subwoofer amplifier 'rules' and phase - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 13 Old 11-30-06, 07:43 AM
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Re: subwoofer amplifier 'rules' and phase

Woke up this morning thinking about this stuff.

I guess I should add that, when applying filters (analog or digital), phase can do strange things when the math becomes complex (i.e., has an imaginary component). The phase shift will change over the frequency range. Like I alluded to in my previous post, there may well be ways to combat this, especially in the digital realm, but I certainly don't have any practice with it.

I still believe that a time delay or distance change will create a phase shift. This is a simple case, but will still cause different phase relationships at different frequencies. Those phase shifts are different that the ones caused by filtering. I just wanted to say that I know that there are some things I'm glossing over or ignoring, and I'd have to go review some textbooks to better understand it.

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post #12 of 13 Old 11-30-06, 09:52 AM
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Re: subwoofer amplifier 'rules' and phase

I was wondering the same thing a while ago about how to adjust phase on pro amps, and I remember someone bringing up the distance setting. I have a plate amp with variable phase now, so I thought I was all good, and I remember tweaking the phase to get the best output when I set up my sub a couple years ago. Then I read (along with the other posts here) this:

Quote:
SteveCallas wrote: View Post
Start with the actual distance you are from your sub, then play with it until the FR of your crossover range measured at the seat ends up with the flattest response.
And I realized that my tweaking was probably at best uneffective, and at worst, detrimental. For one thing, I didn't do it at the crossover point, I just put on a CD with tones from probably around 30 - 40Hz and tried to dial it in to the loudest. For another, I did this from about 1 foot away from the sub, while I was leaning over it into the corner so I could reach the dial. So it won't be great at the listening position about 8 or 10 feet away. Finally, did I even have the mains on at the time? I must have... I think... who knows. Looks like it's time to retune. At least I have a project for Saturday. And it might even cure the boominess I've been noticing too.

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post #13 of 13 Old 11-30-06, 03:28 PM
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Re: subwoofer amplifier 'rules' and phase

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Right, but aren't delay and phase really the same thing?
I guess you could argue that a wholesale digital delay in time is hardly the same as a phase shift, but phase shift controls do end up as close approximations.

Consider a 10Hz signal that has a calculated period of 100ms versus an 80Hz signal that has a period of 12.5 ms.
If we delay each of these signal by 1msec (1foot), then the phase of each signal is preserved and delayed the same amount and remains a constant, but if I introduce a constant analog phase shift by passing the signals through a filter, the end result is quite different. A degree of phase shift for the 10Hz signal is different than a degree of shift for the 80 Hz signal. But that isn't what phase controls do.

Most proper phase adjustment circuits are of a second order 'all pass filter' design with a low Q, and over the designed frequency range result in a linear phase response, such that they closely mimic a pure time delay. I did a little reading about all pass filters and I think you can safely use time delay as a replacement for phase if you have no control.

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