Acoustic Research AR-M2 hi-rez portable player Measurements

Sidebar 3: Measurements

I measured the Acoustic Research AR-M2 with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Source materials were WAV and AIFF test-tone files, as well as DSD64 and DSD128 files of test tones prepared for me by Malcolm Hawksford. The AR-M2's battery was fully charged at the start of the testing.

The maximum output level was 1.89V from the line output jack and 3.71V from the headphone jack, both with the volume set to its maximum. The output impedance was 100 ohms from the line-out jack and 10 ohms from the headphone jack, both figures across the audioband. Though 10 ohms is a little higher than optimal, with headphones like the AudioQuest NightHawk and Audeze LCD-X, both of which have a constant impedance of 22 ohms across the audio band, this will not be an issue other than a reduction in volume. And the AR-M2 played still louder with these headphones than anyone would want. Both outputs inverted absolute polarity, as can be seen from the impulse response with 44.1kHz data (fig.1). This graph also reveals the M2's reconstruction filter to be a conventional FIR type, with symmetrical ringing before and after the single full-scale sample.

Fig.1 Acoustic Research AR-M2, impulse response at 44.1kHz (4ms time window).

Wideband spectral analysis of the player's output while it decoded 44.1kHz-sampled white noise at –4dBFS (fig.2, red and magenta traces) revealed that this filter rolled off the output very rapidly above the Nyquist frequency (half the sample rate, indicated by the green vertical line). As a result, the aliased product at 25kHz of a full-scale tone at 19.1kHz (blue and cyan traces) is suppressed by 105dB. The distortion harmonics associated with this tone are also low in level, all lying below –90dB.

Fig.2 Acoustic Research AR-M2, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).

Fig.3 shows the AR-M2's frequency response with data sampled at 44.1, 96, and 192kHz. The responses at the lower two rates follow the same basic shape, with a sharp rolloff just below each Nyquist frequency. With 192kHz data, the output continues to roll off smoothly above 48kHz, reaching –12dB at 95kHz. The responses were identical from both line and headphone outputs. Channel separation (not shown) was >100dB in both directions below 400Hz, but decreased with increasing frequency, reaching 66dB at 20kHz, due to the usual capacitive coupling between the channels somewhere in the output circuitry.

Fig.3 Acoustic Research AR-M2, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), 192kHz ) (left blue, right red) (1dB/vertical div.).

The AR-M2 offered low levels of noise with 24-bit PCM data, though with DSD64 data the audioband noise was about 3dB higher. Increasing the PCM bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS dropped the noise floor by 10dB (fig.4), suggesting that the player offers close to 18-bit resolution. With an undithered tone at exactly –90.31dBFS (fig.5), the three DC voltage levels described by the data can be seen, along with very slight degrees of DC offset, but the waveform is overlaid with some HF noise.

Fig.4 Acoustic Research AR-M2, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).

Fig.5 Acoustic Research AR-M2, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

With 24-bit PCM data and a 600 ohm load, the AR-M2 offered low levels of harmonic distortion (fig.6), with the second harmonic lying at –97dB (0.0014%) and the third at –90dB (0.003%). With DSD64 data, the harmonics were all a little higher in level (fig.7)—I have extended the analysis window to 50kHz to show that while the usual rise in ultrasonic noise with DSD data is present, it remains below –83dB. Intermodulation distortion with an equal mix of high-level tones at 19 and 20kHz was very low (fig.8). However, the left channel (blue trace) has a spurious tone just below 19kHz, and pairs of tones at 12.4/13.4kHz and 25.6/26.6kHz can be seen. Yes, all of these tones are at very low levels, but they are nevertheless unusual; while the 1kHz spacing of each pair is obviously related to the difference between the fundamental tones, the absolute frequencies suggest that these are indeed high-order products.

Fig.6 Acoustic Research AR-M2, 24-bit PCM data, spectrum of 1kHz sinewave, DC–10kHz, at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).

