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Please validate my understanding of crossovers and resistance

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  • Please validate my understanding of crossovers and resistance

    So this is a stupid question for most folks around here...I'm a rank amateur at this, and have tried to read through these forums and other places about a question I had, and I think I understand it now but would like for someone to validate it for me...

    Say you were using a 2-way crossover to hook up a woofer and a tweeter, and each of those speakers were 8 ohm. The resistance seen by the amp is therefore also 8 ohms.

    If you were to hook up 2 tweeters to that crossover, along with the one woofer, all 8 ohm units -> the amp would see 8 ohm resistance on the woofer side and then 4 ohms on the tweeter side...right? So I guess kind of a variable load there that depending on the frequency of what's being sent down could vary anywhere from 4 to 8 ohms (changing impedance from the speakers themselves notwithstanding). As long as the amp can handle a 4-ohm load, this is still OK right?

    And if you had 2 8 ohm tweeters and 2 8 ohm woofers hooked up to that crossover, everything would look like 4 ohms.

    I think that's how it works - but if I'm wrong please tell me

  • #2
    Hard to say exactly. Once you hook the wrong impedance to a crossover, all you can really say for sure is it will malfunction.
    Francis

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    • #3
      At a high level, you're correct. A pair of parallel 8 ohm woofers or tweeters will look like a 4 ohm load for their frequency range after the crossover.

      As you allude.. it's not that simple due to the changing resistance of the drivers vs. frequency. Also, the crossover points will change drastically with a different driver load.
      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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      • #4
        Originally posted by wogg View Post
        At a high level, you're correct. A pair of parallel 8 ohm woofers or tweeters will look like a 4 ohm load for their frequency range after the crossover.

        As you allude.. it's not that simple due to the changing resistance of the drivers vs. frequency. Also, the crossover points will change drastically with a different driver load.
        Great, thank you. Some day, with any luck, all of this will make sense to me :D

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        • #5
          Originally posted by Motoman View Post

          Great, thank you. Some day, with any luck, all of this will make sense to me :D
          Yeah, any electronic filter like that cares about the load impedance. When you design them, you have to know what the driver impedance is as a function of frequency.
          Francis

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          • #6
            Impedance is a function of frequency. Don't confuse "Impedance" with "Resistance." Pure simple resistance doesn't concern itself with frequency, because it is measured at no frequency (Zero Hertz). Both impedance and resistance are measured in "Ohms" (not "Ohmage") but impedance can and will change as the frequency changes. Audio frequencies are typically in the range from 20 Hertz to 20,000 Hertz. (20Hz-20KHz)


            A typical 8 Ohm woofer might have an impedance that varies from slightly below 8 Ohms at 1 Hertz to maybe 60 Ohms or more at its resonant frequency peak (fs) and then back down to somewhere near 8 Ohms again. After that a woofer's impedance continues to rise gradually as the frequency extends towards and beyond 20,000 Hertz.


            If you use an 8 Ohm woofer and an 8 Ohm tweeter WITHOUT a crossover, your impedance would be 4 Ohms! But who would use a woofer and a tweeter together without a crossover? A crossover is a frequency dividing network. As long as you are using a crossover with an 8 Ohm woofer and an 8 Ohm tweeter then your impedance will remain at or near 8 Ohms straight across, not 4 Ohms.


            If you use a 4 Ohm woofer and an 8 Ohm tweeter without a crossover, your impedance would be 2.66 Ohms. But with a crossover, your impedance would be 4 Ohms in the range of frequencies covered by the woofer and 8 Ohms in the range of frequencies covered by the tweeter.


            If you were to use a 4 ohm woofer with an 8 Ohm midrange and a 6 Ohm tweeter all with a crossover, then your impedance would be 4 Ohms in the range of frequencies (bass) covered by the woofer and 8 Ohms in the range of frequencies covered by the midrange and 6 Ohms in the range of frequencies (treble) covered by the tweeter.
            Last edited by AEIOU; 09-09-2020, 02:13 PM.

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            • #7
              You are basically correct. A xo essentially isolates the impedance of each driver covering a different frequency range so that even though you are wiring them in parallel (most common but not the only choice), each driver or set of drivers tends to keep its own impedance instead of following the usual parallel or series wiring rules. And yes you are free to combine multiple drivers in each given frequency range which may or may not change the impedance of that group of drivers (that depends on how many drivers and how they are wired) and the xo will still essentially allow your amp to still see the impedance of each (group of) driver(s) separately. Thus the reason it's not a problem to combine 8ohm and 4ohm drivers.

              However things are of course a little more complicated than that.

              For every driver, resistance always changes with frequency. This is the definition of impedance. So you can only refer to a driver's resistance in reference to a single frequency. In other words, impedance is the term you should be using.

              When a driver is labeled as 8ohm or 4ohm this is called its nominal impedance and is referring only to its minimum resistance at a certain frequency. To make it more complicated, in actuality the minimums are usually less than the specified values within a certain percentage. So a nominal 8ohm driver usually has a minimum of 6ohm and a 4ohm usually has a minimum of 3ohm. And when an amp says it can handle 8 or 4ohm loads, they also take this factor into account.

              Now go look up some drivers and look at their impedance curves in their spec sheets. You should see large(ish) increases at both ends of the frequency range and the minimum somewhere in between. So again, the actual resistance of most any driver varies widely depending on what frequency you are looking at but it's the minimum that is usually the most important.

              Next thing to know is that when you combine drivers, the resulting impedance depends on how you wire them up. I'm talking now without a xo. Wire 2 of the same drivers up in parallel and the impedance is indeed halved and SPL is increased by 6dB. But wire them up in series and the impedance is doubled and SPL remains the same. In your example, it is in fact unusual to combine tweeters but not uncommon at all to combine woofers in parallel (the extra 6dB gained happens to very conveniently compensate for the 6dB of baffle step loss when a speaker sits out in a room) and/or midranges too.

