Electrical – Can someone explain this op amp “tone control”


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Its supposed to be part of a guitar effect pedal, why is there positive feedback besides the negative feedback? I was reading about op amps and from what i understand positive feedback is very rarely used. How would this affect the signal?

Best Answer

Quite often the way to analyse these sorts of things is to ask yourself what happens at the extremes (of control position and frequency). The nice thing about this approach is that at low frequency (whatever that means) caps are approximately open circuits and at high frequency they are approximately short circuits.

Ok the 1k and 220nF at the left are a more or less fixed passive lowpass, so forget them, not that interesting.

Now with the pot slider all the way to the left: It just adds 220nF in series with 220 ohms across the existing 220nF so the thing gets a bit more lowpassy (With a bit of a shelf)...

With the slider all the way to the right we get something more interesting: Now at low frequency we model the cap as open circuit so by the usual rules of the opamp (The output is driven such that the two inputs are pretty much equal) we can clearly see that we have a gain of 1 from the opamp stage.

At high frequency however the caps appear short circuit (ignoring that passive input LPF for the moment!), so the opamp now has a gain of about 1 + (1000/220) = ~5.5 times, we can ignore the 20K pot because it is swamped by the 220R resistor, so we have a variable treble boost of about 14dB preceded by a passive lowpass filter.

As to where the action happens, just figure the timeconstants (1K & 220n, 220R & 220n).

It is amazing how often this sort of grossly simplified analysis is all you really need, and it is a ton quicker then disappearing into a mess of S plane bullshit.

Hope this helps.