Here is a short guide to help when selecting resistors to attenuate (reduce the level) of a tweeterin a loudspeaker system. Especially useful if you cant find an original type replacement and have to fit a unit of greater sensitivity.
Tweeter Attenuation Chart.Attenuation is needed to match the output levels ofmidrange and tweeter drive units to match the output ofa woofer. This is especially true when using horn loaded midrange and tweeter units, which canbe 15 to 20 dB higher in sensitivity than thewoofer.
The most common way to match the levels of the drivers is to add an L pad to the output of the tweeter and midrange sections of the crossover, which allows the volume to be turned down, while maintaining a proper load to the crossover. An L pad is designed with 2 resistive elements controlled by a single shaft, and they look similar to the diagram shown below.
The crossover side has a resistive element of 8 ohms (for an 8 ohm L pad), and the driver side has an element of about 80 ohms. The point is, as you adjust the level, the series portion of the 8 ohm element, when added to the parallel value created by the 8 ohm driver in parallel with the driver side element, always adds up to 8 ohms. The purpose ofkeeping a constant 8 ohms is because the capacitor and inductor values that make up the crossover were all chosen based upon them seeing an 8 ohm load. If the load changed, then the crossover point would also change.Many people in the audio field do not like any variable units in the signal path, although they are very useful for testing purposes and can help determining the values required for a high quality fixed resistor network to be used permanently in place of the L pad.
The chart belowgives you a quick look up table to create a voltage divider to give you the desired attenuation. Attenuation in dB for the above circuit R1 R2 (value in ohms) assuming an 8 Ohm crossover design and an 8 Ohm driver. Please note that these are values for resistance, and as with all things loudspeaker the impedance of the actual systems components may subtly alter the attenuation achieved in any particular system combination.
Attenuation in dB R1 in Ohms R2 in Ohms1 0.87 65.56 2 1.6530.90 3 2.34 19.39 42.95 13.68 5 3.5010.28 6 4.00 8.00 7 4.436.46 8 4.82 5.29 9 5.16 4.40 10 5.473.70 11 5.753.14 12 5.992.68
You dont have to have exactly the listed value, althoughdo try to be pretty close!. (within @10 %) For resistors in series simply add the values together for the total resistance. Resistors in parallel are calculated differently. If all of the parallel resistors are the same value, then simply divide the resistor value by the number of resistors. For instance, say you had three 10 ohm resistors in parallel, the total resistance would be 10 divided by 3, or 3.3333For two 8 ohm resistors in parallel, the total would be 4 ohms, and so on Resistance of parallel resistors is ALWAYS less than the lowest value resistor in the parallel circuit. For parallel resistors of different values, this formula is the one to use:How to calculate resistance for resistors in parallel:Where Rt = total resistance
R1= value of resistor 1R2= value of resistor 2, etcAn example. We have 3 resistors, one of which is 2.7 ohms, one is 5 ohms, and one is 12 ohms. Solve 1/R for each resistor, and then add them together, like this:1 / 2.7 = 0.370370371 / 5 = 0.21 / 12 = 0.0833333So you add 0.37037037 + 0.2 + 0.0833333, and you get 0.6537, and now 1 / 0.6537 gives you 1.529 ohms total. By paralleling resistors, you can get to almost any valueAlways be sure to check power ratings of resistors for such networks, we recommend at least 7W ceramic units are chosen.
Chris CooperConnect Audio Ltd
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