NAD Masters M28 Seven Channel Power Amplifier Review

NAD’s Masters M28 Seven-Channel Power Amplifier is an amazing component that delivers a continuous 200 watts per channel into seven channels, drives any speaker load, and does it without breaking a sweat. When I say this, I mean that it doesn’t get hot, or even warm for that matter. And it does this from a relatively small box that weighs less than 50 pounds and is smaller than most AV receivers. It offers a true balanced signal path from a hybrid digital topology called Eigentakt, designed by Purifi Audio in Denmark and manufactured by NAD. It is an amazing achievement that I can’t wait to spend some time with.

Since the dawn of time, it has been an inescapable fact that for a power amplifier to deliver big wattage, it must be heavy. Through much of AV history, the go-to topology for consumer power amps has been Class AB. More efficient and economically practical than pure Class A, this design delivers plenty of clean power with low distortion. Class AB amps require large, heavy power supplies and large heavy heat sinks, just not as large and heavy as Class A. Compared to its predecessor, the NAD Masters M25 (a Class AB 7 channel amplifier with 160 watts per channel and weighing in at 96 pounds) the new M28 is half the weight with significantly more power per channel.

NAD has created many high-quality Class AB amps in the past, but their recent focus has been on Class D. Class D amplifiers use a single analog switching frequency, between about 350 kHz to 1.2 MHz, to reproduce the musical waveform by turning on and off for varying lengths of time, called Pulse Width Modulation (PWM). It is an analog process, but there are some new Class D amplifiers that have digital inputs. They are considered as Hybrid Class D, but the amplification stage is still analog.

The promise of Class D is great – lots of clean power, low distortion, and little waste heat. No giant power supplies or massive heat sinks are required. Like any new audio technology, Class D has gone through many stages of refinement, and today, it can honestly be said that its sound quality is very close to being equal to the best versions of other topologies.

The NAD Masters M28 Seven-Channel Power Amplifier is the latest in a series of Class D amps that have graced my home theater and it might just be the best one yet. Let’s take a look.

• NAD M17 M27 Masters Series Surround Seperates Review
• NAD T-778 AV Surround Receiver Review

For this review, NAD sent me just the M28 power amp. It’s a perfect match for their surround processor, the M17 V2i. I reviewed previous versions of this excellent pre/pro in 2018 and 2014.

The M28 represents NAD’s latest foray into the state of the art in Class D amplifier technology. It and the M33 streaming amplifier are the first Masters Series components to use Eigentakt modules designed by Purifi Audio in Denmark and manufactured by NAD. It’s a logical continuation and refinement of the Class D hybrid digital technology seen in previous NAD products. The goal as always is to push the limits of measured performance at all volume levels from the most subtle ambience to the loudest peaks while maintaining stability into any speaker load.

You can see from the specs that NAD has made an impressive achievement. The M28’s power rating is virtually the same regardless of how many channels are driven. With 200 watts as a conservative number into 8 ohms, it can manage stable peaks of 700 watts into 4 ohms without clipping. Where competing Class D modules might sound a bit harsh, the M28’s sound is more like what you’d expect from a high-end Class AB amplifier. It has all the warmth and character of that genre with none of the heat.

That last bit is hard to describe in words. Suffice it to say that nothing I played through the M28 during my review increased its temperature more than a few degrees above ambient. And this is achieved without the use of large heatsinks, fans, or a boat-anchor-sized power supply. It weighs quite a bit less than 50 pounds and is smaller than many AV receivers. And don’t forget, we’re talking about seven channels here, not five, not two. The M28 weighs about the same as my two-channel Emotiva XPA-200. My XPA-5 is over 75 pounds. Note that I am not saying Class AB and Class A amplifiers are not great products. I am just saying Class D is getting to be as good, without the chassis weight and inefficiency.

The M28’s exterior is styled in the classic Masters Series aesthetic with a black faceplate that appears to float on a brushed silver background. The only other features are two score-lines that separate the plate into thirds and a NAD logo surrounded by a backlit square. In standby mode, it glows orange. During power-up, it’s red for a few seconds, then turns white when its startup sequence is complete. The top has a large array of vents covered by perforated grills. To turn the amplifier on, a small touch-sensitive switch is placed on the top edge in the center.

In the back, you’ll find a full array of single-ended and balanced inputs along with substantial five-way speaker binding posts. They have a small flange set at a 45° downward angle to keep spade connectors from making contact with one another, a nice touch I haven’t seen before. The inputs are selected with tiny toggle switches. Small LEDs, one for each channel, indicate status. Green means go of course. To one side are the grounded power cord receptacle, the main power switch, and a single 12v trigger input.

