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Help understanding electric phase angle and it's effects on amplifiers

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  • Help understanding electric phase angle and it's effects on amplifiers

    Hi Everyone,

    I'm trying to understand the electric phase angles that result from XO designs and the real world problems they can present to the amplifier that's driving the speakers. I have seen folks smarter than I am reference rules of thumb for electric phase, saying it should be within +/- 30 degrees, and have a gentle slope to be nice to your amplifier. There are also some caveats to these rules depending on your impedance at the given frequency being high or low... but this has confused me up to this point. What actually happens to amplifiers that are presented with good vs. bad phase situations?

    Can anyone help set me straight, or point me to a document that has this info in a relatively easy to understand format? I'm a mechanical engineer by trade, so all my crossover design experience has been practical rather than theory-based at this point. The finer points, and most of the "why we do things" still escapes me.

    I've got an example chart attached just to help aid the conversation.

    Thanks in advance!
    Keith

    Attached Files
    Voxel Down Firing with Dayton SA70
    Translam Subwoofers - The Jedi Mind Tricks
    The Super Bees - Garage 2 way
    SevenSixTwo - InDIYana 2018 Coax
    The Defiants - InDIYana 2019 "Bare Minimum" Build

  • #2
    We have some real experts here (not me!)

    Short answer, it causes the amplifier to draw too much current.
    The amp can overheat, or the magic smoke can escape.
    Not limited to crossovers, it's been known for a lone time.

    https://en.wikipedia.org/wiki/Power_factor

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    • #3
      To use a Mechanical Analogy - it's like the difference between Static forces and live loads that are usually unstable or moving loads.
      "Not a Speaker Designer - Not even on the Internet"
      “Pride is your greatest enemy, humility is your greatest friend.”
      "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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      • #4
        As said, the main problem is over heating the amplifier especially for class AB.

        Here's a bit of a pictorial. With the voltage and current in phase (0 degree resistive load), the peak current and peak voltage are in line. The power the output devices are required to get rid of is the difference between the power supply rail voltage and the instantaneos voltage * the instantaneous current. So at peak current, the output device is also dropping minimum voltage so the power requirement is reduced.



        When you have a reactive load, capacitive or inductive, that current starts to fall out of sync with the voltage swing. Now, at peak current output, the voltage is no longer at peak, so at that instant the output devices are required to dissipate a lot more power.


        There's also the risk of an amp that has marginal stability in the first place. The load phase can push the feedback loop phase just enough for the amp to start oscillating. Most amps shouldn't be that marginal, so the heat is a larger issue.
        Electronics engineer, woofer enthusiast, and musician.
        Wogg Music
        Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

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        • #5
          Thanks Don, Syndey, and Wogg! The pictures and analogies are helping. Wogg touched on another detail here... reactive loads. I've heard folks say that one style (capacitive or inductive) is worse than the other, but I can't recall which. So for those times that XO designs are pushing 30 degrees or more of phase shift, what situation should I be avoiding? I would imagine we can't achieve nominally flat XO designs with some drivers without flirting with these +/- 30 deg rule of thumb limits. Is that when folks say that drivers are difficult to work with, or that a pair of drivers is a poor match?

          Thanks again for helping me get this important set of details straight!
          Voxel Down Firing with Dayton SA70
          Translam Subwoofers - The Jedi Mind Tricks
          The Super Bees - Garage 2 way
          SevenSixTwo - InDIYana 2018 Coax
          The Defiants - InDIYana 2019 "Bare Minimum" Build

          Comment


          • #6
            @PWRRYD They are talking about power factor!!!!!!!!!!!!!

            Guys you're in trouble now, I told on you.... Tee Hee! ;)


            Transformer coupled tube amps are most suceptable to extreme reactive loads.

            The rest can vary. With modern amps -/+30 as said is a good rule of thumb as mentioned. Even then if the amp does not have high instantanious current capability, and good dynamic headroom, there the risk of additional THD being introduced as the volume increases, that is before clipping. Music is not a sine wave at a frequency. It is an extremely complex wave of fundementals and Harmonics with extreme, sometimes instant swings.

