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Amplifier harmonics and load stability

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  • #16
    That's a more interesting comparison. May I suggest that you totally ignore (don't even read) any of the words and just concentrate on the graphs. Don't look-up the price or look at the cosmetics. Just look at the graphs.

    Disclaimer: I'm sure that both of these amps have a market and satisfy their users. They are both likely to be technically a lot better than even the best loudspeaker. It is not our intention to criticise either brand.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

    Comment


    • #17
      Again, without looking at any pictures, without reading a single word of the review, trying not to see the brand names or models (not that they would mean anything to me), without being aware of the prices, without knowing anything about the technology (tube? transistor?), solely looking at the published graphs and taking them at face value ..... if I was in the market for an amplifier I would have to select amp C in preference to either amp A or B. That is, unless A or B was much cheaper and I was on a tight budget.

      Can you see why from the curves?

      I'm ignoring such unknowns as brand after-care back-up, brand image, country of origin etc. etc.. Just considering the narrow technical aspects, which would be a rather crude way of making a selection and certainly doesn't include a listening test which could, conceivably be decisive.

      Amp A

      Amp B

      Amp C
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • #18
        Why 100W vs 10W vs 1W???

        Originally posted by A.S. View Post
        ....... if I was in the market for an amplifier I would have to select amp C in preference to either amp A or B. That is, unless A or B was much cheaper and I was on a tight budget.

        Can you see why from the curves?..
        I am not sure if I can make anything out of the graphs.

        distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 4, 8 ohms.

        Vs

        distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 8, 4, 2 ohms.

        Vs

        distortion (%) vs 1kHz continuous output power into (from bottom to top at 1W): 8, 4 ohms


        And Amp C is a power amplifier whereas Amp A and B are Integrated Amps.

        ST

        Comment


        • #19
          Interpreting graphs esp. load sensitivity

          Originally posted by STHLS5 View Post
          I am not sure if I can make anything out of the graphs.
          Imo, think the graphs you are referring to tell quite a bit, regarding how well an amp is designed, the philosophy and discipline. The graphs show distortion levels as more power is required from it, where it peaks out (the “notch” -normally should correlate relatively closely with rated standard specs). It might also tell how much noise there is to begin with. In other words, we can observe where distortion of an amp. begins on the scale, how it travels and starts to deteriorate (close to rated claims).

          Think this thread started on “how load dependent an amp is”. If im not mistaken, the first graphs on any of those 3 pages with the “simulated loudspeaker load” line (the most wavy one) answers the question most well. Some amps have a much more “wavy” line than others.
          But Alan does put it into a beautiful perspective. The ugliness of an amplifier is by far more liveable than the slightest blemish of a speaker.

          Comment


          • #20
            Originally posted by kittykat View Post
            The ugliness of an amplifier is by far more liveable than the slightest blemish of a speaker.
            I am sorry, I do not understand what you mean by this. Could you (re)paraphrase?

            Comment


            • #21
              Meaningless comparisons

              Originally posted by kittykat View Post
              Imo, think the graphs you are referring to tell quite a bit, regarding how well an amp is designed, the philosophy and discipline. The graphs show distortion levels as more power is required from it, where it peaks out (the “notch” -normally should correlate relatively closely with rated standard specs). It might also tell how much noise there is to begin with. In other words, we can observe where distortion of an amp. begins on the scale, how it travels and starts to deteriorate (close to rated ........
              I still don't get you. If you look at THD+N graphs it gave some sort of curves. Why*were the measurements taken using different values? Why not all the amplifier measured using 100W? Why would the magazine use 100W to measure THD +N in one amplifier and a mere 1W in another amplifier? Naturally, the one with 1W looks neat. Whether that's relevant or not is beyond my limited knowledge.

              In another forum, someone posted a graph promoting a certain product. On closer look, I noticed that the measurement for the selected product was given for every 100Hz for certain frequency range and in another graph for another product it was given for every 10Hz ( or something like that). Expectedly, the one with 10 Hz increments showed a wavy plot.

              As a consumer, I want an apple to apple comparison.

              ST

              Comment


              • #22
                Closer look at the curves ...

                Now we have some discussion about this you've motivated me to have a look at the curves in more detail.

                There's not much we can learn from the basic frequency response curves. These amps all look great in that respect; far flatter than our ears and certainly as good as they need to be. But you're right: the (perhaps) tell-tale distortion plots are taken at different power levels, and as we know that amplifiers (just like speakers) exhibit more distortion the harder they are working (the more power they are generating). As you point out, we are indeed not comparing apples with apples. Not remotely so. Why?

