Editor’s Note: Sonoma Acoustics has been rebranded. The new company name is Warwick Acoustics Ltd.
When they see the words “electrostatic earphone,” the first thing that comes into most experienced audiophiles’ minds is Stax Ltd. Since 1960, when Stax introduced its SR-1, it has owned a hefty percentage of the electrostatic headphone market. Sure, there have been attempts by Koss, Jecklin, and others over the years to field competitive offerings. But despite competitors’ best efforts and Stax’s own decidedly laid-back marketing and distribution, no one so far has given Stax any serious competition. That may now have changed. Sonoma Acoustics’ M1 electrostatic headphone system ($4995) may not eat all of Stax’s lunch, but it seems poised to sit down at the table and take a major bite out of it.
Sonoma Acoustics is a new company formed by several people who worked together at Sony on DSD and SACD projects, including David Kawakami, Gus Skinas, Andrew Demery, and David Walstra. Dan Anagnos, the principal designer, works for Warwick Audio Technologies and was also part of the SACD launch. The name Sonoma comes from the DSD recording-and-mastering workstation originally developed by Sony and now owned by Super Audio Center LLC in Longmont, Colorado. The Sonoma Acoustics headphone’s design goals are “to deliver high-resolution audio in unparalleled sound quality, and to provide the ultimate in listener comfort.” If realized, the Sonoma Acoustics M1 system would be a step forward toward the ultimate headphone transducer, capable of producing an uncolored, reference-quality sound with any source.
The Sonoma Acoustics headphone system has two parts—the headphones themselves and the energizer/DAC that powers them. Currently, the two units are only sold as a package and are not interchangeable with other manufacturers’ electrostatic headphone designs.
The Sonoma Acoustics system boasts some unique proprietary technologies, beginning with the electrostatic panels themselves. This patented High-Precision Electrostatic Laminate (HPEL) audio transducer was developed in the UK by Warwick Audio Technologies Ltd. (WAT). Instead of a thin membrane coated with a conductive material suspended between two electrically conducting metal grids, as in a conventional electrostatic panel, the HPEL uses a thin flexible metalized laminate film for the “front” grid. The laminate is attached to an insulating spacer, and the film is machine-tensioned in both the x and y planes. The octagonal design of the spacer essentially creates eight separate “drum-skin” diaphragms from one piece of film. Finally, a stainless-steel mesh forms the back grid.
Unlike a traditional electrostatic panel, the sound does not pass through a grid. According to Sonoma Acoustics, “WAT was able to fine-tune the characteristics of the ‘drum-skins’ such that they have different resonant frequencies. Each cell is acoustically independent, but driven in parallel. As a result, the sound from each cell combines in acoustic space, but the independent resonances average out, avoiding any large resonant peak in the audio band (as can happen with a single driver area).”
The M1 enclosure, which Sonoma Acoustics calls “earcups,” is made of injected magnesium that Sonoma Acoustics claims has one-third less weight than—and superior sonic characteristics compared to—aluminum. The earpads are top-grain Cabretta sheepskin, hand-sewn in Germany, and tanned by Pittards in the UK, which has been tanning leathers since 1826. The headband is made from Nylon 12 (aka Polyamide 12). This material allows the headband to flex and still be strong. The Nylon 12 also acts to dampen noise and vibration. Internally, the headband employs stainless-steel parts with a vapor-deposited titanium coating.
The M1 system’s headphone cable uses fine strands of silver-plated, oxygen-free, high-conductivity, ultra-pure copper (OFHC), insulated with foamed polyethylene. There is no shared ground between the left and right channel signal cables and the fiber filler material in the jacket keeps the conductors as far apart as possible. For strength, two Kevlar fibers are woven into the cable.
