Understanding the Evil Duo: Gain Knobs and Master Volume

Discussion in 'Amps and Cabs' started by epluribus, Feb 12, 2009.

  1. epluribus

    epluribus Member

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    I originally posted this on another thread, which is well worth the read for all the other posts, but IMHO I see these concepts so often misunderstood on TGP that I thought it might be best to catalog it under its own heading.

    (Caution: Big subject, so I did some generalizing here and there. Plz feel free to QC my tech stuff accordingly, I always appreciate it. I'm a ton more rookie than this post might suggest. :beer ...or maybe not. :rolleyes:)

    Additional remarks and links follow below, but here's the post:

    **********


    Gain is not dirt. Saturation isn't just dirt, it's much more. Distortion means the signal comin' out doesn't look like the signal that went in. Volume isn't just for volume, and Master Volume controls control Saturation, not just volume.

    What?...

    For starters, you need to define Gain if you're going to control for it as an amp user. Very simply, it's how much the signal strength grows by having passed through an amplifying device like a tube or transistor. Gain is not, however the same as distortion, saturation, drive, or any other popular industry misnomers.

    Each place you take the signal and amplify it is called a Gain Stage. So that's each one of your amplifying tubes or transistors.

    If you push any gain stage hard enough, it will distort simply because it runs out of capacity to accurately amplify. Distortion, which we more commonly refer to as Saturation, can take many forms, including clipping (aka dirt), altered frequency response, or a variety of other rather subtle effects like Phase Shift. The king daddy, btw, is added harmonics--the secret of power chords.

    The takeaway here is that these are all different phenomena, and the names in proper use are not interchangeable. Trouble is, in the gear biz, we do just that all the time. But for the intelligent knob twister, knowing the difference is what puts you in command of your sound instead of in the back seat.

    In an amp, Gain, Volume, and Master Volume knobs simply control the output of each gain stage as the signal gets passed along. Yup, all three are known as attenuators, and all three do the same thing--it's the circuit around the attenuators that makes them different. Now we find out how...

    With complex amps, it helps to draw yourself a picture so you can see where each gain stage is, and where each attenuator (aka knob) is in the circuit path. It's also extremely handy to note where your tone knobs are, since they're attenuators too. So just f'rinstance, here's a diagram of my Deluxe Reverb II, a rather complex little chameleon of a beast, and one not particularly inclined to suffer fools...

    [​IMG]

    So let's see what your signal does and where you can control how the amp saturates.

    You start by going into the first gain stage (G1) straight from your guitar. (No pedals yet.) Guitar signals are not strong enough to drive a tube gain stage into saturation--not even close. So all G1 does is give your signal a nice clean boost.

    At this point, if all you did was feed that straight into G2, your second gain stage would still be just shy of max output, and hence just shy of starting to distort. Still "clean" undistorted sound. But your signal won't get anywhere close to maxing G2 out because it's going to get reduced by several attenuators...

    ...Those attenuators being the Tone and Volume knobs. Those typically take a lot of strength out of your signal. The weakened signal isn't really powerful enough to drive G2 into saturation either. Hmp. Still clean...

    But if G2 could feed yet another gain stage, G2's signal is easily strong enough to push another gain stage well into saturation...so that's just what Paul Rivera soldered in here for us--a third gain stage. Now if you smacked G3 with a full strength signal hot off G2, this thing would put out dirt for days, plus a ton of compression, a drastically altered freq response curve, lots of various harmonics, and several other things. Cool when you want it, but you don't always want it.

    So Paul gave us a way to control how much signal G3 gets, and how much G3 saturates--another attenuator. And just to confuse everybody, he called it a Gain knob. As you might expect, you can drive G3 into dirt oblivion with this knob, but there's a very important notion you need to think about. That wicked strong signal is goin' right on into everything else left in the circuit path, and it's easily capable of driving every single remaining gain stage into saturation as well. More on that in a minute.

    What's left? At this point we hit the Phase Inverter, which makes the peaks and troughs identical for the power tubes and prevents phase cancellation. But the PI in modern amps is also a very capable gain stage in its own right. Which means that you can take that hot signal from G3 and saturate the PI very nicely. AAMOF, even a fraction of the signal from G3 will cause saturation in the PI, so what does Rivera do to give us some control? Solders in another attenuator of course. This time, again to confuse us, he calls it a Master Volume.

    Lotta people get seriously derailed here, and end up despising Master Volumes cuz they "suck tone." Trouble is, a lot of folks don't understand that what MV really does is control how much saturation occurs in the Phase Inverter and the Power Tubes. (See diagram.) Turning the MV down doesn't just turn down the volume--it actually takes the PI and the Power Tubes right out of saturation. So your dirt sounds different cuz it only comes from the preamp tubes in G1-G3, and you don't get that legendary power section (PI and Power Tube) flavor. You also lose all those cool power section harmonics, the unique brand of compression you'd normally get from the power section, along with the changes in the freq respone curve, and all those subtle saturation effects like Phase Shift. So, yeah, trying to use an MV to simply turn the volume down is a gi-mungous tone suck. The big moral here is: don't use your MV for a volume control. Use it to control for saturation.

