That ideal is not easy to achieve, for many reasons. Even a straight wire of any practical length and structure has properties (resistance, capacitance, inductance) that can affect how the signal is transferred to it on one end and how the signal is transferred from it on the other. These effects can be within the audible range depending on what device is at each end. Insert a complex device like a real-world amplifier (with different wires in and out) and Walker's ideal starts to seem unrealistic, although it should be possible to get close.
Walker further stated that an "audio power amplifier is required to produce an output signal that differs from the input signal in magnitude only." (footnote 1) In attempting to realize that goal, he tried both tubed and transistor designs (although he averred that the latter were superior). Today, with the availability of tube amps and an expanding range of solid state designs, we have a broad range of options but no better standard than Walker's.
The NAD C 298 stereo power amplifier is based on the Purifi Eigentakt class-D amplifier module, the most recent brainchild of Bruno Putzeys and Lars Risbo. The C 298's design incorporates feedback to achieve a linear and accurate transfer function, an approach espoused by Peter Walker, although the Purifi guys are using an approach to amplification—class-D—that even advanced thinkers in Walker's time may not have been aware of (footnote 2). So, let us not obsess about "how" the amplifier does its job and focus on how well it does it, technically (Cue JA!) and how well it plays music.
The amp arrives
Even as I was reviewing the NAD M33 Streaming Integrated Amplifier last year, it seemed obvious that NAD would soon release a stereo power amplifier using the same Purifi technology. I wrote that the M33 has "a switch to bridge the two channels into a >700W mono monster, and the line outputs allow you to add an external matching amp or two. (Do you think that NAD has something like that in the works?)" But no, the next amp NAD introduced with the Purifi technology was the M27's successor, the M28, a seven-channel power amp. (See Sidebar 1,) The M33 and the M28 are, as indicated by their common prefix, members of NAD's generous and stylish Masters Series.
When it arrived, NAD's two-channel Purifi-based power amplifier was observably not in the Masters Series but rather their more affordable, less luxurious "C" (for "Classic") series. The C 298 employs the same Eigentakt modules used in the M33 and M28, "but the power supply and input circuitry is specific to the C 298." The C 298 is heavy for a class-D amp, at about 25lb. Its appearance is plain but clean, and the amp seems well-constructed.
The front panel bears only a Standby button and two small LEDs. When you connect the AC cable and rock the main power rocker switch on the rear—nothing seems to happen. Press the front panel button, though, and after a few seconds' delay, a relay clicks as the LED above the Standby button blinks in amber. In a few more seconds, the LED turns blue and the amp is ready to make music. The slightly larger LED to the right of the Standby button lights blue if the amp is in bridged mode.
There is more going on in the back: both XLR and RCA inputs for each channel with toggle switches to select them; a pair of RCA line outputs to permit daisy-chaining the input to additional amps; a toggle to select fixed or variable gain; a gain control; and a control for the sensitivity of the Auto-Sense function. Above these connectors is a grounding lug to help remove ground-loop hum. Next is a mini-USB port for servicing, a 12V trigger input and output, and a bridged-mode switch that's intentionally hard to get at. Finally, there are two pairs of sturdy, multiway speaker terminals, that power rocker, an AC fuse holder, and an IEC receptacle.
Standard bridging strategies are verboten with Purifi amp modules since neither output terminal is at ground potential and should never be connected to an external ground. Unlike regular bridged amplifiers, where the output is taken from the left and right positive terminals, NAD uses the left positive and right negative terminals and cautions against connecting subwoofers, switches, or headphone adapters with common ground connections. Safety is a good reason for the bridging switch to be inconvenient.
If one uses the C 298 in a multichannel system or to power active multiway speakers or if one wants to mix bridged and stereo C 298s with other amps, it's necessary to adjust the gain—hence the C 298's gain-control knob, which allows you to select gain between 8.5dB and 28.5dB in stereo and between 14.5dB and 34.5dB bridged. NAD could improve this very useful feature by providing a visible index mark on the tiny knob.
Context and listening
Although my system is multichannel, there is no processor or preamplifier. The output of the DACs drives the power amps directly via balanced connections. I used the NAD's XLR inputs in both stereo and bridged modes. When playing two-channel music, I used fixed gain. Fixed gain in stereo mode (28.6dB) also worked when playing multichannel with one C 298 powering the front L/R speakers. However, to use a pair of bridged C 298 amps, my options were to use fixed gain and adjust the channel balance in JRiver or the DAC or to use variable gain to match the other channels. The character of the amp remained constant across all these permutations.