                It seems that now I've scanned the specs that amp C is by far the most powerful. Judging solely from the specs, the technical performance is really examplary: it was evidently designed by a master craftsman who had many tricks available to him to drive distortion down. There is a generation or more of technical skill revealed by these figures. It is not a amateur design. It certainly would have involved a very careful, intuitive positioning of all the components on the pcb, mm by mm consideration of the circuit board track layout, the dressing of the cable loom - let alone the actual circuit design and component values. It really is an exceptional piece of engineering. I have not read the review so I do not know how it was subjectively rated but I'd be very happy indeed to own one, based on reading the graphs alone.

                Picking amp A (the first letter of the alphabet, I could have picked B) informs us that the distortion traces were, as noted, made at a much lower power of only 10W. I can only imagine that this low level was being kind to the amp, and that had it been worked harder, the distortion would have risen, perhaps dramatically. Perhaps there was some discussion between SP and the makers as to what power level represented a typical in-use level, relative to the maximum power available from the design. That would be an entirely reasonable approch: you wouldn't normally measure a cars 100-120mph acceleration; the 30-60mph figure would be much more relavant for the ordinary user in town.

                But the distortion curves are indicative of two very different design approaches, much more than just a comparison of two very different power output capabilities. Let's look at just one type of measurement: the THD + Noise graph. THD is Total Harmonic Distortion, and in the old days using an analogue moving coil meter, some simple circuit would add together the total of all the harmonics and arrive at a number. For example, ignoring the single, precision fundamental test tone, if there was some second harmonic distortion generated in the amp + some third + some fourth + some fifth ...... the final amount would be lumped together into one nice convenient number. Say, "1.5% THD @ 10W into 4 ohms, 50Hz - 10kHz" or something like that. What that didn't tell us at all was how much of each harmonic was present in the 1.5%. And that really matters because odd harmonics like 3rd, 5th, 7th ... sound horrible when excessive to the ear, whereas even order harmonics can actually sound very pleasant. Yes, true: they can warm-up the sound as musical instrument designers have known for centuries.

                Then along came the computer based audio spectrum analyser of the type used to make these graphs and we can clearly see all the harmonics laid out from lowest frequency (the fundamental) to the highest across the audio band, left to right. So we can pick-out visually the 2nd, 3rd and so on. Luckily for us, the fundamental tone injected into this amp has been scaled to use the full vertical height of the graph, and peaks at exactly 0dB. That's perfect because we can now easily read-off the individual harmonics and their contribution will be correctly scaled relative to the stimulus. So, looking along the horizontal line, we can see that the 2nd harmonic of the test signal (100Hz) is a little smaller than the 3rd harmonic, which I read as being at about '-70dB relative to the fundamental'. Make sense?

                Very important point! The test stimulus was a high precision, technically perfect, absolutely pure, computer generated 50Hz sine wave. It contained no harmonics at all*. So all those harmonics you see spread from left to right at multiples of the 50Hz were generated inside the amplifier alone. They should not be there at all. They will add some character to the music.

                *The AP test set used is absolutely state-of-the-art. It's purity of tone generation would produce (a few, tiny, irrelevant) harmonics that are below the bottom of the vertical graph scale. We can safely say that what we see here is the amp alone, not the amp + test set together.

                >
                Attached Files
                Alan A. Shaw
                Designer, owner
                Harbeth Audio UK

                Comment


                • #23
                  Harmonics

                  Originally posted by A.S. View Post
                  .... For example, ignoring the single, precision fundamental test tone, if there was some second harmonic distortion generated in the amp + some third + some fourth + some fifth ...... the final amount would be lumped together into one nice convenient number. Say, "1.5% THD @ 10W into 4 ohms, 50Hz - 10kHz" or something like that. What that didn't tell us at all was how much of each harmonic was present in the 1.5%. And that really matters because odd harmonics like 3rd, 5th, 7th ... sound horrible when excessive to the ear, whereas even order harmonics can actually sound very pleasant. Yes, true: they can warm-up the sound as musical instrument designers have known for centuries.....
                  Mmmm....very interesting. So if I have an Amp with THD of 1% but with predominately odd number harmonics and the other one with the same percentage of distortion but with predominately even number harmonics, then the latter would sound better than the former?

                  Could this mean, even if specs wise both Amps may be identical but sound wise they could be different? It cannot be, I have compared several amplifiers, and the difference only heard when the Amps driven to its limit.

