Interesting insights about the M2TECH Mitchell analog electronic crossover. “For this example Mitchell shapes the frequency response of a medium-sized woofer (20-25cm) with a somewhat problematic response. Suppose we use it in a three-way system, cutting it at about 300Hz with 12dB / oct of slope. First, we select the desired filter type from the drop-down menu of via A and activate the automatic filter design function, choosing a 300Hz Butterworth filter, as shown in the first figure. The configurator will activate the first state variable filter, correctly setting a cutoff frequency of 300Hz and a merit factor of 0.707.”
“By pressing F1 we activate the display of the simulated graphs of the frequency response. The red graph represents the frequency response of the newly designed filter.
Let’s suppose that, from the loudspeaker data or from measurements, it appears that the cone has a breakup at about 800Hz, a frequency at which the driver, albeit already in the transition band, is still heard. It is necessary to “tame” that breakup which, among other things, is still outside the useful band of the filtered driver. For this purpose, we exploit the second state variable filter, using it as a notch filter (otherwise called “line eliminator”) centered at the offending frequency (third figure). So select “Notch 1” in the configurator and set the frequency. By playing a little with the merit factor, we optimize the selectivity of the notch. A new look at the graph (fourth figure) shows the effect of the notch.
But our loudspeaker is far from perfect, or maybe it is the cabinet in which we have inserted it that does not behave well: the measurements show a “hump” of the frequency response around 100Hz that dirties the mid-bass. We can compensate for this by using the “shelving” function, available thanks to the fact that the chosen filter has a slope of 12dB / oct and therefore does not use either of the two 6dB / oct sections. So let’s choose a new type of filter from the upper drop-down menu: “LowPass 12dB / Oct + Shelving”: the textboxes relating to the low-pass and high-pass sections at 6dB / oct now appear. Since we want to attenuate a band around 100Hz, we have to set the low-pass cut-off frequency to a value below 100Hz and the high-pass frequency to a value above 100Hz. The values chosen in the fifth figure achieve an attenuation of about 2.5dB at 100Hz, as shown in the graph of the sixth figure.
Obviously, given the structure of the filter block of each channel, it is not always possible to use all these functions with any type of filter. For example, if the woofer were cut at 18dB / oct, shelving would not be available as the 6dB / oct low pass section would be coupled to the 12dB / oct state variable filter to achieve the desired cutoff slope. Similarly, with a 24dB / oct filter both state variable filters are used for clipping and no notch would be available. However, it must be said that with high slopes, breakups represent a minor problem, unless they occur in the passband.
The limitations of the single filter block, however, can be overcome by connecting two ways in cascade. In this way, for example, it is possible to achieve a filtering at 18dB / oct having three notch filters and unon shelving available.”