freestompboxes.org

Hello everybody,

First of all, sorry in advance if this question has already been answered, I've looked around but to no success.
I've been looking at the corner frequency formula that's generally accepted for the RC filters with a resistor and a capacitor in series going to ground from the feedback loops, of e.g. the RAT (2 filters) or the Tube Screamer(1 filter). The usual formula of 2*Pi*R*C with R the value of the filter's resistor is used everywhere I've looked. Does anyone know how this is derived in that particular topology?
To me, R and C being in series, the original voltage divider formula can't be used for these two as it is for regular high or low pass filters, and instead the gain formula should be used, but this should in turn include the feedback resistor value.

Thanks a lot,

Pierre.

My quick analysis would be to calculate the crossover frequency (2 *pi * R * C )

At frequencies far below the crossover frequency consider the cap to be a open circuit and calculate the gain.

At frequencies far above the crossover frequency consider the cap to be a short circuit and calculate the gain.

The Rat has the complication of two RC combinations in the gain setting leg of the op amp - and they are electrolytic which have wide tolerances and can drift in value over time.


Andrew.

The electrosmash site has a good discussion of the Rat

http://www.electrosmash.com/proco-rat-a ... rtion.html

Geofex has a good tube screamer discussion (and electrosmash as well)

Andrew

Hello Andrew,

Thanks for your replies. They're great sites, and they both use the 2 Pi R C formula. The thing is I don't see how they get at it, the res and the cap being in series below the inverting input. In a usual high pass filter, active or passive, the voltage is taken between the cap and the res, so that you have a voltage divider topology and you can get at the Fc formula via the 3db loss for Vo/Vi or via setting Zc=R. But in the present case, how do you get the Fc formula? I can derive a number but it depends on the feedback res value too.

Thanks!

Pierre.