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 recognised 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).

Custom 64-bit Fixed-Point Digital Signal Processing

Anyone who is familiar with loudspeaker or amplifier measurements will undoubtedly have seen plots of flat frequency response from a few tens of Hz all the way up to 20+ kHz. A flat frequency response is the ultimate goal for those components, and is quite easily achieved in an amplifier, but more difficult to achieve in a loudspeaker, especially in a real-world listening room (as opposed to an anechoic chamber).

Unfortunately, the situation with headphones is a lot more complicated. The way in which a sound field interacts with our ears gives rise to a non-flat frequency response known as a Head-Related Transfer Function (HRTF). Worse, the HRTF changes depending on the direction from which the sound arrives.

Further complicating matters, there is no universal agreement on what a headphone target frequency response should be. Historically, there were two options: free field or diffuse field. Free field is akin to listening to a pair of loudspeakers in an anechoic chamber, something very few listeners do. In contrast, diffuse field is akin to listening to a pair of loudspeakers in a highly reflective (reverberant) room. While this may be a bit closer to most listening conditions, it is still less than ideal.

“Frequency response is the single most important aspect of the performance of any audio device. If it is wrong, nothing else matters.”Floyd Toole

Studies have found that listeners actually prefer something different, and more like listening to a pair of ‘flat’ loudspeakers in a good listening room. This is the approach we have taken with the M1, and the target frequency response is termed a ‘modified, pseudo-diffuse field response’.

Helping us to achieve the desired response at the output of the headphone, we digitally process all signals using custom 64‑bit double-precision fixed-point arithmetic, running within a high performance, multi-core XMOS processor. It is well‑known in the field of audio processing that fixed-point arithmetic is best, and the 64‑bit arithmetic used in the Sonoma M1 exceeds the performance of most professional audio workstations.

All the filter responses within the DSP are minimum-phase, slow roll-off and are optimised for excellent time-domain response.

Taking further advantage of the incredible accuracy of the 64-bit DSP, we were able to implement within the amplifier a fully-digital interpolated volume control which dramatically outperforms all the pure-analog, stepped attenuators we evaluated. The benefits include:

1. no loss in fidelity and no loss in dynamic range
2. perfect left/right channel matching
3. no potentiometer/attenuator non-linearities at the end of range
4. no noise issues like ‘zipper’ noise, clicks/pops, etc.

In short: it sounded better!

AKM 32‑bit/384 kHz Premium ADC

Due to the need for DSP to achieve the target frequency response, all incoming analog signals must first be converted to digital. This is undertaken in a multi-channel 32-bit/384 kHz AKM Premium ADC chip. As the amplifier has both low-level (3.5 mm) and high-level (RCA) inputs, separate ADC channels are used depending on the input selected. That is, there are two independent, fully-optimised signal paths, one each for the low- and high-level inputs. In this way, the full dynamic range capability of the ADC is achieved regardless of the input selected. The measured SNR of the ADC stages exceeds 120 dB.

Crystek Ultra-Low Phase-Noise Oscillator

The ultimate performance of any digital system is determined by the quality of its master-clock. To guarantee the highest performance regardless of the input signal format, the Sonoma M1 is built around an ultra-low phase-noise oscillator from Crystek. Operating at 100 MHz, it features extremely low close-in phase-noise (<90 dBc/Hz) and an industry-leading rms jitter level, at 100 MHz, of 82 femtoseconds (82 x 10-¹⁵ s). All clocks used throughout the M1 are derived from this exceptionally accurate master oscillator via a sophisticated clock distribution scheme using Texas Instruments frequency dividers.

Optimum PCB Layout

The use of premium parts alone does not guarantee high quality performance. In the design of the M1 system, particular attention was paid to circuit board layout to ensure the lowest noise and distortion levels possible. Through careful component positioning, we were able to maintain an ultra-low noise-floor of -129 dB!

Precision CNC-machined aluminium case

In contrast to the M1 headphones, where every effort was made to minimise weight, the M1 amplifier is a very substantial unit. Starting with a solid billet of high purity Aluminium 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 5/16^th 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.

Superior USB Data Cable

As USB is the only connection which accepts all the high-resolution digital audio formats which the Sonoma M1 supports (PCM up to 32‑bit/384 kHz and DSD64/DSD128) it was vital to ensure that there was no compromise in performance when using this connection. Consequently, in collaboration with Straight Wire Inc., we developed the supplied USB cable. Featuring gold-plated connectors and a silver-plated data path, it is the ideal connection between your digital music source and the Sonoma M1 system.

Custom Universal Power Supply Unit

It should, perhaps, be obvious that stable, clean power is essential to the operation of any audio circuit. For the M1 we have opted for a two-part power solution in order to keep as much noise as possible out of the sensitive audio path.

The first-stage involves a custom-designed, universal voltage, outboard switch-mode power supply unit. While it may look similar to the units provided with laptop computers, etc., internally it is very different. To avoid any headroom issues, our device 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 85 kHz) to avoid the possibility of the switching-frequency dropping into the audio band as the power draw changes (i.e., the switching-frequency remains above 85 kHz under all operating conditions!). This unit is designed with improved internal filtering to yield extremely low noise and ripple (<50 mV peak-to-peak). It is connected to the amplifier via a custom, high-quality, shielded cable (with braided cover to match the headphone cable), and, to ensure an ideal connection, it is fitted with a high-performance, Switchcraft locking DC power connector. A massive ferrite core filter has been added to the DC cable to reduce EMI.

Inside the amplifier, all audio circuitry is supplied by multiple stages of ultra low-noise, high-current linear regulators from Analog Devices. Isolated power regulation stages are used for both analog and digital sections as well as high- and low-level circuit stages.

This two-part power solution is expensive, but, we feel, necessary to meet the design goal of delivering audio performance without compromise.

The Bravura headphones will only use the Sonoma Amp.

Specifications