Can Someone explain this simple Circuit for Me?

[Guitarist1983]

Why are there two high-pass RC filtering branches in the RAT's op amp feedback loop? Couldn't one resistor/cap (RC) do the same job?
Specifically C5 + R4 || C6 + R5

I have played with a on-off-on switch at C5 to get noticeable midrange content changes.  And this set me thinking about C6, too.  What's going on here?

[GibsonGM]

Here you go, guitarist, pretty good analysis of what's going on there!   

https://www.electrosmash.com/proco-rat

Each pole has a different purpose...

[Sooner Boomer]

The Electrosmash web site is great for breaking down the elements in a pedal's circuit, and explaining it with numbers.

[Guitarist1983]

Thanks, guys.  So if I read ElectroSmash right, the parallel RCs effectively combine to double the roll-off below 60Hz.   -40 dB/decade (-12 dB/octave).   Is that right?   Basically, if I removed the 60Hz RC filter (560ohm, 4.7uF), rolloff would still occur below 60Hz but at  -20 dB/decade as a result of the remaining RC filter (47ohm, 2.2uF) which begins roll-off at 1.5K Hz.  ??  Yes?

[PRR]

It doesn't double the roll off, and who cares what happens below 60Hz?

C5 gives a 6db/oct slope from 1kHz to 100Hz. This alone would be bass-shy. R4 terminates the roll-off, sets gain flat 100Hz to 30Hz. The combination of slopes changes the slope from general 6dB/oct to 4.5dB/oct, a small but musical change.

The C6 R5 and C1 R2 pairs *seem* to only work sub-sonic. I doubt they are critical.

[antonis]

Nice diagram mode, Paul.. 

[Steben]

It doesn't double the roll off, and who cares what happens below 60Hz?

C5 gives a 6db/oct slope from 1kHz to 100Hz. This alone would be bass-shy. R4 terminates the roll-off, sets gain flat 100Hz to 30Hz. The combination of slopes changes the slope from general 6dB/oct to 4.5dB/oct, a small but musical change.

The C6 R5 and C1 R2 pairs *seem* to only work sub-sonic. I doubt they are critical.

A similar yet a bit different function is found in the Bluesbreaker.

[Guitarist1983]

C5 gives a 6db/oct slope from 1kHz to 100Hz. This alone would be bass-shy. R4 terminates the roll-off, sets gain flat 100Hz to 30Hz. The combination of slopes changes the slope from general 6dB/oct to 4.5dB/oct, a small but musical change.

This is interesting; thanks for your input and the chart is great.  Can you point me to an explanation of how the R4 resistor terminates the roll-off and how 100Hz is determined to be the end of the roll-off?

[Digital Larry]

I have a rule of thumb or two for these situations.

a) at "low" frequencies capacitors are "not there"
b) at "high" frequencies capacitors act like a short circuit
c) The "action" frequency for a capacitor is that point at which the capacitor's impedance equals the load impedance on the cap.  That last part can be tricky to figure out sometimes but you'll get the hang of it if you do it a few times.

SPICE or something equivalent is pretty easy to use these days.  Wasn't like that when I was a sprout.