Technology Reference Guide

BD-HPEL Transducer

The single most crucial component of any headphone system is the transducer. The ultimate performance of the complete sound reproduction system will be determined and limited by the acoustical performance and fidelity of the transducer. 

The APERIO utilises a new Balanced-Drive HPEL Transducer (BD-HPEL) – a symmetrically-driven variant of Warwick Acoustics’ High Precision Electrostatic Laminate (HPEL) transducer, originally developed for the Sonoma Model One headphone system. The BD-HPEL was developed specifically for applications that demand the highest level of audio resolution and greater output capability. 

The BD-HPEL requires symmetrical, high voltage drive signals to generate a differential, electrostatic force that moves the diaphragm. The BD-HPEL has twice the excursion capability, greater linearity and lower distortion than Warwick Acoustics’ first generation HPEL. 

This next-generation HPEL transducer is built in the United Kingdom, at Warwick Acoustics’ state-of-the-art R&D facility. The BD-HPEL is manufactured under strict laboratory conditions using proprietary, semi-automated, cell-based production techniques. We control all aspects of sourcing, production, testing and quality assurance. Every transducer produced is tested in our lab, graded and matched within 0.5 dB to a complementary unit to form a matched-pair. Only the best-performing matched-pairs of BD-HPELs are installed in APERIO headphones. 

The core technology of our BD-HPEL has been refined over several years to be impervious to environmental changes such as temperature and humidity variation, stable in performance and sound quality over time (even after years of daily use), and extremely durable. These characteristics have been achieved because of the simplicity and robustness of the design, utilisation of only the finest materials available and Warwick’s proprietary manufacturing processes. 

The BD-HPEL retains Warwick Acoustics’ patented cellular structure, which acts to significantly reduce and distribute resonant phenomena, and extends bandwidth to over 60 kHz. Subdivided cells act as smaller, phase-aligned planar sources, whose parallel-driven outputs combine acoustically to form a cohesive planar wave front. 

Injected Magnesium Ear cups

Major structural components of the headphone, for example, the ear cups, are fabricated of injected Magnesium – a rigid material lighter and significantly better-damped than aluminium and most polymers. Because the Magnesium is injected, complex and precise components that typically would require CNC-machining are possible. Key structural fasteners are fabricated from stainless steel for strength and durability. Gaskets are formed from closed-cell foams that are well-damped and free from compression-set. 

The mechanical design is robust and employs the finest materials available for each functional purpose. Even the two-stage finish was selected for its excellent damping properties and durability. The result is a mechanical foundation that is extremely rigid, well-damped and free from resonances. 

The development team carefully designed the ear cup components to minimise acoustical reflections and diffraction, on both sides of the transducer. Similarly, acoustic impedance is optimal on each side of the transducer. The APERIO is extremely “open” to the surrounding air, with a minimum of obstruction between the transducer diaphragm and the listener’s ear. Unlike most headphones, the APERIO does not have regions over the transducer covered with acoustical materials that impede air flow or spectrally filter the sound (aka, passive equalisation). This common approach is not needed for the APERIO and has deleterious effects on sound quality, especially with regard to detail, resolution and sonic neutrality. 

Ear Cushions

In order to achieve outstanding low frequency performance, the APERIO requires a small sealed air volume between the transducer and listener’s head. The APERIO maintains this air volume and requisite air seal through key design features that eliminate air leakage, even with variations in head and ear shape. 

And in order to minimise acoustical standing waves within the enclosed air volume, which in turn create resonances, the ear cushions feature acoustically-absorbent foam around their inside perimeter. The ear cushions maintain the necessary air seal to the user’s head while absorbing damaging acoustical reflections within the enclosed air volume. 

The ear cushions are designed to improve cooling around the user’s ears via perforated top surfaces and a copper thread mesh that dissipates heat. The ear cushions are covered in the finest, natural, top grain Cabretta leather that is sustainably sourced. 

Headphone Comfort

The APERIO addresses comfort by minimising weight and optimising geometry. The advanced materials utilised in the design, such as Magnesium and Engineering polymers, result in a very low mass of 405 grams.

The headband has been designed around the ideal geometry for a wide range of head sizes, with the optimum clamping force to ensure a good air seal and stability on the head, without excessive pressure that could be problematic during long listening sessions. The headband maintains this optimum clamping force over the full range of adjustability (extension of the headband and ear cup position). 

Because high voltages are required for operation, the APERIO is designed with a high degree of electrical isolation and multiple safety features. The BD-HPEL transducer is electrically isolated from the conductive Magnesium components. The headphone body is electrically grounded and features double-insulated electrical connections; internal wiring is insulated with High Voltage Silicone and Teflon. High performance, polarised, self-latching, stainless steel connectors with gold-plated copper, multipole contacts are utilised for electrical connection to the headphone cable. 

Cutting-edge components

Because the APERIO is a complete sound reproduction system, the electronics are optimised specifically for the APERIO headphone. Every element of the sound reproduction chain between raw audio data input (from an analogue or digital source) and output to the headphone is included and fully optimised within the APERIO electronics. 

