cancels some of the two sources' mutual contributions, thereby diminishing the loudness, and this loudness diminution makes an easily audible cue that you have strayed from optimal alignment. If you're really fussy, you'll notice that the maximum loudness tends to occur over a small span of the phase control knob, about 10 degrees worth (since small phase discrepancies produce only slight cancellation of amplitude). The best alignment point is simply at the center of this maximum loudness span.
      When we placed the M6 atop the W1, with their front edges aligned (as discussed above), we found that the maximum loudness point (and optimum sonic integration point) was achieved when the X1's phase control knob was rotated to near maximum, at about 80 degrees as indicated on the dial (equivalent to an actual 100 degrees). At this setting, the X1 is effectively advancing the W1's signal by 100 degrees at the crossover frequency, relative to the signal the X1 feeds the satellite M6 (and relative to the X1's input signal, which is relevant in case you want to run the M6 full range and bypass the X1).
      Since the W1 and M6 are physically aligned (at their front edges) in this example, why is the optimum phase setting 100 degrees, rather than 0 degrees? The W1 intrinsically lags behind the M6, so it needs to be electrically effectively advanced in order to be correctly temporally aligned, even though the cabinets appear to be already physically aligned in the same location. The W1 lags behind because its drivers are actually farther away in its cabinet (especially when the drivers are optimally facing sideways, as discussed above) than the frontal M6 drivers are in their cabinet. And the W1 lags even further behind because its driver is slower, and because its input signal is band limited to lower frequencies. The X1's phase control compensates for all this W1 lag, whatever the cause, by simply effectively advancing the signal it feeds the W1 by 100 degrees.
      A footnote for the technical among you. Since the X1's phase control cannot literally advance the phase of its output signal to be ahead of its input signal, what it actually does is to advance the phase of the bass signal ahead of an imaginary point that is 360 degrees (one full cycle) behind the input signal. Thus, when you synchronize the satellite optimally with the subwoofer, via the X1's adjustable phase control, you are really synchronizing the satellite with the previous bass cycle, that came down the pike, being fed to the subwoofer. This staggered mismatch by one cycle, at the bass crossover frequency where both satellite and subwoofer are contributing, is not of serious consequence for many bass notes, which repeat substantially identical cycles for some time. It could slightly degrade the accuracy of bass transient attacks, where the first cycle sounds and looks different than the second cycle, but the audible effect of this inaccuracy would be to quicken and sharpen the bass transient attack (since the overtones would lead the fundamental by one cycle), so this slight inaccuracy might well be euphonic, since we are always struggling to speed up the attack of our sluggish bass systems. Thus, the X1's electronic phase control is a very practical and efficacious solution, which also cures the far worse sonic degradation heard when satellite and subwoofer are not in synch at all, as is the case with most other loudspeaker systems employing separate subwoofers.

