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Old 5th April 2009, 09:40 PM   #1
bondo is offline bondo  United States
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Default phase shift of second order filter

im having a hard time figuring out my crossover design for a 2 way system im building, which includes a 6.5" tangband driver along with a fountek ribbon crossed at 2.8k with a second order butterworth filter. ive read that second order filters have 180 degree phase shift, and i am wondering if this is corrected by switching the polarity(positive and negative connections). im asking this because i read that phase and polarity are different but i do not now why. also one of the calculators that i looked at showed that the wiring for the tweeter was switched but not the woofer, why is this?

http://www.carstereo.com/help/Articles.cfm?id=9
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Old 5th April 2009, 09:47 PM   #2
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Hi bondo,

No technical talk: The 180 degree shift means the two drivers are effectively firing at opposite times so to correct that you reverse the leads on the tweeter only. If you were to reverse them both you have canceled the reason for the switch. If you were to take the speakers somewhere and get them tested, you will see a dropout at the XO point if they are connected both positive to positive. Little else changes. There's a lot more to it but I thought you would be able to swallow that pretty easy to start.
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Old 5th April 2009, 10:48 PM   #3
tinitus is offline tinitus  Europe
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With 12db its normal to reverse tweeter polarity
But only if drivers are correctly time allignet on front baffle
This is usually not the case
From what I have seen, it seems like 18db is often used on tweeter, to compensate fore time allignment, and still 12db on woofer
Not sure about driver polarity when doing that

Another way is to tilt the box a bit

Mind you, its way more complicated than that, as it is the actual acoustical slopes that matters most
Calculaters wont tell you much about that

To me, THE most critical part of doing crossovers IS the phase issues
Its not very difficult to make a speaker sound right, but phase is a whole different thing
But a 2way should be doable, with proper drivers
And you should be able to hear when phase is ok

Lots of designs around to study
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Old 5th April 2009, 11:12 PM   #4
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bondo,

The reason the woofer's polarity stays the same and the tweeter's polarity switches is because the high-pass filter has a 90 degree phase-lead at the crossover point, and the low-pass filter has a 90 degree phase-lag at the crossover point. So the phase-lead with the phase-lag is what causes the 180 degree phase shift.

By wiring the tweeter in reverse your woofer and tweeter are in phase at the crossover point relative to one another but still 90 degrees out of phase relative to the input signal.

Roger
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Old 6th April 2009, 08:57 PM   #5
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That's the theory.

The reality is not so nice.

You have to test the actual acoustic response of the drivers to see what is really happening. If you are trying to crossover very near to the LF rolloff point of the tweeter, you will find that the actual slopes and phase shifts are quite different.

It is essential to work with the actual acoustic response + xover, and not just the theoretical.

Fwiw, I generally avoid 2nd order and 3rd order xovers if at all possible. Personally I feel that first order (often not practical) or a version of a 4th order seem to end up sounding the best. There are exceptions, but that is what seems to work out generally speaking.

Also, there is some very real difference between the practical implementation of a 2nd order parallel and a 2nd order series network.

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Old 6th April 2009, 09:17 PM   #6
tinitus is offline tinitus  Europe
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Problem is that "6db" doesnt really exist, or at least hard to achieve with ordinary drivers

I have just tried to convert my 3way into a "6db" design, and that didnt work at all...I struggled with severe phase issues

Though, I have done small very simple 2ways with just a coil and cap(6db), and drivers just a 5" and cheap cone tweeter and they worked out very nicely without much effort,, maybe not the last word in "highend", but very easy to listen to, and made the young lad very happy fore a couple of years

Maybe I could have used them now, with a small ribbon on top and a good big woofer on the bottom
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Old 6th April 2009, 09:43 PM   #7
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Quote:
Originally posted by bear
That's the theory.

The reality is not so nice.
Ok, let's not scare him away. I think he had a question that needed a rather elementary answer. Come to think of it he hasn't been back. Maybe he's scared of bears.

Quote:
Originally posted by tinitus
Though, I have done small very simple 2ways with just a coil and cap(6db), and drivers just a 5" and cheap cone tweeter and they worked out very nicely <snip> and made the young lad very happy fore a couple of years
I think that's the league we are in with this poster. He just wants to get what he can right. Bondo you still there?
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Old 8th April 2009, 12:28 AM   #8
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Sorry... Cal you are right...

Bondo, the fact is that anything you make is better than not making anything.

Even if it comes out wrong, it is a learning experience that will only make it better next time around... The good part is that speakers always make sound and even if the sound doesn't seem quite like what you want, you can change some things around and hear what happens... no big deal.

You can also do something, be unsure and ask questions here. You'll probably get as much information and input as you can handle - just let it wash over you and take out of it whatever you can grab and try to use it...

Have no fear and go for it!

_-_-bear
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Old 8th April 2009, 01:42 AM   #9
MJK is offline MJK  United States
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I have used 2nd order passive and active crossovers on many of my recent designs and have never reversed the polarity of the high frequency driver. I always connect the drivers in-phase. Designing a crossover is more then just looking at the textbook filter magnitude and phase as if it were driving a pure resistor. Bear's first post was exactly right, you have to look at the driver's/system's actual acoustic response and impedance to design the crossover.
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Old 8th April 2009, 02:24 AM   #10
pooge is offline pooge  United States
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If you can get a hold of this paper, it will explain a lot:

http://www.aes.org/e-lib/browse.cfm?elib=3978
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