    Whew! At this point you know that dirt, compression, EQ coloration, and lots of other useful things happen when you have a strong enough signal to push a gain stage into saturation. Nice that you also know that gain simply means a stronger signal, not distortion or saturation. And armed with all that, you can look at the diagram and see how each knob lets you control for saturation in each gain stage. So now we've got one more step to Nirvana--cascading effects...

    As you revisit the diagram, you'll note that whatever you do to signal strength flows downstream to all the gain stages that come after it. You may have also noted that the tone knobs not only control signal strength and saturation, they control which frequencies saturate. Finally, you may now have noticed that if you turn one knob up early in the circuit, you can still run the later gain stages as clean as you want, because you can turn all the other knobs down.

    So in the diagram you can see that you have enough control to make almost any single gain stage in the entire amp either clean or dirty, and exactly how clean or dirty. You can blast a single stage or balance/distribute moderate distortion between every stage in the circuit. You like power section harmonics better than the preamp flavor for your power chords? No problem, you build exactly the harmonics you want. In other words, you have gigantic tone shaping power--and that's why audio engineers are so hot on this holy-grail subject...Gain Structure.

    So go back to your amp and draw yourself a diagram like mine. Think about which gain stages are clean or dirty as you dial your controls. Distribute the saturation in all the different ways you can think of.

    Bet you never dreamed you had so many cool amps hidin' in that one little box.

    --Ray

    ps...now to really work this circuit...draw yourself a boost pedal in front of the amp. That's right, another gain stage. Not a dirt pedal, just a clean boost. Remember how G1 and G2 always run clean, and you have only G3, the PI, and the Power Tubes makin' dirt? Not any more... :dude

    Happy tweakin'!

    ***********

    Hokay, and now for the Sources and links for Further Reading... (asleep yet? :))

    What is Gain?

    Power Tube vs. Preamp Tube Distortion

    In Pursuit of The Real Deal on MV

    Types of MV

    Gain Structure in Hi-Gain Amps


    Some related subjects:

    Do Small Amps Really Sound Different? Part I

    Do Small Amps Really Sound Different? Part II

    Do Small Amps Really Sound Different? Part III

    A Word On Amp Myths--Controversial

    Are Amp Tone Controls Worth It?

    Whither FX Loops

    The Real Deal On Swirl...Almost

    "Bad" Distortion: Ghost Notes


    Oh, and one link not from TGP...tons of good info in a small space, and an excellent glossary, all courtesty of TGPs own Randall Aiken...

    Aiken Amplification's Tech Pages.


    HTH, and happy pickin'! :beer

    --Ray

    Big PS...Huge thanks to the massive list of people who post here and have contributed to this entire body of knowledge. I heard it here first. :)

    (Search tag: Jabberwoky)
     
    Last edited: Jul 11, 2011
  2. garyrogue

    garyrogue Gold Supporting Member

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    Thanks for the informative post. Hope I can put it to use, time to lay down and digest.
     
  3. sqadan

    sqadan Member

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    Lots to digest... but very informative!

    Bravo sir.... This makes things much clearer. :AOK
     
  4. andrekp

    andrekp Member

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    Nice post, but you are also just confusing the issue further. The control you label as the "master volume" then note that it is improperly blamed for sucking tone, is NOT what is actually blamed for sucking tone by non-MV amp advocates. In a non-MV amp, as I presume you know, what you label here as the MV, is really just the Volume on a non-MV amp. So what I, as a non-MV advocate say sucks tone on a MV amp is NOT the MV in your drawing, but the OTHER volume/gain controls. For a non-MV amp advocate, the only volume you WANT is the one you label the MV.

    It may be that for MV amp fans, that incorrectly using a MV can make a MV amp suck tone, but it is incorrect to say that people who say MV amps suck tone, say so for the reason you specify, and to say that people who don't like MV amps improperly blame what you label the MV for sucking tone is either incorrect, confusing, or misleading.
     
  5. stratzrus

    stratzrus Silver Supporting Member

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    Great post Ray!

    It gives you something to think about instead of just grabbing the knobs and twisting away.
     
  6. ~Abstract~

    ~Abstract~ Member

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    Where did you get the picture?

    I'd love to see some of the grail amps topology. SLO and such...


    thanks for the write.
     
  7. Tone_Terrific

    Tone_Terrific Member

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    Since we're all critics here-
    Separating the term gain from distortion is accurate. Using the term saturation for distortion (?)... needs a better definition.

