Z-1 Technology Overview

BG Corporation was founded in 1994 and is based in Carson City, Nevada. Up until late last year, they went by the brand name Bohlender-Graebener but today prefer to be simply known as BG. Their focus has been on permanent-planar magnetic loudspeaker systems.

About half of their planar drivers are manufactured at their Carson City facility. The rest are proprietary designs to their specification. The process begins with steel frames built in-house, planar drivers are assembled using their proprietary process for tensioning and securing of the planar film in the speaker. All drivers are quality tested for impedance, 'buzz' tests and frequency response.

There are two sets of five-way binding posts, allowing for bi-amp or bi-wire setups (if that sort of thing really floats your boat). A grill is also provided and fits flush into the front baffle via four pegs that attach to receptacles in the baffle. Four rubber feet are permanently attached to the bottom of each cabinet, this would help reduce energy transmission to/from the stands and save the finish from scratches.

Planar-magnetic drivers are based around a thin film (planar meaning flat or lying in a single geometric plane) either made of a conductive material, or with a conductive filament embedded across its surface. This film or ribbon is suspended between two magnets. The signal is passed through the conductive material in the film/ribbon and causes it to oscillate between the magnets. This movement of the film/ribbon displaces air and thus creates sound. This approach is not a panacea, and as with everything else has its pros and cons.

On the plus side; the films are extremely light (compared to conventional drivers) and theoretically have better transient response due to lower inertia, as claimed by proponents of that school of thought. Also, the flat structure of the film/ribbon avoids the standing wave and phase issues typically faced by cone designs.

According to BG, its not the speed in general understanding that provides high resolution, but how fast and clean the decay is. Well designed planar ribbons can have advantages over conventional driver counterparts in this respect. This is because mass of a conventional driver is much higher than a ribbon, and subsequently the inertia is also higher and resonances in general are more pronounced and delayed.

On the negative side; these drivers have a limited excursion and are not efficient at moving large volumes of air. They are thus often relegated to high-frequency duty only, though in some larger designs even the mid-range and bass. They would produce a compressed sound in the lower frequencies, unless the drivers were of massive proportion. Also, if the backwave of the drivers were not somehow eliminated, placement would need a lot of attention. Dispersion suffers with these drivers as the surface area increases.

Editorial Note about Driver Speed by Mark Sanfilipo
From an analytical point of view, how "fast" a driver is, or its "speed" is canonically defined as its "rise time". Rise time (in this case) is the time it takes for the driver's acoustic output to increase from 10% to 90% of its final amplitude, in response to an electrical step-function input. Rise time is a function of the driver's bandwidth. In actual practice, a driver's bandwidth is usually constrained or limited at the high frequency end of its acoustic response by the low pass crossover circuit feeding it signal. Although various mechanisms, such as the driver's voice coil inductance, do indeed affect a driver's bandwidth, once its part of a complete system, it's the crossover that typically has the largest bandwidth-constraining effect on the driver. Thus, for example, a big woofer and a small woofer, having identical band-limited, acoustic low pass characteristics will have the same transient response.

Large drivers have large Mmd (total moving mass, including air load) values. They also have large magnet structures, which mean they can apply large amounts of force moving the mass. Since acceleration is the applied force divided by the mass, for a given mass, by varying the force, acceleration can be varied. Thus a small, lightweight driver could actually prove slower than a heavyweight counterpart IF the available accelerating force is too low. Mass or inertia are therefore, by themselves, no indication of a driver's speed or lack thereof.

Having said the above, it is only fair to mention that planar products have diehard followings who often hold on to their speakers for decades. In 2004 BG was awarded a patent for innovation in this technology, the inventor was Igor Levitsky, VP of Engineering for BG Corp. I spoke with Igor about his invention and here is a summary of the improvements the patent was awarded for.

The original ribbon drivers used an aluminum foil. This foil itself was conductive and thus did not need conductive filaments to be fused to the film. But aluminum foil had several drawbacks; it was very delicate and could be easily damaged or deformed, dynamics and SPL capabilities were somewhat limited. Mylar and Kapton emerged as successful replacement materials, but also had their own set of limitations. BG settled on Teonex, which according to Igor can operate at much higher temperatures than Mylar, and has better damping and is less expensive than Kapton.

A common issue with planar drivers is the tradeoff between frequency bandwidth and dispersion. Generally, to extend the bandwidth into lower frequencies the driver has to be made larger, but the wider the driver is the greater the beaming at high frequencies. The foil in the BG PDR tweeter is considerably wide and allows a specified frequency extension down to 1.8kHz (from 25kHz, +/- 2.5db), the foil width extends to the four columns of slots seen on the faceplate. Normally, a driver this wide will have very limited horizontal dispersion in the higher frequencies; BG came up with an ingenious twofold solution to combat this. The process of etching removes the metal and creates gaps between aluminum conductors. First, the magnetic field is induced on the planar is designed to be stronger in the center and weaker to the sides. Second, a special felt covering the outside two columns progressively filter frequencies as they increase. The combined effect of these two design features results in the radiating width decreasing as the frequencies increase, to the point that the two side columns have no output above 10kHz. So above 10kHz the radiating width is only 15mm (0.6"), which would result in excellent horizontal dispersion.

The patent and design of the tweeter is impressive indeed, but in the end the only thing that matters is how the thing sounds. Well, let's find out.