Gain, what it is, what it isn’t?

Discussion in 'Amp Tech Center' started by peteb, Jan 5, 2019.

  1. trouserpress

    trouserpress Tele-Holic

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    Looks like Gain can sometimes clean things up instead of only adding dirt.
     
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  2. peteb

    peteb Friend of Leo's

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    Can we agree that the most meaningful way to measure and compare the gain in two different amps that agrees with our idea of what gain sounds like is to compare the gain in the pre amp?
     
  3. Mexitele Blues

    Mexitele Blues Tele-Meister

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    Does gain even have a sound? It is a unitless numeric value. More gain just equals more volume until clipping occurs, and the point when that happens is a function of the circuit.

    It also isn't meaningful to compare the gain between amps as the value is only relative within the same circuit.
     
  4. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    No way. Cascaded AU's are not going to sound the same as AX's, and an small signal pentode preamp arrangment (which will give you tones of very high gain) is so dependant on the surrounding circuitry that you can get many very different results from them...the level of gain being varyable in several different ways.

    One can not expect to find any meaningful way, let alone a most meaningful way, empirically or subjectively, to compare the gain in two different amps by anything other than by either a vague assessment of the circuitry to assume its potential at extreem settings, or, experientially plugging in, and twiddlin'....Then one could judgmentaly assign the amp to the pigeonhole hight of your choosing in the dovecote hall of gain, knowing that others might lodge it elsewhere.

    Many screaming Marshall stack sounds on recordings are probably just Champs...and in this digital age...who can agree on anything.

     
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  5. Wally

    Wally Telefied Ad Free Member

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    Pete, when I read your query about agreement, I knew a couple of things. You are mellowing out if that search for agreement is an indicator, and you are still on the search for the unobtainable.
    It is still....Happy New Year!!!!! I still have nothing to lose and everything to GAIN!!!!!!
     
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  6. DougM

    DougM Friend of Leo's

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    Brian Wampler has a video where he explains the difference between gain and clipping. I like his technically oriented videos, and find them very educational and entertaining.
     
  7. Old Tele man

    Old Tele man Friend of Leo's

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    DUMMY 101 -- Gain is the "m" variable in the linear equation: Y = m*X + b

    SIGN: Could be positive (+), could be negative (-)
    MAGNITUDE: Could be BIG, could be small.
     
  8. peteb

    peteb Friend of Leo's

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    its understood that there are many variables involved and that is why no one seems to use a number to describe the gain of the entire amp. Gain of sections of amps are much more understood, and that's where I am going. There is no real documented standard other than Vout /Vin, but this does not help us to know where the Vout is measured.





    I refuse to assign any meaning to the gain measured at the speaker for two reasons. the gain increases on its way to the power tube plate and then lowers dramatically at the speaker. How is this going to account for a signal that goes way up at the plate and then way down at the speaker versus up a small amount and down a small amount? It wont. second reason is that the impedance of the speaker greatly affects the size of the voltage on the speaker.


    My idea is to eliminate these variables.





    that's why I conclude that the only meaningful gain measurement meant to describe the performance of an amp is going to be measuring the voltage gain in the preamp, or the preamp and the power amp, but not the OT/speaker.
     
  9. peteb

    peteb Friend of Leo's

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    That is a very nice answer. Thank you




    You are saying that there is no way to quantitatively measure and compare the gain between two amps, or two different amps? Are you sure? I disagree. I understand that there are many variables that make it difficult. My idea is to eliminate as many variables as possible in order to make the best comparison.




    My point is that measuring the preamp gain will get you the best answer that will also give you an idea about the sound or gaininess of the pre amp, which is mostly how the amp sounds. There is the power tubes, but more signal out of the pre amp not only gives you an idea about the pre amp sound but it will also tell you how hard the power tubes are getting driven.


    Example A



    Compare the gain of the 4x10 blackface concert and the blackface pro amp. The preamps are AB763, the power section is the same but one drives an 8 ohm 15 inch speaker and the other drives I assume a 2 ohm load like the other 4x10s.





    The amps will sound close to each other. If the gain were calculated as Vspeaker/Vinput, the pro is going to have a higher gain because Vspeaker is higher for the same power out because it is driving a bigger load




    The gain test should compare similarly in the two amps. If the test measured the signal at the plate of the PI or the grid of the power tubes, the test should compare close, there shouldn’t be much difference.


    If the test compared the signal at the plates of the power tubes, I am not sure if they would test out the same, since one is driving a bigger load. Maybe the different OTs would compensate and they would test the same? I don’t know.





    I’m confident that measuring the gain at the PI plate/input will give just about the best answer possible.






    Then again, nobody tests to compare gain between amps, so it really doesn’t matter.
     
    Last edited: Jan 9, 2019
  10. peteb

    peteb Friend of Leo's

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    Good point Wally.


    Thanks!



    Happy new year!
     
  11. clintj

    clintj Friend of Leo's

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    If you measure the signal at the PI input on a Mesa Dual Rectifier with the channel volumes matched (clean channel plays as loud as the lead channel), guess what? They'll be really close to each other. Must be the same amount of gain in both channels, I guess.

    Sent from my Pixel 2 using Tapatalk
     
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  12. WelshBluesMan

    WelshBluesMan Tele-Meister

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    The low gain amp goes "ding"...

    The high gain amp goes "ARRRRGGHHHHOOOOOOHHHHAAARRGGGRRRHHH!!!!!!!!"
     
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  13. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    This was answered in the first or second reply to your ORIGINAL post!

    You MUST set fixed maximum THD. If either amp exceeds that, it must be dialed back to within that limit or the test is invalid.

