at powers between 20W and 180W into 8 ohms and between 40W and 70W into 4 ohms. (Below those regions, the traces in these graphs are dominated by noise.) The C 298 is one of the lowest-distortion amplifiers I have measured, rivaling the less powerful Benchmark AHB2 that KR reviewed in November 2015.


Fig.5 NAD C 298, stereo mode, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.6 NAD C 298, stereo mode, distortion (%) vs 1kHz continuous output power into 4 ohms.

In bridged-mono mode, the NAD's rated power is specified as 620W into 8 ohms (27.9dBW). Again, the C 298 exceeded its specified power, clipping at 980W into 8 ohms (29.9dBW, fig.7). NAD doesn't recommend using the amplifier in mono mode to drive impedances below 8 ohms. Nevertheless, I measured a clipping power of 880W into 4 ohms (26.4dBW, not shown, footnote 1).


Fig.7 NAD C 298, mono mode, distortion (%) vs 1kHz continuous output power into 8 ohms.

I examined how the THD+N percentage in the C 298's output in stereo mode varied with frequency at 14V (equivalent to 24.5W into 8 ohms, 49W into 4 ohms, and 98W into 2 ohms). I tried a higher output level—20V—but the amplifier went into protection mode after a short while driving the higher frequencies at 2 ohms. The distortion into 8 ohms was very low (fig.8, blue and red traces), though it started to rise in the top octave. The distortion hardly rose into 4 ohms (cyan and magenta traces) and only by a little into 2 ohms (green and gray traces).


Fig.8 NAD C 298, stereo mode, THD+N (%) vs frequency at 14V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (left green, right gray).

The NAD amplifier's distortion signature at high power in both stereo and mono modes into 4 ohms was primarily third harmonic (fig.9). (The distortion was below the noise floor at lower powers, especially into 8 ohms.) Spectral analysis confirmed that this harmonic was slightly higher in level in the right channel (fig.10, red trace) than the left (blue trace), though at –124dB ref. 100W into 8 ohms (0.00006%), it is negligible. Note the commendable absence of higher-order harmonics in this graph. At the same output voltage into 4 ohms, which is equivalent to 200W, the third harmonic rose to –110dB in the left channel and to a still very low –104dB in the right (fig.11). In bridged-mono mode at 100W into 8 ohms, the second and third harmonics both lay at –120dB (0.0001%, fig.12), though the third harmonic rose to –100dB (0.001%) at the same voltage into 4 ohms (fig.13).


Fig.9 NAD C 298, stereo mode, left channel, 1kHz waveform at 200W into 4 ohms, 0.0031% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.10 NAD C 298, stereo mode, spectrum of 50Hz sinewave, DC–1kHz, at 100Wpc into 8 ohms (left channel blue, right red, linear frequency scale).


Fig.11 NAD C 298, stereo mode, spectrum of 50Hz sinewave, DC–1kHz, at 200Wpc into 4 ohms (left channel blue, right red, linear frequency scale).


Fig.12 NAD C 298, mono mode, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 8 ohms (linear frequency scale).


Fig.13 NAD C 298, mono mode, spectrum of 50Hz sinewave, DC–1kHz, at 200W into 4 ohms (linear frequency scale).

With an equal mix of 19 and 20kHz tones and the signal peaking at 200W into 4 ohms, intermodulation in stereo mode was extremely low (fig.14). Surprisingly, the higher-order products were even lower in mono mode at 00W into 8 ohms, though the difference product at 1kHz still lay below –120dB (fig.15).


Fig.14 NAD C 298, stereo mode, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 200Wpc peak into 4 ohms (left channel blue, right red, linear frequency scale).


Fig.15 NAD C 298, mono mode, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 8 ohms (linear frequency scale).

Going back to the days when NAD's amplifiers were designed by the late Björn Erik Edvardsen, I have always been impressed by the company's conservative and competent engineering. The NAD C 298 continues that tradition but, with its "Eigentakt" class-D output modules, sets a new standard for combining very high power with supremely low distortion.—John Atkinson

Footnote 1: This impressively high power could not however be sustained for more than 30 seconds or so. The owner's manual says that running the C 298 in bridged-mono mode to drive impedances lower than 8 ohms "may cause the amplifier's thermal cut-out to operate if played at high levels." Very likely, a bridged NAD 298 would perform well into loads less than 8 ohms at normal listening levels—and it would then have impressive short-term reserves to draw on for transient power. Indeed, while NAD does not recommend bridged C 298 for use with loads below 8 ohms, it specifies the bridged amp as having an impressive 1100W of "IHF dynamic power" into 4 ohms when bridged.

