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Hello peoples, it's been a while.
I'm intersted in possibly modifying the presence control on one of my amplifiers to try to tune it's voicing, so I'm trying to come to an understanding of how it works. It is my hope that by fully understanding the presence control, I can desgin my mod intelligently, instead of just blindly swapping parts and praying that the mojo gods will smile upon my guess.
To be clear, I do have a basic understanding of how the presence control works: Most often, it is a low-pass filter in the feedback loop tied to a potentiometer. Turning the pot reduces or increases the effect of this filter, in turn increasing or decreasing the negative feedback above the filter's cutoff frequency. This changes the power amp's voicing by altering its gain above said frequency. I do know that some amplifiers are exceptions to this, e.g. several Mesa amps.
I've done some searches on Google and done some RC filter calculations based on the component values I see in various schematics, and this is where I am running into trouble. Per most of the articles and forum posts I've found with Google, the presence control affects the upper midrange and high frequencies. On many amps I've played with, my ears tend to agree. However, I must be reading these schematics wrong. When I calculate the RC cutoff frequency of the components I see as most likely to form a low-pass filter, I'm getting cutoff frequencies in the range of 10 to 40Hz!
For example, take the presence control in the Marshall 1959, 1987, 2203 and 2204 circuits. If I'm understanding this correctly, the negative feedback enters into the PI at the bottom of the tail resistor, from the voltage divider created by the 100k resistor attached to the output transformer, and the 4.7k resistor to ground. Parallel to the 4.7k resistor, is a .1µF capacitor in series with a 22k pot wired as a variable resistor. Okay, so it seems that the low pass filter would be formed by the .1µF cap and the 100k resistor. But when I calculate the cutoff frequency, I get 15Hz! That covers the entire bandwidth of a guitar, not just the high frequencies! There must be other factors involved to make the presence control effective only over upper mids and highs, but as much as I look at these schematics, I'm not spotting them.
Where else should I look, and what other things should I be considering?
I'm intersted in possibly modifying the presence control on one of my amplifiers to try to tune it's voicing, so I'm trying to come to an understanding of how it works. It is my hope that by fully understanding the presence control, I can desgin my mod intelligently, instead of just blindly swapping parts and praying that the mojo gods will smile upon my guess.
To be clear, I do have a basic understanding of how the presence control works: Most often, it is a low-pass filter in the feedback loop tied to a potentiometer. Turning the pot reduces or increases the effect of this filter, in turn increasing or decreasing the negative feedback above the filter's cutoff frequency. This changes the power amp's voicing by altering its gain above said frequency. I do know that some amplifiers are exceptions to this, e.g. several Mesa amps.
I've done some searches on Google and done some RC filter calculations based on the component values I see in various schematics, and this is where I am running into trouble. Per most of the articles and forum posts I've found with Google, the presence control affects the upper midrange and high frequencies. On many amps I've played with, my ears tend to agree. However, I must be reading these schematics wrong. When I calculate the RC cutoff frequency of the components I see as most likely to form a low-pass filter, I'm getting cutoff frequencies in the range of 10 to 40Hz!
For example, take the presence control in the Marshall 1959, 1987, 2203 and 2204 circuits. If I'm understanding this correctly, the negative feedback enters into the PI at the bottom of the tail resistor, from the voltage divider created by the 100k resistor attached to the output transformer, and the 4.7k resistor to ground. Parallel to the 4.7k resistor, is a .1µF capacitor in series with a 22k pot wired as a variable resistor. Okay, so it seems that the low pass filter would be formed by the .1µF cap and the 100k resistor. But when I calculate the cutoff frequency, I get 15Hz! That covers the entire bandwidth of a guitar, not just the high frequencies! There must be other factors involved to make the presence control effective only over upper mids and highs, but as much as I look at these schematics, I'm not spotting them.
Where else should I look, and what other things should I be considering?
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