    Non-mv amps don't have the MV where you have it in your diagram, of course, and increasng the drive of the power stage on a non-mv amp always invlolves adding volume (and power) to the output.

    Overdriving any component, including clipping diodes, produces distortion and tubes are inherently not quite linear so the lines of distortion/saturation/compression at various frequencies are blurry, at least, to me.

    Clipping the signal with drive to the first stage is possible, but may not sound good, either. Lots of
    variables, eh?

    BTW MV got a bad rep from back in the 70's when they were simply added to a lot Fender amps to allow preamp od. Rather unrefined and undeveloped, resulting in thin, clippy od tones. The pesence of the mv, even on full, in that circuit, seemed to kill a bit of the clean tone, too, so it was a lose/lose situation.
     
    Last edited: Feb 12, 2009
  8. Jay Mitchell

    Jay Mitchell Member

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    No. In a typical non-MV amp (e.g., classic Fender, Vox, and Marshall designs), the volume control is placed in the position labeled "V" in the graph in the OP.

    Check this for yourself. Here are some sample schematics:
    http://schematicheaven.com/fenderamps/deluxe_reverb_aa763_schem.pdf
    http://schematicheaven.com/voxamps/ac301960.pdf
    http://schematicheaven.com/marshallamps/jmp_super_bass_100w_1992.pdf

    And you are mistaken.

    Oops. Then all the classic non-MV amps suck tone. Bummer.:huh

    Seriously, you are the one who is confusing the issue here. The OP is correct.
     
  9. epluribus

    epluribus Member

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    Umm...maybe. Trouble is, MV is confusing, and for the reasons you point out. Actually, MV comes in two flavors, pre-PI and post-PI. Worse, some mfgrs call an attenuator in that position Volume instead of MV, and also the other way around.

    Try the links to the MV threads. Several very respected designers contributed to those and do a nice job of addressing this very question.

    A distinction well worth bringing up, esp for guys with gear labelled differently than the DR II. 'Preciate it!

    --Ray

    BTW, Jay Mitchell, thanks for the links to the schemos. Nuthin' like lookin' under the hood to be sure we know what we're talking about.

    Oh yeah, and just for the record...I'm not pickin' on anybody's taste in circuits. If anyone dislikes MV, then so be it, it's a big world and there's room for everybody in the pool. All I'm saying, however, is that if you don't like something, it can be useful to know exactly what it is you don't like, and why. I 'spect we'd save a lot of GAS that way. :)
     
    Last edited: Feb 12, 2009
  10. epluribus

    epluribus Member

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    Tell ya what Stratz, the DR II was one frustrating varmint till I finally sat down and drew myself a picture. Now the doggone thing's a little tone laboratory.

    Glad you like the post--kind of a digest of all the stuff people told me that helped me out. Happy to pass it on.

    --Ray
     
  11. epluribus

    epluribus Member

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    The pic was done in an old copy of CorelDraw, and painstakingly so I might add. (aka, yours truly has ten thumbs when it comes to that stuff. :))

    As for the amps down in the fancy seats, I agree, I'd love to see some schemos there as well. As for the SLO, I did this very thing with it too, and then perf-boarded the preamp circuit in what used to be a poor little '66 Kalamazoo Model II. Worked good--hilarious! Also very educational--learned a bunch about hi-gain gain structure. Some day I gotta cook up my own flame-thrower circuit and solder that poor little amp solid once and for all. :rolleyes:

    --Ray

    BTW, it's been a while, but IIRC that Soldano schemo has an error in the Crunch/Clean switch. I never bothered to build the switch anyway, so I can't say for sure how it should look. Well...I suppose it should look like Angelina Jolie if it was my circuit, but...
     
  12. ~Abstract~

    ~Abstract~ Member

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    thanks!
     
  13. epluribus

    epluribus Member

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    Wow, great observations. These are a few of the compromises that went through my head when I wrote this. And as you dig into the way the circuit actually responds to the knobs and the guitar, they get pretty important if you really want the goods.

    Saturation vs. distortion. +1, this is such a useful subject that IMHO it needs not just a better definition, but an entire thread. However, how 'bout if I try a starter definition?

    Saturation is technically reached when a gain stage has reached the limit of its output and can deliver no more. The parts of the signal just below full saturation make it through (more or less, till we get into distortion) unscathed, and the parts beyond saturation get clipped and all kinds of other things. In tubes, saturation is depicted on the characteristic curves as a line beyond which further amplification isn't possible. However, a lot of distortion and clipping occur well before the tube reaches saturation...

    [​IMG]

    The red line is a load line drawn over the graph. The blue circle identifies the point of saturation on the load line. The green dot is one of many places you could set your bias--it's known as an Operating Point. On the low end of the load line the tube enters Cutoff, btw. How do we know how to plot the load line? Determined by the builder's choice of Load Resistor and Plate Voltage.

