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Why high gain amp use multiple stages but attenuate before driving the next stage?

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  • Why high gain amp use multiple stages but attenuate before driving the next stage?

    Like the tittle, all the higher gain amps be it 3 or 4 stages, output of each stage is followed by resistor divider ( sometimes with high pass caps). So it's like, you amp the signal up, then divide it down, then amp it up again. You can get as saturated output from a 3 stage with less of the divider as a 4 stages. I know it's about the harmonic contents. What is the theory behind this? What is the characteristics of the sound difference? I don't have the best ears, to me, the 3 stages with less divider gives a more grinding Marshall Plexi sound vs more liquid sound of the 4 stages.

  • #2
    Originally posted by Alan0354 View Post
    Like the tittle, all the higher gain amps be it 3 or 4 stages, output of each stage is followed by resistor divider ( sometimes with high pass caps). So it's like, you amp the signal up, then divide it down, then amp it up again. You can get as saturated output from a 3 stage with less of the divider as a 4 stages. I know it's about the harmonic contents. What is the theory behind this? What is the characteristics of the sound difference? I don't have the best ears, to me, the 3 stages with less divider gives a more grinding Marshall Plexi sound vs more liquid sound of the 4 stages.
    It comes down to a number of factors.
    1. One is the order in which the stages break over into overdrive, which changes the dynamic of the sound as a note fades.
    2. One is the amount of overall gain desired.
    3. One is how much tone shaping you want along the way, to change the way the later stages grind up the input signal. Generally you want to trim some bass to prevent "farty" tone....


    HTH!

    C ya,
    Dutch

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    • #3
      Attenuation between stages is good mitigation for blocking distortion What Is "Blocking" Distortion?
      With reasonable settings, all stages other than the 1st will have maybe 60Vac at the plate, and the following grid will enter grid conduction when the instantaneous Vg1 >Vk (eg Vac >~2).
      So given RC coupling, something is needed to avoid them being pushed into cut off.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #4
        ^^^^^^^^ that's a BIG factor.

        Another is that tube stages distort different from SS for the first 10/15 dB , but afterwards they start to plain squarewave and in the end they are exactly the same as transistor clipping.

        So if you like that "intermediate" clipping tubey sound and you want more clipping than just 1 stage can give you, it's better to add more moderate/attenuated stages working close to the "sweet spot" than driving just one "balls to the wall".
        Juan Manuel Fahey

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        • #5
          Originally posted by joe_bfstplk View Post
          It comes down to a number of factors.
          1. One is the order in which the stages break over into overdrive, which changes the dynamic of the sound as a note fades.
          2. One is the amount of overall gain desired.
          3. One is how much tone shaping you want along the way, to change the way the later stages grind up the input signal. Generally you want to trim some bass to prevent "farty" tone....
          I'll add a 4th, which is keeping signal to noise ratio down.
          "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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          • #6
            also because bone headed guitarist often pay per tube (pre and power)

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            • #7
              Alan,
              May I recommend a book purchase - this will answer this and many other questions you may have regarding tube preamp design for Guitar.
              http://www.amazon.com/Designing-Tube...5154481&sr=1-1
              Get the 2nd edition, it has some stuff not covered in teh 1st edition. I have both editions.
              Cheers,
              Ian

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              • #8
                Originally posted by tedmich View Post
                also because bone headed guitarist often pay per tube (pre and power)

                I hope that's a photoshopped April's Fools joke !!!!!!
                Juan Manuel Fahey

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                • #9

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                  • #10
                    Just read a few reviews.
                    Crazy or not, looks like it sounds very good, according to reviews I found, but many claimed Mesa Boogie sued the hell out of it, so they pulled it out of the market.

                    Most ridiculous thing is that MB didn't sue about anything *electronic* but on having built a rack preamp with the word "Quad" on the front panel.
                    These guys think they own the Universe.
                    Juan Manuel Fahey

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                    • #11
                      Well since Carvin got mentioned....

                      ...we must not overlook one important part of this topic, which is the importance of making a design that makes pattern of harmonic distortion interactive with signal envelope in preferred manner. When overdriven, many tube preamp designs don't just statically clip tops off from both negative and positive halfwaves, instead the clipping in each stage is usually deliberately designed quite asymmetric. When the stages are capacitively coupled the asymmetry creates gradual DC bias shifts within the gain stages and the bias shift further on creates varying symmetry/asymmetry when the signal is further on clipped. If the gain stage inverts the signal (like typical tube gain stages) the signal asymmetry also leads to duty cycle modulation. As a result we get harmonic pattern of distortion that interacts and changes with note envelope instead of a "static" harmonic pattern where amplitude of each harmonic just rises or decays according to signal amplitude.

