AXPONA 2017: Sonoma M1 Headphone System


Sonoma M1 headphone system.
I’ve written about the Sonoma M1s before but what made their AXPONA room different this time was that these were no longer in prototype form.  In fact, the Sonoma team had formed a partnership with Chad Kassem’s Acoustic Sounds and the production versions were available for sale right in the marketplace. Present at the show were some big names in pro and consumer audio: David Kawakami, familiar from his leadership on the Sony SACD launch team, Gus Skinas a “jack of all trades” engineer who ran the Super Audio Center and does incredible work on Sonoma and Chad’s new analog tape offering and Dr. Andrew Demery one of the Super Audio launch team regulars who has authored many discs amongst other engineering tasks.  Essentially there have been two aspects that have attracted me to the Sonoma M1 product:  The pedigree of the principals and the sublime sound they have achieved.  While the Sonoma M1 is not cheap at $5K, it does represent a real bargain as it compares favorably – based on my listening – to a full Stax 009 system at roughly half the price.  The system includes both the headphones with state of the art HPEL panels and the DAC/amp that powers the HPELs. It can take analog and digital sources and hi-res files in both PCM and DSD.  In addition to the Super Audio crew, Warwick Audio Technologies developed some of the key technology including the new HPEL panels.

So what the heck is an HPEL?

DAC/Amp with HPEL panels.
HPEL stands for “High Performance Electrostatic Laminate.”  Somewhat like electrostatic panels found in a loudspeaker system, there is a panel between two electrically conducting metal grids which, when an audio signal is applied, move the panels in a way to precisely create music.  HPELs are also a transducer that have a unique characteristic of being a “single ended” design. When applied to headphone terminology this means that the sound-creating panel that bias is applied to to make it move back-and-forth goes from more tension to less tension which is easier to control that have a zero-crossing point and going positive to negative.  It’s not the more common “push-pull” technology.  This makes the panel more accurate in sound reproduction, or more transparent if you will.  The membrane is super thin as well: 15 micrometers, less than the width of a human hair.

Pretty cool stuff, but here is where things get more different and interesting.  The HPELs have a polycarbonate frame that looks like grouped hexagons.  This forms what look like tiny drum heads around an inch or smaller in diameter.  A 1350V DC bias is applied and the drum skins vibrate in a way to produce sound.  Now Warwick, being quite clever, has tuned each of these drums to have different resonant frequencies.  The drums are acoustically independent but driven in parallel.  These hexagon drums are then optimized by digital signal processing, or DSP.  As Dr. Demery says, this “gets the panel to act as it should act.” That allows the combined inventors of Warwick and the Sonoma team to get even more natural and transparent sound.  The panels create a full frequency range that is linear to 60khz!  State-of-the-art manufacturing provides exceptional channel matching as well to within +/-0.8db.  The panels are very thin and light so the polycarbonate frame ensures rigidity and protection but allows the other major sonic benefit: terrific transient response.

Now this new technology took a while to develop so I asked Gus Skinas and Dr. Demery what went into the final push to achieve production-ready status.  There were essentially four areas leading to the final product:

  1. Headband – Sonoma wanted to get a really good bass response and that depends in part of the degree of “seal” on the earcups.  Sonoma found a better clamping approach via a comfortable metal band.
  2. Refinement of the HPELs and DSP modules.  The engineering team refined the DSP software to get the most from the hexagon drums in terms of improved sound.
  3. Additional refinements to the DAC/amp modules.  I will offer details on the design herein.
  4. USB inputs.  Further refinement of USB interactions to assure better performance.

All guts and glory.
Overall refinements relied on Dan Agnanos from Warwick.  Dan is the designer behind some of these newer Sony loudspeakers that sound superb.  Frequency response on headphones is tricky as most “cans” deviate from linearity above 8 khz due to the way our ears hear.  There is some voicing needed to get a neutral response at the upper-end where measurements become less useful.  This is referred to as the HRTF or Head-Related Transfer Function.  To address this, Sonoma/Warwick used a custom XMOS chip of their own design using 64-bit, double-precision arithmetic.  Benefits include no loss of fidelity and excellent channel matching.  Dr. Demery says the goal was to recreate “listening to good speakers in a good room.”

DAC/amplifier module
The DAC/amp module is pretty fascinating as well.  As one would expect from a team of Super Audio pioneers, the DSD performance is very solid, as is PCM.  According to Dr. Demery, the SPDIF can handle up to 24/192 and the USB allows up to 32/384 khz PCM and DSD 64 and DSD 128.  Since DSP must be used for the HPELs, the DSD reproduction gets converted at a very high PCM rate of 384 khz. On the analog inputs, this requires an ADC to convert to h-ires PCM for playback as well.  Sonoma uses an AKM 32-bit/384khz chip for this.  The DSP allows another key benefit as well, you can control the volume entirely in that domain making for effectively a 100% perfect volume control and avoid certain negative effects from potentiometers and attenuators.  Power was a big consideration and the team looked at several options.  The power system is a fixed-frequency switching power supply with a frequency above 85 khz.  There are multiple stages of voltage regulation with very low “ripple” and the power supplies are split into separate digital and analog supplies.  This results in a laptop-sized power brick that you can hide behind the desk.

Attention to detail.
The amplifier is a FET-based, single-ended, Class-A design that employs FETs from International Rectifier. The case is CNC-machined aluminum with EMI/RFI shielding.  As hopefully my photos below show, it is quite beautiful to look at on the desk.  There are wave-like vents on top for heat dissipation but it doesn’t run very hot in my experience.

How does it sound?

Courtesy Chad Kassem.
Spectacular.  This system produces some of the most natural sounding music I have heard on full-size headphones.  It seems a bit more refined than even a top-of-the-line Stax package, while sharing several of the strengths of electrostatic reproduction.  Gus, who seems to have access to the best source material anywhere, brought in a big studio Sony reel machine and had a sampler from Chad’s Ultra Tape Series, that Gus does the mastering for.

Reel to reel source.
It was a perfect source to show off what the M1s were capable of.  One track I listened to twice was the sublime Steely Dan cover Show Biz Kids by Rickie Lee Jones.  The guitar and vocals sounded perfect to me and I often use this as a reference track for the stereo at home.  The music just flows through the M1.  I forgot about “gearhead” considerations like hi-res format particulars, associated gear, mastering heritage, and other distractions that generate currency in audiophile circles but often take away from musical enjoyment.  I just wanted to sit there all day to be honest.  Every other track I heard was exceptional as well.  Dynamics?  Check.  LF response? Check.  My all-important midrange purity? Check.  Resolution? Check.  It literally ticked all the boxes for me.  The product appearance itself is beautiful too.  Something you would show off to friends if they came over with a good bit of pride.

There’s a lot of technology in this package but it would not have meant much if the resulting sound quality was not excellent.  Stax makes excellent gear as well but I would be nervous if I were them.  The Sonoma M1 is serious competition for a lot less money.


About Lee Scoggins 118 Articles
A native of Atlanta, Georgia, Lee got interested in audio listening to his Dad’s system in the late 70s and he started making cassettes from LPs. By the early 80s he got swept up in the CD wave that was launching which led to a love of discs from Mobile Fidelity Sound Labs. Later while working on Wall Street in the 90s, Lee started working on blues, jazz and classical sessions for Chesky Records and learned record engineering by apprenticeship. Lee was involved in the first high resolution recordings which eventually became the DVD-Audio format. Lee now does recordings of small orchestras and string quartets in the Atlanta area. Lee's current system consists of Audio Research Reference electronics and Wilson Audio speakers.