Sub amp phase w/ back loaded horns (again) - diyAudio
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Old 3rd August 2010, 04:35 PM   #1
wicked1 is offline wicked1  United States
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Default Sub amp phase w/ back loaded horns (again)

I always have trouble pairing subs to my back loaded horns. What sounds good seems illogical.

Most recently, I gave up on feeding the subs their own unamplified signal, and hooked them directly to the output of my amp, along w/ the speakers.. (they're daisy chained off my horns).

So, logically I think the phase should be 0. It's a straight line.. negative from amp goes to negative speaker terminal, to negative sub terminal (and positive to positive to positive).
But, I just did some testing, flipped the phase switch, and the sound was much more full w/ it reversed to 180.
And then even more strange is that I have 2 subs on my right channel, and one of the left. Both on the right sound best switched to 180, and the one on the left sounds correct at 0.

Alright, so I guess the basic question is, do some sub amps reverse the phase? Or do the back loaded horns revers the phase of the bass they produce? (it does come from the back of the cone)..

One of these subs is one I built w/ a plate amp recommended by someone here.. The amp is wf-100k. So, if anyone knows if that reverses polarity, let me know. The other one I have switched to 180 is a boston acoustics. The one that I leave on 0 is a sony.

Why is this so difficult!?!?
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Old 3rd August 2010, 05:32 PM   #2
TerryO is offline TerryO  United States
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Quote:
Originally Posted by wicked1 View Post
I always have trouble pairing subs to my back loaded horns. What sounds good seems illogical.

Most recently, I gave up on feeding the subs their own unamplified signal, and hooked them directly to the output of my amp, along w/ the speakers.. (they're daisy chained off my horns).

So, logically I think the phase should be 0. It's a straight line.. negative from amp goes to negative speaker terminal, to negative sub terminal (and positive to positive to positive).
But, I just did some testing, flipped the phase switch, and the sound was much more full w/ it reversed to 180.
And then even more strange is that I have 2 subs on my right channel, and one of the left. Both on the right sound best switched to 180, and the one on the left sounds correct at 0.

Alright, so I guess the basic question is, do some sub amps reverse the phase? Or do the back loaded horns revers the phase of the bass they produce? (it does come from the back of the cone)..

One of these subs is one I built w/ a plate amp recommended by someone here.. The amp is wf-100k. So, if anyone knows if that reverses polarity, let me know. The other one I have switched to 180 is a boston acoustics. The one that I leave on 0 is a sony.

Why is this so difficult!?!?
I'm pressed for time, but here's my take on it.

The phase of the sub should match the phase of the speaker at the frequency that it crosses over to the sub. The driver on your back loaded horn is probably already 180 degrees out of phase with the signal coming out of the horn's mouth. If the sub's output is in phase with the driver itself (it would be if wired in series with the driver) then the sub's output is 180 degrees (more or less, realistically) out of phase with the mouth's output and would tend to cancel theose frequencies shared by the horn's mouth and the sub, leaving a big hole in the total response.

Hope this helps.

Best Regards,
TerryO
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Old 3rd August 2010, 05:48 PM   #3
wicked1 is offline wicked1  United States
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I have my subs wired in parallel to my speakers.. does this make a difference?

And right, what you are saying is what I am experiencing. So I need to figure out the lowest reasonable frequency from my horns, and xover there...
and it sounds like it most likely will be at 180 degrees. (I wish I could remember the alt code for the degree symbol) assuming the higher frequencies coming directly off the speaker cone are at 0. I did a mathcad worksheet on my speakers and that's basically what I came up with, though the frequency switched phase back and forth, and I dont have test equipment, so can't be precise w/ this. (and can't really trust the frequency scale on the plate amps)

But, then why is the left channel sub sounding best IN phase.. does room geometry play that much of a factor.. (it's way off to the side in a corner)
Or, do some subs reverse phase? I wouldn't think so.....
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Old 3rd August 2010, 11:38 PM   #4
GM is online now GM  United States
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Originally Posted by wicked1 View Post
(I wish I could remember the alt code for the degree symbol)
0176: Windows Alt Key Codes

As for the rest, boundary loading affects acoustic phase as does different path-lengths of the various acoustic sources to the LP, so unless the time of flight for each one can be accurately measured and then digitally corrected about all one can do is either position the subs wherever in the room they sound best when wired in phase with the mains and/or use whatever phase control or switch setting sounds best overall.

Note that for 'normal' speakers, subs positioned ~on the same plane as the mains should in theory always be wired in reverse polarity to them. BLHs/TLs being 1/4 WL fundamental resonators will be around -90 at Fp in 2pi space if the driver, horn mouth have the same path-length to the LP, but acoustically small rooms muck with them so much that its mostly moot.

GM
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Old 3rd August 2010, 11:48 PM   #5
wicked1 is offline wicked1  United States
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Would building bass horns be a good idea/better match? Now they're either sealed or vented boxes. (Not sure if 'base horns' is actually what they are called.. but I think u know what I mean) Actually, the one's I've seen have the driver buried inside the box, and just the horn outlet coming out of the box.

Thanks for the alt codes!!!
I used to run a BBS, just before the internet. Was a hub for usenet and fido. I spent much time w/ ascii codes back then. All the fancy menu's for my BBS were keystroke codes. I sadly lost my backup, but someone else got a copy.. I had a 'sysop' somewhere out in the proto-cloud who also took full backups. No idea who that was, these days.. (holy fudge that was 18 years ago).. But I digress.. I better go find some old geezer activity

Last edited by wicked1; 3rd August 2010 at 11:55 PM.
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Old 4th August 2010, 01:28 AM   #6
GM is online now GM  United States
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You're welcome! Actually I didn't have a clue how folks made those symbols, but when you mentioned ALT codes, I Googled it out of curiosity and it was the first link, so thanks for cluing me in. Hmm, 18 yrs ago, the sum total of my computer skills consisted of inter-office, corporate email and using the basic math functions of Lotus 123 to do my various work, hobby, calculations.

Anyway, I assume you're referring to the tapped pipe, horn, TQWT 'subs' currently in vogue and since these are extreme BP alignments, a good match to back or front loaded horns if digitally controlled.

GM
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Old 4th August 2010, 04:52 PM   #7
TerryO is offline TerryO  United States
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Quote:
Originally Posted by GM View Post
0176: Windows Alt Key Codes

As for the rest, boundary loading affects acoustic phase as does different path-lengths of the various acoustic sources to the LP, so unless the time of flight for each one can be accurately measured and then digitally corrected about all one can do is either position the subs wherever in the room they sound best when wired in phase with the mains and/or use whatever phase control or switch setting sounds best overall.

Note that for 'normal' speakers, subs positioned ~on the same plane as the mains should in theory always be wired in reverse polarity to them. BLHs/TLs being 1/4 WL fundamental resonators will be around -90 at Fp in 2pi space if the driver, horn mouth have the same path-length to the LP, but acoustically small rooms muck with them so much that its mostly moot.

GM
GM has it right, as usual. The phase difference, in nearly all BLH that are competently designed, is 90 degrees, not 180 like I posted earlier. I should think before I type.

Good catch!

Best Regards,
TerryO
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