Designing EQ Correction Circuits

[elwood]

Hey all,

I'm pretty experienced with the classic filter types and putting together various active and passive filters for simple things. But here's something I've recently run into that I haven't experienced before: converting a known (desired) complex EQ curve into the simplest circuit possible.

Following the advice of something I read, I've been using a 31 band graphic EQ to shape the sound going into the power amp section of my project. I've also found a great windows program called Tombstone for measuring and printing response curve of real circuits. But I'm having trouble figuring out a systematic (automated, programmatic?) way of producing a circuit that has roughly the response of my desired curve. Do I make a gyrator for every peak and valley and tweak each one like in a graphic EQ? Or should I make multiple stages, one for each boost/cut? Simple rolloffs are easy, but I need a few bumps and dips in the response here and there.

What method to people use for this sort of thing? Any insight would be greatly appreciated.

[Marek]

Elwood,

For simple filters I would look at: http://www.herby.kielce.pl/~piter/hexenew2/ie.php?c=cabsims_e&l=diy_e
(translated by Teemu and me  ).
For more complex filters I would use any software that allows sound tweaking (e.g. SoundForge) to create required frequency response and then, having all required parameters, you can set up your analog filters or alternatively, you can implement the filters as a DSP circuit. For DSP you may take a look at: http://www.spinsemi.com/products.html or http://www.wavefrontsemi.com/products/AL3101pop.html

Marek

[J M Fahey]

Dear Piter, Marek and Teemu: thanks for the incredible study on cabinet simulators. I see a lot of work was done there. It´s a very important part of SS amps design, trying to get them away from the "cold, sterile" label given them by tubeheads. Cabsims show the way to go. Thanks.

[PzP]

Hello Guys!
Regarding the speaker simulations on my site (thanks again fot the translation!), here are some thoughts that i had over the last years:

To get more peaks, deeps, more unregular frequency response - how about to use the phase shift stages, just as in the Phasers. Instead of controlling the phase shift with the LFO let's just use a simple constant voltage to set the notches at desired points in the frequency domain. I think it could lead to some new interesting sounds. I'm going to try this idea soon, ma latest phaser project has the ability to manually set the position of the overall phase shift. So, i'll just feed the phaser with already 'cabsimed' signal and try to find some interesting sounds.

Don't be afraid to use the EQ's with cabinet simulators. Often, they are just a simple filters and an additional, even one band parametric EQ can be very helpful. I think that the analogue speaker simulation is quite difficult and i tend not to name a specific sounds like "1x12 simlation" or "4x12 sim". There's so much factors influenting on the entire sound experience called a "guitar sound" that it's better to use the basic highpass+lowpass, which describes the main frequency responsce of the guitar cab and combine it with anything that will create the frequency curve in the useful range. And this is only a partial picture of the guitar sound. Add to this the room accoustics, reflections, reverberation etc.

This leads to, in my opinion of course, the best method of speaker simulation. The method that gives quite convincing results and it is used more and more commonly by the software amp simulators. A convultion.