What's going on? In effect, bridging an amplifier cuts the impedance the amplifier sees in half, so while the amp has more power, it needs more power to drive a more difficult load. The same phenomenon is at work in all bridged amplifiers.—Editor

NAD Electronics International
633 Granite Ct.
Pickering, Ontario L1W 3K1
(905) 831-6555

Charles E Flynn's picture

There is a pdf of the December 1975 issue of "Wireless World" here. The article "Current Dumping Audio Amplifier" is on pages 560-563. There are responses at the end of the issue. Search on the page for "walker".

tonykaz's picture

Certainly, you are that high integrity listener that was impressed.

What will the next Decade bring?

Tony in Venice Florida

Ortofan's picture

... collaborate with Bob Carver to develop a Purifi amp module with sound quality indistinguishable from a Conrad-Johnson tube amp.
The NAD amp using those new modules will be the C 298t.

JRT's picture

Some recordings deserve high purity in the playback signal chain. Some others might be better with some added manipulation, coloration, masking. Subjective opinions vary.

There are existing products that purport to emulate by various means the nonlinear behavior of various desirable guitar amplifiers, microphone preamplifiers, and other audio gear utilized in the creation of music. One of the more promising paths forward utilizes the processing power of the digital audio workstation computer's graphics processing unit (GPU). It is early days, with lot of room for improvement going forward.

More to the application of playback gear, I would expect to see software applications providing the consumer with varied subjectively euphonic nonlinear distortion colorations specifically mapped to playback material. There could be a separate market for creative works providing those mappings separate from the original work, such that they would not have to license rights to the music to provide the remastering.

Example of the technological development toward what I am referring to (pdf download):

donnrut's picture

As I got older I wished someone made a preamp that had a setting or two for age related hi freq hearing loss, primarily to boost the hi freq in a gentle curve up. In the last 5 years or so of digital wizardy, I don't know if any audio brand hardware or software has done it, I am woefully ignorant of current digital software.

But, lo and behold, I got new hearing aids last year, Widex Moment. The audiologist was happy to answer my question about listening to my home stereo. They are very programmable, and the Music program sets them to deliver to my brain their max physical limit, calculated with my loss, of a flat freq range from about 100 hz up to around 8 khz. And it has bass, mid, and hi freq volume adjust in the software on my phone or computer. In case I need some slight adjustment.

I am happy as a lobster before you turn up the heat. Meaning, I love this investment, $6K, every nickel and dime. And then, I was surprised to read that 2 years ago, M. Fremer got the same devices. See, I am way behind in catching up. I listen to both 180 gram Jethro Tull as well as 45 rpm Scheherezade. Lily Pons and Grace Slick. On a VPI Prime ttbl, a VPI Fat Boy arm, Lyra Delos cartridge, and Bohlender Graebener "ribbon" bookshelf & woofie speakers.

Wishing you all the best

tonykaz's picture

What do Conrad-Johnson tube Amps sound like ?

I was once a CJ Full Line Dealer, I can quite recall what was/were the good points of the Product Range. Hmm. ( except the sweet MV45a )
Tony in Venice Florida

Ortofan's picture

... "Carver Challenge", which was chronicled in a certain audiophile publication?
(See footnote 3 on the first page.)

tonykaz's picture

Sure, I recall.
Mr. Bob Carver is a clever guy, maybe more clever than our typical High End Manufacturer.
I was manufacturing Turntable accessories at the time. ( acrylic Mats and Arm bits )
Back then, we didn't have attentive listening folks like Mr.HR, Mr.Dudley and a few others able to discern differences in Cabling, Amps, Phono Carts., Arms, etc...
Back then we did have an abundance of review writers promoting product with Broad Stroke Endorsements. ( I brought in touted electronics and loudspeakers that were not at all Good enough to carry as a product line ) I was especially critical & leery of TAS endorsements.
Overall our highest Integrity Authority has always been Mr.JA of Stereophile although he hasn't the articulate ear of his Mr. Herb R. ( who does ? )
Mr.Carver was always a sharper Spoon in the drawer than most of our tweaky manufacturers. ( many of which I seemed to represent at one time or another ).

I have never owned any piece of Carver Gear.

Tony in Venice Florida

Kal Rubinson's picture

Some recordings deserve high purity in the playback signal chain. Some others might be better with some added manipulation, coloration, masking.

I would say preferred rather than better.


Subjective opinions vary.

Indeed, they do.

JRT's picture

Your wording better conveys the notion I had intended.

And ... Thank you for another interesting review on another interesting component.

NIkos Razis's picture

I was wondering how come KR could distinguish the piano provenance in the two Proust recordings but heard a cello in the Harmonia Mundi one… Mixing up the players is one thing, obviously a typo, but mixing up the instruments?

