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Passive crossovers and speaker impedance

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Anonymous
February 26, 2005 5:07:05 PM

Archived from groups: rec.audio.pro (More info?)

Hi,

what would be the likely effect of someone having replaced a 4 ohm bass
driver with an 8 ohm one in a 2 way 4 ohm passive crossover speaker?
Specifically, this is a Peavey (sorry about that) 1 x 15 plus compression
driver box where the original Black Widow 4 ohm is now an Eminence Kappa 8
ohm. I'm most concerned about what happens to the compression driver
crossover point.


Thanks,

Gareth.
Anonymous
February 26, 2005 5:07:06 PM

Archived from groups: rec.audio.pro (More info?)

Gareth Magennis <sound.service@btconnect.com> wrote:
>Hi,
>what would be the likely effect of someone having replaced a 4 ohm bass
>driver with an 8 ohm one in a 2 way 4 ohm passive crossover speaker?

The crossover point changes by a whole octave, totally screwing things up.
Also the efficiency of the driver will be different which will also totally
screw the response up.

>Specifically, this is a Peavey (sorry about that) 1 x 15 plus compression
>driver box where the original Black Widow 4 ohm is now an Eminence Kappa 8
>ohm. I'm most concerned about what happens to the compression driver
>crossover point.

You can think of most crossovers as being independant low pass and high
pass filter that (with a perfect amp and cable) don't interact much. So
putting the wrong load on the low end will screw up the low pass point but
it won't change the high pass point much.

BUT, you also have problems in that the T-S parameters for those drivers
are very different, so the box interactions will not be right either and
your low end will just generally be wrong in addition to the hole at the
crossover region from the wrong impedance.

I don't think you'll damage anything operating the speaker like this, but
it must sound really horrible. Change the driver.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
February 26, 2005 5:07:06 PM

Archived from groups: rec.audio.pro (More info?)

> what would be the likely effect of someone having replaced a 4 ohm bass
> driver with an 8 ohm one in a 2 way 4 ohm passive crossover speaker?
> Specifically, this is a Peavey (sorry about that) 1 x 15 plus compression
> driver box where the original Black Widow 4 ohm is now an Eminence Kappa
> 8 ohm. I'm most concerned about what happens to the compression driver
> crossover point.

Assuming "conventional" crossover design, and assuming the crossover has a
low-pass filter ahead of the woofer, * and assuming both woofers had an
essentially resistive impedance, the crossover frequency would rise one octave,
producing increased output in that octave.

Scott is incorrect in assuming the driver's "efficiency" (he means sensitivity)
would be different. It might or might not. Also, you wouldn't have a hole in the
crossover region, but increased output. He _is_ correct about differences in
driver parameters further screwing things up.

Nothing would happen to the crossover point for the compression driver. That's
not what you should be worrying about.

* It might not. Some systems use the woofer's natural HF roll-off as the
low-pass filtering. This saves the cost of a bulky inductor and improves
electrical damping.
Related resources
Anonymous
February 26, 2005 5:07:07 PM

Archived from groups: rec.audio.pro (More info?)

William Sommerwerck <williams@nwlink.com> wrote:
>
>Scott is incorrect in assuming the driver's "efficiency" (he means sensitivity)
>would be different. It might or might not.

Given the two specific drivers he mention, it will be different.

>Also, you wouldn't have a hole in the
>crossover region, but increased output.

Yes, that's correct. My goof.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
February 26, 2005 6:09:22 PM

Archived from groups: rec.audio.pro (More info?)

Thanks guys, inductors, impedances and integrating complex numbers were
never my strong points at uni.
Anonymous
February 26, 2005 9:28:55 PM

Archived from groups: rec.audio.pro (More info?)

"Scott Dorsey" <kludge@panix.com> wrote in message
news:cvq4pj$ch$1@panix2.panix.com...
> William Sommerwerck <williams@nwlink.com> wrote:
> >
> >Scott is incorrect in assuming the driver's "efficiency" (he means
sensitivity)
> >would be different. It might or might not.
>
> Given the two specific drivers he mention, it will be different.
>
> >Also, you wouldn't have a hole in the
> >crossover region, but increased output.
>
> Yes, that's correct. My goof.

I think you were right the first time. With an 8-ohm woofer replacing a
4-ohm, the rolloff point will be an octave below where it's supposed to be,
say 400 Hz rather than 800 Hz. The compression driver still rolls off at 800
Hz. Hence, you have a hole from 400 - 800 Hz.

Peace,
Paul
Anonymous
February 27, 2005 4:37:27 AM

Archived from groups: rec.audio.pro (More info?)

"Gareth Magennis"
>
> what would be the likely effect of someone having replaced a 4 ohm bass
> driver with an 8 ohm one in a 2 way 4 ohm passive crossover speaker?
> Specifically, this is a Peavey (sorry about that) 1 x 15 plus compression
> driver box where the original Black Widow 4 ohm is now an Eminence Kappa 8
> ohm. I'm most concerned about what happens to the compression driver
> crossover point.
>


** The compression driver will not be directly affected - however the bass
driver will operate up to a higher frequency.

There may be some impedance anomalies in the load as seen by the amp -
depends on the details of the x-over.

The level balance between the bass and the horn may change - for the
better or worse.





............. Phil
Anonymous
February 27, 2005 2:54:50 PM

Archived from groups: rec.audio.pro (More info?)

"Paul Stamler"
>
> I think you were right the first time. With an 8-ohm woofer replacing a
> 4-ohm, the rolloff point will be an octave below where it's supposed to
> be,
> say 400 Hz rather than 800 Hz.


** Series inductors for 4 ohms are half value compared to ones for 8 ohms.

Ergo - the x-over frequency goes up when the load impedance is
doubled.


The compression driver still rolls off at 800
> Hz. Hence, you have a hole from 400 - 800 Hz.


** Nope - there will be an overlap instead.





................. Phil
!