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viability of using a fet as a voltage controlled resistor in a HPF

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  • viability of using a fet as a voltage controlled resistor in a HPF

    One of the big obstacles in using an amp/effect designed to have the level of overdrive and clipping controlled predominantly by the guitar's volume pot is trying to achieve the desired frequency response in the non-overdriven low signal levels, throughout the rotation into heavy clipping. Unfortunately, this isn't easily overcome, because the amount of low frequency gain needed for a full, "fat" clean tone can muddy up the overdriven sound, causing the guitar to lack clarity at that level. So, IMO, there's the challenge that the desired bandwidth changes throughout clean to overdriven range.
    Two examples I which illustrate this well are in the Tim/Timmy overdrive pedal designed by Paul Cochran, and the venerable Fuzz Face circuit (see schematics below).
    Timmy overdrive pedal, is a fantastically simple overdrive circuit, which is as good as anything I've played at taking on the characteristics of the guitar, player, and amplifier (Hence, why Danelectro branded it the "Transparent Overdrive", when it copied the exact circuit in their Cool Cat series). The magic lies in that it has a pre-clipping bass control. BUT, the eq doesn't work the way it does in most other ODs. In order to get the most out of the pedal, you have to make adjustments to the bass/treble settings, when you make adjustments to the overdrive level.
    The Fuzz Face.. I never understood Fuzz pedals for the longest time. It seemed every pedal maker had a fuzz pedal (or several), and I didn't get it. Until, my friend was troubleshooting one (Fuzz Face circuit) one day, and handed me the guitar so I could play it while he worked something out. So, I dimmed the guitar volume knob, and played through the heavy, saturated type distortion. Then I just rolled the volume off a little; down to like 7 or 8, or something, and all of a sudden there was this simmering clean tone with a little bit of a bite. That was when I instantly got it. I finally got Fuzzes, and I was in love.
    But, it's not perfect. You can tell that as the volume control is turned down, the loading mismatch of the input causes a heavy loss of bass just when you need it most. The challenge is keeping the guitar from thinning out too much in the stock circuit.
    Now, back to the initial query. One solution, would be a custom type dual pot which would act to cut bass as the volume/gain was turned up. I'd imagine some experimentation would be necessary to find iron out the balance of bass roll off to overdrive setting. But that doesn't solve the problem us Fuzz guys face in the quest to have the perfect everything using one control, right at our fingertips at all times.
    This got me thinking about using a fet as a active level control, using the input voltage as a voltage controlled variable resistor in a high pass filter. I have some basic circuit ideas for how it could be used, but I wanted to run it by some of you heavies and see what you thought?


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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  • #2
    Interesting subject.

    As far as using a JFET as a variable high pass filter, the signal will need to be very small, on the order of a few hundred millivolts peak to peak. There are a few exceptions but they are hard to find and only operate on DC signals.

    An interesting amp is the Trainwreck Express. Basically: Fender two stage Blackface preamp -> High pass filter -> Cold clipper stage -> EL34 power amp. The high pass filter is one place Ken would fine tune each amp by adding caps in parallel to the HP filter. I used that circuit as a blueprint to make a solid state preamp. It was interesting to use a variable high pass filter before the third gain stage. In the end, I just found a rolloff that sounded good and used fixed components. I didn't think it would make a good front panel control.

    Edit: for some reason the forum software changes "{shift} period" to ">". WTF colon left parenthesis ?
    Last edited by loudthud; 01-05-2022, 10:51 AM.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      Originally posted by SoulFetish View Post
      So, I dimmed the guitar volume knob, and played through the heavy, saturated type distortion. ]
      Sorry - I take "dimmed" to mean "rolled down". Do you mean you "dimed" the volume knob??

      cheers,

      Jack Briggs

      sigpic
      www.briggsguitars.com

      forum.briggsguitars.com

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      • #4
        I the US a dime is 10 cents hence the word dimed. Like a dimebag.
        nosaj
        When I die I want to go quietly in my sleep like my grandpa did. Not kicking and screaming like the passengers in his car.

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        • #5
          Originally posted by loudthud View Post
          Interesting subject.

          As far as using a JFET as a variable high pass filter, the signal will need to be very small, on the order of a few hundred millivolts peak to peak. There are a few exceptions but they are hard to find and only operate on DC signals.

          An interesting amp is the Trainwreck Express. Basically: Fender two stage Blackface preamp -> High pass filter -> Cold clipper stage -> EL34 power amp. The high pass filter is one place Ken would fine tune each amp by adding caps in parallel to the HP filter. I used that circuit as a blueprint to make a solid state preamp. It was interesting to use a variable high pass filter before the third gain stage. In the end, I just found a rolloff that sounded good and used fixed components. I didn't think it would make a good front panel control.