Kal Rubinson's picture

Guilty on both counts. I do know better, enough not to make an issue of comparing the "two cellos." Mea culpa. (Now fixed. Thanks to Jim and you.)

BluesDog's picture

An exciting turning point for Class D when purchased by Kalman Rubinson and excelling at the stringent testing of John Atkinson. Both no small feats. I have followed Class D development, which showed promise with the ATI Ncore amps of a few years ago. Superb progress by Bruno Putzys, et al. Light weight, cool running powerful amps that, in some cases surpass Class AB. What’s not to like?
Is the M22 V2 the 2 channel equivalent of the M28 or is that 2 channel offering yet to come?

Kal Rubinson's picture

Is the M22 V2 the 2 channel equivalent of the M28 or is that 2 channel offering yet to come?

In the most fundamental way, yes but not exactly. NAD uses the same amp modules in both (as well as in the M33) but the input stages, power supplies, features and packaging vary.

Long-time listener's picture

The Absolute Sound (excuse me) characterized the C298 as being "forward sounding" in the upper mids and treble, and noted that care in system matching might be called for as a result. Likewise, if the comparison here is to the PS Audio that Michael Fremer reviewed, I note that he was far from complimentary about some aspects of its sound: "The piano [in its upper registers, had a] a slight "ringy" quality, a glare or glow around the notes, like a parasitic halo. The vibraphone had it throughout its range, combined with a blunt and less-than-satisfying roundness to what should be a shimmering bell tone." Kalman Rubinson seems to have found the two roughly comparable in sound quality, though different in character.

So: Aren't we still pretty much in Class D territory here? Or not? Please comment.

curmudgeon47's picture

Perhaps a better comparison would be to the PS Audio M700s, which are closer in price and power output than the M1200s. I can commend an audition of the M700s to anyone seeking high powered amplifiers. I think
they are wonderful.

georgehifi's picture

Why are some of the bench test sins of Class-d hidden by the use of the low pass AUX-0025 filter just because the input of the SYS-2722 analyser can't handle the residual switching noise on the amp being tested speaker terminals.

Wouldn't it be best to get the input of the analyser headroom raised to take the residual switching noise, so all can then see what's really comming out of Class-D's speaker terminals?

Cheers George

tabs's picture

I get what you mean about presenting the raw data, but are you suggesting that you can hear ultrasonic noise or that it has any relevance? Forget 25khz or even 50khz which is already outside your ability as a human to perceive. With Purifi we’re talking a narrow bit of switching frequency noise at 500khz at -20db and a harmonic at 1mghz at -65db (1khz test tone, per AudioScienceReview measurement of the Purifi 1ET400A module). The rest of the ultrasonic noise is remarkably clean and sub-100db down. Even if your ears could hear that, do you think your loudspeaker is responding to that signal at all? Impossible. Put it out of your mind.

If you want to talk about sins then talk about the amps that cost multitudes of the C298 which show horrendous distortion and noise ALL within the actual audioband! The C298 presents as clean of an audioband as anyone can hope for at this power output and at any price. The fact that this level of performance is now as attainable and mainstream as a $2k unit from NAD is something to celebrate.

Kal Rubinson's picture

The C298 presents as clean of an audioband as anyone can hope for at this power output and at any price. The fact that this level of performance is now as attainable and mainstream as a $2k unit from NAD is something to celebrate.


georgehifi's picture

"Even if your ears could hear that, do you think your loudspeaker is responding to that signal at all? Impossible. Put it out of your mind."

READ!!! I had to replace a friends pair of Watt Puppy 7's tweeter diaphragms, because the the voice coils turned blue with heat after 6mts of constant little too high level of this "ultra sonic bombardment", from a very well known reviewed here Class-D monoblocks.

So let the warts and all be seen, and get the AP analyzers input changed so it doesn't over load so we can see everything.

Linear amps in the past were always shown by Stereophile if they oscillated, why not these too, even though it's for different reasons, but still ends up at the speaker output terminals

Cheers George

tabs's picture

Your reply makes it sound like I lack reading comprehension, but nowhere did you claim to have first-hand experience with damage to speakers from Class D. I don’t even want to get into asking you how you know it was definitely ultrasonic noise that fried your voice coils, but consider me dubious.

Name names. Which amp?

georgehifi's picture

Nuforce Reference-9SE V2 or 3 monoblocks with newest Nuforce updated output filters, and the owner never pushed his speakers.
Just the tweeters started to sound off over time, he sold them after I showed him the bluing of the old voice coils when (like I said) I replaced them with the new ones, and after that he went back to his Halco DM68's

Stereophile used to show warts and all with and without AP filter with no troubles to the AP analyzing gear in the past, even with the massive powerfull 2kw 4ohm Anthem Statement Reference Class-D monoblocks

Cheers George