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I'm trying to roughly replicate the sound of the per-stereo-channel 3 band equaliser conttols of an old Citronic Pro-8 / Pro-10 DJ mixer. If I'm correct had a fairly steep curve so bass and treble controls didn't affect middle frequencies much, and were very effective at cutting bass/treble as required.

As long as the sound is all flat when tone controls are centred and there are no odd peaks /resonances.

What type of filter might it have used? Googling 3 band audio equaliser brings up circuits that I don't think would do it? Technically they all seem to be first order if Im correct? OK I should try it first but in principle something to try next anyway? Second order filters? Or even a half way house of technically easy in analogy circuits?

My mixer wasn't parametric but I guess a single parametric tone control (not thought this out at all mind you) could give the same effect on top of a simple first order set of tone controls.

Ideas:

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    \$\begingroup\$ Could be anything. Have you considered looking up a service manual for the circuit diagram -- or maybe even telling us what the mixer was? \$\endgroup\$ – Scott Seidman Sep 26 '18 at 16:44
  • \$\begingroup\$ @ScottSeidman I didn't realise things could get that creative! I tried but failed, maybe it's too recent? They probably still use similar circuits in their newer studio mixers and like to hold onto their IP \$\endgroup\$ – CL22 Sep 27 '18 at 15:06
  • \$\begingroup\$ music-filibe.com/webshop/product_info.php/language/en/info/… \$\endgroup\$ – Scott Seidman Sep 27 '18 at 17:08
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There is a real problem when using higher-order filters as tone controls at audio frequencies. The phase response and delay that those filters introduce become additive and subtractive at various frequencies as the controls are adjusted.

Early graphic equalizers are good example of this problem. The frequency response changes in odd fashions as you make what should be minor changes to the different filters. This is both audible and easily measured.

On the other hand, both peaking and shelving high-pass and low-pass filters use at the very ends of the audio spectrum don't have as much problem with this. These high-pass and low-pass filters are often 2nd order filters (12 dB / octave) but I have seen even higher-order filters in older analog audio mixing consoles.

I suspect that what you are calling "bass" and "treble" controls on the DJ mixer you are referring to are, in fact, shelving high-pass and low-pass filters.

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