                  ST

                  Comment


                  • #24
                    A hypothesis about harmionics? Over to you ....

                    Originally posted by STHLS5 View Post
                    ...Could this mean, even if specs wise both Amps may be identical but sound wise they could be different? It cannot be, I have compared several amplifiers, and the difference only heard when the Amps driven to its limit. ST
                    That's a bold statement, and now you rather than me are going to have to do some thinking to justify it! I'm sure that you didn't expect my last post of a mere few hundred words would define the entire process of how amplifiers may/can/will/should sound different to the human ear. This is a multi-faceted issue as all things are relating to human perception.

                    Certainly, the specifications are only a snapshot of the amplifiers capability - they cannot (yet) define enough of the performance envelope to be comprehensively descriptive. But they are a useful starting point. The point you have driven amplifiers to when you've noticed more characteristic distortion is shown on the attached graph: at and beyond the 'knee' in the power/distortion curve where the distortion can shoot up to infinity (total) over a very small increase in output power. In short, the amp has just run out of steam but you are pushing it onwards. Like whipping a horse that is already galloping. The heart can only pump so much blood around. It would be interesting to compare this very well defined knee with a graph of a tube amp.

                    To recap: I've shown how an amplifier (although it could have been a pickup cartridge, a microphone, a speaker .....) has amplified a pure incoming signal and then, for whatever reason, generated, unasked for, a whole series of harmonics that are mathematically related in frequency to the incoming stimuli but are definitely not present at the input. We call these harmonic distortion(s). Even good loudspeakers produce plenty of these distortions. But you say that you are audibly unaware of these harmonics superimposed onto the music until the amplifier is driven to the upper limit of its power potential, when we would expect all the distortions to increase, perhaps dramatically. I assume that you could also comment that you are not audibly troubled by the harmonics generated in the loudspeaker until it is pushed hard, or from the pickup cartridge until it is tracing a really loud groove or from the microphone unless someone is screaming into it at close range.

                    How would you weave these observations into a cohesive working hypothesis?

                    >
                    Attached Files
                    Alan A. Shaw
                    Designer, owner
                    Harbeth Audio UK

                    Comment


                    • #25
                      Amps and their sound v. speakers

                      Originally posted by Haligonian View Post
                      I am sorry, I do not understand what you mean by this. Could you (re)paraphrase?
                      Imo, Probably pretty difficult to find an obnoxious “sounding” amplifier, one you really couldn’t live with, even if it measures poorer as indicated by the graphs. In fact, some perverse measuring amps find followers. For a speaker however, an audition should reveal the slightest blemishes relatively quickly, which will magnify, the longer you sit/ stand in front of them.

                      @ST. Regarding harmonics. in my experience, amps. don't necessarily need to reach the limit for harmonics to be discernable.

                      Comment


                      • #26
                        The audibility of distortion in amps (HiFi News 1977)

                        From my scanned archives, July 1977 just as the Harbeth company was producing its first speakers, an article covering the very subject of audibility of harmonics. In 1977 the fancy computer graph plotting system was many years away so engineers photographed the screen of their oscilloscope to show the fundamental and harmonics. Having become familiar with the new system (previous posts) it should be possible to interpret these photographs.

                        In those days, six years before CD was released, there was a real expectation that measurement and science would drive-up audio quality. But then the plot was well and truly lost.

                        >
                        Attached Files
                        Alan A. Shaw
                        Designer, owner
                        Harbeth Audio UK

                        Comment


                        • #27
                          Loudspeakers distortion is masking Amplifiers distortion

                          Originally posted by A.S. View Post
                          That's a bold statement, and now you rather than me are going to have to do some thinking to justify it! .......
                          Originally posted by A.S. View Post
                          ......But you say that you are audibly unaware of these harmonics superimposed onto the music until the amplifier is driven to the upper limit of its power potential, when we would expect all the distortions to increase, perhaps dramatically. I assume that you could also comment that you are not audibly troubled by the harmonics generated in the loudspeaker until it is pushed hard, or from the pickup cartridge until it is tracing a really loud groove or from the microphone unless someone is screaminginto it at close range.
                          How would you weave these observations into a cohesive working hypothesis?
                          >
                          Do I hear the harmonics generated by loudspeakers or amplifiers?Let us get past loudspeakers distortion first. I would very cautiously say I don’t even hear them in my 16 year old SHARP mini-compo,which serves me as my bedroom musical companion for late listening of oldies in FM Mono (through the centre speaker only). Coloration – Yes but to say they produce some sort of harmonics to the point it distorts the sound…..well….I have not been consciously aware of them. I usually listen at a very low level, i.e about 20 of60 of the digital volume unit. Probably, about 40 to 50dB. Of course, I hear all sorts of distortions from crackling sound to radio waves hiss at higher volumes but that’s usually in the early morning rush listening to the news where the message takes precedent over quality and it never bothered me but probably will do so for next few days since I am reminded of it in this post.