The M28 interfaced with my Integra DHC-80.1 processor and powered up when I turned on the system. If you prefer to toggle power manually, touch the tiny switch on the top-center edge of the front panel.

Installing the M28 is a simple affair. After dragging my Emotiva XPA-5 out of the rack, and being thankful it was on the bottom shelf, I slid the NAD into place and connected the seven XLR cables that service my Axiom speaker system. The mains are LFR1100s and require two amp channels each while the VP180 center and QS8 surrounds receive one apiece. My banana plug terminated 10AWG Blue Jeans speaker cables went into the binding posts and were locked in place. I highly recommend locking bananas because they will fit securely into any binding post. I plugged the power cord into a Clarus Duet Power Block with wall power fed by a Clarus Crimson Power Cord.

I was fortunate in that my M28 sample had already been broken in by another reviewer, so I set about watching and listening right away.

What better way to inaugurate my review than with a viewing of Captain Marvel. Marvel films’ soundtracks are always filled with bombastic effects and subtle ambience, sometimes in the same scene. I quickly found I could turn up the volume a little higher than usual because of the M28’s super-clean presentation. The soundstage was unsurprisingly large and deep, but transitions and layering were clearly superior to my Emotiva amps. The dialog was also crystal clear and sharp. I was able to understand every word regardless of background sound intensity. The detail brought out by the M28 was stunning.

Transformers: Revenge of the Fallen is one of my favorite home theater demos. Its opening sequence is loaded with impactful sound effects that will challenge any audio setup. My first impression was one of impeccable clarity with each and every element of the soundtrack perfectly delineated and occupying its own space as well as contributing to the whole. Dialog, even when amidst violent chaos, was easily understood no matter what its register. Even the background music could be clearly heard to where I could pick out what instruments were being played. I turned up the volume to the limit of safety and the M28 never faltered. If anything, it was even cleaner. Heat? Non-existent.

The Mandalorian is the standout original series from the Disney Plus streaming service. Through my Integra processor, I was able to hear the core Dolby Digital soundtrack in 5.1. It is also available with Dolby Atmos. It completely brings the feel of the original three films (Episodes IV-VI) to its presentation and the M28 had no trouble making the most of this superb encode. Though the Mandalorian’s dialog is a bit muffled and processed through his ever-present helmet, I had no trouble understanding every word. I also enjoyed the wide dynamic range with quiet moments that put me on the edge of my seat followed by punctuations that made me jump. Surround effects panned with precision in all directions and were perfectly synced with the on-screen action. Despite the compressed format, the NAD Masters M28 made it sound fantastic.

The restoration of Star Trek: The Next Generation on Blu-ray includes masterfully done DTS-HD Master Audio 7.1 sound which greatly enhances and improves the original stereo recordings. Here again, I was blown away by the clarity of dialog which came across realistically in every register. It was never chesty or shrill, just perfectly balanced. Ambient effects like computer blips or the constant low-level rumble of the Enterprise could be heard in the surrounds and subwoofer, also in perfect balance. This classic TV show has never sounded better.

I started by setting my Integra processor to its direct mode so I could listen to a few two-channel CDs. The Brahms Symphony cycle from Sir Georg Solti and the Chicago Symphony is a true torture test. The recording is very compressed and can often sound grainy. It has little dynamic range when compared with modern performances. The M28 removed that grain and let me turn up the volume to where I could broaden the sound stage and more easily hear individual instruments. A Brahms symphony might have two or three things going on at once, and occasionally four or five. I had no trouble picking out every line and nuance Brahms wrote.

This was taken further when I cued up Anne-Sophie Mutter’s performance of Alban Berg’s Violin Concerto. This work is incredibly complex, even when just a few instruments are playing. It goes from bombastic brass fanfare to intimate chamber music in the same few seconds. The M28’s clarity when playing this CD was simply stunning. I felt as though I was sitting in the audience and in the middle of the orchestra at the same time. Only two channels were active, but the music completely surrounded me. I’ve never heard this recording sound better.

