NAD M23 Stereo Amplifier Review

NAD M23 Class D Amplifier

The NAD M23 is a 200wpc stereo amplifier utilizing the SOTA Class D Purifi 1ET400A to push the envelope of performance and efficiency. Is this the best amplifier we’ve reviewed in our 25 year history?

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Note: Original NAD M23 test report was published on 09/20/22

The NAD M23 amplifier utilizes the PuriFi Class D topologywhich is a high efficient (95% at full load), state of the art design. NADinformed me that they manufacture the Class D module and don’t simply just buy it directly from PuriFi. This allows them some leeway with regards to matching with their custom designed input stage. NAD alsodesigned their own SMPS power supply. This is quite a lot of effort on NAD’spart to build a reference amplifier from the ground up based on a SOTA Class Dtopology. Much could go wrong if care isn’t taken to ensure a clean layout andimplementation. Knowing this, I was even more excited to test the M23 amplifierto see if NAD was able to eek out every bit of performance this design offers.Will the attractive package that the M23 dons befit its performance both objectively on the test bench and subjectively listening to music? Let's find out.

The Purifi Eigentakt circuit shares with Bruno Putzeys’previous designs (UcD and Ncore) the use of full global feedback. This meansall of the loop gain is used for the entire circuit, including the outputfilter.

The big step forward for Eigentakt is that Brunodeveloped a much more thorough understanding of the large signal behavior ofself-oscillating modulators which has allowed him to push loop gain up tonearly 80 dB across the audio band, a 25 dB improvement over his previous work.This has allowed Bruno to relax the switch timing and still get a large netimprovement in performance.

The switching frequency of the Purifi 1ET400A designis a fairly standard 550kHz and the output filter physically cuts off at 35kHz,which is actually lower than many competing products. But a 10uH output coilwith a loop gain of 10,000 around it means that the output impedance is morelike 1nH. That’s less than the inductance of the binding posts!

The high switiching frequency and excellent Low Pass Filterdesign allows for this unit to be tested without a precondition filter on myAudio Precision that lesser designs would often need for accurate test results.In fact, the PuriFI Class D design is theoretically capable of lower distortionthan my Audio Analyzer is capable of measuring. The data sheet shows a THD+Ndistortion spec of .00017% or 115dB at 100 watt drive and full bandwidth! As aresult, there are some measurements that may not realize the true performanceof this product and it will be noted within this test report. All measurements wereconducted using our Audio Precision APx585 8 Channel HDMI Audio Analyzer.

NAD M23 Testing - I had to remove the alligator clips and measure at the amp side for more precision.

Editorial Note About M23 Testing:

Becausemy APx585 Audio Analyzer actually has less performance than the Purifi 1ET400Aamp module, I do my best to ensure my test gear is NOT the limiting factor inthe accuracy of sensitive distortion tests and note it within this test reportwhere applicable. In fact, during theinitial testing procedure, it was determined that the long leads and magneticalligator clips on my probes were causing higher distortion than NAD specifiedwith this amplifier. When I jerry-rigged new test leads, I somehow blew up thefirst M23 test sample and sent it back to NAD to investigate the cause offailure. The second M23 test unit performed flawlessly and the measurements inthis report have come from that unit but I also attained similar power andnoise performance on my first unit prior.

Formore information about how we measure power amplifiers, please see:

Basic Amplifier Measurement Techniques

NAD M23 Amplifier Top View

TheNAD M23 is a thing of beauty both inside and out. You can see great care was takenin the layout of this product to ensure minimal noise coupling. They didn’t usebus bars to route power and instead used heavy gauge twisted pair wire. This ismuch preferred for better signal integrity and reduction of crosstalk coupling.The amp modules are collocated in their own area with short connection paths tothe output binding posts to minimize losses. The SMPS power supply is locatedon it’s own board with the capacitors isolated from heat sensitive areas.

