Sonus Faber Extrema loudspeaker
The Extrema represents one of the fullest expressions of the art of speaker design: performance rules absolutely, price only being a minor consideration. (If you're building the speaker equivalent of an Italian supercar, why should cost be important?) The Extrema's remarkable styling includes a deep front-to-back ratio, all slopes and curves, with heavy sculpting in solid wood. It certainly has an Italian "designer" quality. Standing on a massive six-pillar iron platform of prodigious weight, the whole assembly beds down like the Rock of Gibraltar.
This analysis leads with a feature which, as far as I can tell, is unique (Sonus Faber has applied for a patent). Most designers would not have given it a second thought, since classical engineering doctrine concerns the primacy of efficiency. Sonus Faber, however, has rethought the concept of the high-pass crossover filter, ignoring the efficiency dictum and daring to waste a small proportion of an amplifier's output power to achieve a new crossover realization without using any capacitors. If the speaker were to be used in a battery-powered arrangement, the idea would be impractical, but for a mains-powered amplifier, the consumption of a few more watts is likely immaterial—probably less than the idle power of the higher-current-enriched class-A or full class-A models generally available.
This crossover begins with the design approach adopted for the system, namely the use of first-order rolloff slopes of 6dB/octave; a single reactive component is used to divide each driver's working frequency range. This appears to be an attractively simple concept requiring just two components: an inductor in series with the low-frequency driver to roll out the high frequencies, and a capacitor in series with the tweeter to roll out the mid and bass. Advantages include a complete absence of electrical overshoot or ringing, a smooth load impedance, and, with suitable drivers and an appropriate physical alignment, a close approach to minimum-phase performance.
Minimum phase means that the phase varies linearly and smoothly with frequency in a manner mathematically related to it by the Hilbert Transform. [The phase response offers "just the right amount" of deviation for the frequency response.—Ed.] Perhaps more important, a "minimum-phase" component will reproduce a complete acoustic waveshape correctly, with all phase relationships preserved (at least to within the accepted limitations of the loudspeaker's defined overall bandwidth). The concept of "linear" phase, however, is misleading; it implies that the speaker cannot have a defined bandpass characteristic. Reasonable bandwidth limits are entirely sensible, however, and are found in nature; furthermore, they are not in themselves a cause of significant aural distortion.
The slow electrical rolloff rate of the simple first-order crossover unavoidably results in a large overlap of response between the defined working ranges of the drive-units. This mandates the use of very-wide-bandwidth drivers of very uniform amplitude and phase responses. Rolling out the bass/mid unit through the low treble is usually no problem with a modest series inductor or "choke," and the electrical matching works well.
However, this simple crossover often gives a problem with the tweeter. The usual series capacitor has the unfortunate property of reacting with the tweeter's varying load impedance in its lower range due to the unit's motional impedance. A severe and unwanted modification of the crossover rolloff occurs with this interaction (fig.A, dashed curve). More complex, higher-order crossovers are often used, together with complex correcting networks and/or resistive loadings and dampings in order to get a better match between the actual filter performance and the theoretical. Such complication goes against the desire for simplicity and its associated rewards of low phase shift, good dynamics, and, for Sonus Faber, an expressed requirement for signal transparency.
Fig.A High-pass filter slopes: theoretical 6dB/octave (dotted/dashed); "Sine-Cap" resistor/inductor filter (solid); and conventional capacitor filter (dashed).
If, however, the tweeter has an extra-wide bandwidth, exceptional power handling, and higher-than-necessary sensitivity, another route is available to the designer. A first-order crossover can be achieved, not with a series capacitor but with a single inductor, this connected in parallel with the tweeter. The parallel inductor connection terminates the high-frequency driver well, helping to control its natural fundamental resonance. For the inductor to give a defined filter characteristic, however, there must be a series impedance; in the case of the Extrema, a resistor (fig.B). This resistor/inductor combination appears wired across the speaker input terminals; it will dissipate power over the speaker's entire frequency range, mainly according to the resistor value.
Fig.B Conventional first-order high-pass crossover filter (top) vs "Sine-Cap" filter (bottom).
In the case of the Extrema, the series resistor for the treble section is typically 20 ohms. This provides the required source impedance for the crossover inductor, and also correctly attenuates the treble unit by the required 4dB or so. Yet with the Extrema's nominal 8 ohm drivers, the total system impedance does not fall below a 4 ohm minimum—it is nominally a 5 ohm-rated design. Fig.C shows the high-pass filter's impedance curves, both for a normal crossover with a series capacitor (upper trace) and for Sonus Faber's "Sine Cap," parallel inductor case (lower trace). The former's variations are normal and show a typical 8 ohm system, one without that 20 ohm power in parallel over most of the range.
Fig.C Sonus Faber Extrema, electrical impedance with "Sine-Cap" crossover (solid) and conventional crossover (dotted). (5 ohms/vertical div.)
Footnote 1: Other products include a 50Wpc integrated amplifier and the Gravis, an active 70Hz filter allowing the use of the smaller speakers with subwoofer systems.—Martin Colloms