    You would measure amplitude at the input jack (port) and at the output jack (port).

    Compare the results from the two amps, and the one with the highest amplitude at the output (assuming equal input amplitude) is the amp with the highest gain.

    THAT is the difference in gain of two different audio amplifiers.

    ANYTHING else is a different subject. Add a preamp level control and master volume and play with them - results are void. different subject.

    Try to measure some type of distortion generated by the preamp - results are void - different subject.

    And so on ad nauseum, because in this thread you have now asked about the same basic definition of amplifier gain twice - and the first time avoided, ignored and/or misunderstood the results.

    THIS time I answered YOUR question without reading any of the (sigh) second group of followup. If it goes the same way I'll probably just ignore the rest of the thread. IMO it was disrespectful of you to ignore the answers to your original question that dealt with it directly.

    It would - IMO - indicate some kind of attention problem, or just a habit of rude behavior, to do it twice in your own thread.
     
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  14. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    No.

    Go back to your post where you stated "let's finish her off" in reply to my request to get this thread back on ANY single track.

    In that thread...well, just read the quote in my post above.

    Your "Can we agree..." thinking brain is off in the Magic Land of Allkazam again....except your wishes don't all come true! (only certain "boomers" will get THAT one!).

    You've gone from one point....to several tangents...back to ONE point....and now are going back to THE SAME tangents again.

    What a stunningly odd, pointless waste of everyone's time - i.e. you asking a question, rejecting and ignoring various results that don't fit your instantly-changing criteria...and eventually asking the SAME thing again and repeating the cycle.

    I'll award this thread the TDPRI "Mobius Strip Thread Award for 2019." And it's only January 9.

    I can't even imagine that could be any challengers.

    o_O
     
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  15. DADGAD

    DADGAD Tele-Afflicted

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    Gain is turning an ant into an elephant. :D
     
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  16. Asmith

    Asmith Friend of Leo's

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    I'm going to have one more swing, as someone else has said gain is simply a number, it's the out voltage divided by the input. It seems your set on moving the final measuring point before the power amp and now we're focused on the preamp/phase inverter.

    You also seem focused on equating the abartairy number that is gain with sound but you can't. The trouble is that the are so many ways and technologies to apply gain to a signal, to name the common ones, triodes, pentodes, opamps, bipolar transistors, JFETs and MOSFETs. Then there are countless different types of these devices which have varying maximum/minimum gain and audio qualities, then they can be attenuated and amplified again like in effects loops, tone stacks and cascading gain stage designs.

    If your listening for a comparison then you have to keep all parameters the same other than the preamp designs. Since your limiting power and distortion and all else is the same that means your limiting the max output of the gain stages by the position of the gain level knob(s) found in the circuit. This in turn should also keep the preamp clean as the power amp would saturate before the preamp.

    So basically it's a clean signal amplified to a set max voltage out to the power amp achieved by any of the previous methods mentioned at a set input voltage as the input would need to be the same for comparison sake. This fixes your gain to a set number regardless of it's preamp design so how is a design high gain or low gain in this thought experiment you have created? And how can we compare the high and low gain sound characteristics when we don't know what type of amp is in play.

    Gain is a 1 dimensional quality and one that you have fixed. There is no way to compare the impact on sound, especially if were looking at gain before it gets out of the amp.
     
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  17. Asmith

    Asmith Friend of Leo's

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    Also a 4x10 is likely to be an 4, 8 or 16 ohm load as you can wire 4 X ohm speakers in a series parallel network to get a total load of X. Also when we hear the sound out of a speaker we don't hear the potential difference we hear the decibel level which is a function of power in these situations not voltage.
     
  18. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    upload_2019-1-10_21-23-1.png

    Here is a guitar amplifier schematic from the Australian "Radio & Hobbies" magazine......as they tell us, this is a fine example of a high gain guitar amp.
    In case you want to find it, it is in the December 1940 issue, page 49.
    upload_2019-1-10_21-30-38.png upload_2019-1-10_21-32-13.png
     
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  19. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    Beautiful example that technically varies the amplitude immediately AT the input port - and again *after* the output tube. The first adjustment is identical to a signal generator's ;level control...or guitar volume control.

    Pete, there's the essential definition of gain in an audio amp (as already explained by several) - the difference in amplitude between the input and output ports.

    It'll have "maximum gain" with the input level control raised to any random level and the output level control raised to maximum headroom (based on whatever level of THD is acceptable for "clean" - say 3%?).. THAT is the amp's gain.

    Until you raise the input level control and that amp exceeds the 3% THD limit and you have to lower the output level control. THAT is the amp's gain.

    Then fiddle with one...or the other...maintaining amplitude that doesn't exceed 3% THD. EACH time the measurement is the amp's gain.

    You can ALSO vary the supply voltage - whoops! the rulebook is tossed.Or change ONE tube that isn't perfectl;y identical in specs to the previous one and pushes an extra volt. Or one less.

    It's a high gain amplifier, and you have your two dials Pete - but there's no actual change of the preamp gain as far as distortion goes, so we don't agree.

    And "amplifier gain" measurements have to be based on a "not to exceed" THD standard.

    Otherwise you are NOT discussing amplifier gain - you might be discussing preamplifier gain. Or power amplifier gain.

    But if you are going to MEASURE ANY type of audio amplification gain you have to set standards somewhere.

    Without any specific baseline standards this will be a neverending thread with no point/purpose - because there can easily be 15 participants using different "standards/
     
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  20. Old Tele man

    Old Tele man Friend of Leo's

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    In this example, "high gain" refers to the high MU values of the pentode pre-amp and output tubes.
     
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