NADM23 Gain Structure

TheNAD M23 has 3 different gain settings which are specified as follows:

Mymeasurements verified the 19dB for low gain, 23dB for mid gain (vs 23.9dBspecified), 29dB for high setting. Oddly, the gain was the same for unbalancedor XLR. Usually in home theater systems (mandatory in THX), unbalanced wouldhave 6dB less gain on the amp side since the balanced outputs of a preamp areusually 6dB hot. This wasn’t the case here but it’s understandable since theM23 provides 3 different gain settings to suite your needs. I measured 30dB ofgain in bridged mode for the mid setting which is what NAD specifies.

Frequency Response

NAD M23 FrequencyResponse under Various Load Conditions

Imeasured the NAD M23 frequency response in 3 load conditions (open circuit, 8ohms and 4 ohms). Lesser designed Class D amplifiers will alter the amplituderesponse significantly at high frequencies but we don’t see that here. Thanks to the global feedback strategy mentioned earlier and the verylow output impedance that this produces, we see NO appreciable interaction here. Thismeans, the NAD M23 is a load invariant amp as good as some of the best linearamp designs in this regard. Under all 3 load conditions, I measured the -3dBpoint of 65kHz which is what NAD specs for this amplifier. Simply excellentmeasurement!

FFTDistortion Analysis

NADM23 1 kHz FFT @ 2.83Vrms (1 watt, 8 ohms)

TheNAD M23 produced the absolute best FFT distortion measurements I’ve seen in anamplifier. You can see when measured at 1 watt (2.83V, 8 ohms), there were nomeasurable harmonics at least within the limits of my Audio Precision testgear. When I ran both channels, I found a slight 3rd order harmonicbut quickly realized it was my other resistor load causing it. Measurements ofthis sensitivity can vary greatly just due to minor test set up differences.Note there is no ground hum in this measurement either. Simply amazing!

NAD M23 1 kHz FFT @ (200watts, 8 ohms)

Atrated power (200 watts/ch, both channels driven, 8 ohms), we see an incrediblyclean FFT with the 3rd harmonic being -113dB below the fundamental. Theamp is rated it’s cleanest according to the data sheet at 150wpc, but at 200watts, this is better than virtually any other amplifier I’ve measured to date.Bravo!

NAD M23 FrequencyResponse vs Distortion @ 1 watt and Full Rated Power (200 watts, 8 ohms)

Withthe AES17 (20kHz) filter employed, I measured the NAD M23 frequency response vsdistortion at 1 watt and full rated power (200 watts) into an 8 ohm load. Whatis remarkable about this measurement is how closely distortion tracksregardless of power level. We see even at full rated power and at 10kHz, thedistortion peaks at about 0.005% or 106dB SINAD. Simply incredible!

I also tested with a wider bandwidth filter to measureout to 20kHz but the harmonics beyond that show excessively high distortionsince it’s out of the optimized bandwidth of the amplifier feedback loop. Sincethis is irrelevant to the perceptual audio performance, I decided NOT topublish this test data. I did spot check the amplifier using a two-tone test at18.5kHz and 19.5kHz and saw similarly low distortion products. I noticed PuriFi published distortion specs on this very test which I show below for completion. The results are as expected, negligible distortion products below the two-tone test frequencies (18.5kHz and 19.5kHz).

SNR& Crosstalk

NAD M23 SNR @ 1 Watt(a-wt)

Ialways measure amplifiers at 1 watt so that apples to apples comparisons can bemade between different products that have different maximum outputcapabilities. If you want to know theSNR at rated power, then you simply take the 1 watt rating and add 20* log (V*R)^1/2/ 2.83) where V = Vrms and R is the load. In this case, that would be: 102.7dB+ 20*log(40/2.83) = 99dB + 23dB = 125.7 dB (a-wt). I also measured the amp at rated power and itproduced 126dB (a-wt) as expected.

NADspecifies the M23 at 101.7dB so I actually measured 1dB better than their spec!

NAD M23CH-CH Crosstalk (1CH, Undriven) @ 1 watt & Full Power

Thechannel-channel crosstalk was excellent for a stereo amplifier whether drivenat 1 watt or full power. I measured > 84dB of channel to channel separationat 20kHz at full rated power and below 100dB at 1 watt!

