diy speakers

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marvin
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diy speakers

Unread post by marvin »

I built a pair of 250 watt 3-way speakers using a 10 inch bass driver, an 8 inch mid-range and a bullet tweeter. the bass and mid-range drivers are separated by a sealed baffle and there are ports in both sections. I've used them in a couple of different small venues and I am told that they sound very good but I've been reading that the mid should be wired up with reverse polarity. Does anyone know why this might be and should I swap the positive and negative on the mid?
I don't know if this is the place to ask such a question. Please advise.


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Re: diy speakers

Unread post by parametric »

Hmm . . . .

I can't think WHY that would be an advantage?

Normally, ALL the units should be in-phase (The cones moving in the SAME direction at the SAME time)

Making the MID move forward when the others are moving backwards seems weird to me . . . .

I'd love to know what audible effect it has . . . . . . ?

Interesting.

I'll inquire of a BBC Engineer I know and see what he has to say . . . . (Y)

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Re: diy speakers

Unread post by marvin »

Here's the quote about reverse polarity. It may make more sense to you than to me! :)
(Quote from another page):
Reactive XO components (caps & coils) that have an impedance that varies with freq, actually change the phase of the driver that they're connected to. The mid is wired reverse polarity to bring it BACK so it moves WITH the woofer and tweeter.
P1050085.JPG
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Re: diy speakers

Unread post by SysExJohn »

Maybe because the mid will have both low cut AND high cut filter components within the crossover, it changes the polarity one more time than either the bass or the tweeter which just have one filter each. ???

Maybe?


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Re: diy speakers

Unread post by parametric »

Just had a reply from my Engineer Mate . . . .
That sounds like pretty flaky logic to me Chris, I can't think why you might want the mid out of phase with the boom and tizz. It does sometimes happen by accident (I'm sure you have heard it once or twice. In fact one very well known studio I was in had one of their main monitors with an oop mid driver - they hadn't noticed and were very embarrassed when I pointed it out). The whole phase change with electronic component thing is a red herring I'm afraid. It is what we would normally call good design! That's what crossovers are all about. Now where it gets really clever is the phasing in the acoustic components in ported enclosures, where the design is such that the port and the driver are consistent in phase no matter what the frequency. That is proper clever that is.
[The Italics are mine - just for emphasis ;) - <para>]

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Re: diy speakers

Unread post by marvin »

That sounds much more reasonable to me. When I assembled the speaker components I went positive/positive throughout. I'd never heard of reverse polarity in mids before and I've done a bit of reading on the subject.
Last edited by marvin on Sun Jan 05, 2020 11:53 pm, edited 1 time in total.


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Re: diy speakers

Unread post by david.krause63 »

Well, I feel to comment this from a technicians point of view! If the crossover network is 2nd order, it results in 180 degrees total phase shift between the speakers at the crossover frequency (in a ideal situation). That sometimes requires reversing the polarity of one speaker to prevent a cancellation dip centered at that frequency.

You may also be interested in what wikpedia says, below the headline "second order": https://en.wikipedia.org/wiki/Audio_crossover
It is commonly thought that there will always be a phase difference of 180° between the outputs of a (second order) low-pass filter and a high-pass filter having the same crossover frequency. And so, in a 2-way system, the high-pass section's output is usually connected to the high frequency driver 'inverted', to correct for this phase problem. For passive systems, the tweeter is wired with opposite polarity to the woofer; for active crossovers the high-pass filter's output is inverted. In 3-way systems the mid-range driver or filter is inverted. However, this is generally only true when the speakers have a wide response overlap and the acoustic centers are physically aligned.
You can hear the difference if you put some pink noise to the speakers and change the polarity. There are quite a few commercial speaker system, which have one drivers polarity changed therefore!

David


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Re: diy speakers

Unread post by marvin »

Thanks for this, David!
Is there any way to tell which crossover I might have? I've built several 3-way systems and when I order the hardware from my supplier I just use what they send me. Even though my specs are always quite similar the parts I get are quite often different than the ones before so I'm never quite sure if what I received is what I need. The parts have always worked, though.


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Re: diy speakers

Unread post by david.krause63 »

Do you have any schematics or a picture of the crossover network at least?

David


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Re: diy speakers

Unread post by marvin »

This is about all I have at the moment. I'll have to open a cabinet to get better ones. This is from a few weeks ago when I started this project. I had some cosmetics to complete on one cabinet in the 2nd photo.
SPEAKER PARTS.jpg
SPEAKER PARTS2.jpg
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Re: diy speakers

Unread post by david.krause63 »

Sorry for the delay (i was on vacation last week)...

Well, the networks are partly covered by the speakers on the pictures. But they look as one of the better kind! So they are at least 2nd order or maybe more...

I guess the easiest way, would be to change the mid-range speakers polarity of only one of the cabinets, place them beneath each other and use the pan pot of one channel of your mixer to test which sounds better! They should sound awful if you feed them both in the middle position of the pan pot, as the mid frequencies will be canceled almost completely. But one or the other should cancel only the crossover frequencies if you put the pan pot to the left or right position....

You may also do the pink noise test I mentioned before: https://www.audiocheck.net/testtones_pinknoise.php

David


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