Shelf filter, variable freq and gain?

[Tubis]

Any ideas on how to design a shelf filter with variable freq and gain of the shelf?
I can design the shelf with either freq variable or gain variable but i dont understand how to combine the two..........

I would like to be able to construct low shelf and high shelf, opamp based....

[Steve Conner]

Have you seen Douglas Self's design for this?

Self on Audio - Google Books

"Self on Audio", Page 72 onwards. The complete schematic may not be in the Google Books preview. It was originally published in Wireless World, if that helps.

[Tubis]

Have you seen Douglas Self's design for this?

Self on Audio - Google Books

"Self on Audio", Page 72 onwards. The complete schematic may not be in the Google Books preview. It was originally published in Wireless World, if that helps.

Not realy, no. By my understanding thats just bas and treble tone controlls thats able to sweep the freq up and down. Not a shelf filter?

[Steve Conner]

Not realy, no. By my understanding thats just bas and treble tone controlls thats able to sweep the freq up and down. Not a shelf filter?

That's what a shelf filter is. The standard Baxandall tone control is a low shelf and a high shelf with fixed frequencies. Self's circuit is the same, but with variable frequencies.

If you want something different, it's not called a shelf filter. Did you maybe mean a 2-pole resonant filter or something?

The free preview only shows part of the circuit, the bits he shows hook up to boost and cut controls. And the response plots are not of the full circuit. He wants you to buy his book.

[Tubis]

This is what im after and that circuit doesnt do that, no, because its dependent on the settings of the other filter. You often se the shelf filter as a part of a parametriq eq. Maybe its not the correct name for it....??
A baxandal is dependent on that the high filter is very close to the low filter to make the shelf and totaly dependent on the settings of the other filter.
I dont want the high filter to be dependent on the low filter , in fact, i want them to be totaly separated.



I tried to just make i side chain to a 2nd order filter but it made a bump in the graf as illustrated below.
Dont realy get it? Why the bump? The side chain is totaly flat and i want the shelf to be so as well.


Not real life grafs, spice simulations.

Oh by the way, a first order filter doesnt make a good shelf. I think a second order or greater is a must. If a first order would be enought i wouldnt have any problem constructing it.

Sorry if my english is bad...im from sweden....

[Tubis]

Found a good image to illustrate. Dont realy need to be able to change the slope but a 6dB slope i think is a minimum?

[Steve Conner]

The above picture is what the "bass" part of Self's circuit would do if you built it and turned the level control to -18.

Well, except that he returns the boost to 0dB at very low frequencies to avoid amplifying subsonic trash.


These are 1st order filters: that's what 6dB/octave implies. A second order filter is 12dB/octave, and resonant, and that's probably where the bump in your graph came from: the resonant peak.

[Tubis]

Well i spiced it and it realy dosent make a shelf. It acts like a band-stop filter when cut and bandpass when boosted..

Like this:

[Steve Conner]

Well, except that he returns the boost to 0dB at very low frequencies to avoid amplifying subsonic trash.

So, that's how it's supposed to work Over the audible range, the effect is identical to a shelf.

[Tubis]

So, that's how it's supposed to work Over the audible range, the effect is identical to a shelf.

Yea, but i want to raise the freq and that doesnet work for me.. But i managed to remove that effekt from the circuit and now it works like a shelf. But i think the slope looks realy shallow. I think a steeper slope would be what im looking for.......