                      Which gets us to Carvin. Their engineers, like many others, discovered this phenomenon rather quickly after the typical "tubes produce even order and transistors odd order distortion" idea had been proven blatantly incorrect. The following image portrays operating principle of Carvin's tube emulator, which emulates the behavior of few AC coupled triode gain stages one could typically find from a tube guitar preamp.

                      The block diagram illlustrates the overall idea of interactively varying the signal asymmetry rather well. And as said it applies to tube amps too, in fact for solid-state designers this is an important aspect to mimick from tube amps if they wish to mimic some of that "tubey" touch sensitivity. Naturally the designer may wish to tweak relative clipping levels of each stage and this is done by adjusting either stage gain, attenuation or both. If you take a deeper look you likely notice that generally speaking gain stages in tube guitar preamps don't always have same component values (deliberately) and both gain altering and attenuation takes place within the circuit, whether in obvious forms like with resistive dividers or in less obvious froms like with various filters. Here lies a major part of "gain staging", which hugely affects the overall response and tone of the amp. The other major part... well it's in those "various filters" but that's another story. ;-)

                      The Quad-X's "cascade" design was sort of "culmination point" of the whole idea: How ridiculously many gain stages one can employ to create some subjectively idealistic interactive harmonic pattern to distortion. If you heed that it was John Murphy's design team that created -both- the aforementioned tube emulation circuit and the Quad-X amp the latter design may seem to gain some slight sense. As said, it was the culmination point of the overall design idea and yes, you don't generally need that many gain stages to achieve certain design goals (like modern hi-gain tone, etc.). But this is simply one of those things where I guess "artistic" part of guitar amp's design process simply shows its head more blatantly. I have to admit, there is definitely some idea behind all that.

                      In the end the Quad-X also showed that many isn't always significantly different from less. The resulting tone of Quad-X wasn't remarkably different from other similar hi-gain amps, at least not any more different than hi-gain amps are from each other in the first place.

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                      • #12
                        Fully agree
                        John Murphy is an unsung design hero, way more advanced than others who just endlessly tweak the same old designs.
                        He's so naïve that he didn't even patent the "variable duty cycle with signal level" circuit, which then was used to death by another unjustly despised amp: Crate.
                        It sounds so good that very heavy Metal players (think Doom style and such) prefer Solid State amps for their sound, if possible Ampeg VH140 and if not, Crate GH130 (which is roughly the same, and both come from the same factory )

                        I remember old Guitar Player reviews (by Craig Anderton, none the less) about the first Carvin SX amps (SS incorporating the distortion shown above by Teemu) where they commented: "it sounds like a Tube amp, only more of it, has all the best of Tube amps and the best of SS ones" .
                        Of course then the Tube Retro craze started and fully swamped it.
                        Another great designer which got swamped by it was Pearce, of Pearce and Gibson L series amps fame.
                        Juan Manuel Fahey

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                        • #13
                          Originally posted by pdf64 View Post
                          Attenuation between stages is good mitigation for blocking distortion What Is "Blocking" Distortion?
                          With reasonable settings, all stages other than the 1st will have maybe 60Vac at the plate, and the following grid will enter grid conduction when the instantaneous Vg1 >Vk (eg Vac >~2).
                          So given RC coupling, something is needed to avoid them being pushed into cut off.
                          I have seen blocking distortion in the preamp section when the input is trying to swing above the 0 grid of the next stage. The coupling cap got charged up and push the tube in the next stage to class AB or even class B and it sounds like cracking. But this can easily be fixed and a lot of amp already have it. It's the grid leak resistor. If you put a 100K grid leak resistor that pretty much reduce the problem to inaudible.

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                          • #14
                            'If you put a 100K grid leak resistor that pretty much reduce the problem to inaudible'
                            Yes, definitely that's one way of dealing with it.
                            But might it be seen as a form of implementing interstage attenuation, using the output impedance of the source stage as 'R1'?
                            My band:- http://www.youtube.com/user/RedwingBand

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                            • #15
                              Originally posted by pdf64 View Post
                              But might it be seen as a form of implementing interstage attenuation, using the output impedance of the source stage as 'R1'?
                              No doubt it is.

                              Every different method of getting around the block has its own advantages and disadvantages. Lowballing the grid leak resistor has a nice low parts count, but there's got to be a reason you don't see it in virtually any overdrive circuit. The reason could be nobody's tried that yet, or that they have, and found it sounds like doo-doo....

                              C ya,
                              Dutch

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