Warwick Acoustics APERIO electrostatic headphone system

Headphone amps and amp/DACs
Warwick Acoustics APERIO
Warwick Acoustics APERIO electrostatic headphone system

It seems as if it was just yesterday that I received a review sample of Warwick Acoustics’ Sonoma Model One (M1) electrostatic headphone system (£4,995, or $4,995 US). I reviewed the system favourably for Hi-Fi+, while my colleague Steven Stone praised it in our sister publication, The Absolute Sound. Steven’s and my comments closely paralleled one another; we both appreciated the Sonoma system for its accurate, natural, and uncoloured tonal balance, its uncommonly fast transient speeds, its overall subtlety and nuance, its wise-range frequency response, and for its versatility.

Given how good the Sonoma M1 system was and is, the last thing I expected was Warwick Acoustics’ decision to create an even higher performance electrostatic headphone system—one whose capabilities promise to surpass those of the Sonoma system in every way. That super-system is here and is called the APERIO electrostatic headphone system (APERIO, says Warwick Acoustics, derives from a Latin word that means to “uncover, open, or reveal”). Naturally, such an all-out attempt to redefine the state-of-the-art in headphone performance does not come cheaply and accordingly Warwick Acoustics will be selling the APERIO system at £20,000 (or $24,000 US). However, the APERIO system aims to deliver sound quality rivalling (or surpassing) that of loudspeaker-based systems selling in the six- and even seven-figure range.

As a declaration of its intent, Warwick Acoustics states, “The APERIO is designed for the demanding professional audio market, as a reference studio monitor headphone system for High-Resolution Audio production, mastering, mixing, and recording applications,” but also for “ultra-high-end home consumer applications.” With these ends in mind, Warwick Acoustics has followed what it terms a 'Complete System Design' approach, meaning that the system’s analogue and digital front ends, its powerful electrostatic amplifier, and its intensely revealing headphones were designed from the ground up to complement one another in every way. 

The first indication of how different the APERIO system is to the Sonoma M1 system comes when the system arrives in a beefy watertight, crushproof, and dustproof polypropylene travel case. The next comes when you first see the APERIO’s preamp/amp/DAC and realise that it is roughly three times wider than the Sonoma M1's amp/DAC. The reason for the size increase is that the APERIO preamp/amp/DAC supports a much wider range of digital and analogue inputs than the Sonoma amp/DAC did and features circuitry specifically optimised for each input type. Further, the APERIO amp also provides considerably more power output and a more elaborate and robust power supply than the Sonoma M1 amp/DAC did, and it is fully capable of serving as a preamplifier in high-end audio systems.

In the analogue domain the APERIO provides single-ended and balanced analogue inputs and outputs, with High/Low gain switches for both analogue inputs. Digitally, the APERIO offers a very flexible set of inputs including USB, coaxial S/PDIF, AES3, and a fully DLNA compliant Ethernet interface. In turn, the APERIO DAC section, which is based on dual 32-bit, 8-channel DACs arranged in a dual mono configuration, can decode PCM files with sample rates to 384kHz and DSD files (native or DoP) up to DSD256. One crucial point, says Warwick Acoustics, is that, “all audio signals are kept in their native domain and format: analogue always remains analogue; DSD stays DSD until its final conversion to analogue; PCM samples are never converted.”

Warwick Acoustics uses the highest quality parts throughout the APERIO, leading to some impressive performance specifications. The APERIO’s costly clocking circuitry, for example, provides very low jitter (82 fSec RMS @ 100 MHz) and an extremely low noise floor (-168 dBc/Hz). Warwick Acoustics notes that any DSP performed on PCM audio data is “double precision, 64-bit, fixed point, at native sample rates—equal to the best Digital Audio Workstations (DAWs).” The APERIO’s dual DAC/dual mono architecture yields a signal-to-noise ratio of 131 dB. Separate EMI-shielded chambers enclose the DAC sections for each channel and are fed by quiet, noise-isolated power regulators.

The APERIO amp/DAC uses separate, domain-specific volume level controls. A fully differential, analogue attenuator, based on parallel, laser-trimmed, resistance ladder networks, is used for analogue and DSD signals. In turn, a DSP-based digital attenuator is used for PCM signals. Warwick Acoustics claims the attenuators are “calibrated and closely matched”. 

The APERIO’s amplifier section uses eight discrete 1000V MOSFET devices per channel in a fully balanced configuration that is based on “a proprietary topology based on single-ended Class A operation.” The upshot is an amplifier that delivers a noise-free 1800 V DC bias voltage for “charging the (APERIO’s) BD-HPEL transducers” and serves up 15 Wpc of power output with very low distortion and noise. 

Finally, Warwick Acoustics has gone to great lengths to maximize the performance of the APERIO’s power supply and power regulation sections, its PCB layout and construction, the design and construction of its noise-isolating chassis, and its heat management system, which features four very low-noise cooling fans. In short, Warwick Acoustics has explored virtually every design and construction detail to give the system state-of-the-art performance.

At the same time, the APERIO electrostatic headphone offers plenty of innovations of its own. The original Sonoma M1 headphone used a proprietary, single-ended electrostatic driver featuring the firm’s patented HPEL (High-Precision Electrostatic Laminate) technology. In contrast, Warwick Acoustics has used Multi-Physics Finite Element Analysis modelling to create all-new BD-HPEL (Balanced Drive High-Precision Electrostatic Laminate) drivers for the APERIO headphone. The BD-HPEL drivers are a symmetrically-driven (not single-ended) variation on the drivers used in the original Sonoma headphone. 

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