            99.9% of amps out there at normal listening volumes (70dB or so depending on the weighting) will never have an issue. Flea-watt amps, escoteric push/pull class A solid state *may*

            Years ago, impedence compensation in "high end" speakers was extremely common since amps were snowflakes. It still possible to add those cicuits into a design but it adds quite a bit of complexity.
            .

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            • #7
              I know nearly nothing compared to you guys, all I can do is regurgitate info I picked up around here in the past, so take my input with a grain of salt..

              I have heard and follow a general rule that within 30deg is optimal, within 45 is acceptable (my builds are usually within 45), and 60 is as far as you would ever want to take it and should be avoided.

              I believe when phase does get to a higher degree it is more acceptable if the same frequency is of a reasonable impedance, whereas a questionable phase lining up with a low impedance dip will potentially cause amplifier strain.

              Comment


              • #8
                Originally posted by KEtheredge87 View Post
                ... Is that when folks say that drivers are difficult to work with, ...
                Often the problematic loads are those that exhibit a large negative phase, with a low impedance, in the bass range - because the power demands of typical music are largest in the bass spectrum.
                This (as was mentioned earlier ) causes excessive current draw which can exceed SOA of the semiconductors.
                And a capacitive load can cause high frequency instability.
                "Not a Speaker Designer - Not even on the Internet"
                “Pride is your greatest enemy, humility is your greatest friend.”
                "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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                • #9
                  Get to know ELI the ICE man, a mnemonic for capacitance, inductance, voltage and current relationships.

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                  • #10
                    Both a lot of negative phase shift and a low impedance dip are current hogs. Where you can really run into problems is when they both occur at or near the same frequency, so that's what you want to avoid.
                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

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                    • #11
                      Originally posted by billfitzmaurice View Post
                      Both a lot of negative phase shift and a low impedance dip are current hogs. Where you can really run into problems is when they both occur at or near the same frequency, so that's what you want to avoid.
                      Thanks Bill, So in the example charts I included in my first post, the lowest phase is ~ -60 degrees at the just after the port tuning dip on the woofer. That happens at around 17 ohm impedance, so that's not as big of a concern? It doesn't seem like there's much I could do about that since that's tied to the resonant frequency of the driver.
                      Voxel Down Firing with Dayton SA70
                      Translam Subwoofers - The Jedi Mind Tricks
                      The Super Bees - Garage 2 way
                      SevenSixTwo - InDIYana 2018 Coax
                      The Defiants - InDIYana 2019 "Bare Minimum" Build

                      Comment


                      • #12
                        I would not worry about the phase relationship until above the woofer Fs/tuning relationship. This is inherent to the design, and isn't something that is induced. If you have multiple woofers at that point to make the impedance low then it becomes an issue for multiple reasons, but at a safe nominal impedance this should not be an issue. You are between the 30s when you should be between the 30s, so you have done well.

                        And agreed... -60 deg and low impedance is to be avoided.

                        For what it's worth, Having the Rs resistor for a tweeter out in front of the xover will reduce the negative swing. Values might need adjusted elsewise, but this is a common practice for me.

                        Later,
                        Wolf
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                        • #13
                          Originally posted by KEtheredge87 View Post
                          the lowest phase is ~ -60 degrees at the just after the port tuning dip on the woofer. That happens at around 17 ohm impedance, so that's not as big of a concern?
                          Not particularly. The usual source of trouble is when the crossover frequency is where there's a lot of negative phase shift and the crossover Q is too high, causing impedance to drop too low. I don't see any evidence of that on your chart.

                          www.billfitzmaurice.com
                          www.billfitzmaurice.info/forum

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                          • #14
                            Another perspective though is that there are plenty of speakers you can find on say Stereophile whose measurements show both very low impedance and unfriendly phase angles. So it's not like it's considered dangerous practice, so much as you need to make sure you've got an amp that can handle it. So get a good amp.

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                            • #15
                              What would happen If your phase angle at 3000hz was -77 but the Impedance was 20ohm? Could that cause problems with a low powered (20watt per channel) Class A/B amplifier?

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