Using ourAudio Precision APx585 8-channel HDMI analyzer, we conducted a full barrage ofmulti-channel amplifier tests on NAD M23 per our Amplifier Measurement Protocol. We tested power usingthree methods all of which were taken at < 0.1% THD + N:

Keep in mind most review publications don'tdo continuous power measurements and they usually publish power measurementsinto clipping at 1% THD + N. Our measurements are very conservative as we use adedicated 20A line with no Variac to regulate line voltage. We constantly monitor the line to ensure itnever drops more than 2Vrms from nominal, which in our case was 120Vrms.

Formore info on amplifier measurements, see: The All Channels Driven (ACD) Test

NAD M23 FrequencyResponse @ Full Rated Power (2CH driven, 8 ohms)

When driven at 200 watts/ch (8 ohms, 2CH driven), the NAD M23 producedruler flat frequency response with -3dB pt at 65kHz, same as it was at 1 wattdriven. Bandwidth uniformity is excellent and distortion was under 0.01% THD+Nfor the entire sweep.

NADM23 1kHz PSweep (2CH) - 8 ohms

The NAD M23 produced 241 watts/ch at 0.1%THD+N and 259 watts/ch at 1% with both channels driven into 8 ohms. Thedistortion was exceedingly low hovering around .0003% THD+N or 110dB for SINADnerds below 150 watts. NAD rates the M23 to .00069% THD+N (103dB SINAD) atrated power which is about what I’m seeing here as well. Again, this is thelowest distortion I’ve measured from an amplifier at rated power. Excellent!

NAD M23 1kHz PSweep (2CH) - 4 ohms

When driving 4 ohm loads, the NAD M23 wasable to produce 444 watts/ch at 0.1% THD+N and 529 watts/ch at 1% THD+N withboth channels driven. This exceeds NAD’spower rating of 380 watts/ch into 4 ohm loads with both channels driven. Distortionwas again exceedingly low at 0.0003% THD+N at 250wpc and an impressively low0.0005% THD+N at 380wpc.

NADM23 1kHz PSweep (Bridged) - 8 ohms

The NAD M23 is a powerhouse when bridgeddriving an 8 ohm load. It was able to muster 898 watts at 0.1% THD+N and 1038watts at 1% THD+N. NAD rates the M23 at 700 watts when bridged into 8 ohms, sothis rating is exceeded by quite a margin in this test scenario. It's rare when Igotta break out my Big Bertha power resistor to test these power levels, but itwas an awesome site to see this amount of clean power produced by an amplifierthat wasn’t even warm to the touch delivering it. Distortion performance isn’tas stellar when bridging this amplifier as you can see the SINAD drop from 100dBbelow 100 watts to about 85dB at max power. This is still a good figure,however. I checked the frequency response bandwidth in bridged mode and it remained similar to regular 2CH operation.

NAD M23 Frequency Response - Bridged Mode, 8 ohm load

NADM23 1kHz PSweep (Bridged) - 4 ohms

CAUTION: NAD Clearly states the M23should be used with 8 ohm nominal speakers when bridging. I still couldn’tresist testing it into a 4 ohm load to see what it could produce. Be warnedthat when bridging an amp in a Bridge Tied Load configuration like this, eachamplifier sees ½ the load impedance. Thus, each of the two amplifiers in theM23 are really driving a 2 ohm load which as you can see in this test scenariois a bit too much. I had to decrease the sweep time to avoid tripping theprotection circuits which still kicked in at the very end of the power sweep.Still, I was able to measure 1,182 watts at 0.1% THD+N and 1,399 watts at 1%THD+N into a 4 ohm load. This is one of the most powerful amplifiers I’vetested to date! However, note the SINAD drop from 94dB below 100 watts to 70dBat max power. In all honestly, I do NOT recommend bridging this amplifier IFyour speakers impedance profile dips below 5 ohms. Using this amp in stereoconfiguration will provide better distortion performance and increase longevityof the product for difficult speaker loads. IF for some reason you need morepower, go with dedicated high power monoblocs instead.

