THE RL FILTER

Since in almost all cases a tweeter will require a corrective action on its emission level as well as the cut, to opt for this solution allows us to kill two birds with one stone. It is to put a resistor in series with the speaker and an inductor in parallel. Such a configuration allows the elimination of the whole highpass filter as it itself operate a 6dB/oct. highpass filtering. This is the electric representation of the RL cell:

In this case too the value of the resistor is given from

R = Z_tw*(10^a/20-1)     [Ω]

but pay attention to its wattage because being lacking of the filter ahead it has to handle the whole power sent by the amplifier and it is interested in a very high flow of current. It also gets very hot so take care of its positioning.

The value of inductor is calculable through

L = (R*Z_tw)/(R+Z_tw)/2πf_c     [mH]

A filter of this kind is not free from side effects. In combination with lowpass row it constitutes a very critical load for the amplifier, that sees an impedance formed by the parallel of the woofer with the resistor. Speaking from experience, the RL filter requires a great amount of knowledge and ability to experiment. Don't all of you begin to build them starting tomorrow unless you have some idea of what you are doing.

In these last lessons we have described a series of corrective actions to implement on the speaker so that it correctly interfaces itself with its dedicated filter. As a matter of fact, with all we've done till now, you should be able to design and build a passive crossover for any speaker system and even expect it to work. Nevertheless, before we go to the end, we would like to return on the topic filter to develop a concept that in this tutorial we have only touched on: the filter "Q"