    Distortion, OTOH, indeed has very blurry boundaries, and in tubes many types of distortion begin almost at the lowest signal levels. The ones that come to mind are harmonics and freq response to name two. I'm hopin' somebody jumps in on some of the more esoteric types of distortion. What we usually refer to as distortion...dirt...is clipping that doesn't occur until much nearer saturation than some of the other types of distortion. Ultimately, distortion is any amplification behavior that doesn't represent a straight-line extrapolation of the initial signal--popularly known as Non-Linearity.

    Good start or am I taking an oblique approach to the idea? Not useful?

    Yeah, might not have made that clear. MV has two functions, the obvious one being volume--and it does indeed very effectively control volume. The other function, however, is to control the extent to which the downstream components distort. Better distinction?

    Very much agreed. BTW, on the Hi-Gain Design Theory thread, this is exactly the question we were getting into. I'd love to see it explored more--at least so I can understand it better and design for it better.

    Agreed. I do indeed love to use clean boost on my amps, but only sparingly. Easily overused. Better to build a purpose-built gain stage to make it sing right. But then you just replace V1 with another V1 don't you? :) Incidentally, lacking an O-Scope, I presume clean boost doesn't cause much distortion in the first gain stage (G1 in the pic) if you have the volume knob up appreciably--most of the distortion you get from a clean boost occurs in the second gain stage. But if you turn the volume or tone knobs down sufficiently, it's entirely possible to blister G1 and still have G2 running pretty much clean. Choices indeed. :)

    So I understand, 'preciate the brief history. John Phillips and I got into a discussion of proper ways to design MV, and not all mfgrs. do it. Properly done, when the MV is wide open it electrically disappears from the circuit entirely, and any impact is inaudible.

    Anyhoo, the above isn't meant as an "answer" or a rebuttal, it's more just a provisional statement of the issues as I understand them thus far, esp in light of the issues you bring up so well. Better and more useful discussions than mine are definitely welcome. Thanks TT!

    --Ray
     
    Last edited: Feb 12, 2009
  14. ~Abstract~

    ~Abstract~ Member

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    Then what's transformer saturation?
     
  15. epluribus

    epluribus Member

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    Hooooh, good one! I stayed out of this subject for the purposes of this post. However, in a power-limited amp like the DR II, part of the above diagram is aimed at allowing you to picture how you're choosing to use your available power. You can't twist all the knobs at once on a DRII and expect it to stay focussed.

    For starters, it depends on which transformer, the PT or the OT.

    PTs and rectifiers generally do things like sag and make the low end flabby...like in the DR II and Deluxes of all stripes. (My infamous list of links is in Post #51. Got coffee? :))

    OTs do other things, most notably high and low-frequency rolloff at high output.

    This subject is a terrific one and definitely is worth another thread. Also, it's been talked about around here a lot already, so I'll make a request...How 'bout somebody posting links to good transformer behavior threads? I for one want to learn a lot more about it.

    --Ray
     
  16. rockon1

    rockon1 Member

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    To each thier own. To some NMV amps are the only way to go. To me MV amps are the only way to go. Simple.Bob
     
    Last edited: Feb 12, 2009
  17. stratzrus

    stratzrus Silver Supporting Member

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    You've really got me thinking now.

    My Rivera KHR 100 can be really frustrating sometimes, yet at other times it sounds great. Your post made me realize that I haven't really understood or mastered the interaction of the controls yet in order to get exactly what I want out of it. I was largely just twisting knobs until it sounded good (not blindly, but you know what I mean).

    Interesting that they are both Rivera designs as well...a little care in setting it up can go a long way.
     
  18. epluribus

    epluribus Member

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    What's really eye-opening is to play a Rivera alongside a II-series Fender. I tell people to visit Paul's site and listen to his clips to get a feel for how he hears tone and designs for it. Lotta great DNA bein' shared there.

    On the good-day/bad-day thing, the DRII is also like that. I cured most of that by running it through a computer UPS power supply with a voltage regulator. They put out really steady voltage by virtue of those big batteries. My theory is that you can put such a delicate balance in the gain structure in these amps that a routine change in wall voltage throws the whole thing off. Anyhoo, go steal the SO's power supply and run your amp with it, and see how things work out. :)

    But anyhoo...quite the balancing act isn't it? And we haven't even talked about the impact of hot pups on a carefully balanced hi-gain amp...
     
  19. epluribus

    epluribus Member

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    You too huh? :)
     
  20. LSchefman

    LSchefman Supporting Member

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    So much good info here.

    And it explains why a few minutes in a store with an amp isn't going to give you the opportunity to know all that much about its capability (though often you can weed out the ones you don't like at ALL quickly).

    I've always said that I have to spend a few months with an amp to really have a solid feel for what it can do.
     

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