                          In the case of my car audio system, resolution aside, it performs alright up to a certain level and starts to sound cacophonous above that. Not sure if that is related to THD. And yes, due to old age of the speaker, I hear speaker distortion at a very low volume level, but it gets masked the moment, I increase the volume to my usual listening level. In this case, there’s a very audible distortion but still below and gets masked when the music is louder than the distortion noise. I am sure this distortion is thousand times higher than amplifiers distortion.
                          ST

                          Comment


                          • #28
                            Can't hear speaker distortion

                            Originally posted by STHLS5 View Post
                            I hear the harmonics generated by loudspeakers or amplifiers?Let us get past loudspeakers distortion first. I would very cautiously say ....
                            Ummm. That looks like more observations not a worked-up theory of audibility to me. You're sending us two messages in one post:

                            You are saying ...

                            A) That you are not audibly aware of loudspeaker distortion even in a cheap mini-system and
                            B) That loudspeaker distortion "is a thousand times higher than amplifier distortion".

                            We seem to have reached a dead end here. Please, as you've cut across my train of thought, I offer you to take-over the subject from me. I obviously had a full, incremental explanation in mind. Walk us through to a reasonable understanding of distortion in your own, words. For starters what do you mean by the word "masking" which you have introduced without explanation.

                            The essence of explaining science to non-scientists is to avoid distractions, even well intentioned ones. I'm extremely busy and if you can take on this 'education' role to the same or better effect please do so. Please keep sharply in focus. Over to you ....
                            Alan A. Shaw
                            Designer, owner
                            Harbeth Audio UK

                            Comment


                            • #29
                              Originally posted by A.S. View Post
                              To recap: I've shown how an amplifier (although it could have been a pickup cartridge, a microphone, a speaker .....) has amplified a pure incoming signal and then, for whatever reason, generated, unasked for, a whole series of harmonics that are mathematically related in frequency to the incoming stimuli but are definitely not present at the input. We call these harmonic distortion(s). Even good loudspeakers produce plenty of these distortions. But you say that you are audibly unaware of these harmonics superimposed onto the music until the amplifier is driven to the upper limit of its power potential, when we would expect all the distortions to increase, perhaps dramatically. I assume that you could also comment that you are not audibly troubled by the harmonics generated in the loudspeaker until it is pushed hard, or from the pickup cartridge until it is tracing a really loud groove or from the microphone unless someone is screaming into it at close range.

                              How would you weave these observations into a cohesive working hypothesis?

                              >
                              Well, here is a limited hypothesis I think I can extract from the information presented thus far:

                              1. The audibility of harmonic distortion is dependent on a combination of:

                              a. overall signal level;

                              b. amplitude of the distortion relative to the amplitude of the musical signal.

                              The corollary I suppose would be that, at a sufficient low level in either absolute or relative terms, distortion would be inaudible.

                              If that's correct, I can think of two reasons that it might be so (and I recognize there may well be others):

                              1. When speaking of distortion relative to amplitude of a musical signal, I would conjecture that the "masking" principle that's been discussed in other posts would operate here as well; that is, a low level of distortion would be masked, i.e. rendered inaudible, by a higher-level non-distorted signal occurring at the same time.

                              2. There may also be a threshold of audibility depending on environmental factors, e.g. background noise.

                              Sorry if I'm off base; that's my best stab at it.

                              Comment


                              • #30
                                The audibility of harmonic distortion ...

                                Originally posted by EricW View Post
                                1. The audibility of harmonic distortion ...
                                Will it be helpful to first ascertain if we have an idea what introduced harmonics sound like? The electric guitarists (and synthesizer musicians) amongst us, I suspect, might be most familiar with this phenomenon.

                                This practical test I believe might answer most, if not all the questions here…If its possible to find an electric guitar and a harmonics stomp box/ effects machine to play with, slowly dial this phenomenon in without altering the volume and making sure its not distorting in the traditional sense. Switch off harmonics and what do we hear?

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