              Now what every xo does is alter a driver's impedance at certain frequencies. That means that the power the driver receives at those different frequencies is also changed and that means that the frequency response, the SPL at those different frequencies is also changed. So when you add a xo to your selected drivers, the resulting impedance of the speaker is again going to vary wildly with frequency and again what's important in respect to your amp is the impedance minimums. Certain components and typologies in the xo will raise the impedance and certain other ones will lower them. It's the latter that you have to be most concerned with.

              Lastly, something else that needs to be accounted for is the different drivers' sensitivity. Typically tweeters end up being louder than woofers and so the xo also has to do the job of fixing this. One of the ways of doing this is to add series resistance to the tweeter and so guess what? That 4ohm tweeter (nominal impedance) may have just been increased to 5 or 6 or 8ohms or perhaps even more after the xo does its work and the levels of the different drivers is better matched. Or in other words, speaker impedance doesn't just depend on the impedance of the drivers (or the group of drivers) that you combine, but also depends on what the xo does with them.

              Perhaps that helps?

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              • #8
                Click image for larger version

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                m-man, this is a plot of how a B4N's impedance changes (the "8 ohm" woofer from the "Overnight Sensations") w/respect to freq.
                (LOOK at the bottom line.) This chart is readily avail. to you from PE - you've just got to hunt it down.

                It's really only "8 ohms" at 3 frequencies: near 20Hz, and again near 200 and 600 Hz.
                It's actually LESS at 10Hz (we don't normally care about that) and around 300-400 Hz.
                But... it's also approx. 40ohms (that's FIVE TIMES higher) at about 65Hz (its resonant freq. - or "Fs") and again up @ 20kHz. (That impedance "rise" is driven (partly) by the driver's "Le" parm, and is part of the reason why its output drops off at higher freqs., making it a bad tweeter).

                Also note that the bottom end (below about 300Hz) will change depending on the size and style(/tuning) of the box it's put into.

                If you ran a pair of these in parallel (for a 4-ohm load) you could basically HALVE all the values along the curve.

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                • #9
                  Usually the woofer's resistance will be more important to match for your amp because its lowest resistance will stay about the same regardless of the crossover you use.
                  Tweeter/s resistance can often be heavily changed by the crossover when needed. A pair of 8ohm tweeters wired together in parallel (to make 4ohms) will probably be WAY too loud watt-per-watt compared to the woofer, so their high-pass crossover will ideally tame them down to balance nicely with the woofer....and that taming can easily include increasing the resistance of that tweeter section from its crossover.

                  It's pretty common for tweeters (even one at a time) to be much louder watt-per-watt than most woofers, so it's also really common for the crossover to increase the resistance going to the tweeter if you need it higher to pair well with a particular amp.
                  My first 2way build

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                  • #10
                    The driver is only resistive where the impedance levels out on the graph. Elsewhere, it's at least partially reactive. So the fact is, resistance is futile!
                    Francis

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                    • #11
                      Usually, you will add resistance to the tweeter circuit, so even if you had two tweeters in parallel, ( don't know why) you probably add a couple of Ohms.

                      Remember, there are three things with the "Ohms" parameter:
                      DC resistance which the smallest value measured with DC.
                      You then have the impedance, that is a complex number and described in a graph. It includes resistance, inductance and capacitance vectors.
                      Lastly is the "nominal" value which is somewhat arbitrary but usually about the lowest impedance.

                      A point many here miss is the difference between tube amplifiers with transformers, linear transistor amplifiers, and class D switching amplifiers.
                      Transformer output tube amps had taps for 4, 8 and 16 Ohm "nominal speakers" so it could match to the load. The tubes saw the same design impedance on the primary side.
                      Transistor amps much prefer higher impedance. They would rather provide higher volts with lower current. This gives much lower distortion. Hint: do not design a 4 Ohm speaker unless you have a very special amp that has twice the outputs and cost big bucks. Almost all AVR or receivers do not want to see a 4 Ohm nominal load.
                      I do not know about the difference in loading of class D. Logically it would have some effect on the integrator.

                      CAR stereos went to 4 Ohm drivers before the invention of cheap DC-DC converters so they could produce some power with only 12V supply. No reason to still be doing that, but kind of SOP.

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                      • #12
                        It could help a lot, search online for many of the free basic AC/DC electronics courses. In just a couple of hours, you can really get a jump on understanding the question and save you a lot of confusion in the future. Easy stuff.

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                        • #13
                          Originally posted by Motoman View Post
                          So this is a stupid question for most folks around here...I'm a rank amateur at this, and have tried to read through these forums and other places about a question I had, and I think I understand it now but would like for someone to validate it for me...

                          Say you were using a 2-way crossover to hook up a woofer and a tweeter, and each of those speakers were 8 ohm. The resistance seen by the amp is therefore also 8 ohms.

                          If you were to hook up 2 tweeters to that crossover, along with the one woofer, all 8 ohm units -> the amp would see 8 ohm resistance on the woofer side and then 4 ohms on the tweeter side...right? So I guess kind of a variable load there that depending on the frequency of what's being sent down could vary anywhere from 4 to 8 ohms (changing impedance from the speakers themselves notwithstanding). As long as the amp can handle a 4-ohm load, this is still OK right?

                          And if you had 2 8 ohm tweeters and 2 8 ohm woofers hooked up to that crossover, everything would look like 4 ohms.

                          I think that's how it works - but if I'm wrong please tell me
                          Here's a 3-way using 8 ohm drivers. The x-over choices change the final impedance. I made a sim for this speaker that was pretty much 4 ohms most of the range. The builder wants higher impedance. This is his xo, and is typical.

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