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Old 1st January 2013, 04:56 AM   #1
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Default L-Pad attenuators

I'm going to build a crossover and add a tweeter to my full range speakers. The tweeters will need attenuating so I was going to make a simple L-Pad with 2 resistors for my 8ohm tweeters.

I then thought that if the tweeter was over or under powering the music, it would mean I would need to change the resistors again. I've seen rotary L-Pad attenuators on eBay and they all say that they are 8ohm. Does this mean each winding is 8ohm?

When using various online L-Pad calculators, a 3dB attenuation requires 2.33ohm series and 19.33ohm parallel resistance. A 1dB attenuation requires 0.87ohm series and 65.7ohm parallel! What is the actual resistance of the windings of these attenuators?
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Old 1st January 2013, 10:57 PM   #2
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It means that the amp/crossover will see 8 ohms. Calculaors will get you in +/- 20% of actual values, I have tested suggested L-pads and they deviate from reality. L-pad is a voltage divider, also, the shunt resistor will reduce the impedance peak at Fs for the driver thus changing the slope and phase of your crossover.

Last edited by Mario Pankov; 1st January 2013 at 11:00 PM.
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Old 1st January 2013, 11:04 PM   #3
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I need an adjustable L-Pad for a 4 ohm speaker, but I cant find any anywhere. The 8ohm version would not work, would it?
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Old 2nd January 2013, 11:21 AM   #4
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Quote:
Originally Posted by portreathbeach View Post
I need an adjustable L-Pad for a 4 ohm speaker, but I cant find any anywhere. The 8ohm version would not work, would it?
It has different series/parallel resistance values, so no. Why not build an L-pad with resistors? Do some calculations on how much you need to pad, get a bunch of cheap wirewound resistors around these values and build your L-pads.
In another thread I see you`re looking at the XT25SC as a possible option - shunt resistor will decrease the resonance of this tweeter to some extent. The XT25 is around 3ohms DC, get L-pads/resistors after you have the tweeter.
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Old 2nd January 2013, 09:32 PM   #5
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Hi Mario. Thanks for your answer. The reason I was asking about adjustable L-Pads is I would like to have the option to adjust the tweeter when needed. But I suppose if I size everything correctly and test it, I shouldn't have to do any adjustments, just sit back and listen to them.
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Old 2nd January 2013, 09:50 PM   #6
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adjustable L-pads tend to oxidize over time, and become scratchy & unreliable. If you do this a lot, it's a good idea to buy just one, solder crocodile clips to it, & use it while setting up the xover. Measure the value of each arm, & replace with fixed resistors.
(although it's not much hassle to tweak a fixed resistor L-pad, you can usually get it pretty good after one or two series resistor changes. I usually use cheap 1/2 or 1 w resistors until I've found the final values, & you don't need to be too precious about the values, e.g. the 3dB example you give above, I'd use 2.2 ohms & 18 ohms, susbtitute the 2.2 with a 2.7 to see how that sounds, & tweak from there)
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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