Eminent Technology LFT8B’s reversing woofer polarity

My tech disconnected the tweeters and placed the speakers with the tweeters facing the outside - they sound and image much better. 

His own speakers may have had their phases reversed. Subwoofers can be tricky. If you are crossing down low without a full two way crossover and your main speaker's bass is on the fat side it may well sound better with the phase reversed. Back in the day with dipoles there was a strong movement towards wiring subwoofers out of phase. If you are crossing under 80 Hz with a full two way crossover running a steep slope you will have to go back and forth a few times to pick up on the difference. Crossing up at 100 Hz the difference is very noticeable. 

One thing to remember about dipole planars is that the rear wave reaches the front wall (behind the speakers), is reflected off that wall at the same angle but in the opposite direction, eventually reaching the listeners ears delayed in time in relation to the same sound coming from the front of the speaker. The front and rear waves may reach the listener’s ears in perfect phase, 180 degrees opposite each other, or anywhere in between. That creates the possibility (probability, actually) of comb filtering. And that’s just with the planar section of the ET LFT-8b (and all other hybrid designs).

The x/o in the LFT-8b is symmetrical 1st order, so the bottom of the Linear Field Transducer’s (hence it’s model nomenclature) passband and the top of the sealed dynamic woofer’s have a broad area of overlap, centered at 180Hz. You want the output of the two drivers to sum rather than cancel, which brings into play the relationship between the front and rear waves of the LFT drivers, as discussed above.

One cool benefit of using the GR Research/Rythmik OB/Dipole Sub in place of the stock ET woofer is that the Rythmik plate amp (included in the Sub kit) has a phase/delay control that provides continuously-variable phase rotation/delay, from 0 to 16ms/180 degrees rotation. This allows one to position the LFT-8b and the OB/Dipole Sub side-by-side for maximum coherence and imaging, while simultaneously allowing optimum phase alignment regardless of speaker/sub distance from the front wall. Another benefit is with both speaker and sub being dipole, their relative outputs remain constant regardless of their distance from the listening position (the output of dipole and direct radiators fall off at different rates as listening/measuring distance changes).

Regarding any perceived brightness in the LFT-8b, the midrange/tweeter x/o (again, symmetrical 1st order filters) is located at 10kHz, well above the brightness range. The LFT-8b allows for three tweeter output settings, but that won't help with brightness. What will help is a superior tube amp (a Music Reference RM-9 MK.2 or RM-200 Mk.2 will work nicely), one reason being the LFT drivers, unlike other magnetic-planars (I'm looking at you, Magnepan) are an easy 11 ohm load, almost purely resistive. The LFT-8b is very insensitive (low-to-mid 80's), but not so much with the woofer run separately. Another factor is toe-in; try the LFT facing straight ahead, or with just a little toe-in. I use diffusion behind mine (Vicoustic 3D), but I have a warm room. For colder rooms, try absorption on that wall (or the side walls) instead.

For those who missed it: the LFT-8b reproduces 180Hz to 10kHz from a pair of LFT drivers, one above the other. Now THAT'S a wide bandwidth driver! Superb coherence and timbral uniformity. As the pianist's hands move up and down the keyboard, every note has the same timbre as every other. My Infinity RS-1b failed THAT test (and the GRR/Rythmik Servo-Feedback Sub is a considerable improvement over that of the RS-1b).

As long as I'm here ;-) :

I'm assuming every LFT-8b owner bought and is using the Sound Anchors stands specific for the speaker. If so, and you intend to keep the ET's for a while, an accessory to consider is the Seismic Pod by Townshend Audio.

While many speakers are candidates for the full Seismic Platform or Podium (I forget which is for loudspeakers), the LFT-8 mounted on the dedicated SA stand is not one of them. The SA stand is a T-shaped outrigger design, and Seismic Pods installed in place of the stock spikes works great. The LFT's center of mass is such that a "C" Pod bolted onto each of the two front legs and a single "D" Pod on the lone rear leg balances perfectly. I had John at Townshend send me Pods made with a bolt hole through the top "cap", which allowed me to bolt the Pods directly onto the bottom surface of three legs (using thread adapters I got from those provided with the IsoAcoustic GAIA footers ;-). The top of the cap is the same diameter as the width of the SA legs, so the pairing looks they were made for each other. Whether the sonic improvement justifies the expenditure I'll leave up to others. I'm not going back to the spikes ;-) .