-- Spaced Locations for Subwoofer and Satellite

      You can also use the X1's phase control to compensate for placements of the W1 at different locations than the M6. As noted, you would not want to place the W1 in front of the M6, partially blocking the path from the M6 to you, so in practice you'd rarely have the subwoofer much closer to you than the satellite, which means that you would not have much use for the range of the X1's phase control below 100 degrees. But you might well want to place your subwoofers farther away from you than the satellites are, perhaps to position your subwoofers in room locations where they won't trigger room resonance modes. In this case, you'll probably be exploring the phase control's range above 100 degrees, to advance the subwoofer phase even more, in order to compensate for the fact that you have now moved the subwoofer to be physically farther away from you than the satellite is.
      The rotating knob actually achieves phase rotation from 0 to 120 degrees (the label says 0 to 90 degrees, but it is overly modest). To access the phase control's range above 120 degrees, simply flip the small switch adjacent to the rotating knob, from 0 to 180. In this position, this switch adds 180 degrees to the setting of the knob. So, for example, to get 200 degrees worth of phase advancement, flip the switch to the 180 position and set the knob to 20 degrees (about 15 degrees on the modest dial)
      The X1's phase control gives you considerable flexibility in positioning the subwoofers to the side of and/or to the rear of the satellites, while still preserving excellent sonic integration between subwoofer and satellite. But there are limits on how far the phase of the subwoofer feed can be advanced electronically by the rotating knob (at reasonable circuit cost). The true range of the rotating knob is 0 to 120 degrees, and flipping the small switch can add 180 degrees to this. Thus, the X1's phase control completely covers the following phase ranges (of effective advancement of the W1), where it can achieve optimal alignment: 0 to 120 degrees (by rotating knob, with switch flipped to 0), 180 to 300 degrees (by rotating knob, with switch flipped to 180); 360 degrees (which equals 0 degrees again, but with the bass now two cycles back instead of one cycle back) to 480 degrees; and so on. You can see that this progression can extend to cover a large differential in distance, between your path length to the location of the satellite and your longer path length to the location of the subwoofer that you want to synchronize with this satellite.
      You can also see that there are also gaps within this progression, each gap consisting of 60 degrees. Fortunately, these gaps are not especially pernicious sonically. Why? If a subwoofer and satellite are out of synch by up to 90 degrees, the subtle sonic phenomena of true integration won't be quite optimized, but at least you will hear good performance in the more obvious sonic phenomenon of tonal balance. In other words, tonal balance, which is much more aurally obvious than the subtle effects of true sonic integration, won't be degraded that much, so long as the synch error between subwoofer and satellite stays below 90 degrees. That's because, if two sources radiating the same frequency are out of synch by 90 degrees or less, the resulting interference will produce a tonal balance error of less than 3 dB. Note that the X1, even in these 60 degree gaps, can always keep the synch error to 30 degrees or less (by setting the phase controls at either the bottom or top border of this gap), and this 30 degree synch error is well below the tolerable 90 degree threshold.
      On the other hand, if synch error were to get higher than 90 degrees, severe tonal balance anomalies can be produced by the resulting interference (reaching the point where, at 180 degrees of synch error, the joint frequency response of the jointly contributing and jointly interfering sources can plunge down an astounding infinity dB). Other loudspeaker systems with separate subwoofers, lacking the Evolution's phase control, can easily tread into this forbidden region above 90 degrees of synch error, and they thereby often produce horribly ragged frequency response where the subwoofer and satellite overlap in their mutual crossover region.
      We've just seen which phase regions the X1's phase control can optimally compensate for, and which phase regions are gaps where the X1's compensation is not optimal, but at least is never worse than 60 degrees out of synch. Now, what distance differentials do these phase ranges correspond to? That depends upon the crossover frequency you have chosen. At the centrist and common crossover frequency of 80 Hz, an acoustic wavelength, which contains 360 degrees of phase rotation, is 14 feet long. From here, simple arithmetic tells us the correspondence between phase degrees and offset distance (the difference between your distance to the satellite and your distance to the subwoofer).
      Our starting zero offset point is with the M6 atop the W1, and with the front edges of their cabinets aligned. And we have to take account of the fact that, with these two units thusly physically aligned, we had to dial in about 100 degrees of effective W1 advancement on the X1's phase control, in order to achieve optimal sonic integration. This means that the zero distance point corresponds to 100 degrees already used up of the X1's phase adjustment capability. And thus we should subtract 100 degrees from the above phase ranges, before we translate the phase into distance at the 80 Hz crossover frequency. Therefore, the phase ranges above, reset to our new zero distance baseline, are: 0-20 degrees; 80-200 degrees; 260-380 degrees; and so on. These are the phase ranges within which the X1 phase control can achieve optimal alignment electronically (note again the 60 degree gaps where alignment is not quite optimal).
      At a crossover frequency of 80 Hz, these phase ranges correspond to offset distances of: 0-.75 feet; 3-7.75 feet, 10-14.75 feet; and so on. If the offset distance (the distance differential) happens to be within these distance ranges, then you can achieve optimal sonic integration using the X1's phase control (assuming you have chosen 80 Hz as your crossover frequency). If the offset distance happens not to be within these ranges, you won't achieve optimal sonic integration, but you'll still get pretty good sound, since the X1's phase control will still get you close enough to optimal alignment so that you won't experience the gross tonal balance anomalies that most other loudspeaker systems with separate subwoofers produce.
      If you choose some other crossover frequency, the distances above should be scaled accordingly, with a proportionally higher frequency having proportionally shorter wavelengths, thus proportionally shorter offset distances. For example, if you choose a crossover frequency of 110 Hz instead of 80 Hz, multiply the above distances by .72 (80/110). This also suggests that the X1's phase control will in effect interact with the X1's crossover frequency controls, since every time you change crossover frequency you will be changing the effective distance offset in terms of wavelengths, and that will automatically require you to re-optimize the phase control setting.
      The above discussion of phase and distance ranges is presented to give you full knowledge and information about the product. But you should not worry about restricting yourself to the optimal distance ranges when you position your various loudspeakers within your room. It's much more important to position your subwoofers where they minimally trigger room resonances, and to position your satellites where they afford you the best spatial imaging. Then, let the chips fall where they may as to whether the resulting distance differential happens to be within one of the optimal ranges of the X1's phase control or happens to lie in one of the 60 degree (2.25 feet at 80 Hz) gaps where its phase correction is still very helpful but not quite optimal. Likewise, feel free to choose your crossover frequency where, after weeks of experimenting, with various program sources, you think it sounds best, regardless of the implications for the distances covered by the X1's phase control.
      When it comes to setting the X1's phase control, you could get out your tape measure, measure the path lengths from your seat to the satellite vs. to the subwoofer, calculate the distance differential, and then, based on your chosen crossover frequency, calculate the required phase compensation (remembering to add 100 degrees), and dial it in on the X1's phase controls. Or you could instead just use your ears. With each new loudspeaker position you try and each new crossover frequency configuration you try, during your experimental weeks of setup, simply rotate the phase control knob over its full range, then flip the 180 switch and again rotate the knob over its full range, until you find the setting that gives you the loudest, richest sound in the bass and warmth region from bowed double bass viols. Presto! You've got it set, as good as it can get. Who cares if it happens to be within one of the X1's optimal ranges or not? This is as good as it gets, period.