Editorial Note AboutAmplifier Bridging

Examining the data sheet for 1ET400A PurifiAmp module, it seems they are current limited to about 24 amps which is alimiting factor when bridging two of these amplifiers together like what isdone in the M23. Since each module essentially sees half the load impedance, it’s beingasked to deliver 26 amps into a 4 ohm load to produce 1400 watts that Imeasured which exceeds the safety limits of the module.

It appears that the increased distortion in bridgedmode is due to the fact that each module is scrupulously controlling thevoltage that appears between its own output terminals. Afterall, this is wherethe speaker is normally connected. In bridged mode however, the speaker getsconnected between two modules’ positive terminals. Any potential differencebetween the two modules’ negative (ground) terminals thus gets added to the voltageseen by the speaker. Older generations of class D amplifiers had high enoughdistortion for this problem to go unnoticed, but the native performance of the1ET400A is so good that any distortion caused by wiring sticks out like a sorethumb.

It turns out that the modules actually have aprovision to mitigate this. The feedback connection can be detached from themodule output and reconnected elsewhere. In the case of bridging, distortioncan be reduced by tying the cold terminals of the feedback lines to some commonground point. That way, the sum of the voltages seen by both feedback loopcorresponds to the voltage across the two positive speaker terminals.Unfortunately, it doesn’t appear that NAD has used this trick in the M23.Simply trying to minimise the resistance between the grounds of the moduleisn’t good enough if you want to maintain SOTA performance in bridged mode.

Acloser look at the Purifi 1ET400A Spec sheet confirmed my concerns aboutBridging:

Similarly, bridging two 1ET400A Designs willresult in performance degradation as the circuit is not configured to sense thevoltage differential that exists between the (now unused) OUT- terminals of thetwo 1ET400A Designs. Bridging is therefore NOT recommended. Alloperation and performance specs are void in this configuration.

NAD Response about Bridging the M23Amplifier

With regards to the trick of lowering the distortionby re-wiring of the feedback signal for bridge mode; we’re well aware of thispossibility but decided against this. We decided to optimise the M23 for normalstereo use. With distortion levels as low as they are, just adding switchingfor re-routing the feedback signal will affect distortion in normal operationmode.

We channel-match the Left and Right modules to be asclose as possible; this helps reduce distortion in bridge mode also. Indeed, ifbridge operation would be permanent and thus not switchable, it’s logical toapply re-routing the feedback loop.

As performance in bridge mode still is as good as someof the best amps out there, we decided this was an acceptable compromise.

NADM23 CEA 2006 Dynamic Power – 4 ohms

The CEA-2006 burst tests simulate musical program material toillustrate dynamic capabilities of the amplifier. The NAD M23 was able toproduce 248 watts/ch at 1% THD+N into 8 ohms and 460 watts/ch at 1% THD+N into4 ohm loads with both channels driven. It’s a bit odd to see lower powerfigures in the dynamic power test than the 1kHz power sweep tests but I canonly guess that has something to do with the SMPS power supply limitations or because it's regulated unlike a linear power supply.Still, these are good power figures.

NAD M23 Power MeasurementTable

*NAD does not recommendbridging the M23 when driving 4 ohm speakers. The amp will current limit andpotentially shut down. I had to reduce sweep times to even test the amp inbridged mode when driving a 4 ohm load so the power figured I measured hereinare not sustainable.

