Shelving EQ variable frequency

[PBE6]

Does anyone have a schematic for either a shelving or Baxandall-type EQ that allows one to vary the frequency using a potentiometer but keep the gain (relatively) constant? I'd prefer not to switch capacitors in and out if possible.

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[samhay]

You have to vary the gain somewhere or else it won't be a filter, so I suppose you mean the gain of the shelf?
High pass or low pass?

[merlinb]

Do you mean a parametric version of Baxandall? Simplest way is to use a James stack instead, but make R3 variable:


Here's a simple-ish active parametric control:
http://research.microsoft.com/pubs/102099/Malvar_ParmEq_ElMag0381.pdf
(Malver, Electronics, Mar 10, 1981)

[PBE6]

Yes, I should have been more clear. This will be for a pre-clipping bass cut control. I'd like the gain of both the shelf (lows) and the passband (highs) to remain relatively constant while sweeping through cutoff frequencies using a potentiometer. May or may not be possible without switching the capacitors, but hopefully there is a solution.




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[PBE6]

Great!! Thanks merlinb! I will check this out.


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[PBE6]

Now that I know what I'm looking for, I found a version of the James that uses 2 caps instead of 4 (see Fig.3):
http://sound.westhost.com/articles/eq.htm#s4

I will try implementing this with fixed resistors and adding a 1M pot in series with the 2k2 resistor (R3).


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[robthequiet]

Somewhere on Rod Elliot's site is also a variable frequency shelving EQ that is very constant across the sweep. Something like Variable High-Pass And Low-Pass Filters It's probably more complicated than you might like in a stompbox but looks like it might produce a good result.

[lion]

Now that I know what I'm looking for, I found a version of the James that uses 2 caps instead of 4 (see Fig.3):
http://sound.westhost.com/articles/eq.htm#s4

I will try implementing this with fixed resistors and adding a 1M pot in series with the 2k2 resistor (R3).

I'd be interested in your progress.

Erik

[FiveseveN]

Here's my first thought: RC high-pass with variable R give you control of cutoff frequency (variable v in the sim). Mixing back the original signal gives you "gain" control (variable w). Of course in the real world you need to mind the impedances, maybe buffer a branch or both.

[Brisance]

you cant just mix a filtered signal with the original, it will be out of phase

[FiveseveN]

Well it's a starting point that's easy to try out.
But on the subject of pre-clipping high-pass, I've tested a lot of variations and settled on a simple RC filter with variable frequency. The Tim(my) design is also useful. But I'm interested in high-gain; for cleaner sounds, more control might be better.

[PBE6]

Welp, I finally got around to building this. Interesting, but not earth shattering. I'll post some sound clips later tonight if I can.


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