Footnote 2: Class-D amplification was invented in the 1950s, and by the mid-'60s there was even a commercial product from Sinclair Radionics, which put out a whopping 2.5W. It's likely true, though, that most designers in the hi-fi space weren't aware of it.—Editor
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COMMENTS
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".
https://worldradiohistory.com/UK/Wireless-World/70s/Wireless-World-1975-12.pdf
Certainly, you are that high integrity listener that was impressed.
What will the next Decade bring?
Tony in Venice Florida
... 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.
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):
https://arxiv.org/pdf/1804.07145
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
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
... "Carver Challenge", which was chronicled in a certain audiophile publication?
https://www.stereophile.com/content/carver-challenge
(See footnote 3 on the first page.)
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
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.
Your wording better conveys the notion I had intended.
And ... Thank you for another interesting review on another interesting component.
I fully get that classical music is KR's decided preference but does this mean that Beatles, Beach Boys and loads of other 60s and 80s pop and dance classics-even issued on CDs by major labels-are likely going to need EQ, tube preamping or other "manipulation" to sound however pleasing when played through amps like this? We will never know for sure of course because I can't recall KR reviewing any DAC, amp or preamp where he ever played any such recordings via that hardware. Would I therefore end up hating what I may hear from these recordings a lot less if I feed them via an Exasound or Okto DAC to an old Levinson, Marantz or other such Class AB solid state amp?
I listen to older recordings from the 60s to the 80s (and older), using the same system as for modern recordings and, for the most part, they sound better than ever. I would not want them to sound as they did when we had inferior methods of reproduction. Do you surmise that different music genres demand different playback systems?
Oh, I have mentioned the Beatles and other non-classical classics on occasion but they really ain't my thing. My relative lack of interest and exposure to the popular repertoire doesn't qualify me to say much more about it.
As per JFT's comments, my concern is that the system you said that you play everything on-which I assume is virtually the same hardware listed in this review-would mercilessly reveal every flaw in those old recordings as to perhaps make them way less pleasurable. Again, there's no telling to what such degree that may be as you're likely the only professional reviewer of the gear of interest-and the only sources in your reviews are high def recordings classical recordings-hardly helpful to readers with wider tastes in music.
I'm just praying that IF I'm right and most Class AB solid state amps would be a better choice for me that most of them have variable input sensitivity like those NAD and Benchmark amps, so that I can feed the DAC directly to them.
I understand your concern and, as I responded to JRT, there is no sin in making adjustments/corrections for flawed recordings. I do it all the time.
However, unless you listen only to such recordings and they are all flawed in the same way, it makes no sense to have a system with complementary flaws when corrections/adjustments are easily made. It makes more sense, to me, to have a system that is as accurate and transparent as possible for the best recordings and to feel free to make compensations for the others. I have a library of such filters on tap (no pun intended) for that purpose.
For the most part I tend to agree with you about the advantage of playing mid-fi quality recorded music on transparent wide band systems-at least if your restoration software truly can render them enjoyable enough to make you light up when you hear them. Do you use something like this? https://www.izotope.com/en/products/rx.html
Also, do you use the same or another MCH system for DVD, BD or streaming movie playback? If yes, are they the Revel F206 mains and subs and Benchmark amps for movies? And JRiver for DVD/BD movie or movie streaming playback?
And with old movies and/or those with less than pristine movie sound-even if it was transferred cleanly to formats like lossless DTS MA-what, if anything do you use to tweak the sound in real time or otherwise? Usually, I find that the most common flaw in sound from vintage movie and TV shows is restricted dynamics due to what likely was standard amounts of applied compression. Sadly, I'm not aware of anything which can undo this.
But for other kinds of sonic damage, please specify any hardware/software and basic remedial techniques you may have used to treat problems with movie sound played over your MCH system.
No movies. Only music on this system. All correction filters are based on the GEQ in JRiver.
Why so? Because even the lossless DTS MA track on too many BDs has limited frequency response and/or is often too compressed, distorted, noisy to sound pleasing on your MCH system?
Why so? First, because there is no large screen display or video projector in the room and, second, because my HT system is elsewhere.
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?
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.)
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?
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.
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.
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.
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
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.
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.
Amen.
"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
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?
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 https://ibb.co/cbj2mhf
Cheers George
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