Guitar Tube Amp RC Filter frequency fluctuation

filtertube

In a simple LP or HP RC filter, is there a scenario where the centering frequency can fluctuate? So say the filter is a low pass and its set to pass everything up to 800hz, is there a situation where it may cut off at 750hz, or perhaps at a different time cut off at 825hz, etc.?

Maybe the guitarist picks the strings harder/softer, air pressure, ambient temperature, etc. could cause this? I am researching simulation techniques and trying to understand if factors like these could make an amp model a little more realistic in terms of tonal quality and dynamics.

The "Feel" of an amp plays a major role in defining the characteristics of how one plays and changing frequencies that are allowed to pass into a non-linear function will alter the feel quite drastically (at least in the digital amp sim world). The difference could be relatively small too, 120hz changed to 100hz will add a more "saturated" type feel. You may not notice much difference in tone, but you will in "feel". In the analog world I have noticed this "difference" in cut off only added to tone(allowed for more bass), but had little effect on feel.

So, can (or do) RC Filter cut off frequencies fluctuate?

Thank you..

Best Answer

A low pass filter is pretty much defined by the R and the C value: -

\$f_C = \dfrac{1}{2\pi R C}\$

The same applies for a simple RC high pass filter and the steepness of the transition from "pas" to "reject" frequencies is very shallow: -

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Even if the cut-off frequency did fluctuate by (say) 1% (and likely very slowly due to warming and cooling effects of the capacitor) the perceived change in tone would be minimal over that long period of time (at least several minutes.

Think about a slow 1dB change in volume - this could occur in a space of 2 seconds and you just wouldn't notice it let alone over a period greater than several minutes.

And there is certainly no dynamic effect of tone shifting due to the way the guitarist picks or strums the strings.

I don't think looking at CR filter changes is going to yield results that you think. I play and record guitar and have done so over many years. I do all sorts of things to the recorded guitar sound from basic equalization (messing with tone controls) to complicated effects and the least clearly obvious things that effect the sound of a guitar is movement of tone unless it is something dramatic like the wah-wah effect.