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MultiWay Conventional loudspeakers with crossovers 

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19th January 2004, 03:49 AM  #1 
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Location: Calgary, Alberta

Question on Lpad theory/formula
For the Lpad:
from this site: http://lalena.com/audio/calculator/lpad/help.shtml I see that R2 = function (impedance, attenuation) and R1 = function (impedance, R2). Does this mean that varying R1 has relatively little impact on attenuation? The reason I ask is that I am trying to understand how to estimate the amount of padding present in a published crossover design, which of course often have very different R1 values or no R1 resistor at all.
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19th January 2004, 04:44 AM  #2 
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R1 is the the constant impedance to the amplifier example "8 ohms"
R2 is the "variable" and provides attenuation to the driver while R1 maintains the impedance necessary for the amplifier 
19th January 2004, 04:54 AM  #3 
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R2 does the attenuation by providing a parallel current path. Regardless of the value of R2, the total impedence for R2 and Z will alway be less than Z. Therefore R1 is used to bring the total circuit impedence back equal to Z to theoretically provide a constant impedence to the Xover or amp.
Since the Z of a driver varies with frequency, the LPad is at best an approximation of a constant impedence. Therefore, the actual amount of attenuation will be frequency dependent.
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Rodd Yamashita 
19th January 2004, 05:10 AM  #4 
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Thanks for the replies.
So, as you didn't actually say it in these terms, does this mean that for a published crossover, I can estimate the attenuation merely by knowing driver impedance Z and resistor R2, and ignore R1?
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19th January 2004, 05:19 AM  #5  
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Quote:
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Rodd Yamashita 

19th January 2004, 06:19 AM  #6  
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Quote:
Without R1 R2 would simply reduce the impedance as its in parallel with the driver, R2 would not attenuate at all. R1 forms a voltage divider with R2driver. Both R's affect attenuation. The series R is necessary for attenuation but used on its own the impedance always rises. R2 is introduced with a lower value of R1 such that the attenuation is the same and the impedance is ~ the same. To estimate attenuation estimate R2driver, then estimate the the effect of the voltage divider R1 and R2driver. As an example consider a tweeter crudely modelled by a 6R. R1 = 6R only, attenuation = 6dB, Z =12R. (R2 = 6R only, attenuation = 0dB, Z =3R.) R1 = 3R, R2 = 6R, attenuation = 6dB, Z = 6R. sreten. 

19th January 2004, 03:13 PM  #7  
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Quote:
Since there can often be no R1 resistor at all in some crossovers, does this mean that there is little attenuation and only impedance adjustment, or that for a proper estimate, the DCR of all inductors must be considered in the calculation?
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19th January 2004, 04:08 PM  #8  
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Rodd Yamashita 

19th January 2004, 06:39 PM  #9  
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Quote:
If R1 is missing R2 is not for attenuation. Its probably there for impedance correction only. If R1 is missing the tweeter has the correct nominal sensitivity. This is common in commercial speakers where a low sensitivity tweeter allows savings in the tweeters magnet size. And saves the cost of a resistor in the crossover. And yes high resistance coils are often used in commercial speakers to allow the coils to to be cheaper when they are used in combination with a series resistor or they are the series resistor as well. Quote:
peak and R1 used for attenuation, the target being a higher Z than the nominal speaker rating, as high Z at HF is good for amplifiers. sreten. 

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