-- Re-Optimizing Phase Control Setting

      As you go through your first few weeks of fine tuning with the Evolution system, you'll be tweaking this phase control many times. Fortunately, that's no problem, because it's so quick and easy to do (just tweak it for maximum bass and warmth volume each time). When you first set up the satellites and subwoofers at their preliminary locations, you should optimize the setting of this phase control. And of course every time you move the position of a subwoofer or satellite, you should re-optimize the phase control on the X1 feeding that subwoofer or satellite. It's important to re-optimize the X1's phase control each time that you make any fine tuning change to the system, so that you're always listening to each new system change at its optimum, with the best sonic integration and the fullest bass and warmth (from aligned, coordinated summed contributions by both satellite and subwoofer).
      Since the X1 is a two channel unit, it obviously makes sense to dedicate each X1 to matched pairs of loudspeakers that will be physically moved in unison (e.g. the two front main satellites, or the two side surround satellites, etc.). In this way, the re-optimizing of the X1's single phase control for both the X1 channels will maintain the alignment of both the loudspeakers in each matched pair, as you experiment with various room locations for them.
      There might also be a slight interaction between the X1's phase control and its controls that change crossover frequency, so you should likewise re-optimize the phase control setting immediately after you make each of your many experimental changes in crossover frequency, in order that you may then be able to judge and compare each new crossover frequency configuration fairly and at its sonic best.