Fig.7 Acoustic Research AR-M2, DSD64 data, spectrum of 1kHz sinewave, DC–50kHz, at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).

Fig.8 Acoustic Research AR-M2, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 600 ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).

With 16-bit J-Test data (fig.9), the odd-order harmonics of the low-frequency tones are all very close to their correct levels (green line), and no jitter-related sidebands are present. While a small amount of spectral spreading can be seen at the base of the 11.025kHz tone, this is minimal. With 24-bit J-Test data (fig.10), the noise floor was absolutely clean.

Fig.9 Acoustic Research AR-M2, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Fig.10 Acoustic Research AR-M2, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

The Acoustic Research AR-M2 offers generally excellent measured performance.—John Atkinson

Acoustic Research
3502 Woodview Trace
Indianapolis, IN 46268
(844) 353-1307

spacehound's picture

I wonder if it can drive a typical 'line' input to reasonable levels? My iPad can't. Nor will its 'Lightning' output at a high enough level into a USB port. You can't really tell from the specs.

So for the future it might be helpful if you could try this and tell us. (I was too tight-fisted to order the optional Burmester audio in my car so now need an external player.)

AR? I've still got, and use, their turntable, later copied by Linn. Bought it from the local branch of Laskys. Do you remember them too?

John Atkinson's picture
spacehound wrote:
I wonder if it can drive a typical 'line' input to reasonable levels? My iPad can't.

It will have no problems driving a line input, especially from the headphone output.

spacehound wrote:
AR? I've still got, and use, their turntable, later copied by Linn.

That's unfair. Yes, both were belt-drive, suspended-subchassis turntables, but that's where the comparison ends. The original AR was still a good turntable but was let down by a poor tonearm.

John Atkinson
Editor, Stereophile

spacehound's picture

Castle Engineering was a small machine shop and copied the Ariston turntable design that they were contracted to make some parts for, right down to the lid, then sold it as their own, changing their name to Linn at the same time. That's how Linn got started in the audio business. There was a court case about it but it never came to anything for or against the two opponents.

Thanks for the AR output data. I tried it and it's fine. But only a demo model, the dealer hasn't got any stock at the moment.

jhanken's picture

In terms of function this is approaching perfect, with PCM, DSD and Tidal, but the omission of MP3 and AAC to me is a bit heartbreaking, I still have some music in those formats that I cannot get otherwise. I am really hoping to have just one portable player for everything. In the review, you said one AAC file actually played fine, any chance you could please test a 320KB MP3 and verify that it would play properly? Thanks, and very grateful for this review!

John Atkinson's picture
jhanken wrote:
In the review, you said one AAC file actually played fine, any chance you could please test a 320KB MP3 and verify that it would play properly?

The AR-M2 player has long since been returned to the manufacturer, but I did play 256kbps MP3s with it without any problems.

John Atkinson
Editor, Stereophile

robfol's picture


The M2 plays almost every format under the sun, including MP3 of all shapes and sizes and AAC, plus FLAC, ALAC, WAV, AIFF, APE DSD64/DSD128 & DXD. Sensible future formats will be addressed by our regular firmware updates.

Unfortunately an error on our website led JA to believe that these base formats might not be supported.

Regards Rob Follis for Acoustic Research

Long-time listener's picture

I spent about an hour listening to the M2 at a local dealer, with my own headphones and music. Aside from generally good clarity and lack of obvious grain or sibilance, its most appealing aspect was its spacious soundstage, which is hard to give up once you've gotten used to it. But at the same time, the soundstage seemed sort of artificially "inflated," and its excellent bass likewise seemed a little boomy or artificially inflated, and the sound was slightly "dry." (And the Alps potentiometer was making noise as it turned.) The guy in the store also told me that downloads of gapless capability aren't available yet for the general consumer, even if they are for John Atkinson--and gapless is a VERY fundamental feature that should be standard on every player.

