How low can I XO?

fortyquid · 11th May 2004, 01:10 AM

Now I have heard that it is necessary when passively OXing to HP about two octaves or so above the tweeter's Fs. I assume that this has chiefly to do with the impedence bump exhibited at Fs, so is such a great margin necessary when XOing pre-amp?
To be slightly more specific....
can I XO this tweeter at 12dB/oct @ 1500Hz (right about at Fs), or will it over-excurd?

http://www.apexjr.com/SDTGraph.htm

Gregm · 11th May 2004, 08:28 AM

From a quick look, I'd try for ~3kHz at least. It looks like it would distort impolitely below that with yr x-over. Indeed, maybe a LR 4th at ~3k would be indicated

jomor · 11th May 2004, 01:02 PM

Actually you need more than 2.5 octaves distance between the crossover cut-off frequency and the driver's Fs, for a 6dB/oct slope crossover. The goal is for the driver to "see" the smallest possible impedance towards the crossover near its Fs. If you need to get closer to the Fs, the highest the x-over's order, the better.

Ouroboros · 11th May 2004, 03:39 PM

But if the tweeter is a lot more efficient than the bass unit, then it is wise to have an 'L' attenuator on the tweeter so that the crossover still sees an 8 ohm (or whatever is required) load. This attenuator will help a lot to prevent the crossover output seeing a big rise in impedance at the Fs of the tweeter.

Whatever you do, don't attenuate the tweeter with a single series resistor, this doesn't help at all!

rabbitz · 12th May 2004, 02:40 PM

Quote:

Originally posted by Ouroboros
Whatever you do, don't attenuate the tweeter with a single series resistor, this doesn't help at all!

Why? For this particular application or in general? Just curious and not being a smartass.

Cheers

Ouroboros · 12th May 2004, 02:49 PM

In general it's not a good idea. For the XO to work properly, it really needs to see a resistive load. Now a tweeter has a rise in impedance at resonance, which means that the XO attenuation will be lower (perhaps much lower) at the Fs of the tweeter, just when you want it to be rolling off nicely.

If you attenuate the tweeter with the correct L-pad, then the L-pad will be designed to provide a nominal 8 Ohms (or whatever is required) on the output of the XO high-pass section. If you do the simple calculations you will see that the more attenuation you have in the pad, the smaller change there will be to the impedance presented to the XO as the impedance of the tweeter changes.

If you simply attenuate the tweeter with a series resistor, then the XO will need to be designed to give the correct filter response with the higher Z load, and the load impedance will increase at the tweeter FS, meaning that the filter will not be meeting the design parameters at this frequency.

rabbitz · 12th May 2004, 04:19 PM

Thanks. Nice explanation.

I do use a single resistor in my crossovers (1st order series) instead of an L-pad as it just sounds better (don't know why). Normally it's only to attenuate about 1-4dB.

So, if I'm reading correctly, as long as the design takes the extra impedance into consideration with the actual impedance (not nominal) of the tweeter at the xo frequency it should be cool but you get less attenuation at the Fs than with an L-pad.

Cheers

Andy G · 12th May 2004, 10:24 PM

Ouroboros is right about the use of just a series resistor for helping the power handling of tweeter.. which I think is how he is referring to it in this case...it does help a bit, but if you want to use a tweeter nearer its Fs and the sensitivity is a lot higher, then L-padding can be very helpful.

In the series x-o things are a bit different... here we often use the series resistor as a balancing resistor and it is particularly useful for this when there is not too much difference in sensitivity. You need to remember that using a series resistor on the tweeter in a series resistor actually pushes a bit more through the woofer because of the impedance balance change. In other words, we are actually using the impedance imbalance to shape the response !!

If we do have a big difference between sensitivities in a series x-o, an L-pad on the tweeter is still recommended.

fortyquid · 13th May 2004, 12:01 AM

No, no, no, Pre- Amp. Impedence no object, right? I'm talking about a LR between the CD player and the amp, thus Active XO; custom vs. out-of-the-box

Now, would that be a problem?

Ouroboros · 13th May 2004, 07:04 AM

Soory, I jumped in without realising you were refering to a XO between the pre-amp and the power amps.

Why not use an active XO here? It makes things a lot easier.