The amplifier/energizer unit of the M1 system uses a high-performance, single-ended, discrete-FET Class A amplifier, which was matched to the HPEL. The amplifier was specifically designed to drive the HPEL’s inherently capacitive load. Its output stage operates at a high bias level, so it can deliver a very high slew-rate and improved linearity. The drive signal has a maximum amplitude of 145V (RMS), which is superimposed on the 1350V DC bias. The output devices in the M1 are field effect transistors from International Rectifier, spec’d for linear amplifier applications. Passive amplifier components are all optimized for their specific application in the design and sourced from AVX, Bourns, and Vishay. The chassis itself is encased in a completely shielded, machined-aluminum enclosure.
The Sonoma Acoustics M1 system has provisions for four inputs—USB 2.0 digital, SPDIF coaxial digital, RCA single-ended stereo analog, and a 3.5mm stereo analog. The USB 2.0 input accepts all hi-res audio formats up to 32-bit/384kHz PCM and DSD via DoP (DSD64/DSD128), while the SPDIF input accepts all PCM formats up to 24-bit/192kHz. Two stereo ESS Technology 32-bit Reference DAC chips are used in a special mono mode to deliver a measured 129dB signal-to-noise ratio (SNR).
The Sonoma Acoustics system digitally processes all signals using custom 64-bit, double-precision, fixed-point arithmetic, running within a high performance, multi-core XMOS processor. All the filter responses within the DSP are minimum-phase slow-roll-off and are optimized for excellent time-domain response. The 64-bit DSP also allowed Sonoma Acoustics to implement within the amplifier a fully digital interpolated volume control.
Due to the need for DSP to achieve a target frequency response, all incoming analog signals are converted to digital by the Sonoma Acoustics M1 system. The A-to-D employed for this is a 32-bit/384kHz AKM Premium ADC chip. Because the amplifier has both low-level (3.5mm) and high-level (RCA) inputs, Sonoma Acoustics uses separate ADC channels depending on the input selected. There are two independent fully optimized signal paths, one each for the low- and high-level inputs. The measured SNR of the ADC stage for either analog input exceeds 120dB.
Because the quality of the power supply has a major effect on overall performance, the first stage of Sonoma Acoustics’ two-part power supply employs an outboard custom-designed, universal voltage, switch-mode unit. To avoid any headroom limitations, this PSU is capable of delivering about 3.5 times the maximum power the amplifier is designed to draw under steady-state conditions. In addition, the unit uses a fixed-frequency switcher (operating at over 85kHz) to avoid any possibility of the switching frequency dropping into the audio band as the power draw changes. It also has improved internal filtering to yield extremely low noise and ripple (<50mV peak-to-peak). The other power supply components are inside the amplifier chassis itself where all audio circuitry is supplied by multiple stages of low-noise, high-current linear regulators from Analog Devices.
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.
The M1 system’s midrange is as close to harmonically neutral as any headphone I’ve heard recently. Not too dry or too lush, the M1’s midband characteristics are ideal for its primary intended use—as a reference monitor. Listening to my recording of the acoustic band, Mr. Sun, from a live concert at the Salina Schoolhouse through the Sonoma Acoustics system was both a delight and a very accurate rendition of the event, instantly bringing me back to the moment I made the recording.
And how does the Sonoma Acoustics handle bass? Very well, indeed. Even Mike Posner’s synth assault on “Silence (Sluggo x Loote Remix)” doesn’t ruffle the M1’s feathers one bit. Although not as bassalicious as the Sony MDR-Z1R, the M1 system pays bass its due with spot-on pitch control and fine definition. For deep bass and pulse control listen to Everything is Recorded’s “The Rhythm of Life and Death” from Close But Not Quite. Through the M1 system, even these low bass transients have definition and solid, visceral impact. This bass weight is one particular sonic attribute that separates the Sonoma Acoustics M1 from the Stax Lambda Professional series earspeakers. The Stax bass response is fast, but somewhat ethereal when it comes to in-your-face lower-octave weight. Through the Stax you are slapped instead of punched.
