The Sonoma Acoustics M1 headphone system can be connected either by its USB 2.0 digital, SPDIF coaxial, or two analog inputs—one pair of RCA single-ended analog connectors and a mini-stereo connection. The USB digital input became my preferred connection after a few days of listening to (and hearing some distortion during high-level signal peaks on) the analog inputs. If you intend to use the Sonoma strictly via its analog inputs, I strongly recommend that you try to limit the output levels so they do not exceed 2.1 volts on the RCA inputs and 850mVs on the mini-stereo input. The system is optimized for signals up to these maximum levels. Once analog levels edged up over those maximums, I noticed distortion on the bass transients, and the higher the level, the worse they became. Since the M1 drive unit has more than enough gain to drive the headphone’s volume well past my own comfort level with lower, friendlier voltage peaks, I do not consider this a deal-killer. But anyone planning to use the analog inputs should be aware of these limitations so he can work around the problem. I would advise keeping a sharp eye on your analog output levels, especially with DACs or preamplifiers whose specifications indicate they can easily produce a higher-than-the-“normal,” 2-volt, single-ended RCA output level.
I used several different manufacturers’ cables during the review period, and settled on the Play JCat reference cable for the USB connection and the Kimber KCAG cable for the analog feed, which during most of the review was provided by the Mytek Manhattan II DAC/pre. Although the Sonoma amplifier is a Class A design—a type known to throw off a large percentage of its consumed energy as heat, the Sonoma Acoustics amp remained only warm to the touch even after it had been powered on, continuously, for several days.
With headphones, the most important ergonomic feature is their fit. The Sonoma Acoustics M1 headphone has a somewhat higher-than-average side-pressure (which helps ensure a good tight fit). I have a slightly smaller-than-average head size (71/8 hat size), and I found I could wear the M1 headphones for an hour on average, sans break, with no discomfort whatsoever, due in large part to their soft and dreamily comfortable earpads. But after 1½ hours I needed to take a break due to the side-pressure. For comparison, I found the HiFiMan HE1000 V2 had less side-pressure and did not require periodic comfort breaks. The other “pressure-point” on the M1 headphones was at the top-center of the padded headband. Again, it took a long period of uninterrupted wear to be noticeable, but after a long session there was one small contact point that forced me to take a break from the pressure, bless my pointy little head.
The headband itself has a wide range of adjustability. I ended up at two clicks shy of minimum extension, leaving plenty of range for larger heads. I appreciated the fact that once extension is set it takes some effort to change the settings, so you don’t have to go through the ritual of readjustment each time you put on the M1s. Also, there isn’t a great range of horizontal adjustability at the pivot on the side gimbal, so don’t expect the M1 headphones to fold flat, but the adjustability was more than adequate to ensure a correct seal and an adaptable fit.
Other ergonomic considerations are the cable and its connectors. The cable itself is flexible without being floppy, and is removable and replaceable. My only complaint with it is the cylindrical metal connectors that protrude almost a full inch from the headphone enclosure. The rubber strain-relief collar that protects the junction of metal connector and cable will receive the brunt of abuse. I would make sure your headphone stand is high enough to provide clearance without crimping the cable connection points. On the amplifier side, I found the rubber strain-relief collar came off periodically during regular use and needed to be pushed back into its proper position almost daily. The prototypes I tried several months ago had the same connection scheme and the same issue. I was hoping that it might have been changed and improved for the production model, but it seems to be identical.
One additional comment on the cable: While it was of sufficient length for desktop use, I would like to see another version offered for those who require a longer tether. Given the M1 system’s probable popularity, I suspect it won’t be long before audiophiles have an option of third-party custom-made cables from Moon Audio and Audience (to name a few likely suppliers).
A certain car company has been using the slogan “professional grade” recently. The same pithy phrase certainly applies to the M1 headphone unit. Unlike some headphones that I would be hesitant to hand over to someone without a bit of supervision, it doesn’t matter where you grab the M1s. Even if you hold their back and inner grille in a pincer grip, you can’t damage the drivers because they are well protected, as they should be.
The control unit’s ergonomics are impeccable. The large centrally located volume knob turns easily and has attenuation settings that are set in firmware. At the low end the adjustments are in 8dB steps, then 6, 4, 2, 1, and 0.5dB steps near the top of its range. Besides the volume knob, the only control on the front panel is the analog/digital selector, which switches from one setting to the other with a firm click. There are no provisions for L/R balance adjustments.
One last ergonomic detail: The M1 headphone design is an open enclosure that generates nearly as much sound out into the room as into your ears. If you need a headphone that can isolate you from the listening environment and provide some degree of privacy, any open-enclosure design, including the M1, should not be your first choice. The leakage was sufficiently robust that I could still hear output at what I consider normal listening levels when I was 15 feet away from the M1s.
The two words that kept popping up in my listening notes during my time with the M1 system were clarity and cohesion. That “whole cloth” quality that I’ve only heard from full-range transducers that use a single driver (planar or dynamic) and have no crossover circuitry is very much in evidence here, even though the reality is that the M1 headphone diaphragm has eight separate sections. Also, the speed and clarity that electrostatic headphone technology is known for is definitely present in the M1 system. Listening into a mix, no matter how dense, seems easier through the M1. Want to focus on the second or third guitar part? No worries, mate. Through the M1, once I located a part within the soundstage, it was easy to follow it throughout the performance. I suspect this was due to a combination of factors, including the M1’s low-noise electronics section and the M1 driver’s native abilities.
Reference-quality headphones create just as complex and three-dimensional an image as loudspeakers do, although it’s different. Many years ago, David Wilson reviewed several cartridges in a survey for The Absolute Sound. In the review, he included illustrations of each cartridge’s soundstage. If I could do that for the Sonoma Acoustics M1 it would be a perfect one-half globe. With some headphones that shape would look more like one-half a dirigible turned sideways. But the Sonoma Acoustics M1 system doesn’t laterally exaggerate or stretch the soundstage like some open-enclosure headphones. On my own recordings, the image placement was identical to what I’ve come to expect and matches the soundstage placement I get from the Ultimate Ears RR IEMs, which are my primary on-location reference monitors.
I am, technically, an old guy. My treble range currently extends to 13.5k (and I don’t expect it will increase). But I’m as sensitive to treble nonlinearities as I was when I was younger. The M1 system is extremely smooth and detailed in the top octaves. I was hard-pressed to discover any hint of over-emphasized peaks in the treble. My venerable pair of Stax Lambda Pro earspeakers can sound slightly sterile and matter-of-fact, especially when coupled with the Stax SRM-1 Mark II solid-state driver unit (though much of that edge vanishes when the Lambdas are connected to the SRM-007t tube unit). The Sonoma Acoustics M1, practically right out of the box, had a less antiseptic and more organic character, especially in the upper midrange and treble region than my vintage Stax, regardless of which energizer was connected to them.