          Edit: for some reason the forum software changes "{shift} period" to ">". WTF colon left parenthesis ?
          I think I could make a small signal work. I could tap the voltage off the cathode of an unbypassed gain stage, or even run the input signal to a paralleled source follower. That would probably be ideal anyway, as the idea would be to track the level coming from the guitar.
          Thoughts?

          Also, I use something similar to Fisher's 3rd preamp stage, but use a 100k pot in series with a 68k fixed resistor to vary the HPF.
          If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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          • #6
            Originally posted by jack briggs View Post

            Sorry - I take "dimmed" to mean "rolled down". Do you mean you "dimed" the volume knob??

            cheers,
            by the way, Briggs, you make a fine looking instrument, my friend.
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            If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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            • #7
              Originally posted by jack briggs View Post

              Sorry - I take "dimmed" to mean "rolled down". Do you mean you "dimed" the volume knob??
              I believe that was a typo and he meant dimed in the first post. Dimmed and dimed in guitar/amp world mean roughly the opposite (turned down vs turned up). Probably best to avoid either term for that reason alone.
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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              • #8
                Originally posted by g1 View Post
                I believe that was a typo and he meant dimed in the first post. Dimmed and dimed in guitar/amp world mean roughly the opposite (turned down vs turned up). Probably best to avoid either term for that reason alone.
                yes. sorry, typo. I'm a notoriously bad speller, so if the red squiggly underline doesn't warn me, I'm screwed. In this case, It was simply a mistake caused by fast typing with gloves on.
                I meant "dimed".
                If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                • #9
                  Originally posted by nosaj View Post
                  I the US a dime is 10 cents hence the word dimed. Like a dimebag.
                  nosaj
                  ...but my amplifier goes to 11. That's one bigger, innit?
                  If it still won't get loud enough, it's probably broken. - Steve Conner
                  If the thing works, stop fixing it. - Enzo
                  We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                  MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                  • #10
                    There are solid-state versions of the vintage Magnatone vibrato phase shift circuit. Bjorn Isheden used jfets instead of valves, and BAS70 silicon diodes operating at low current instead of SiC varistors to make the SurfyTrem pedal. The diode introduces a variable resistance with the required V-I curve, although in this case a high level low frequency modulating signal moves the operating point along the V-I curve, and the audio signal is a smaller signal level that interacts around that LF modulated operating point. Perhaps another way to skin the cat.

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                    • #11
                      There are also OTA solutions. For example, the LM13700 or the old CA3080 (you can still find them). They basically use a control current as a gain control.
                      They're great for a harmonic tremolo, oscillators, filters, etc.
                      “If you have integrity, nothing else matters. If you don't have integrity, nothing else matters.”
                      -Alan K. Simpson, U.S. Senator, Wyoming, 1979-97

                      Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

                      https://sites.google.com/site/stringsandfrets/

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                      • #12
                        uneumann, im gonna have to look into those.

                        the other thing is that in order for it to work as intended, it cant just dump bass at peaks of the waveform responding to transients caused by pick attack.
                        so I think the guitar reference voltage should probably be rectified with an RC filter to somewhat smooth out and average the reference voltage for a more stable rds.
                        Yes?
                        If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                        • #13
                          Originally posted by SoulFetish View Post

                          by the way, Briggs, you make a fine looking instrument, my friend.
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                          very kind of you - thanks!

                          cheers,
                          Jack Briggs

                          sigpic
                          www.briggsguitars.com

                          forum.briggsguitars.com

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                          • #14
                            Originally posted by SoulFetish View Post

                            by the way, Briggs, you make a fine looking instrument, my friend.
                            Not to derail the thread, but...

                            I'll second this and add:

                            Do a *oogle search and choose "images" to see past models and custom work. And visit the site for current models. Wow! Real nice stuff.
                            "Take two placebos, works twice as well." Enzo

                            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                            • #15
                              Originally posted by SoulFetish View Post
                              uneumann, im gonna have to look into those.

                              the other thing is that in order for it to work as intended, it cant just dump bass at peaks of the waveform responding to transients caused by pick attack.
                              so I think the guitar reference voltage should probably be rectified with an RC filter to somewhat smooth out and average the reference voltage for a more stable rds.
                              Yes?
                              Yes. Look up envelope followers.
                              If it still won't get loud enough, it's probably broken. - Steve Conner
                              If the thing works, stop fixing it. - Enzo
                              We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                              MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                              Comment

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