John Atkinson wrote about the Step Plus in April 2019 (Vol.42 No.4):
When Ken Micallef reviewed the Audio Physic Step Plus ($2599/pair) in our February issue, he found it relatively difficult to optimize the positions of this two-way stand-mounted speaker. However, once he'd done so, he noted that the Step Plus could produce "serious lower-register heft for such a small speaker," and that the pair of them "disappeared" as the sources of the sound, leaving only the instruments in space before him. Ken summed up by saying that the Step Pluses "proved overachievers to the nth degree," and concluded: "If you're in the market for a soundstaging and imaging champ that produces clear upper-range frequencies and solid bass fundamentals with almost any amplifier, the Audio Physic Step Plus should be numero uno on your list."
The reason for this suckout on the tweeter axis is that while the drive-units are both connected in positive acoustic polarity, they're out of phase in the crossover region. Usually, with second-order crossover filters, either the tweeter or the woofer is connected in inverted polarity, to compensate for this lack of integration. (An alternative solution was featured in a speaker B&O marketed in the 1970s, which featured a "filler driver" to compensate for the missing on-axis energy.) But with both drivers wired in the same polarity, listening to the Step Plus above the tweeter axis pushes the woofer's output back in time compared with the tweeter's, and the two drivers now correctly sum in the crossover region.
In the lower midrange and below, the Audio Physic has a little less output than the KEF, which suggests, as I said in the Measurements sidebar accompanying the original review, that the speaker needs to be placed relatively close to walls to best balance its lower frequencies with its upper midrange. The Step Plus both produces more energy between 1 and 2kHz than the LS50, and significantly more energy in the top two audio octaves. And while there's a small dip in the Audio Physic's crossover region compared with the KEF's response, this is not as deep as the anechoic response in fig.1 suggests. As long as you don't sit in the nearfield—in my room, the Step Pluses were 10' from my listening chair—the crossover suckout will tend to be filled in, as the missing on-axis energy is still radiated off axis.
So how did the Audio Physics sound in my room? Leaving them in the positions at which I'd measured their spatially averaged response and driving them with an Ayre Acoustics EX-8 integrated amplifier, I found their low frequencies indeed on the lightweight side. I then experimented with their positions, and ended up with each speaker some 15" from its sidewall; this brought up the bass. The low-frequency warble tones on Editor's Choice (CD, Stereophile STPH016-2) were reproduced at full level down to 80Hz, with then the 63Hz tone suppressed a little, and the 40Hz and lower tones inaudible. Even so, the bass drum and the left-hand register of the piano in Trevor Pinnock and the Royal Academy of Music Soloists Ensemble's recording of Erwin Stein's chamber-orchestra arrangement of Mahler's Symphony 4 (24-bit/192kHz ALAC files, Linn CKD438) were reproduced with just-sufficient weight accompanied by excellent definition.
The Step Plus does seem optimized for articulation, not just in the bass but also in the upper midrange, where the image of the voice of Sónia Grané, the soprano soloist in the Mahler's fourth movement, was both palpable and pushed a little forward at the listener. When a recording's balance already has forward upper mids, this becomes too much of a good thing. Monty Alexander's piano in "Just in Time," from The Good Life: Monty Alexander Plays the Songs of Tony Bennett (24/96 FLAC, Chesky SACD340), and Bill Evans's instrument in "Autumn Leaves," from Live at Art Lugoff's Top of the Gate (24/44.1 ALAC, Resonance RES 2012), were both too upfront at my preferred listening level. With the KEF LS50s, both pianos sounded in better balance with the supporting instruments.
But what about that crossover-region suckout? The dual-mono pink-noise track on Editor's Choice did sound hollow if I slumped in my chair—I needed to sit up very straight for the treble to sound evenly balanced with this signal. Compared to the KEF LS50, the Step Plus's top octaves were more airy. Billy Drummond's ride cymbal in my recording of the Jerome Harris Quintet playing Duke Ellington's "The Mooche" (16/44.1 ALAC, from Editor's Choice) sounded suitably swishy. This track also revealed the stability and precision of the Step Pluses' stereo imaging, the small differences in the positions of the trombone and alto saxophone readily apparent—as was the way the image of the vibes stretched from far left to center stage, as I'd intended when mixing this track. Surrounding the musicians, the subtle ambience of Chad Kassem's Blue Heaven Studios was unambiguously evident.
So while the quasi-anechoic frequency measurement did reveal a potential problem, it won't become an audible problem as long as the Audio Physic Step Pluses are used on stands that place their tweeters sufficiently below the listener's ear height. The speaker's forward upper midrange might make system matching more tricky than usual, but as long as the Step Plus is placed close enough to walls that its lightweight low frequencies can benefit from some boundary reinforcement, it offers just enough bass. Ken Micallef basically called it correctly.—John Atkinson
Fig.1 Audio Physic Step Plus, anechoic response averaged across 30° horizontal window on tweeter axis at 50" (red), averaged across 30° horizontal window centered 10° above tweeter axis (blue), both corrected for microphone response, with complex sum of nearfield port and woofer responses plotted below 310Hz.
However, when I measured the Step Plus, I found that there was a major suckout in the crossover region in the farfield response on the tweeter axis (fig.1, red trace). This suckout disappeared when the response was measured 10° above the tweeter (blue trace). If the Steps are placed on stands low enough that the tweeter is at least 8" below the listener's ears, the treble balance will then be even and flat.
Fig.2 Audio Physic Step Plus, spatially averaged, 1/6-octave response in JA's listening room (red); and of KEF LS50 (blue).
Given the contrast between Ken's praise and this anomaly in the Audio Physic's measured performance, I set up the review samples in my listening room on 24"-high single-pillar stands, which placed the Step Pluses' tweeters 34" above the floor and 4" below my ear height when I sit up straight. I began with the speakers in the positions where my usual KEF LS50s sound best. The red trace in fig.2 shows the Audio Physic's spatially averaged response in my room, generated by averaging 20 1/6-octave–smoothed spectra, taken for the left and right speakers individually using a 96kHz sample rate, in a vertical rectangular grid 36" wide by 18" high and centered on the positions of my ears. For reference, the blue trace shows the spatially averaged response of the KEFs in the same positions, with the levels normalized at 750Hz, where both speaker models have their highest output.
