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Output transformer Harmonics Cancellation ?

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  • #1

    Output transformer Harmonics Cancellation ?

    I've heard that running unbalanced tubes in a push pull amp with a shared cathode resistor emphasizes Even order harmonics in the output signal. How does that get done ? By what mechanism ? Is it in the resistor, the output transformer, or both ?

    Is it a phase cancellation that normally reduces even order harmonics with balanced output tubes, but they become more prominent when the pair is unbalanced, or is it something more esoteric ? (Please explain in laymans terms, but I don't mind a bit of mathematics, and if I don't understand the nomenclature or the varables, I will ask about that as well).

    Does it require the use of a shared cathode resistor, or will an unbalanced pair of output tubes run with seperate cathode resistors do the same thing ?

    Does the cancellation take place only in the output transformer ?

    Thanks for your help ! This is a mystery to me that I have accepted at face value... Up until now that is !
    " Things change, not always for the better. " - Leo_Gnardo
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  • #2
    Originally posted by HaroldBrooks View Post
    Is it a phase cancellation that normally reduces even order harmonics with balanced output tubes, but they become more prominent when the pair is unbalanced
    This is my understanding. It has to do with the gain of the tube, nothing more. Unbalanced gain -> asymmetrical waveform when reproducing a sine wave -> 2nd-order harmonics are created.

    I could be wrong, but I think the stipulation of a shared cathode resistor is simply to eliminate the bias voltage as a factor. If there were separate cathode resistors, then the asymmetry (and 2HD) could be controlled by varying the resistance. Hmmmm. I bet Kevin O'Connor has done exactly that, somewhere, at some time.
    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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    • #3
      Originally posted by eschertron View Post
      ... Unbalanced gain -> asymmetrical waveform when reproducing a sine wave -> 2nd-order harmonics are created...[/I]
      How does an asymmetrical waveform favor and increase 2nd order harmonic amplitudes / creation ?

      That's something I am missing in my understanding, regarding waveform symetry and harmonics, and the propensity to favor one (even order) over the other (odd order).

      Thanks for the help so far !
      Last edited by HaroldBrooks; 11-19-2019, 02:34 PM.
      " Things change, not always for the better. " - Leo_Gnardo

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      • #4
        Originally posted by HaroldBrooks View Post
        How does an asymmetrical waveform favor and increase 2nd order harmonic amplitudes / creation ?

        That's something I am missing in my understanding, regarding waveform symetry and harmonics, and the propensity to favor one (even order) over the other (odd order).

        Thanks for the help so far !
        I did some experiments on waveshapes and harmonics a while back since I was asking myself similar questions and I wanted some better intuition about all of this... Maybe it will help - maybe not - but it's found here... https://sites.google.com/site/string...rm-vs-spectrum
        “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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        • #5
          well, internet lore suggests that a signal through a single preamp tube distorts in a way that adds 2nd harmonics. The mechanism is that one half of the waveform is distorted differently than the other half, typically as the signal approaches cutoff it 'flattens' a bit to become asymmetrical. I took a quick look around to see if there was a graphing calculator or other web resource that showed this behavior, but nothing that could verify that this waveform is rich in 2nd harmonics.
          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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          • #6
            Originally posted by uneumann View Post
            I did some experiments on waveshapes and harmonics a while back since I was asking myself similar questions and I wanted some better intuition about all of this... Maybe it will help - maybe not - but it's found here... https://sites.google.com/site/string...rm-vs-spectrum
            I should also mention I did some experiments on clipping and harmonics produced by the DCCF circuit. The common DCCF is an asymmetric clipper that produces strong even harmonics - however, you can virtually eliminate the 2nd harmonic (if you wish) by forcing the DCCF circuit to clip symmetrically. That makes it sound much more like push-pull output stage clipping (IMO).
            That experiment is described here.... https://sites.google.com/site/string...dccf-harmonics
            Last edited by uneumann; 11-19-2019, 05:46 PM. Reason: clarification
            “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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            • #7
              I seem to remember that asymmetric distortion results in odd and even harmonics, symmetric distortion odd harmonics only.
              I think that the whole ‘even = good, odd = bad’ harmonics concept is nonsense; it’s the higher order harmonics, a product of sharp transitions of the waveform, that can sound harsh, especially as some as dissonant to the fundamental.
              My band:- http://www.youtube.com/user/RedwingBand

