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In Reply to: RE: Tweeter update posted by Tadlo on December 16, 2011 at 07:26:44
I have not read any bad advice here. All good points.
There is a simpler way to go about this, however, and not all will agree. Many crossover designers insist that padding be AFTER the crossover and the resulting impedance is used to calculate crossover values. When you insert padding then you've upset the balance of the crossover. If you keep changing the padding resistor(s) after the crossover, you have to make sure that the values you use keep the resulting impedance the same. That takes a lot of calculating if you don't have a program to do it for you. If you have access to David Weems' original book on speaker building which he did for Radio Shack, he has the necessary formulas listed to calculate resulting impedance when using a pad with a series and a parallel resistor. That's help me many times.
Here's what I would do: If you can isolate the lead going to the tweeter BEFORE the cross over take two 1 Ohm Ten Watt resistors and stack them in parallel. That gives you a 2 Ohm, Twenty Watt resistor to put in line before the crossover. (I already hear the disagreements.) If you need more padding, add another 1 Ohm resistor. Then you'll have a 3 Ohm Thirty Watt resistor. An even better way is to do what was suggested and buy an assortment of values. I like to stack resistors for this application because the higher Wattage value is better when you're tapping off the speaker input and going to the tweeter before the crossover.
The next best thing is a variable L-Pad and if you get a quality L-Pad you'll get many years of good service out of this.
Follow Ups:
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When connecting resistors in Parallel, THEN combined resistance does not sum but instead REDUCES in value (two same R-value divide by 2 & three same R-value divide by 3, etc). Parallel resistors Do sum their Wattage rating, but not their resistance values, their combined R value shall Always be Less than the smallest-value resistor of the combo. OTOH Series-connected resistors Will sum their R-values but Not their wattage ratings (which will only be what individual resistors are rated). He needs to re-visit the combined resistance formulas also present in Weems' Little Red Book.
Edits: 12/22/11 12/22/11 01/01/12 01/01/12
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