Sonoma Acoustics Model One Headphone System

The Electrostatic Headphone Reborn

Equipment report
Integrated amplifiers,
Sonoma Acoustics M1 Headphone Amplifier,
Sonoma Acoustics M1 Headphones
Sonoma Acoustics Model One Headphone System

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.

Technological Tour
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.