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Gain, what it is, what it isn’t?

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

  1. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Suppose that in the era of the PA and in the direction of simplification and standardization, all amps will be 15 W into an 8 ohm load and only 10” eight ohm single speakers will be used.





    In this parallel existence, what is a high gain amp? What is a low gain amp? Or, will there only be one amount of gain used by all amps?
     
  2. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    I would be interested in hearing what people consider a high gain 15 W amp versus a low gain 15 W amp.



    I will give my own ideas but I would like to hear from others first.




    I would specify the configuration of a 15 W amp with playing characteristics that I was looking for, kind of like designing a new amp.


    Still on the table:

    Tube type
    # of tubes
    Class of operation
    Bias
    Phase inversion
    Rectification
    Feedback
    Global feedback
    # of channels
    # of inputs
    Voltages
    Controls
    # of stages
    # of controls
    Effects?
     
    Last edited: Jan 5, 2019
  3. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    FACT -- Gain is ±: positive gain or negative gain, so can be bigger or littler!
     
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  4. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    Amplifier gain is a well established, defined term - an increase in signal between input and output. If you have decided to "fix" the power the increase is between the input and power stage ("negative" gain mentioned above isn't applicable to practical audio circuits except *between* stages, so it's not applicable to the OP).

    I'm not sure what you're trying to "get".
     
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  5. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

    Mar 26, 2014
    Northern Germany
    If we take gain as the ratio of output to input, and the output maximum is set, then we can only obtain a higher gain by making the input smaller.......seems like an odd goal.

    Another way to look at it may be, the ultimate low gain set-up will take a very large input, impart no increasive decibel amplification to it, yet still obtain said set output....so it is no longer an amplifier in the positive sense, infact it could be a mute.
     
    Last edited: Jan 5, 2019
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  6. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    This is the amps only spec


    Output Test:

    15 Watts R.M.S. Into 8 ohms Resistive load ar 5% T.H.D., 1 KHZ. Measured at rated line voltage.
     
  7. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    And if it measures 16 W at 5% thd, then it’s a 16 W amp, and it’s disqualified.
     
  8. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    If it can be bigger or smaller, does it matter?
     
  9. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia


    Are you saying if the overall amount of power is fixed then the overall amount of gain is fixed?


    I agree with that.



    I think you are also saying that if the overall gain is fixed, the balance between pre amp gain and power amp gain is not fixed.


    I agree with that.


    Are you also saying that if the overall gain is fixed, then the ‘gain’ is considered the gain in the pre amp.



    That sounds reasonable.
     
  10. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia


    Good point.


    The output power and I think the output voltage is fixed.



    The amp that can reach the fixed out put power with the smallest signal would be the amp with the highest gain.







    That is not what I was thinking, but you are right.



    I think that’s the best answer.
     
  11. Lynxtrap

    Lynxtrap Tele-Holic

    644
    Feb 29, 2016
    EU
    Often when people talk about high gain amps, they are talking about preamp gain. You could cascade six 12Ax7 stages and send it to your 15W output stage. There will be a massive amount of gain in the circuit, but to no avail if measured between input and output.

    Then again, you could find the most efficient 10" speaker there is and get more volume than the next guy out of the 15 Watts. Technically, it might still not be regarded as more gain though...
     
  12. JL_LI

    JL_LI Tele-Afflicted

    Age:
    68
    May 20, 2017
    Long Island, NY
    OK. Gain is the increase in output from the input signal. If an amp is rated 15 watts at 5% THD, and an increase in gain, say from the preamp, causes an increase in THD, then it is necessary to reduce preamp or power stage gain to get back to 5%. This describes some usable range of clean settings. If you want preamp stage distortion but want to stay at 15 watts, increase the gain control for the preamp and reduce volume, the volume control working on the power stage. If you want a loud distortion tone, use maximum volume and increase gain to taste realizing that such a setting will produce more than 15 watts power. This will require a speaker rated for more than 15 watts. My understanding of amplifiers is limited. Is what I stated above correct? If my understanding is even close to correct, I don’t get the point of the exercise. I’m not trying to be difficult or contrary. Someone please help me understand this better. Thank you.
     
