THE CIRCUITRY PATTERNSWe will examine the circuit diagrams of filters up to fourthorder, the most used in crossover circuits. Higher orders don't bring substantial benefits to listening, rather they introduce more than a few complications. A firstorder lowpass filter is simply formed by an inductor in series with the speaker. It produces a rolloff of 6dB/oct.: Do you remember what we said about the behavior of capacitors in the fourth lesson? Well, replace the inductor with a capacitor and you'll get a firstorder highpass filter with the same attenuation slope: Now try to add a capacitor to a firstorder lowpass, but in parallel with the speaker this time. The capacitor — which is totally transparent to the highs — actually diverts them toward ground, helping the inductor to reject unwanted frequencies with better effectiveness. This way you'll have gotten a secondorder lowpass filter, with a characteristic rolloff of 12dB/oct.: Now start from a firstorder highpass filter and reverse the above statements. There, the secondorder highpass you asked for: Schematically you can think of a thirdorder lowpass filter as a secondorder one with the addition of a second inductor in series. Its characteristic rolloff is equal to 18dB/oct.: Inversely, the thirdorder highpass will be formed by a capacitor in series, an inductor in parallel, plus a second capacitor in series: The rolloff of a fourthorder lowpass is equal to 24dB/oct. It is formed by an inductor in series, a capacitor in parallel, plus a second inductor in series and a second capacitor in parallel. Surely the most attentive of you will realize it is similar to two secondorder lowpass cascading filters: The list is one short without the fourthorder highpass, but at this point of the story you should be able to guess what it is by yourself: Who's not clever enough to solve the problem is strongly suggested to take a deep breath and start all over again. In the meantime we'll be drawing the numbers