11th May 2004, 01:10 AM	#1
fortyquid diyAudio Member Join Date: Apr 2003 Location: Southern Virginia	How low can I XO? Now I have heard that it is necessary when passively OXing to HP about two octaves or so above the tweeter's Fs. I assume that this has chiefly to do with the impedence bump exhibited at Fs, so is such a great margin necessary when XOing pre-amp? To be slightly more specific.... can I XO this tweeter at 12dB/oct @ 1500Hz (right about at Fs), or will it over-excurd? http://www.apexjr.com/SDTGraph.htm

12th May 2004, 10:24 PM	#8
Andy G diyAudio Member Join Date: Apr 2002 Location: Newcastle, Australia	Hi Peter !!! Ouroboros is right about the use of just a series resistor for helping the power handling of tweeter.. which I think is how he is referring to it in this case...it does help a bit, but if you want to use a tweeter nearer its Fs and the sensitivity is a lot higher, then L-padding can be very helpful. In the series x-o things are a bit different... here we often use the series resistor as a balancing resistor and it is particularly useful for this when there is not too much difference in sensitivity. You need to remember that using a series resistor on the tweeter in a series resistor actually pushes a bit more through the woofer because of the impedance balance change. In other words, we are actually using the impedance imbalance to shape the response !! If we do have a big difference between sensitivities in a series x-o, an L-pad on the tweeter is still recommended.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

11th May 2004, 08:28 AM	#2
Gregm diyAudio Member Join Date: May 2003 Location: Wherever I leave my hat	From a quick look, I'd try for ~3kHz at least. It looks like it would distort impolitely below that with yr x-over. Indeed, maybe a LR 4th at ~3k would be indicated

11th May 2004, 01:02 PM	#3
jomor diyAudio Member Join Date: Feb 2004 Location: Athens	Actually you need more than 2.5 octaves distance between the crossover cut-off frequency and the driver's Fs, for a 6dB/oct slope crossover. The goal is for the driver to "see" the smallest possible impedance towards the crossover near its Fs. If you need to get closer to the Fs, the highest the x-over's order, the better.

11th May 2004, 03:39 PM	#4
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	But if the tweeter is a lot more efficient than the bass unit, then it is wise to have an 'L' attenuator on the tweeter so that the crossover still sees an 8 ohm (or whatever is required) load. This attenuator will help a lot to prevent the crossover output seeing a big rise in impedance at the Fs of the tweeter. Whatever you do, don't attenuate the tweeter with a single series resistor, this doesn't help at all!

12th May 2004, 02:49 PM	#6
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	In general it's not a good idea. For the XO to work properly, it really needs to see a resistive load. Now a tweeter has a rise in impedance at resonance, which means that the XO attenuation will be lower (perhaps much lower) at the Fs of the tweeter, just when you want it to be rolling off nicely. If you attenuate the tweeter with the correct L-pad, then the L-pad will be designed to provide a nominal 8 Ohms (or whatever is required) on the output of the XO high-pass section. If you do the simple calculations you will see that the more attenuation you have in the pad, the smaller change there will be to the impedance presented to the XO as the impedance of the tweeter changes. If you simply attenuate the tweeter with a series resistor, then the XO will need to be designed to give the correct filter response with the higher Z load, and the load impedance will increase at the tweeter FS, meaning that the filter will not be meeting the design parameters at this frequency.

12th May 2004, 04:19 PM	#7
rabbitz diyAudio Member Join Date: Jul 2003 Location: Newcastle, Australia	Thanks. Nice explanation. I do use a single resistor in my crossovers (1st order series) instead of an L-pad as it just sounds better (don't know why). Normally it's only to attenuate about 1-4dB. So, if I'm reading correctly, as long as the design takes the extra impedance into consideration with the actual impedance (not nominal) of the tweeter at the xo frequency it should be cool but you get less attenuation at the Fs than with an L-pad. Cheers

13th May 2004, 12:01 AM	#9
fortyquid diyAudio Member Join Date: Apr 2003 Location: Southern Virginia	No, no, no, Pre- Amp. Impedence no object, right? I'm talking about a LR between the CD player and the amp, thus Active XO; custom vs. out-of-the-box Now, would that be a problem?

13th May 2004, 07:04 AM	#10
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	Soory, I jumped in without realising you were refering to a XO between the pre-amp and the power amps. Why not use an active XO here? It makes things a lot easier.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)