RAT High Pass Filter Question

[GüdPedals]

Hello all,

I'm relatively new to circuits and was hoping someone here could explain the active high pass filter in the Rat's (and any other op amp distortion's) feedback loop. I understand how to calculate the cutoff frequencies, I just don't understand how it constitutes a high pass. My understanding was that a capacitor to ground indicates a low pass, so what is different in this case? I feel like I've seen other topographies where the cap and resistor were reversed and it was still a high pass filter. I've tried to google active high pass filters and I just get the typical passive filter into an op amp explanation.

Any help is greatly appreciated!

https://www.electrosmash.com/images/tech/pro-co-rat/pro-co-rat-clipping-stage.png

[GibsonGM]

Hi Gud, welcome.   You might start here:  https://www.electrosmash.com/proco-rat

Then come back and ask some more questions    If the circuit doesn't branch, the order of passives like caps, resistors is unimportant.  What is important here is where the filters are....in the inverting side of the opamp   

[antonis]

I understand how to calculate the cutoff frequencies, I just don't understand how it constitutes a high pass. My understanding was that a capacitor to ground indicates a low pass, so what is different in this case?

You can face it in some different ways..

One of them is: whatever "normally" works as LPF, say, exhibit inverersed action (HPF) inside a negative feedback loop..
(same stands for C4 where, while it shunts Distortion pot, it forms a LPF toghether with the last..)

To face it more practically:
C5 is set in series with R4 and connects it to GND.. So, total branch value is R4'=R4+X_C5 (where X_C5 is C5 capacitive reactanbce)..
Current flowing through R4' to GND sets NFB gain (1 + Dist/R4').
The more the current through R4' the lower the apparent value of it, so the higher the gain.. But the more current flows when X_C5 value is low..
XC5 is lower for higher frequencies..

P.S.1
Exactly the opposite stands for C4 & 100k pot.. You can face it as C4 "leaks" current out of 100k pot, lowering stage gain..
In the extreme case of X_C4 value reaches zero (infinite frequency) op-amp stage turns into unity gain buffer..

P.S.2
Welcome, also.. 

[Fancy Lime]

Hi Güd and welcome to the forum.

It is as Antonis said. The caps to ground C5 and C6 really are part of LOW PASS filters. But they sit in the negative feedback loop of an opamp. So they make it so that more lows and less highs make it to the negative input. Everything that goes to the negative input is subtracted from the signal. Therefore: Low pass (high cut) in the negative feedback loop means means high pass (low cut) in the output of the opamp.

Hi Gud, welcome.   You might start here:  https://www.electrosmash.com/proco-rat

Then come back and ask some more questions    If the circuit doesn't branch, the order of passives like caps, resistors is unimportant.  What is important here is where the filters are....in the inverting side of the opamp   

Hi Mike. Considering the OP got their circuit image from that very same electrosmash article, I think that link may have been slightly less than helpful It was my first impulse to recommend that as well, though, before I checked the image.


Andy

[iainpunk]

hey, welcome to the forum,

the way i remember it is as follows:
if you take the network in the feedback loop out of the circuit, and just look at the frequency response, all you have to do is flip the response curve upside down.
- if the network cuts a deep cut around the 1kHz region, if you put it in the feedback loop of an opamp, it boosts a lot around 1kHz
- if its a low pass filter (taking off all the high frequencies), it will boost those high frequencies with the same curve,
- if a network boosts the mid range, in the feedback loop it will cut those midrange frequencies
etc.

note that the input of the network is the side that connects to the output of the op amp and the output of the network is the side that connects to the input of the op amp

[GibsonGM]

The article explained it well, though, ha ha.   Good point, Andy.  Intention was for him to do some reading rather than 'picturing'...

Yes, like Iain says, just flip the low pass response to high pass when it's inside the negative feedback loop, and vice versa.  It's a mind game in a way, at its simplest.

[GüdPedals]

Thanks for all the responses!

So, essentially, the output receives the inverse of any filtering done in the negative feedback loop? Actually makes a lot of sense now that I've said it out loud. Funny how that works. 

And yeah, I had read through the electrosmash page, but for whatever reason that part just wasn't sticking. I do appreciate the push to figure it out for myself though!

I'm very much at the stage of "knowing just enough to be dangerous," so to speak. Definitely a fun adventure!

Cheers, folks!

[tsili]

If you are willing to make a clone of Rat, I would suggest you to use the tone control of big muff rather than the original of RAT. In most situation the "filter" knob is pretty useless. Its usable range is a lot smaller (0-3 I would say for metal situations) and the rest of it cuts off a lot of frequencies.

[iainpunk]

If you are willing to make a clone of Rat, I would suggest you to use the tone control of big muff rather than the original of RAT. In most situation the "filter" knob is pretty useless. Its usable range is a lot smaller (0-3 I would say for metal situations) and the rest of it cuts off a lot of frequencies.

nah, i wouldn't use a big muff tone control with a rat, maybe when the values are flipped around and there is a mid hump instead of a mid scoop... but imho, it would sound the best to just use the original rat filter, if you play anything but metal, its a good thing to take out a lot of high frequencies, especially when playing rhythm on low gain settings, like when playing blues, or any open jam session where high gain is frowned upon, you don't want to fill up the mix too much