I couldn’t finish my review without a little rock n’ roll, so I pulled out my trusty copy of Foo Fighters’ Wasting Light. For me, this is a benchmark rock recording. It’s Dave Grohl’s first use of the Neve console that he liberated from Sound City Studios after its closing. As an expression of analog warmth, honesty, and quality, it has no equal. It gave the M28 a chance to really show off its character. Not only is every song presented with unfailing accuracy, it never sounds cold or digital. There is no grain or blockiness, just expression, and musicality. I haven’t found any other Class D amplifiers that offer the M28’s level of tactility and realism and this album showed that trait to its maximum.

Bench tests and technical analysis by John E. Johnson, Jr.

For the bench tests, I used the RCA inputs. Using the XLR inputs will reduce any electrical noise that accumulates in the circuit during amplification through common-mode rejection. It does not reduce the noise that is inherent to Class D circuitry.

A strong low-pass filter, called the AES17 Filter (AES is the Audio Engineering Society), is sometimes used in spectrum analysis of Class D amplifiers. It is a series of resistors, capacitors, and inductors that are placed in between the speaker output and the input of the analyzer. A low-pass filter is already in the amplifier’s output circuit, but it is not as powerful as the AES17. There is some controversy in its use, suggesting that it can cause increased distortion.

I found a more modern circuit for this purpose that is DIY, so it is much less expensive.

The designer of the circuit did mention that he found an unexpected roll-off at 20 kHz. I may build one of these in the future just to show what it does (and does not).

The AES17 design can also be implemented as a digital filter, but you would need the software required to use it. The roll-off starts just above 20 kHz.

I am not comfortable with inserting a bunch of resistors, capacitors, and inductors in the Class D amplifier signal path for bench tests. They will induce artifacts regardless of how sophisticated the design is. I understand the concern about the possibility of the switching frequency “confusing” the bench test analyzer, as the analyzer might consider a 400 kHz peak as a harmonic of the test signal. This is why I implemented the use of restricted bandwidth tests in the spectra shown in this section of the NAD M28 review. The results indicate that the switching frequency (and high-frequency noise) was not a contributor to the test results. For example, THD+N at a 1 kHz sine wave at 20 Volts output into 8 Ohms or 4 Ohms was 0.005% with a bandwidth of 80 kHz. This is very close to the rated specification of THD at 0.003% which does not include noise in the calculation.

So, let’s proceed.

At 1 kHz and 8 Ohms load, 2 Volts output, distortion is 0.026%. Noise peaks are centering at 6.5 kHz and 16.7 kHz. Smaller ones above 20 kHz.

At 4 Ohms, THD+N actually went down a bit, to 0.024%. The noise peaks are more pointed (sharper).

At 20 Volts output into 8 Ohms, THD+N is a low 0.005%. It is lower than with 2 Volts output because the signal peak is higher, but the total noise is about the same.

At 4 Ohms and 20 Volts, the THD+N is still 0.005%, but the 2nd harmonic peak has decreased, and the 3rd has increased.

The Hum Spectrum is shown here with the test signal (1 kHz) on. Peaks at 60 Hz and 180 Hz are barely visible.

With the test signal off, the 60 Hz peak is still a bit visible, but that is about all. This represents superb hum rejection.

Using 19 kHz and 20 kHz sine waves, 2 Volts output into 8 Ohms, the spectrum shows very little IM.

At 4 Ohms, there is one peak at 18 kHz and one at 21 kHz.

Raising the output to 20 Volts, with an 8 Ohms load, there are six (maybe seven) peaks on either side. There are also some peaks in the 37 kHz to 41 kHz range. This is expected at the higher voltage.

At 4 Ohms, the number of visible peaks is the same, but they are about 5-8 dB higher.

Using the standard 60 Hz and 7 kHz sine wave test tones for Intermodulation Distortion (IMD), 2 Volts output into 8 Ohms yielded 0.013% IM.

At 4 Ohms, IM rose only to 0.16%.

At higher voltage, 8 Ohms load, the measured IM was actually a bit lower than with the lower voltage. Harmonics are higher, however.

At 4 Ohms, 0.011% IM. But, again, harmonics are higher. I consider the IM between the two impedance outputs and the two voltages to be insignificantly different. Whatever IM that is there, I believe it to be complicated by noise peaks in the region immediately adjacent to the 7 kHz sine wave test signal.

Here is the Frequency Response (FR) at 8 Ohms (red traces) and two voltage outputs. It is flat to 20 kHz, down about 0.2 dB at 30 kHz, and then rolls off steeply by 40 dB at 100 kHz. This is due to the low-pass filter in the amplifier’s output circuit which is used to remove (reduce) the switching frequency peak. For the M28, it appears to be a very strong low-pass filter (rapid roll-off above 30 kHz) which may be why I got such good results without using an AES17 filter in addition. I will have more info about this in future Class D amplifier measurements.