The Fiio x7 doesn't have that wide open soundstage, but it has a slightly more liquid, full-bodied, and rich character, with equally good clarity and freedom from glare or sibilance. Bass is equally good, and more natural. It also comes gapless out of the box. At about two-thirds the price, I have to consider the Fiio X7 an equally good player and a better value. Combined with the clarity, balance, and full-range response of my single dynamic driver NuForce NE-800M, it sounds marvelous.

robfol's picture


The 2.5.5 firmware with gapless capability and a lot of other tweaks and upgrades will be available within the next 10 days

Regards Rob Follis for Acoustic Research

Long-time listener's picture

And all else being equal, it has to be said that the player with the better soundstage certainly brings the listener one step closer to true realism in sound. And the M2 certainly has the soundstage, so it must be doing something right... If I had known about the M2 sooner, I believe it would be my player now instead of the Fiio.

dce22's picture

Chord Electronics Mojo DAC + Samsung Galaxy S7
4.8 Volt 0.7 Ohm Output Impedance
(Will work perfectly with all the headphones on the market)

Acoustic Research AR-M2
3.7 Volt 10 Ohm Output Impedance
(Will work properly with 100 Ohm or more Headphone Impedance)

Chord Mojo has 10db less distortion and 10 db less noise at all frequencies has better oversampling filter will work with every headphone there is, can be a DAC for your home system and for the same amount of money you can buy top of the line smartphone and Mojo DA instead AR-M2

PS. I am no way affiliated with Chord Electronics or Acoustic Research I am just parroting reality.

robfol's picture

Well, if you are a fan of expensive rubber bands, dangling cables and pocket destroying lumps? then that would be a great solution. In the meantime the M2 is an amazing performer at half the price of relevant competition. Cheers, Robert for Acoustic Research

TheNoose's picture

I wish all reviews would identify if the unit will output balanced audio and to what extent/format. Esp at this level of player. For example the Pono provides this functionality at 1/4 of the price here...and yet this was not tested or used as a benchmark. I wish it was.

robfol's picture


Balanced out adds complexity and cost for in our opinion, little benefit on a portable device. The M2 does not have balanced output.

Regard Robert Follis for Acoustic Research

TheNoose's picture

Acoustic Research is such a renowned brand. I wish there is an opening for me/customers in your statement Robert. It feels like talking to Apple, we know what you need best and aren't listening to you anyway. I wish it wasn't so.
On the ummmm...upside...why don't you buy a couple of balanced out portables say from Astell and Kern and Pono for example and using proper balanced cables and headphones give them a try? I'm sure like me and thousands of other customers and experts you'll hear a difference worth investing in. You could really redefine this facility with the power of your brand...Sincerely.

The Federalist's picture

I was very interested in the AR product when I first stumbled across it on Head-fi, and came very close to dropping $800 on the 2nd gen AK100 but the sheer number of units (from dozens of different manufacturers) up for sale on Head-fi classifieds, on a given day, leads me to believe the product type, while attractive, doesn't have much staying power with people once the whole consumer cycle has been completed and the drugs have worn off.

I would love to see something along these lines that was a larger screen, like an audiophile tablet so to speak, but with the same mechanical transport controls like these smaller units have.

So far the closest I have seen is the Nativ player on Indiegogo but that one is tethered to the wall via mains power so is a different animal.

I do think a lot of people have an aversion to using their phone or their tablet for music delivery. It's pretty clear considering how robust this segment is. I just think a bigger screen would draw you in a bit more. The phone form factor doesn't do it for me.

Either way. A very interesting device and a very good review. I do appreciate your take on this.


The Fed

Boogie6301's picture


Thanks for the review. I'm considering the AR-M2 primarily because of Tidal support. I currently have the QP1R and I'm more than satisfied with its sonic abilities albeit with crappy UI, no streaming and doggy scroll wheel.

Since your review of the AR-M2 came after the QP1R I'm interested to know your comparisons purely from sound qualities. Bluetooth, WiFi and streaming are "nice to have" for me so I'm not willing to compromise sonic qualities with these features.