My investigation of the LX55 responses under stock and two different modifications has shown:
1) Some, perhaps most, but not all, LX55's benefit from the tweeter phase (or polarity) "correction," that means changing the connections (at the crossover) so that the tweeters are nominally in phase with the woofers, instead of nominally out-of-phase as the crossovers were originally wired. I first saw this clearly after applying the phase correction to unit #1. This usually makes a big difference in high frequencies above 2kHz-10kHz, with the wrong phase connection having very depressed highs which sounds very dark, recessed, etc:
I found 2 exceptions. In one case (unit #6) the high frequency response is already so flat that changing the phase could not have possibly helped. I already hung the unit to the wall before getting into this experimentation, and I decided to leave it alone. Here's the final response of all units, with two phase inverted (stock) and two phase corrected units--in each case with the flattest response. Unit #6 (inverted) tracks Unit #3 (corrected) above all else until the highest frequencies. Then the other two units (one inverted and one corrected) more or less track each other at a slightly lower level. This shows that not all the variation among unmodified units comes from tweeter phase misidentification, though the graphs above and below show that a lot of it can.
In another case, "correcting" the tweeter phase actually reduced output 3-7kHz, in contrast to what it did for other units. This was the shock after I had modified unit #5, the high frequency response got sucked out even more than before (it was a bit lower than the others which had already been fixed). I had to revert unit #5 to the original inverted connection of the tweeter to get decent highs. This would probably also have been the case for unit 6, for which I did not make any changes, because it was already the best.
2) In the two LX55's that were sold to me with choke removed, I found response considerably flatter with the choke restored. With no choke, those two units there was an annoying upper midrange boost that tended to obscure the highs above, giving the speakers a kind of "car radio" sound. If the car radio salesman has his hand on the volume control, he could sell you either one by not correcting the level difference correctly.
Now, one thing that "simpler is always better" fans might not guess, is that the step response WITH the choke is far better than without it. Without the choke, the step has an undesireable notch right around 1mS--corresponding to the beginning of the midrange bulge at 1000 Hz. With the choke, the step response smoothly tapers down through that region as it should, and correspondingly the frequency response is relatively flat.
In unit #1, it's hard to say, the speaker sounds so dark with the choke having no choke might be better. Here is #1 in all 3 conditions. I'd still go for the midrange-flatter choke-plus-phase correction blue line. That's actually similar to units #3 and #5, in their final configuration, which I decided to use in preference to unit #1.
3) The higher value choke from the LX5 may not be an advantage. It may cause some dropoff between 3-4k, and still does not depress the 5k peak much.
With the tweeter phase connected so as to provide maximum output 3-10kHz, and with the original factory chokes these are very nice sounding speakers, though with more than desirable variance. I've arranged them so the highest HF output units are at the sides (the on-axis gives less tweeter output with the Lineums) and next best HF output unit is in the corner (which depresses HF response the most). From right to left, the units are 6, 5, 3, 2.
In listening, the surround engineering on Wish You Were Here (SACD multichannel version) and Islands (King Crimson 40th anniversary edition multichannel DVD-Audio) finally makes audible sense to me for the first time because the surrounds are sounding close enough to the virtually flat Revel M20's in the front. Until this month, they were wildly different.
For 5 channel discs, I'm now simply duplicating the signal from the sides in the backs. This seems to add a desireably "rearness" to the 5 channel sound, which otherwise is right at your sides. I found the SSI unit I had been using was chopping out the middle (for center channel) and the sides (for steering) leaving me with only phase noise in the back.
1) Some, perhaps most, but not all, LX55's benefit from the tweeter phase (or polarity) "correction," that means changing the connections (at the crossover) so that the tweeters are nominally in phase with the woofers, instead of nominally out-of-phase as the crossovers were originally wired. I first saw this clearly after applying the phase correction to unit #1. This usually makes a big difference in high frequencies above 2kHz-10kHz, with the wrong phase connection having very depressed highs which sounds very dark, recessed, etc:
I found 2 exceptions. In one case (unit #6) the high frequency response is already so flat that changing the phase could not have possibly helped. I already hung the unit to the wall before getting into this experimentation, and I decided to leave it alone. Here's the final response of all units, with two phase inverted (stock) and two phase corrected units--in each case with the flattest response. Unit #6 (inverted) tracks Unit #3 (corrected) above all else until the highest frequencies. Then the other two units (one inverted and one corrected) more or less track each other at a slightly lower level. This shows that not all the variation among unmodified units comes from tweeter phase misidentification, though the graphs above and below show that a lot of it can.
 |
| Final response of all LX55 in use, May 2019 |
In another case, "correcting" the tweeter phase actually reduced output 3-7kHz, in contrast to what it did for other units. This was the shock after I had modified unit #5, the high frequency response got sucked out even more than before (it was a bit lower than the others which had already been fixed). I had to revert unit #5 to the original inverted connection of the tweeter to get decent highs. This would probably also have been the case for unit 6, for which I did not make any changes, because it was already the best.
 |
| Stock (after restoration) is green, phase "corrected" is purple |
Now, one thing that "simpler is always better" fans might not guess, is that the step response WITH the choke is far better than without it. Without the choke, the step has an undesireable notch right around 1mS--corresponding to the beginning of the midrange bulge at 1000 Hz. With the choke, the step response smoothly tapers down through that region as it should, and correspondingly the frequency response is relatively flat.
In unit #1, it's hard to say, the speaker sounds so dark with the choke having no choke might be better. Here is #1 in all 3 conditions. I'd still go for the midrange-flatter choke-plus-phase correction blue line. That's actually similar to units #3 and #5, in their final configuration, which I decided to use in preference to unit #1.
3) The higher value choke from the LX5 may not be an advantage. It may cause some dropoff between 3-4k, and still does not depress the 5k peak much.
 |
| Green unit #2 with 0.9 mH choke, Yellow #3 with 1.0 mH choke |
With the tweeter phase connected so as to provide maximum output 3-10kHz, and with the original factory chokes these are very nice sounding speakers, though with more than desirable variance. I've arranged them so the highest HF output units are at the sides (the on-axis gives less tweeter output with the Lineums) and next best HF output unit is in the corner (which depresses HF response the most). From right to left, the units are 6, 5, 3, 2.
In listening, the surround engineering on Wish You Were Here (SACD multichannel version) and Islands (King Crimson 40th anniversary edition multichannel DVD-Audio) finally makes audible sense to me for the first time because the surrounds are sounding close enough to the virtually flat Revel M20's in the front. Until this month, they were wildly different.
For 5 channel discs, I'm now simply duplicating the signal from the sides in the backs. This seems to add a desireably "rearness" to the 5 channel sound, which otherwise is right at your sides. I found the SSI unit I had been using was chopping out the middle (for center channel) and the sides (for steering) leaving me with only phase noise in the back.
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