Taking the 3-Way Center Plunge

XEagleDriver

XEagleDriver

Senior Audioholic
After seeing so many forum rec's for larger three way center speakers . . .

Receiving a large three-way center speaker tomorrow (706c).

To compare to / potentially replace a smaller two-way MTM center from same maker & similar series as the rest of a 5.2.4 base layer.

Areas to subjectively compare:
1) HT impact
2) Multi-channel music SQ
3) Dialogue improvement

At worst, free return, curiosity satisfied.
At best, HT improvement for relatively modest $430 (item+tax) cost.
XEagleDriver


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H

Hetfield

Audioholic Samurai
Very interesting, I'll be very interested in what you think as I have become a bit fascinated with getting a 3 way center channel myself.

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ryanosaur

ryanosaur

Audioholic Warlord
As always with these things it can be hard to determine just how much of an improvement you may hear. Of course I say that as a strong proponent of 3-way Speakers! ;) You seem to know what you're about, so it's somewhat pedantic to remind you about taking care to find proper placement and whatnot. :)

I do hope you hear an improvement, especially across the seating area where the MTM would be lobing. I would expect, as I think you do, that the dedicated Mid Range Driver will improve performance.

Look forward to your follow up!
 
TLS Guy

TLS Guy

Audioholic Slumlord
After seeing so many forum rec's for larger three way center speakers . . .

Receiving a large three-way center speaker tomorrow (706c).

To compare to / potentially replace a smaller two-way MTM center from same maker & similar series as the rest of a 5.2.4 base layer.

Areas to subjectively compare:
1) HT impact
2) Multi-channel music SQ
3) Dialogue improvement

At worst, free return, curiosity satisfied.
At best, HT improvement for relatively modest $430 (item+tax) cost.
XEagleDriver


Sent from my SM-G975U using Tapatalk
You are right to be looking at your center speaker. The center is the most important speaker in a multichannel HT system. It takes the most power, and is by far the biggest design challenge. More should be spent on that speaker than any other.

I hope you meet with success, but I find the design choices strange. The mid is only 3.25" and yet is crossed at 280 Hz, that is right at a place were power demands for a center are colossal. It is alleged that the speaker will handle 250 watts. On that I'm afraid you have to count me skeptical. There will be as much power going to that mid, as both woofers combined. I have a feeling that I would burn out that mid before the day was out!

I continue to be puzzled by the design choices of so many commercial speaker designers.
 
XEagleDriver

XEagleDriver

Senior Audioholic
You seem to know what you're about, so it's somewhat pedantic to remind you about taking care to find proper placement and whatnot.
Don't mind at all. I have a lot more to learn than I have learned in this hobby.

Position is set, on AV furniture under TV which puts tweeter near ear level. Angled foam for fine adjustment.

Thanks to a X4500H & app, I can store EQ profiles for old and new.
Not a blind or rapid switch by any means, but allows serious listens with as equal a footing as I can.

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XEagleDriver

XEagleDriver

Senior Audioholic
New center arrived.
It is a monster, the only heavier speakers I have are subs LOL.

Ran Audyssey and made other recommended adjustments on the MultiXT App.
Only oddity was Audyssey flagged a polarity error. Wiring was same as for previous speaker, which never had this error? Internally reversed?
Easy enough fix; red to black and black to red and on it went. Weird.

Early listening, so far, so good, but too early to tell.


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Pogre

Pogre

Audioholic Overlord
How much is that center? I ask because I see the RAAL tweets on your l/r speakers. Ascend Acoustics make a really beefy center channel with the larger 70/20 RAAL tweet, which would be a great match with your mains.


It's not cheap, but it's a beast and would be a great match. I have one and love it.

bm-ribbon-front-large.jpg
 
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XEagleDriver

XEagleDriver

Senior Audioholic
How much is that center? I ask because I see the RAAL tweets on your l/r speakers. Ascend Acoustics make a really beefy center channel with the larger 70/20 RAAL tweet, which would be a great match with your mains.


It's not cheap, but it's a beast and would be a great match. I have one and love it.

View attachment 50489
$400 vs $1600 was the decision maker.
Would love a Horizon, but too pricey ATT.