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              • #8
                Originally posted by pdf64 View Post
                I seem to remember that asymmetric distortion results in odd and even harmonics, symmetric distortion odd harmonics only.
                I think that the whole ‘even = good, odd = bad’ harmonics concept is nonsense; it’s the higher order harmonics, a product of sharp transitions of the waveform, that can sound harsh, especially as some as dissonant to the fundamental.
                I basically agree about the good/bad attribution and higher dissonant harmonics argument, but if you accept that, you should also see an argument for pursuing harmonic control and variations (IMO). I've come to think that sparsity of harmonics is something that does matter. By eliminating/reducing even harmonics, for example, the result is fewer overall harmonics including the high dissonant harmonics and fewer IM products that result from mixing all those harmonics in later stages.

                Of course good/bad is still in the ear of the listener, but there are ways to control harmonics beyond simply hanging a cap somewhere to filter the highs.
                “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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                • #9
                  I think that the whole ‘even = good, odd = bad’ harmonics concept is nonsense
                  Completely agree.

                  that can sound harsh, especially as some as dissonant to the fundamental.
                  Real harmonics can't be dissonant by definition as they are just octaves of harmonic No.1 = fundamental. Of course intermodulation products of not harmonically related frequencies will be.
                  - Own Opinions Only -

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                  • #10
                    Originally posted by Helmholtz View Post
                    Real harmonics can't be dissonant by definition as they are just octaves of harmonic No.1 = fundamental.
                    I'm not sure about that. I've just looked at a table of note frequencies and it looks like this. 2nd harmonic is an octave, 3rd harmonic is a musical 5th (power chord interval), 4th harmonic is 2 octaves, 5th harmonic is a major 3rd.

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                    • #11
                      Originally posted by Dave H View Post
                      I'm not sure about that. I've just looked at a table of note frequencies and it looks like this. 2nd harmonic is an octave, 3rd harmonic is a musical 5th (power chord interval), 4th harmonic is 2 octaves, 5th harmonic is a major 3rd.
                      Yes. Harmonics are whole number multiples of the fundamental frequency, so if the fundamental freq is defined as 1 cycle per (pick your units), then the harmonics are 2, 3, 4, etc. Ratios of small whole numbers ( 5/4, 3/2, 7/4 for ex) are considered 'harmonious' or are at least identified as musical intervals by the ear; other ratios (17/16, 82/81, etc) are microtones and can lend a 'buzzy' hash to the sound. Think sawtooth waves. And these are the harmonic intervals. IMD is a completely different beast.
                      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

                      • #12
                        Originally posted by Dave H View Post
                        I'm not sure about that. I've just looked at a table of note frequencies and it looks like this. 2nd harmonic is an octave, 3rd harmonic is a musical 5th (power chord interval), 4th harmonic is 2 octaves, 5th harmonic is a major 3rd.
                        Sorry, you're right, only even order harmonics are octaves. Nevertheless I think that the main reason for dissonance is equal temper tuning and intermodulation as well as the fact that e.g. string overtones are not real harmonics but tend to be out-of-tune with increasing order.
                        - Own Opinions Only -

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                        • #13
                          Originally posted by Helmholtz View Post
                          I think that the main reason for dissonance is equal temper tuning (snip)
                          I agree with that as written. In fact, I'd love to discuss the relative merits of equal temperament vs. the temperaments that Bach, et. al., favored; alas that ship sailed over 250 years ago. My loss.

                          string overtones are not real harmonics but tend to be out-of-tune with increasing order.
                          Are you referring to an actual deviation from predicted value due to string properties? Please elaborate.
                          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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                          • #14
                            Originally posted by Helmholtz View Post
                            only even order harmonics are octaves.
                            What note is the 6th harmonic?

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                            • #15
                              Are you referring to an actual deviation from predicted value due to string properties? Please elaborate.
                              I understand that the finite stiffness of the strings causes overtones to increasingly sound sharp with higher order. My main source for this kind of stuff (and much more) is Prof. Zollner's book "Physics of the Electric Guitar" aka "POTEG", for english version please inquire here: https://www.gitec-forum-eng.de/landi...-news/contact/
                              - Own Opinions Only -

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