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  13. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Here is my idea of a low gain and high gain 15 watt amp.




    For the low gain one I would start with the blackface Princeton which is 12 Watts.


    Keep the plate voltages low, maybe raise them enough to squeeze 15 clean Watts out of the amp.

    The amp is a good starting point because it is known to stay clean all of the way up the volume dial.

    It would be fine tuned so that right as it reached 10 on the volume, the power would reach 15 Watts.







    For the high gain 15 W amp, and to make it much different than the low gain amp, I would start with the twin reverb using a quad of 6L6 tubes.


    The amp would develop 15 clean Watts with the volume set low, around 2.5 to 3. Once 15 clean Watts we’re reached some part or some parts of the amp would reach their limit and the signal would distort just enough to increase the THD above 5%. What these parts are, I don’t know.


    This amp, like Bendyha’s high gain 15 W amp, would be capable of reaching the test power level with a very small signal.
     
  14. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia


    If you cascaded 6 stages of 12ax7 in the preamp, would it still be below 5% total harmonic distortion, or would it still be clean? How does that work? Its potentially going to generate a high signal voltage. What is this high signal voltage going to do when it hits the power section? Is it still going to be low power out?
     
  15. Lynxtrap

    Lynxtrap Tele-Holic

    644
    Feb 29, 2016
    EU
    I missed the part about a limit for distortion :) It will absolutely not be clean but a total mess, depending on interstage attenuation of course.
    The output stage will still set the limit, so all that gain will have to be "wasted" one way or another.

    And if a distortion free signal is a demand, you'd have to attenuate it by all sorts of means throughout the circuit and it would no longer be a "high gain amp" by anyone's definition, I guess...
     
    Last edited: Jan 5, 2019
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  16. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    That’s a great answer, thanks!




    Just trying pin down and make more tangible what it means when one amp has more gain than the other, all other things being equal.
     
  17. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    I probably added that after you posted.
     
  18. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    This is what I’m thinking. There is a yin and a yang. There is a balance.




    The low gain and the high gain amp could be had by shifting the gain to the output or preamp section of the amp.
     
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  19. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Ok, I think I can take away a tangible point from this thread.



    The power test tests clean power.

    When amps get tested for full power output, their operating point is probably fairly typical. It’s probably hard to get very far from the standard recipe.





    However, the character of an amp or how it sounds is probably more dependent on how the amp sounds at various power levels including full power, irregardless of what the THD is. This more represents the amps full potential.
     
  20. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    NO! Maximum output power may be fixed but gain variable. Read the definition of gain again and if it helps on clarity add "port" to each point of measurement.

    What? What "balance? And I never mentioned preamp gain. I never mentioned a preamp at all. I provided the technical definition of amplifier gain in brief. You seem fixated on "preamp" vs "power amp". But they are not separate items in defining "amplifier gain".

    Nope. I DID NOT USE THE WORD PREAMP. You're making statements about and trying to interpret information not given. Forget "preamp" and "power amp" - UNLESS you want to discuss the gain of each separately. That involves a huge amount of variables and oven overlaps in some designs - subjects for a non-theoretical thread.

    You asked about amplifier gain - not "preamp" anything. Why do you continue to bring up preamps, or "balance"?

    How an amp sounds at various power levels, including full power, is influenced to a HUGE amount by THD. The amp's "full potential" is a non-defined standard. It means different things to different players - but from a design standpoint it's generally maximum RMS, output power at a specific THD limit. Remove THD as a factor and you could have the equivalent of the sound of a garbage truck bouncing down a granite cliff.

    We've discussed this repeatedly. I recommend you do some serious reading about Total Harmonic Distortion, amplifier output, and amplifier specifications. In this thread it seems clear you don't understand it.
     
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