Originally an engineering project conducted at the University of Warwick, HPEL was an entirely new way of making electrostatic headphone drive units. HPEL, short for High-Precision Electrostatic Laminate, allows the manufacture of light and highly responsive electrostatic drivers in large multi-layer sheets, from which multiple drivers of virtually any desired size or shape could be cut and trimmed. The limits are more about the size of the laminate sheet and the equipment used to make the laminates, but we will soon see the possibility of making HPEL drivers large enough to be used as full-range domestic electrostatic loudspeakers, or shaped into car door panels, parcel shelves or footwells to act as active noise cancellation systems. It’s still early days, but the options are legion, especially as HPEL production methodology offers exceptional driver-to-driver consistency and longevity— something not easily achieved with conventional electrostatic driver manufacturing techniques.
The HPEL driver has a unique form of electrostatic driver construction; where most electrostatic designs have two metal grids, one on each side of the highly-charged diaphragm, the Sonoma M1 Headphone driver only has a single grid and a diaphragm that is structured into hexagonal cells. This significantly reduces distortion.
In essence, each driver consists of a three-layer ‘sandwich’ comprising a stainless steel mesh grid that faces the rear side of the ear cup, a centrally-positioned open-cell insulating spacer made of a high spec engineering grade polypropylene, and then a machine-tensioned, 15μm-thick, flexible film laminate diaphragm that faces the front (or ear) side of the ear cup. The diaphragm is made of bi-axially oriented propylene film (BOPP) with a vapour deposited aluminium surface sealed with a synthetic lacquer. This diaphragm is then clamped within a two-piece protective cassette made of 40% glass-filled polyphenylene sulphide (PPS) from which the driver is isolated by precision-made PORON™ microcellular urethane foam gaskets. This cassette drops into the headphone ear cup. When an audio signal is superimposed on a 1350V DC bias voltage, the ‘drum-skins’ formed by the flexible membrane vibrate, producing sound. Gaskets and protective cassette frame aside, there is nothing between the flexible film diaphragm and the wearer’s ears.
The Sonoma M1 Amp features a single-ended, FET-based Class A amplifier capable of a maximum amplitude of 145V. This is powered by an outboard, very high-quality switch mode power supply with extensive filtering and a fixed frequency switcher that operates at over 85 kHz. This connects to the energiser via a custom made, shielded ‘umbilical cord’ fitted with locking connectors. Internally, the energiser incorporates multiple low-noise, high-current linear regulators, with separate regulators feeding both analogue and digital sections of the energiser, as well as high and low-current circuit stages. The energiser has a built-in DAC based around a 32-bit/384 kHz ESS DAC chip for processing digital sources, and an 32bit/384 kHz ADC chip for digitising analogue inputs. It also uses DSP to produce a pseudo-diffuse field frequency response curve; this explains the digitisation process for analogue sources because they later need to be passed through that digital signal processing. This does effectively lock a Warwick user into the brand’s equipment, because no third-party energiser maker will use DSP to modify the response curve of the headphone, and no energiser will make the Sonoma M1 Headphone sound quite as good as the matching amplifier. However, that also ties Warwick Acoustics into making electrostatic headphones that work with that DSP engine (even in subsequent generations of the energiser), meaning a headphone like BRAVURA is inherently more evolution than revolution.
In truth, ‘evolution’s is probably all you need here. The Sonoma M1 was generally extremely well-received, and praised for its mid-band and top end clarity. However, it wasn’t without its criticisms, most of which were levelled at the Sonoma M1 Headphone. In particular, the bass was criticised for being ‘thin’ and ‘light’ sounding, and its sound pressure levels were on the ‘quiet’ side too. The Sonoma M1 system wasn’t the ‘headbanger’s friend’ was the general conclusion.
Aside from some sneak previews at Rocky Mountain Audio Fest 2016 the Sonoma M1 system began arriving in people’s hands in the next year. Four years is a long time in headphone design and a lot has happened at Warwick since those early days. Most importantly, as you might expect from a very new technology, the intervening years have seen several step-changes in the manufacturing techniques.
The BRAVURA is the product of years of HPEL development to refine Warwick’s core technology, in the process improving upon the Sonoma M1. The latest evolution of the single-ended HPEL uses a new stator design and more advanced materials that are suggested to result in ultra-low distortion, increased SPL and a wider bandwidth.
Critics will, of course, see this as both a vindication of their views on the original Sonoma M1, and see this as response to their criticism. In fact, it’s technological development on an inherently new concept; the point in a technology where the delta of change is at its steepest. That being said, Warwick has reacted to customer feedback of the Sonoma M1 Headphone, especially in the use of a new headband and revised cabling. The former helps a lot, because the BRAVURA feels less ‘clampy’ than before, but the latter helps a hell of a lot ; the older model’s cable system was prone to ‘touch it, you hear it’ effects.
Judging by the BRAVURA, this is a subtle, yet significant evolution over the Sonoma M1 Headphone. First, let’s get the ‘artist’s response to just criticism’ part out of the way. These are more comfortable headphones that feel better built than their predecessors. The black finish feels good to the touch (I didn’t have the silver models to hand, so I cannot compare directly) and the more traditional comfort strap across the headband does make the BRAVURA sit considerably more easy on the head. The previous headband was at once more rigid (translation: tighter feeling) and more sonically conductive (translation: if you are a glasses wearer, or the kind of fidget who ends up touching their head a lot during listening sessions, the Sonoma M1 Headphone’s headband was prone to ‘chattering’. The new headband scores well here, both in terms over overall comfort and reducing that plasticky ‘thock, thock’ sound you get when accidentally tapping a bit of ABS.
Then, there’s the cable. The older one was a low-tangle cable, with some very nicely made custom connectors at both ends. It conducted electrical signals quite well too. If this sounds like the cable-equivalent of ‘lovely hair’ praise, well… the other cable blotted its copy book by being more than a little bit microphonic. I’m pleased to say the new cable fixes that (that might have something to do with today’s connections in the BRAVURA headphone ear cup itself, but they look functionally identical, so I am going with the cable). This didn’t make a low thrumming sound if you move around a bit fast, or little thuds each time it hits a shirt button. Instead, it was an almost noise-free (and still tangle-free) cable. It’s not made of spooky headphone magic so it’s not fully immune to sound conduction effects because such immunity is more a myth than a reality, but the cable has gone from a ‘could do better’ C+ to a solid ‘outstanding’ A or even A+.
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