# crossover circuit : 2nd or 3rd order?

#### clargh

Hi everyone, I'm a new member, honored to be part of this community of fans.
(I'm Italian, sorry but I use google translate)
I have a technical question to ask you: I have a Berhinger equalizer and I can not understand the operation of its high-pass filter shown in the figure (the section in the blue box), or rather it is not clear to me how to identify the model that determines the order of the filter. From the bottom of my ignorance it seemed to me a 2nd order and calculating the min and max cutting frequencies you get values equal to about 10Hz and 250Hz. On the potentiometer instead 10Hz and 400Hz are indicated and in the description of the command we read: "The LOW CUT regulator establishes the lower frequency of the limit of the ULTRAGRAPH PRO. The high-pass filter (18 dB/oct) is adjustable in a range of 10 to 400 Hz. If this filter is in the 10 Hz position, let the signal pass through without affecting it in the slightest."
Can you help me understand the pattern and determine the formula to calculate the cutoff frequency? Does the 100 Ohm resistor (R176) have anything to do with it?
Thank you very much if you can clarify this point for me.
(the goal is to replace the potentiometer with a switch and resistors to have preset frequencies)

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#### MarcelvdG

It is a second-order filter and its natural frequency can be tuned from about 10 Hz to 250 Hz. The 100 ohm resistor has only little influence

As to the cut-off frequency: it is a filter with a rather low Q factor - I think Q ~= 1/2, but I may be mistaken. When you define the cut-off frequency as the half power point (-3.010299... dB point), it will be higher than the natural frequency. I don't know by how much, but maybe that could explain the 400 Hz. The lower frequency would then increase by the same factor, though.

That claimed -18 dB/octave makes no sense at all. It's second order, so it must be -12 dB/octave.

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#### clargh

Thank you MarcelvdGH for your explanation especially the Q factor which could explain the max cutoff frequency. However, is it possible that Behringer is wrong in the description (18db/oct) and the information on the min cutoff frequency (10Hz)?

The same information (18db/oct and 10-400Hz) with the same wiring diagram, are reported in the manuals and on the potentiometer of several Behringer equalizers, even the most recent ones (GEQ-3102 FBQ-6200 FBQ-3102 etc.).

And in any case, it would remain how to calculate the exact cut-off frequency by varying VR33A/VR33B... and consequently which resistance values to insert in place of VR33A/VR33B for a given cut-off frequency...

#### ejp

Two reactive elements -> 2nd order, no two ways about it.