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In many guitar amps a presence control is inserted in the feedback loop. In effect this is high pass filter to ground, gradually bleeding more high frequencies to ground and thus decreasing the treble content of the feedback signal, which in turn means that the treble is accentuated in the final output.

To simulate the presence control a simple adjustable low pass filter was inserted into the feedback loop.

/*
	Experiments with Guitarix Components

	Series of experiments with aim of using the "tube" components 
	to build amplifiers as close as possible in topology as real valve amplifiers

	Steve Poskitt 2013

	Blog 1 - single valve stage
	Blog 2 - cascade valves to see effects of overload
	Blog 3 - add some NEGATIVE FEEDBACK and look at effects of filtering feedback loop
	Blog 4 -  Attempt to model simple guitar amp presence control

*/
import("math.lib");
import("music.lib");
import("effect.lib"); 
import("filter.lib");
import("guitarix.lib");

// Slider for the amount of treble bleed to ground.
vol = vslider( "Vol[style:knob]", -20, -70, 10, 0.1 ):db2linear:smooth(0.999) ;
feedback = vslider( "Feedback[style:knob]", -20, -70, 10, 0.1 ):db2linear:smooth(0.999) ;
presence = vslider( "Presence[style:knob]", 0, 0, 10, 0.1 ):*(-6) ;

// This is by experiment seems that main boost should be in the 1-4K area
// Looking ta various graphs online anywhere between 500-1.5kHz seems to be best cutoff point
// Presence uses a fixed frequency and varies the cutoff gain
// Guess from SPICE simulation is that cutoff is around 800Hz-1kHz
freq = 1000 ;
// Find a better low shelf
loop = high_shelf( presence, freq ):*(feedback):lowpass(1,6531.0) ;

// From guitarix stuff put in a tube stage
amp = *(vol):lowpass(1,6531.0):tubestage(TB_12AX7_250k,194.0,820.0,0.840703)  ;
//process = ( sub:amp)~loop;
process = (+:amp)~loop;

And it works!

Some experimentation needed with the filter cutoff but so far a pretty good presence control. Due to nature of the feedback circuit this has quite a different effect from the same filter in the main signal path.

Similarly a high pass filter in the same place produces a "resonance" control, boosting the lower frequencies.