I spent extensive time listening to the NAD M23 stereo amplifier and comparing it to my reference Anthem STR amp in my two-channel music room. The Anthem is no slouch; it has produced superb bench tests and has been a workhorse for me for many years. The preamp used in this listening session was the venerable Anthem STR, which has served me well in my main theater system at my previous house for two-channel applications and in home theater bypass mode, connected to my multi-channel 7.4.4 speaker system. I did my best to level-match both amplifiers, though my testing wasn't entirely instantaneous, nor was it blind, as I knew firsthand which amp I was listening to. However, I ran these tests over several days and lived with each amplifier in my system. The speakers used during this test were the Revel F328be. The cables were Kimber 8TC connecting my Revel's to the amplifier and Belden 1694A from Bluejeanscable.com for all analog and digital connections between my Marantz 30n SACD network player and Anthem STR preamp.The Anthem STR Amp is more powerful than the NAD M23, so this isn’t exactly a fair comparison. However, at no point during this testing did I exceed either amp’s capabilities or drive them into audible clipping. Remember, the NAD M23 has such exceedingly low distortion that it doesn’t produce audible clipping distortion until it reaches well into the vertical region of clipping in the power vs. distortion graph.

Sade - Love Deluxe?

Listening to Sade – Love Deluxe, streamed from Tidal on my Marantz 30n SACD network player, revealed just how beautifully this recording can sound on high-quality audio equipment. With the Anthem STR powering my Revel speakers, the bass in "No Ordinary Love" was powerful and deep, while Sade's vocal presentation felt vibrant and lifelike.Switching over to the NAD M23, I initially thought the bass had thinned out slightly until I realized what was actually happening. Due to the extremely low output impedance inherent in the Purifi Class D design, the woofer cones were likely being better controlled, thanks to the increased damping factor. This can result in what may be perceived as less bass, but it is arguably more accurate. I also noticed the noise floor on the NAD M23 was so exceptionally low that it almost felt artificial—if I may put it that way. The best analogy I can offer is that it’s like comparing the black levels of the best Mini-LED TVs to those of OLED.I’m not criticizing the Anthem amp, which remains one of my all-time favorites, but the NAD's presentation was different and unexpected. There is certainly merit in building an amplifier with the very lowest noise and distortion profile, something that top-tier Class D designs can achieve even better than the best Class AB."I Couldn't Love You More" sounded sublime on the NAD M23, with a remarkable level of precision and a sense of spaciousness and airiness in the percussion. Switching back to the Anthem STR brought slightly heftier bass and a less analytical sound.I can’t say I had a clear preference for either the NAD M23 or the Anthem STR amplifier, as both sounded fantastic, albeit slightly different. At this level of performance, it ultimately comes down to personal preference rather than declaring a definitive winner. There were some listening sessions where I leaned toward the more powerful and dynamic Anthem STR, while in others, I preferred the more precise and delicate presentation of the NAD M23. I would love to revisit this comparison with two NAD M23s in bridged mode to eliminate the Anthem STR's power advantage and see if that would change my perception. In any case, the M23 is a force to be reckoned with, shattering all stereotypes that Class D can’t be taken seriously for the highest-fidelity music playback.

Ihave to pick some nits about NAD’s design decisions with the M23 before I cangush over its performance which I must reiterate is absolutely stellar in every way possible.

Idon’t recommend using this amplifier in bridged mode as previously stated inthe report UNLESS you are assured your speakers don’t dip below 5 or 6 ohms.Using two of these as monoblocs isn’t very cost effective and you lose some ofthe low noise/low distortion benefits of the design when you switch to bridgedmode. So, what’s the point?

Ergonomicallyit’s a challenge to connect any type of fancy speaker cable to the bridged +-terminals that are a whopping 8+ inches apart. I don’t think the engineers atNAD considered this but if you’re rolling some nice Kimber Kable 8TC in yoursystem, good luck reaching the speaker terminals in bridged mode. It would alsobe nice if NAD would have marked the left input terminal as the “bridged” inputon the unit silk screen. There was noway of determining which input to use in bridged mode without consulting theuser manual.

NAD M23 Back Panel View

Editorial Note About Bridging Connector Spacing:

Ergonomicallyit’s a challenge to connect any type of fancy speaker cable to the bridged +-terminals that are a whopping 8+ inches apart. I don’t think the engineers atNAD considered this but if you’re rolling some nice Kimber Kable in yoursystem, good luck reaching the speaker terminals in bridged mode. It would alsobe nice if NAD would have marked the left input terminal as the “bridged” inputon the unit silk screen. There was noway of determining which input to use in bridged mode without consulting theuser manual.

