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MultiWay Conventional loudspeakers with crossovers 

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9th September 2013, 10:48 AM  #1 
diyAudio Member

Lpad v single resistor
I wish to attenuate a tweeter. Assuming a single capacitor for the crossover for the tweeter, there are 2 ways to attenuate that I am looking into.
1) to use an Lpad of 8ohm. 2) to use a series resistor (with obviously a different value capacitor.) Please could you tell me what is the practical disadvantage of using option 2? 
9th September 2013, 11:45 AM  #2 
diyAudio Member
Join Date: May 2011
Location: The South

The trouble with the single resistor problem is that if you have an 8 ohm tweeter, you may need a 30 or 40 ohm series resistor to get your attenuation. There is nothing gained by doing this compared to an Lpad.
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9th September 2013, 01:55 PM  #3 
diyAudio Member

I am only looking for 34 dB attenuation. I am thinking of the series/parallel resistors as an Lpad (not a variable), and I just thought it would be easier to alter just one resistor than two.

9th September 2013, 04:35 PM  #4 
diyAudio Member
Join Date: Feb 2009

A 2resistor Lpad between the crossover and driver has the advantage of allowing you to juggle both resistor values to get the desired attenuation while pretty much maintaining a constant impedance as seen by that crossover, thus not shifting the crossover corner, etc.
Paul 
9th September 2013, 06:17 PM  #5 
diyAudio Member
Join Date: Apr 2003
Location: Stockholm

If you tweak a series resistor you have to adjust the crossover as well anyhow. It is less complex buying pairs of series and parallell resisitors keeping the impedance constant. One additional advantage is that it also flattens the resistance, if you have a high Q tweeter that helps a lot. Especially a first order filter
Philips AD 0160 Tweeter Replacement 
9th September 2013, 06:28 PM  #6 
diyAudio Member
Join Date: Mar 2011

LPAD TUTORIAL
http://www.electronicstutorials.ws/attenuators/Lpadattenuator.html
ENJOY. 
9th September 2013, 07:31 PM  #7 
diyAudio Member
Join Date: Jun 2007

I think the thing to keep in mind is that the impedance of the tweeter is not fixed at its rated 8 ohms. Much like a woofer, a tweeter has a peak followed by a deep valley then a gradually increasing impedance as the frequency rises.
So, with only a series resistor, the ratio of the resistor and the driver is constantly changing. Now, the result of that could be good or bad depending on the circumstances and what you are attempting to achieve. Many people use a series resistor to great effect. However, if you use a series resistor then the impedance load, even the average load, seen by the crossover is no longer 8 ohms. It is 8 ohms plus the series resistor. You would need to design your crossover accordingly. The Series/Parallel LPad provides a constant impedance to the crossover. More so in that the changing impedance of the tweeter is now in parallel with a fixed resistor meaning the shifting impedance as seen by the tweeter is moderated. Using a common low cost Dayton Tweeter as an example, and assuming 4dB of attenuation. The LPad Rs and Rp values are Rs = 3 ohms and Rp = 14 ohms. Using this Dayton Tweeter DC28F8  http://www.partsexpress.com/pdf/275070s.pdf We see that the impedance is 10.6 ohms at 650hz, 6.2 ohms at 3khz, and 10.8 ohms at 20khz. If those impedance are put in parallel with a 14 ohm resistor then in series with a 3 ohm resistor, the resulting impedances are  650hz = 6.03 ohm plus 3 ohm = 9.03 ohms 3khz = 4.30 ohms plus 3 ohms = 7.30 ohms 20khz = 6.03 ohms plus 3 ohms = 9.03 ohms As you can see the result stays closer to 8 ohms. The shifting impedance of the tweeter has less effect on the crossover. This is a tweeter with moderate shifts in the impedance. Other tweeters can be substantially more. The 8 ohm rated Morel MDT29 impedance ranges from about 6 ohms up to 15 ohms. A substantially wider swing than the Dayton. Again, some people use a Series only resistor to great effect. But you have to understand the effect to use it effectively, and you have to design the crossover accordingly. Just a few thoughts. Steve/bluewizard 
10th September 2013, 06:02 AM  #8 
diyAudio Member
Join Date: Mar 2011

Bluewizard, good point. I should add that the calculator I referenced would provide you with a good starting point, but some measurements made at that point would help you see the response with that pad. Adjustments and further refinement (including listening evaluations) could be effected.
I took a several hour tour of the old Glendale JBL works right at the time the L166 was entering the sales phase. An engineer candidly told us that they were working lots of state of the art science and measurements, but always in conjunction with listening and some empirical wisdom. I think it would be safe to say that the results were successful. 
10th September 2013, 08:14 PM  #9 
diyAudio Member

Thank you everyone

13th September 2013, 02:27 PM  #10  
diyAudio Member
Join Date: Oct 2008

Quote:


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