Every functional sub-section of the electronics has been designed to state-of-the-art standards, without compromise. And as a complete system, each section works together seamlessly and synergistically to achieve audio performance superior to individual components, especially within the context of driving the APERIO headphone. 

The APERIO electronics comprise the following audio functions: full-feature preamplifier with line output, digital-to-analogue converter (DAC) and headphone amplifier, including bias voltage “energiser”. The APERIO preamplifier function includes a wide range of inputs: balanced and unbalanced analogue, AES3 (AES/EBU), S/PDIF Coaxial, USB and Ethernet (LAN). The analogue inputs feature independently switchable gain and differential buffer circuitry to maximise audio performance for analogue signals ranging from low-level consumer audio (< 1.0 Vrms) to high-level professional audio (up to 18 dBu). Digital inputs feature state-of-the-art PLL input circuitry and asynchronous clocking for USB and Ethernet sources. 

Within the APERIO electronics, all analogue circuitry and PCB routing is differential (balanced), with gain levels carefully optimised between all stages to maximise Signal-to-Noise Ratio (SNR). The topology is as simple as possible to minimise the number of stages and devices that the audio signal must pass through. All audio routing and switching is performed using the finest signal relays, with gold-plated silver contacts. 

The audio signal is not routed through internal cables and never passes through panel switches or potentiometers. All circuitry in the APERIO electronics, including power supply regulation, employs a Dual Mono topology to minimise crosstalk and interference between channels. 

Only the finest active and passive components are utilised in the APERIO circuitry. Cutting-edge ICs and high voltage discrete transistors are selected and applied in circuitry based upon best audio performance and sound quality achieved. The highest quality 0.5% tolerance metal film resistors and 2% tolerance polymer film capacitors are utilised in critical circuitry. 

The APERIO electronics have been designed to minimise mechanical resonances and the transmission of vibrational energy that could modulate low level audio signals in sensitive circuitry. The aluminium chassis sections interlock and are fastened together using stainless steel hardware with thread-locking compounds (to prevent loosening and vibration over time). The complete assembly rests on specialised mounting feet, formed of a highly-damped thermoplastic compound (IsoDamp C-1002), that isolate the enclosure from external shock and vibration. Internal PCBs also are very well damped due to the glass-epoxy substrate and double-thickness copper. 

Analogue Line Output

The Analogue Line Output section features a novel Class A circuit that can output very high voltages with prodigious current, allowing it to effortlessly drive outboard electronics in any professional or consumer application, even with extremely long cables.

We’ve tested the Line Output section with 150-meter-long balanced cables driven to 15 dBu (12.3 Vp-p) without distortion.

Dual 32-bit, 8-channel DAC's

The DAC section of the APERIO electronics is truly state-of-the-art. Dual 32-bit, 8-channel DACs are utilised – one per channel in dual mono configuration, to achieve a SNR of 131 dB. Each channel’s DAC section is housed within separate EMI shields (“cans”) and supplied by isolated, local, ultra-low-noise power regulators.

The APERIO features a very unique level control (volume adjustment) configuration to preserve audio quality. 

Dual domain level controls are implemented – an analogue attenuator for analogue and DSD signals, and a digital attenuator for PCM digital signals. The analogue attenuator is based on parallel, laser-trimmed resistance ladder networks and is fully differential. The digital attenuator is DSP-based with optimal re-quantisation and dithering to eliminate “zipper noise” and other anomalies. Both attenuators are calibrated and matched. 

Input Cables

Warwick Acoustics supplies two high performance input cables with the APERIO: a USB 2.0 Type A to B cable, and an Ethernet cable. These cables are custom designs, developed by Warwick Acoustics to match the performance and quality level of the rest of the APERIO system. Both cables utilise superior SF/FTP construction, with double overall shielding (aluminium-mylar foil and copper-plated Aramid fibre braiding) as well as internal shielding of each of the double-helix twisted pair conductors. All internal data wiring is formed from 66 strands of micro-fine OFHC copper per conductor, each plated in pure Silver. The wiring insulation is high-grade Polyethylene (PE), with a very low dielectric constant (< 2.0). The outer jackets are formed from low durometer Thermoplastic Elastomer (TPE) covered with a multifilament woven Polypropylene braid for durability and a soft texture. 

The USB cable connectors are high-quality, gold-plated Type A and B plugs. The USB cable is designed to be impervious to noise, interference and emissions while maintaining an optimal 90 Ω characteristic impedance over the full operating bandwidth; it is fully compliant with USB 2.0 standards. The Ethernet cable connectors are the finest quality, ruggedised plugs available: professional, full-metal shielded XLR (RJ45) type from Neutrik and universal, full-metal shielded RJ45 type from Telegärtner. The Ethernet cable also is designed to be impervious to noise, interference and emissions and maintains an optimal 100 Ω characteristic impedance over the full operating bandwidth. While the connectors are certified as CAT6A type, the cable is compliant with CAT7 standards. 

The APERIO electronics are assembled in the Bay Area of California, USA, by skilled technicians using cell-based production techniques. All units are fully tested and measured to verify audio performance after assembly, with individual test results provided for every unit produced.