Back in 2010, when I reviewed the Joseph Audio Pulsar loudspeakers, I concluded, “If I were forced to move into a smaller domicile, one with a modestly sized room for my system, the first speaker and most likely last speaker I’d consider would be the Joseph Audio Pulsar.” Since then I’ve moved into a smaller home and just as my thoughts turned to the Pulsar, Jeff Joseph sent me an announcement about his newest monitor, the Prism. Priced at $3699 per pair, the Prism is Joseph’s attempt to capture the magic of the Pulsar in a less expensive package. As you might expect, I was only too happy to find out how well he had succeeded.
Prism Tech Tour
All Joseph Audio speakers employ the “Infinite Slope” crossover, which ex-McIntosh product designer and senior engineer Richard Modafferi patented back in 1988. During the intervening years, Jeff Joseph has made substantial modifications to Modafferi’s design. The Prism speaker includes Joseph’s latest refinements on the Infinite Slope crossover.
As you might infer from its name, the Infinite Slope crossover has a much steeper slope than a conventional loudspeaker’s crossover. Speakers with “regular” crossovers are more likely to exhibit audible discontinuities between drivers because of the much wider span where the two driver’s frequency ranges overlap. With Infinite Slope, the hand-off from tweeter to midrange/woofer occurs over a much narrower frequency band.
The original Infinite crossover design was symmetrical, meaning that both the tweeter and the midrange driver had the same crossover slope. But the design has evolved—the Infinite Slope filter system currently employed isn’t the same as the one Joseph Audio started with more than twenty years ago. Now the company uses a steeper slope on the woofer than on the tweeter. According to Joseph, “This gives us a more seamless crossover and improves the quickness of the entire system because the decay is cleaner.”
A first-order crossover has a 6dB-per-octave roll-off between drivers. In comparison, the Prism uses a 100dB-per-octave crossover for the low pass and 18dB third order for the tweeter’s roll-off. The low-pass filter drops much faster than 24dB per octave until it hits about -41dB, at which point the attenuation tapers off. This is the benefit of the Infinite Slope technique—it has a very steep initial drop, and reduces wave interference much more effectively than a standard 24dB-per-octave network does. The Infinite Slope crossover allows the two drivers to mesh in a more coherent fashion than most conventional crossovers. This method confines the overlap to where the wavelengths are longer, so the lobing artifacts you see in conventional designs do not occur.
Proponents of first-order crossovers often point to the extreme phase shifts that accompany steeper crossovers as one of the reasons for using a simpler filter. But in the case of the Infinite Slope, phase shifts are not an issue. The Infinite Slope is actually “phase-matched,” because it introduces a full 360-degree phase rotation. But as the actual frequency region of the crossover is narrow and the 360-degree shift puts the two drivers back into linear phase (except for the fact that the woofer is one full cycle behind the tweeter), the audible effect of this phase shift is less than you would hear on a conventional multi-driver first- or second-order-crossover loudspeakers. According to Joseph, “when you use a ‘wrapped-phase measurement’ scheme like we do with the Infinite Slope, you overlay the phase shifts from both drivers so they can be adjusted to sum together nicely. The final result is the phase shift occurs over too narrow a frequency range to be audible.”
Another advantage of an Infinite Slope crossover design is that the off-axis frequency and power response curves of both the tweeter and midrange are far more similar than they are on speakers with conventional crossovers. Jeff explains: “When you listen to live acoustic music you aren’t confined to a narrow and artificial window. You can move around and the sound doesn’t change much. That’s because the power response of live acoustic instruments in a room is more even without the irritating artifacts and incoherencies of speakers with conventional crossover designs.” In theory and practice, the Pulsars’ power response and phase accuracy both on- and off-axis, is more like an acoustic instrument in a real-world space than most loudspeakers.
Now that I’ve covered the overriding technological raison d’être of the all Joseph Audio loudspeakers, let’s look at the Prism’s particulars. It is a two-way, ported, stand-mounted loudspeaker with a 1" Sonotex dome tweeter and 5.5" aluminum-cone midrange/woofer. The crossover point for the Prism is 2.5kHz, which would be on the high side for a conventionally crossed over loudspeaker, but ideal with the Infinite Slope crossover since the midrange/woofer handles most of the “heavy lifting,” while the tweeter has far less in the way of extreme dynamic demands due to the higher crossover point. Published frequency response for the Prism is 45Hz to 20kHz, which rivals many floorstanding “full-range” loudspeakers.
Joseph Audio, as a matter of preference, does not list a sensitivity rating for the Prism or any of its loudspeakers. According to Joseph, “When I try to explain about impedance and a well behaved load to prospective owners, their eyes glaze over and they buy a ‘94dB efficient’ speaker that’s actually 87dB with a 4-ohm impedance and wicked phase angles, and then they wonder why their SET tube amp is distorting. But anyone who tries our speakers on their tube amps are often pleasantly surprised to find they play at a very satisfying levels without strain.”