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Eppie

Eppie

Audioholic General
I'm curious how the timbre of the mains and centre compare. I originally had a v2 centre with my v5 mains and the match was not very good. The v2 was a MTM design while the matching v5 was a 3 way. Getting the matching center was a huge improvement but not because I went from a 2 way to a 3 way. It was getting the same drivers across the front, which you currently have. Looks like you'll have to raise the TV if the Polk turns out better. ;)
 
XEagleDriver

XEagleDriver

Senior Audioholic
I'm curious how the timbre of the mains and centre compare. I originally had a v2 centre with my v5 mains and the match was not very good. The v2 was a MTM design while the matching v5 was a 3 way. Getting the matching center was a huge improvement but not because I went from a 2 way to a 3 way. It was getting the same drivers across the front, which you currently have. Looks like you'll have to raise the TV if the Polk turns out better. ;)
I'll report back.
The TV will need to go up a bit

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Pogre

Pogre

Audioholic Overlord
$400 vs $1600 was the decision maker.
Would love a Horizon, but too pricey ATT.


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Yeah I'm feeling ya... that's a huge price difference. If it was close I was gonna go to work on you, lol
 
lovinthehd

lovinthehd

Audioholic Jedi
I wouldn't trust the polarity message in an REQ program particularly. I'd compare phase with each L/R speaker separately (like with material on many test discs or even the thx/dolby help on some pre-recorded dvd/blurays).
 
XEagleDriver

XEagleDriver

Senior Audioholic
Week 1 report on Polk 706c.
Comparison material used:
Several favorite SACD and FLAC files mainly Jazz, Vocals, Classic Rock and a bit of Classical. Phantom of the Opera and The Mummy.

Areas subjectively compared (thus far):
1) HT impact
- Polk is noticably and impressively better.
2) Multi-channel music SQ
- No improvement or degradation.
3) Dialogue improvement
- Polk is very noticably better here as well.

Other Notes:
I was not able to hear any disadvantage to no longer having matched tweeters across LCR.

The woofers were always different (5.25" in S2 vs dual 4.25" in the Duo) and now are 5.25" in S2 and dual 6.5" in 706c. Although Ascend designed the Duo to play nicely with its older and larger sibling.

Audyssey set the higher sensitivity 706c at about 2dB lower than the Sierra2 LR.
This sounded about right for balance across the LCR.

Used the MultiEQ App to turn Midrange Compensation OFF and used the MultiEQ Filter Frequency Range to limit EQ to below about 3500 Hz on all speakers.

Some say to limit the Audyssey EQ range even lower. Between about 750 and 1000 Hz, which is approximately the room Transition Frequency.
- I tried this lower EQ limit setting and did not like it as much
- Pics of EQ App results for L, C and Crossover settings are below.

All for now, back to the movie!
XEagleDriver


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TLS Guy

TLS Guy

Audioholic Slumlord
New center arrived.
It is a monster, the only heavier speakers I have are subs LOL.

Ran Audyssey and made other recommended adjustments on the MultiXT App.
Only oddity was Audyssey flagged a polarity error. Wiring was same as for previous speaker, which never had this error? Internally reversed?
Easy enough fix; red to black and black to red and on it went. Weird.

Early listening, so far, so good, but too early to tell.


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That phase issue is not an oddity, but a serious problem when using three way center speakers with two way mains.

I ran into exactly the same problem with this system, which I should have foreseen, but due to stupidity did not.



The two mains are two way MTM, with first order electrical low pass, and second order electrical high pass.

The three way center is electrical second order low pass, band pass and high pass. So as often happens, the phase shift is 180 degrees at crossover with a second order LR crossover. So a null is inclined to result at crossover due to speakers being 180 degrees out of phase. This requires a reverse phase to the midrange, to put the speakers in phase at crossover to avoid a null.

So when I listened to this rig, it did not sound right, and Audyssey knew there was a problem. So I soon realized that although the center was correctly phased at crossover by ear and measurement, it was out of phase with the woofer mids of the left and right speakers!

So, fortunately, when I put the mid in phase with the right and left speakers it sounded much better, in fact excellent. Fortunately the null at crossover between the center mid and woofers of the center, was very narrow indeed, and not audible. It sounds excellent keeping the center mid correctly phased with the left and right speakers. but out of phase with the center woofers at crossover.

So I would bet that your mid range of the center is actually out of phase with your left and right speakers, and Audyssey knows it.