Analog adherents may not be completely comfortable with the fact that the M1 system converts all incoming analog signals to digital, but try as I might, I could not hear any sonic hints that the additional A/D and D/A conversion reduced the fidelity from analog sources. I will readily admit that I did not use the analog inputs, as much or as often as I used the digital inputs, primarily due to the input-level limits of the Sonoma Acoustics M1 and the plain fact that the bulk of my listening time is to digital sources, including Tidal and my own DSD recordings.
I recently reviewed the Stax SR-L700 ($1400) coupled with SRM-007tII amplifier ($2150) with a combined system price of $3650. The most notable differences between it and the Sonoma are how they handle bass, their fit, and their ability to withstand moderate abuse. The Stax bass is airier with less “meat and potatoes” impact than that of the Sonoma Acoustics M1. Both were fast and detailed, but the Sonoma Acoustics’ low end was more organic and impactful.
The two headphones fit quite differently, with the Sonoma Acoustics having a lot more side pressure, and the Stax being lighter, larger, and containing a far more delicate yoke/headband system. Having owned the Stax Lambda Pro for many years and having replaced the headband twice during that time, I can vouch for its delicacy. In comparison, I suspect the M1 headphones could survive several long tosses across the room with no lasting ill effects. To the Stax’s credit, I have been able to wear the SR-L700s virtually all day without a break while with the Sonoma Acoustics headphones I felt the need to take a “comfort break” after about 1½ hours of continuous use. Another difference that may be important to some listeners—the Stax amplifiers have provisions for L/R balance adjustments while the Sonoma Acoustics M1 does not. Finally, analog-or-death adherents may wish to remain in Stax’s all-analog domain versus Sonoma Acoustics’ digital one.
If you pair the HiFiMan HE1000 V2 ($2999) with the Inspire Dragon Inspire IHA-1 SE headphone amplifier ($1599) you have a headphone system that costs almost as much as the Sonoma Acoustics M1 system. But, even though both systems employ devices that wrap around your ears and are open-enclosure designs, almost everything else about these two systems, except for the high quality of their sound, is different. Like the Stax system, the Dragon headphone amp lacks any digital inputs and is analog-only. But unlike the Stax (and the Sonoma) the Inspire supports a wide range of headphones via either balanced or unbalanced connections. On the headphone side of things, the HE1000 V2 is easily in the M1’s league in fit, finish, comfort, and build-quality. For listeners who prefer less side-pressure the HE1000’s less intense fit could be far more to their liking. Sonically, connected to the Inspire, the HE1000 V2 bests the Sonoma Acoustics M1 when it comes to bass impact and extension, but the Sonoma Acoustics’ bass response seems more linear. The HE1000 V2/Inspire combo has a “juicier” midrange. In comparison to the HE1000 V2 the Sonoma Acoustics seems to add less embellishment, which could be a good thing or a bad thing, depending on the source material and your mood.
No audio component is perfect, and the Sonoma Acoustics M1 was certainly no exception. Its variations from perfection were small, involving long-term listening comfort, cable hardware, and distortion from excessively high analog input levels. None of these flaws are fatal, so if you are in the market for a superbly neutral, revealing headphone system for critical listening or monitoring purposes the Sonoma Acoustics M1 should be near the top of a very short list.
Specs & Pricing
Type: Open-back, circumaural electrostatic
Impedance: N/A (Closed Electrostatic System)
Sensitivity: 94dB SPL at -10dB full-scale digital input and 75mV analog input (the EN50322 specification)
Weight: 10.7 oz.
Price: $4995 (sold as system with M1 energizing amplifier/DAC)
M1 Headphone Amplifier
Type: Discrete FET, single-ended Class A solid-state
Outputs: 1x headphone connector for M1 headphones
Dimensions: 7.48″ x 2.24″ x 11.42″
Weight: 5.40 lbs.
Price: $4995 (sold as system with M1 electrostatic headphones)