RG Keen Graphic EQ

[tdkrause]

I'm incorporating a two band version of RG Keen's "Simple Easy Graphic Equalizer" into a circuit of mine so I can adjust two particular frequencies in this circuit. I need to know how to extend the level of cut for each band. Is it just a matter of reducing the value of the 33k resistor? I have an MXR 10 band EQ that offers much more cut than I am getting, and that same resistor in the MXR circuit is 100K. It seems like a larger resistor there would decrease the range, so I'm a little perplexed.

[PRR]

> reducing the value of the 33k resistor?

That has almost nothing to do with it.

The maximum cut (or boost) is the tank resonant impedance against the 150K resistors front and back.

The tank resonant impedance is tricky (to me!) because he used equal value resistors (6.8K). As a fast-hack, reduce both by a factor of 2 (3.3K). That will raise the frequency, so to get back to where you wanted you would double the caps (for 1KHz, change 0.0068uFd to 0.0136uFd).

There must be a more elegant method, and R.G. may pop in and explain it. I am more used to gyrator tanks with wildly different resistors (1K and 100K), where the low-value resistor is approximately the tank resonant impedance.

However the stock values have as much cut as musicians would normally want.

> I have an MXR 10 band EQ that offers much more cut than I am getting, and that same resistor in the MXR circuit is 100K.

This too suggests something is not as you think. Could 15K have slipped in where you want 150K? Any change of values will change the response, and I think more than half of all likely changes will reduce the effect.

[tdkrause]

I just double checked all of the components I used. Aside from the capacitors, everything follows the schematic. I had based my assumption about the 33k resistor on the fact that the tube screamer tone control is essentially a simplified version of this circuit. In fact, if you remove all but the highest frequency from this and other graphic eq circuits, it IS the tube screamer tone control. I've used that in a few other circuits and the resistor to ground along with the resistor in the feedback loop sets the gain at full boost and also limits the amount of signal being grounded at full cut. This is why I'm confused. It seems to behave differently here.