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What does the filter knob on a Rat actually DO?

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  • What does the filter knob on a Rat actually DO?

    It isn't just a typical tone control wired backwards, is it? I spent 30 minutes googling it but all I got was "it knocks off the highs," "it works just like a tone knob but backwards," and a really technical electrical discussion that went over my head.
    Does it really just knock off highs? If so, why is it wired backwards?

  • #2
    Re: What does the filter knob on a Rat actually DO?

    It bleeds the highs off

    From Electrosmash: https://www.electrosmash.com/proco-rat4. RAT Tone Control.

    The passive Tone Control is a simple low pass filter


    The cut-off frequency can be calculated as:

    fc=12π⋅(RTONE+R7)⋅C8
    fcmin=12π⋅(100K+1.5K)⋅3.3n=475Hz
    fcmax=12π⋅(0+1.5K)⋅3.3n=32KHz

    So the high harmonics above the fc selected by RTONE are attenuated like the graph below:



    The range of the filter is pretty wide, being able to sweep almost all the audio spectrum from 475Hz to 32KHz. The Tone Control combined with all the other filters placed in the pedal will create a mid-hump, to be analyzed in the Frequency Response Section.



    The clipped waveform will be affected by the low pass filter, as shown above, creating a shark fin shape as the tone control is maxed.

    The true art of the RAT is upstream of this nob though, its in the filtering and clipping in the Op Amp stage.
    “For me, when everything goes wrong – that’s when adventure starts.” Yvonne Chouinard

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    • #3
      Re: What does the filter knob on a Rat actually DO?

      To summarize, it's a tone control wired backwards. Technically a variable low pass RC filter which is simpler than a lot of common tone controls but effective.

      Comment


      • #4
        Re: What does the filter knob on a Rat actually DO?

        So everything below a certain frequency is left untouched and the filter knob reduces everything above that point as you turn it?

        Comment


        • #5
          Re: What does the filter knob on a Rat actually DO?

          It probably was wired backwards to get the effect of spreading out the taper of the pot action, they could have done the same thing by using a reverse audio taper (left handed), pot that worked the normal way, possibly reverse audio taper pots were not available in the quantities they needed at that time. This is just conjecture, but I have seen that some RAT clones use a reverse audio pot that works in the normal way, increasing treble as you turn it clockwise.
          Al

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          • #6
            Re: What does the filter knob on a Rat actually DO?

            IIRC the Rat uses reverse taper pots, so apparently they were approaching it from the opposite direction.

            Originally posted by Dave Locher View Post
            So everything below a certain frequency is left untouched and the filter knob reduces everything above that point as you turn it?
            Yes, I think you get it, it passes everything below the frequency the RC filter is set at, which is determined by the filter pot. The frequencies above that frequency are rolled off as shown in the first graph NegativeEase posted.

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            • #7
              Re: What does the filter knob on a Rat actually DO?

              I tried to like the Rat just like V30 speakers.
              Didn't happen.

              Comment


              • #8
                Re: What does the filter knob on a Rat actually DO?

                Originally posted by JMP/HBE View Post
                I tried to like the Rat just like V30 speakers.
                Didn't happen.
                I just got one and I am using it strictly as a solo boost into a crunchy amp and LOVE it. So easy to get a fuzz-style creamy sustain.

                Comment


                • #9
                  Re: What does the filter knob on a Rat actually DO?

                  There is exactly 3 degrees of the filter control that sounds good on your rig. Everything below that is too bright and everything over it is muddy and dark.
                  Oh no.....


                  Oh Yeah!

                  Comment


                  • #10
                    Re: What does the filter knob on a Rat actually DO?

                    Originally posted by PFDarkside View Post
                    There is exactly 3 degrees of the filter control that sounds good on your rig. Everything below that is too bright and everything over it is muddy and dark.
                    That is 100% correct!

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                    • #11
                      Re: What does the filter knob on a Rat actually DO?

                      Originally posted by devastone View Post
                      IIRC the Rat uses reverse taper pots, so apparently they were approaching it from the opposite direction.



                      Yes, I think you get it, it passes everything below the frequency the RC filter is set at, which is determined by the filter pot. The frequencies above that frequency are rolled off as shown in the first graph NegativeEase posted.
                      Why would they use a reverse audio taper pot and wire it backwards, that would be the same as using a regular audio pot?.
                      Al

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                      • #12
                        Re: What does the filter knob on a Rat actually DO?

                        Like I said, IIRC, which I may not, they may be standard log pots, seems like I've seen it as both on different schematics(?). But, all three pots in a Rat are the same, which is a good manufacturing decision.

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                        • #13
                          Re: What does the filter knob on a Rat actually DO?

                          Originally posted by Zombiwoof View Post
                          Why would they use a reverse audio taper pot and wire it backwards, that would be the same as using a regular audio pot?.
                          Al
                          Honestly, in the late 70s, the Boss DS1, MXRs and Tube Screamer dominated the market -They were trying to make something different -experimenting with all kinds of permutations.

                          I'm surprised more stuff in a RAT isn't weird.
                          “For me, when everything goes wrong – that’s when adventure starts.” Yvonne Chouinard

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