The revolutionary Sonoma Headphone System range has been optimized for today’s High-Resolution Audio formats. Every aspect of the system has been designed to deliver unparalleled levels of performance and sound quality.
Sonoma Model One Electrostatic Headphone System
The ideal loudspeaker transducer would have zero mass, respond infinitely quickly to any signal, be perfectly damped, have no distortion and be perfectly linear. Until now, the technology that best exemplifies those characteristics is electrostatic. Introduced almost 60 years ago, electrostatic headphones have remained the choice of discerning listeners who demand the highest audio quality. Now, a revolutionary new electrostatic transducer, derived from the world of ultrasonics, has been developed in the UK by Warwick Audio Technologies Ltd. (WAT). The patented High-Precision Electrostatic Laminate (HPEL) audio transducer ushers in a new paradigm in the field of electrostatic drivers, and the Sonoma M1 is the first headphone system in the world to use it.
A conventional electrostatic transducer consists of a thin membrane (coated with a conductive material) between two electrically conducting metal grids. There is a small gap between the membrane and grids. The membrane is kept at a high DC potential relative to the grids, and the audio signal is applied across the grids. This results in the membrane moving in response to the audio signal thus creating sound. Clearly, in order for the sound to propagate through the grid/membrane ‘sandwich’, the grids have to be perforated in some way.
In contrast, the HPEL uses a thin (15 μm – less than the thickness of a human hair), flexible laminated film for the ‘front’ grid. The laminate is affixed to the open (cell) structure of an insulating spacer (made of Formex™), and the film is very accurately machine-tensioned in the x-y plane. In this way, small ‘drum-skins’ are created by the cells. A stainless steel mesh forms the ‘back’ grid. When the audio signal is superimposed on a 1350 V DC bias voltage, the ‘drum-skins’ formed by the flexible ‘front’ grid vibrate, producing sound. Unlike a traditional electrostatic panel, the sound you hear from a HPEL does not pass through a grid! To take full advantage of this feature, everything has been done in the design of the M1 to keep the areas in front of and behind the transducer as clear as possible so as not to impede the sound waves.
Thanks to a proprietary Finite-Element Analysis software package, WAT is 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).
Injected Magnesium Ear-cups
Compared to Aluminum, Magnesium is about ⅓ lighter and has superior acoustical damping. Combining excellent strength-to-weight ratio, high stiffness and outstanding EMI/RFI shielding, Magnesium is the ideal material in which to house the HPEL transducer to ensure optimum performance. The ear-cups of the M1 are formed by precision injection molding at very high pressure. The result is a weight of 10.7 ounces (303 grams) which contributes to outstanding headphone comfort. Only high-grade stainless steel screws and fasteners are used in the headphones for high strength and resistance to corrosion.
Handmade Cabretta Sheepskin Ear and Headband Pads
Comfort in a headphone is of paramount importance. Sonoma want you to enjoy the M1 for extended listening sessions. Producing an excellent sounding, lightweight design would all be for naught if the ear and headband pads induced any discomfort. So, to ensure maximum listening pleasure, the pads on the M1 are all handmade from top-grain Cabretta sheepskin leather. Cabretta leather is renowned for its light weight, smoothness, suppleness, and durability.
The leather used in the M1 is sourced from Ethiopia. The hides are then tanned by Pittards in the UK, who have been tanning leathers since 1826. Finally, the pads are hand sewn in Germany.
Nylon 12 Headband
It is crucial that a headband combine sufficient flexibility – to adjust to different head sizes – with sturdiness and a resistance to cracking under stress: just think how many times a headphone gets put on and removed from a head over its lifetime. To meet these criteria, Sonoma have opted for a headband made from Nylon 12 (aka Polyamide 12). This allows the headband to flex as required, while remaining strong over prolonged use. In addition, Nylon 12 acts to dampen noise and vibration.
Internally, the headband contains stainless steel parts which have a vapor-deposited titanium coating to ensure strength and smooth operation.
Custom Low-Capacitance Cable
An electrostatic headphone does not present a normal load to any partnering amplifier. As a result, it is impossible to use just any cable to connect amplifier to headphone. Therefore, in collaboration with Straight Wire Inc., a totally new, ground-up design was done for the M1.
The cable is composed of very fine strands of silver-plated oxygen-free high conductivity (OFHC) ultra-pure copper. The insulation is a foamed polyethylene chosen because it has a suitably high dielectric constant, and is well damped. There is no shared ground between the left and right channel signal cables, and a fiber filler material in the jacket keeps the conductors as far apart as possible (to reduce capacitance and crosstalk) as well as helping to dampen any cable microphonics. To ensure excellent strength, two Kevlar® fibers are woven into the cable.
The resulting cable capacitance is a low 50 pF/m. At the amplifier end, the jack is electrically isolated from the amplifier chassis, while on the headphones high-precision self-latching connectors are used to ensure a secure connection. A sense loop built into the cable runs to each ear cup, and if the cable is disconnected at either the amplifier or headphone ends then the amplifier automatically shuts itself down.
The Energizer and DAC
Like all electrostatic transducers, the HPEL requires a high-voltage drive amplifier in order to function. In the case of the M1, the drive comes from a high-performance, single-ended, discrete FET Class-A amplifier with very low distortion and wide bandwidth which is optimally matched to the HPEL. The amplifier was designed and optimized to drive the inherent capacitive load of an electrostatic transducer, and the Class-A output stage is operated at a high bias level, and delivers a very high slew-rate. Operating at such high bias levels results in improved linearity.
The drive signal has a maximum amplitude of 145 V (rms), which is superimposed on the 1350 V DC bias. Despite operating at low current levels, the high voltages in use translate to significant power (for a headphone amplifier) at the output of the amplifier. Consequently, high quality devices are used throughout which are designed to cope with the power levels.
ESS SABRE Reference DAC
The Sonoma M1 system was developed to deliver true high-resolution performance. For the critical digital-to-analog conversion stage, we turned to an established leader in the field. ESS Technology is universally recognized as the world’s premier DAC chip manufacturer, and we have opted for their 32-bit Reference DAC. Two stereo DAC chips are used in a special mono mode to deliver a measured 129 dB signal-to-noise ratio (SNR).
CNC-Machined Aluminum Amplifier Enclosure
Starting with a solid billet of high purity Aluminum 6063, the material is first extruded, and then CNC-machined to produce a shallow U-shape comprising the base and side walls of the amplifier. The metal walls which remain are 5/16th of an inch thick. A top-plate of the same thickness is similarly produced, along with ½ inch thick end-plates to form the complete enclosure. A special 3D-wave pattern is then cut into the base and top-plate by CNC-machine to facilitate heat dissipation.
To obtain the desired finish in the metal, the machined parts are then blasted under high pressure with fine glass beads before being clear anodized. Finally, all logos and labels are laser-etched into the metal, so will not fade or rub off!
A beautiful case would be of little use if it did not help to improve system performance. The entire M1 chassis is electrically conductive, and acts as an ideal EMI/RFI shield. Care was taken to ensure a low impedance ground path between every mechanical component of the chassis and the power ground plane, which extends to the Custom Universal Power Supply Unit (see below) and earth ground. The signal ground is kept isolated from this protective shield, and all these features together contribute to the ultra-low noise floor and freedom from external interference.