I also measured the FR with no load (yellow traces), and it is the same as it is at 8 Ohms. The reason I did this is that some Class D amplifiers have a different FR at the high end depending on the load impedance. By “No Load”, in this case, I mean that there is no 4 Ohm or 8 Ohm resistor connected to the output. There is, however, the input impedance of the Audio Precision test instrument of 50 kOhms.

At 4 Ohms, the FR is pretty much the same. Older Class D designs would show a decreased 4 Ohm load FR between 10 kHz and 20 kHz compared to an 8 Ohm load. So, the NAD M28 is a very up-to-date design.

THD+N vs. Frequency at 8 Ohms load is shown here. With 2 Volts output, distortion is quite level. At 20 Volts output, distortion increases above 1.5 kHz, drops above 20 kHz and then goes up again due to noise shaping.

At 4 Ohms, 2 Volts output, distortion is about the same (0.02%) as it is at 8 Ohms. At 20 Volts output, it is the same as 8 Ohms at lower frequencies, then starts to rise at 900 Hz compared to 1 kHz at 8 Ohms. The peaks between 10 kHz to 20 kHz and 35 kHz to 40 kHz are higher, however.

Here is THD+N vs. 1 kHz Power Output into 8 Ohms. I performed this test using three different bandwidths: 22 kHz, 80 kHz, and 500 kHz. This is because the additional noise that is incurred with Class D amplification complicates the measurement when the bandwidth is high (I have used 500 kHz with Class A and Class A/B amplifiers to catch any RF noise). So, at 500 kHz, the spectrum is about 32% THD+ Noise, which does not show the true THD. At 80 kHz bandwidth, the distortion numbers are higher than at 22 kHz bandwidth, but not largely different. The 22 kHz bandwidth is the audible distortion and noise. It decreases from about 0.02% at low output to 0.0015% at 50 Watts and then rises quickly beginning at 185 Watts  (0.002% THD+N) and clips (1% THD+N) at 275 Watts. The rated output into 8 Ohms with all channels operating is 200 Watts. These tests only measure one amplifier channel at a time.

At 4 Ohms load, the lowest distortion is about 0.0018% at 32 Hz, using the 22 kHz bandwidth curve. It delivers 370 Watts at 0.004% THD+N, which is approximate to spec (400 Watts at 0.003% THD). Maximum power (at clipping, 1 % THD+N) is 500 Watts. Again, though, this is just with one channel operating. Nevertheless, it does show that power essentially doubles at 4 Ohms, indicating that this Class D amplifier has a substantial power supply.

John Johnson Notes: My conclusions on the NAD Masters M28 Seven-channel Power Amplifier are that it does a more than ample job, has plenty of power, low distortion, and would be an excellent upgrade for any home theater that is currently using the low wattage that comes with most home theater receivers. Class D is certainly, now, a strong competitor to Class A/B amplification.

The M28 is certainly a beautiful industrial design with its twelve transparent rectangles on top.

In my experience, the NAD Masters M28 Seven Channel Power Amplifier is a very good value in terms of performance vs. price. It may be the last multi-channel amplifier you’ll ever buy.

A power amplifier is a relatively simple device. It may use advanced and complex technologies to achieve its goals but, in the end, its job is simply to amplify incoming audio signals into something your speakers can play. I’ve auditioned a fair number of power amps in my time and most have accomplished the task competently. I can’t really say there’s been one I wouldn’t enjoy using every day.

The NAD Masters M28 Seven Channel Amplifier is a cut above the rest. I am a fan of forward presentation and powerful Class AB amps usually provide that. Class D units aren’t as much in my face, but they are always clean and accurate. The M28 is a Class D product that takes unfailing accuracy and neutrality and combines it with character.

That’s an important distinction for me and it’s one that not only impresses but creates desire. Just as fine food makes you want to eat more, or fine art rivets your attention (if you’ve ever seen an original Vermeer painting, you know what I mean), the M28 makes you want to keep listening. It enhances movie soundtracks with impact and detail. It makes music sound…well, musical. And regardless of the set volume, it performs flawlessly. Soft passages come through with delicacy while loud material stays clean and clear. And through it all, the M28 never breaks a sweat. It doesn’t even get warm.

At $4,999.00, the NAD M28 Seven Channel Power Amplifier is a premium product, but, in my experience, it is a tremendous value. It receives my highest recommendation.