Only I have discussed this issue before. However there can often be an issue combining two way left and right speakers, with two way mains.
 
XEagleDriver

XEagleDriver

Senior Audioholic
That phase issue is not an oddity, but a serious problem when using three way center speakers with two way mains.

I ran into exactly the same problem with this system, which I should have foreseen, but due to stupidity did not.

The two mains are two way MTM, with first order electrical low pass, and second order electrical high pass.

The three way center is electrical second order low pass, band pass and high pass. So as often happens, the phase shift is 180 degrees at crossover with a second order LR crossover. So a null is inclined to result at crossover due to speakers being 180 degrees out of phase. This requires a reverse phase to the midrange, to put the speakers in phase at crossover to avoid a null.

So when I listened to this rig, it did not sound right, and Audyssey knew there was a problem. So I soon realized that although the center was correctly phased at crossover by ear and measurement, it was out of phase with the woofer mids of the left and right speakers!

So, fortunately, when I put the mid in phase with the right and left speakers it sounded much better, in fact excellent. Fortunately the null at crossover between the center mid and woofers of the center, was very narrow indeed, and not audible. It sounds excellent keeping the center mid correctly phased with the left and right speakers. but out of phase with the center woofers at crossover.

So I would bet that your mid range of the center is actually out of phase with your left and right speakers, and Audyssey knows it.
TLS Guy thanks,
If I understand you correctly, the bottomline is to reverse the polarity of the Center speaker as Audyssey advises? True?



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TLS Guy

TLS Guy

Audioholic Slumlord
TLS Guy thanks,
If I understand you correctly, the bottomline is to reverse the polarity of the Center speaker as Audyssey advises? True?



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No that is not true. The problem is that ONLY the center mid is out of phase with the left and right mains. The woofers and tweeter of the center speaker are in phase with the left and right mains. So only the center mid needs its phase reversed.

However you really do need measurements to do this. The problem is created by the phase shifts of analog crossovers, both passive and active. Only digital crossovers with DSP can avoid this problem.

The issue is that if you have identical speakers that are three way, with second order LR or Butterworth crossover for that matter, then without reversing the phase of the mid, then the mid is out of phase with woofers and tweeter, but only in the crossover region, where the drivers overlap. This is because the phase shift is 90 degrees for each section and so 180 for the woofers and mid at overlap. So this causes a null in the crossover region, but only in the region where the drivers overlap. The orthodox treatment is to reverse the phase of the mid, to avoid the null. However if you have two way left and right mains, then the mid of a center with reverse phase will be out of phase with the left and right mains, across all of the pass band out of the crossover regions.

The problem is compounded by that fact the the degree and sharpness of the null is highly variable and you have to see what it looks like.

In my case, the null was narrow, and mainly only measurable on, and close, to the axis of the center. So tolerating the null in the crossover region was not a problem in practice.

Usually second order cross overs are the most popular as there is a constant voltage solution, the LR topography. The first order, with 90 degree phase shift generally gives too much power to the drivers out of their pass band, and there are lobing issues at crossover. The third order, with 270 degrees of phase shift, only has a Butterworth constant power solution, also with axis lobing issues at crossover. The fourth order solution has both constant voltage and power solutions, and no lobing issues. However, it has 360 degrees of phase shift, which puts the drivers a whole cycle out of phase at crossover. In practice it usually sounds OK, but the impulse response is not pretty. The big issue is that the part count, and expense, of a fourth order LR crossover is high, and you won't build that for the price you paid for that speaker.

So your solution is likely to reverse the phase of the mid on the center speaker only. However as stated, this should be done, with full data on the effect of it.

In my studio system, I was very careful to carefully design the crossovers of the front three to be compatible as regards to phase and other matters also.

Here is my three way center with the mid driver not phase reversed. The deep null is at the top end of the driver over lap. The black line is the axis response, off axis the response is good.



The room responses are actually very good all around this large space.

Accepting the problem in the bass mid crossover region sounded far preferable to having the mid out of phase with the bass mids of the left and right speakers. So when the center followed normal practice of phase reversal you could not get the center to blend properly with the left and right speakers. The system sounded much better with the center mid in phase with the left and right speakers and accepting the problem in the crossover region.

This is just one more problem with using speakers across the front, that were not designed specifically to work with each other.
 
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