Sonic Quality of X1

      For its low pass output to the Evolution subwoofers, the sonic quality of the X1 is non-critical, since only low frequencies are involved. In any case, you don't have any choice but to use the X1, since it is necessary to have the X1's boosting equalization, from 50 Hz down to 27 Hz, in the signal path to the Evolution subwoofers. And you also will want the X1's many flexible controls at your disposal in driving the subwoofers, especially that wonderful and crucial phase control to sonically integrate your system (not to mention the fun of the buttkicker control). Our only reservation in the sonics of the X1's low frequency output to the subwoofer is that sharp electrical filter peak at 27 Hz and the steep subsonic filter below that, which needlessly degrades the transient response, bass quality, and low bass impact of the W1 to a level considerably below that of which the W1 is intrinsically capable. This should be easy for NHT to fix in a slightly revised X1A.
      Now, what about the sonic quality of the X1's high pass feed to your satellites? Here you do have a choice. You can use the X1 in the signal path to feed your satellites, in which case you also get to use that 3 position slide switch that selects between crossover frequencies of 50, 80, or 100 Hz (all at 12 dB per octave). Or you can bypass the X1 entirely. In this latter case, you could (1) run your satellites full range; or (2) use the crossover built into your surround processor (which is often fixed at 80 Hz, with no other choice of rolloff frequency, when you select small loudspeaker on the processor's menu); or (3) insert a simple capacitor in series with your power amplifier input (which gives a 6 dB per octave rolloff to your satellites, at the frequency you select by the value of this series capacitor working into the shunt input impedance of your power amplifier).
      There are pros and cons to each of these choices, depending on many factors that are particular to your situation, so that lecture will have to wait for another time. But for now, the bottom line question for this review is, if you do choose to employ the X1 in the signal path to your satellites, how good does it sound as a high fidelity component (i.e. as if it were a preamp line section)? Not very good. Our straight wire bypass test revealed that the X1 imposes, upon mid and upper frequencies, a pronounced solid state sound, making music sound artificially electric and crisp (harder and brighter). If you choose to listen to an Evolution satellite along the manufacturer's recommended axis (at tweeter height and inboard of the tweeter side), you are already hearing the satellite at its brightest and hardest, with added artificialities of diaphragm breakup, as discussed above. When we added the X1's solid state artificial electric quality on top of this, the sound became frankly excruciating.
      However, all is not lost. Once again fate smiles upon the Evolution system, turning a lemon into lemonade. For, if you instead listen to an Evolution satellite along our unorthodox recommended axis (at woofer height and inboard of the midrange side), you will instead experience a mellow sounding loudspeaker system, with the obnoxious diaphragm breakup rendered virtually inaudible. And, with this mellow sounding loudspeaker system as the baseline, the addition of the X1's solid state artificial electric crispness is actually very tolerable. Indeed, it could even be said to be a welcome coloration, since it adds a bit of needed sparkle to a mellow sounding loudspeaker system. Ideally, it would doubtless be better if the X1 were more sonically neutral in its high pass output feed to satellites. But, within the context of the fact that the Evolution satellites softly veil the sound (even when listened to along our preferred axis), one can live with this X1 coloration, since the Evolution satellites softly veil the artificial solid state hardening of the X1, even as they also softly veil the music.
      The solid state electric artificiality heard from the X1 at mid and upper frequencies is typical of IC chips, and the X1 is indeed packed with and reliant upon a bevy of IC chips for its many circuit functions. If the X1 were made instead with sonically superior discrete solid state devices, it would have to sell for a higher price (although it is only the high pass circuit that is sonically critical, and this circuit is short and simple, so it would not cost a lot to sonically upgrade just this one portion of the X1's circuitry).
      Since you do have a free choice of whether or not to put the X1 circuitry in the signal path to your satellites, the final judgment call can be yours, as to whether the solid state sonic colorations of the X1's high pass satellite feed are acceptable to you or not. It's pretty easy to try it both ways, so you can make the decision for yourself.

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