NAD claims that the wide spacing ofthe speaker terminals is a consequence of the lay-out chosen by goingfor a dual-mono type configuration of the input stage and keeping themodules away from the input section, yet close to the back panel. Asit so happens, they made a last-minute change to the back panel byreplacing the input/output terminal section, made out of steel, with ananodised aluminium strip. This change, with aluminum being non-ferrousunlike steel, reduced interference between high power speaker signalswith inputs to improve distortion figures yet again, albeit beyond the0.000x% mark.

Ilike that NAD gives you 3 different gain settings on this amplifier. If you’reconnecting this unit to an AV receiver, I recommend using the high gainsetting. Use the medium or low gain setting IF you connect to a dedicatedpreamp or IF you decide to bridge this amplifier. The lowest noise performancewas achieved for the low gain setting (+6dB for low vs high setting in bridgedmode) but the noise was so low in any of these settings that you’ll be fine to choose any as long as yourpreamp has enough voltage drive to reach full rated power (ie. 4Vrms for lowgain, 2.5Vrms for mid gain).

NAD M23 Front Panel View

Thecapacitive touch power button at the top of the chassis is cool but the noveltywears off quickly each time you accidentally touch the case in its vicinity andthe M23 powers on or off when you don’t want it too. I would much prefer a pushbutton power on the front panel NAD logo LED.

NAD M23 Magnetic FootCoasters

Themagnetic foot coasters have me scratching my head. They seemingly look neatwhen placed under the pointy feet but a bit tricky to get lined up properly andarea easy to misalign if you move the amp around in your rack. The discs serve not other purpose but to protect the surface on which the pointy-coned feet (for acoustic isolation) would otherwise leave scratches or dents on the furniture’s surface. This feature was introduced for all full-width Masters Series in this design back in 2014. The NAD isolation feet is a somewhat hip and trendy feature some audiophilesmay appreciate.

Asidefrom my nit picking, the NAD M23 is a gorgeous looking amplifier. From its highquality brushed aluminum casing, to its radiused edges and slick looking ventson the top and sides of the chassis, it’s really a product to behold that makesyou just want to sit still and gawk over. I highly recommend leaving this unitout for display instead of stuffed into a rack in a closet if you can.

Ilove the fact that you get a high power two-channel, audiophile grade poweramplifier that would spank most Class A amplifiers in power and performance,but it’s so light weight at a little over 21 lbs that you can carry it with onearm.

Linear Amps Are Dead, Long Live Class D! - YouTube Discussion

NADmanaged to get most of the ultra-low distortion performance out of the Purifiamplifier module making it a truly SOTA design that has pushed the envelope of performanceand what we can expect from Class D amplification. NAD has proven to me that linearamps are becoming a relic of the past and it’s time to embrace the future of ClassD amplification that offers both superior efficiency and performance! In almostevery category of performance, the M23 produced some of the best measurementsI’ve seen to date regardless of amplifier topology or price.

I absolutely loved my time with the NAD M23, from the precise measurements to the immersive listening experiences. This amp consistently exceeded my expectations, delivering power and performance that seemed impossible for such a lightweight design—I could easily hold it in one arm without breaking a sweat! While the NAD M23 may initially come across as analytical or slightly lean on bass, these qualities quickly become part of its charm. You'll find yourself continually amazed by how quiet it can be when needed, creating an almost surreal experience with sound that's rich, detailed, and incredibly captivating and most importantly, accurate.

Are you considering purchasing this amp or do you already own one? Share your thoughts in the related forum thread below.

The NAD M23 is a 200wpc stereo amplifier utilizing the SOTA Class D Purifi 1ET400A to push the envelope of performance and efficiency. Is this the best amplifier we’ve reviewed in our 25 year history?

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Gene manages this organization, establishes relations with manufacturers and keeps Audioholics a well oiled machine. His goal is to educate about home theater and develop more standards in the industry to eliminate consumer confusion clouded by industry snake oil.