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  • Importance of Tweeter Impedance?

    How important is the impedance of the tweeter to the stability of an amplifier?

    I understand that if the impedance of the speakers is too low for the amp, for example 4 ohm speaker with 8 ohm amp, the amp can overheat and be damaged.

    Typically the "nominal" impedance of a low-mid speaker will be based on the rating in the low-mids around 300hz.

    I understand that the impedance of the tweeter is important for designing the crossover's components.

    However is the impedance of the tweeter plus its crossover important to the amp's stability?

    I ask because I have a situation where the space covered by my speakers is too wide for my tweeters. I'd like to add a pair of tweeters angled towards the sides roughly 60 degrees. The additional tweeter will not be the same model that is in the speaker cabinet and the external tweeter will have its own crossover.

    Unless I go with a piezo, I'll have tweeters that will affect the impedance of the overall speaker. Even if I go with a piezo, I typically like to warm them up by using a parallel resistor--maybe 16ohm--then a capacitor (1st order xover). I don't want the additional tweeters to damage my amp rated at 8ohm.

    Also I've read that changing the impedance of a speaker will change the amount of amp wattage sent to that speaker. Has anyone used this effect to change the loudness of the tweeter relative to low-mid speaker? When I experimented with this concept with a piezo and selection of parallel resistors--10, 16, 25ohm--I did not notice a difference in loudness of the piezo tweeter.
    https://www.speakerimpedance.co.uk/?...age=calculatorhttps://www.speakerimpedance.co.uk/?...age=calculator

    That leads to the possibility of wiring the pair of tweeter plus crossover on each side in serial instead of parallel; however I don't want to change the volume of the tweeter in the speaker cabinet.

    Thanks in advance for any thoughts about this.

    Joe



  • #2
    I believe the impedance in the high frequencies is important for the stability of an amp, BUT typically the crossover and resistor padding (to tame its volume output down) you add to the tweeter will raise its impedance higher than the woofer. A crossover that lets your tweeter reach too low with too much overlapping between the tweeter and woofer or one that's poorly designed might give you a lower impedance than desired though.

    Adding another tweeter to cover a wider area likely means that new tweeter and the old one will both be playing many of the same frequencies so there is some potential for the impedance to drop too low if it's already fairly low before the new tweeter is added. It'll be important to know what your current impedance is at the crossover and above...AKA, what tweeter and midwoofer you're currently using and what the crossover parts are.
    Are you able to tell us what parts your speaker is using right now including its crossover? If not, simply listing as much as you can may still help quite a lot.

    My first 2way build

    Comment


    • goosala
      goosala commented
      Editing a comment
      Hi Lout. I need to pad my B652 Air because I removed the stock resistor when I installed my 2.500 Hz crossover. What value resistor should I install to pad the tweeter down to the 8 ohm stock woofer?

  • #3
    The "nominal" or rated impedance of a speaker is according to the IEC standard - The value of a pure resistance which is to be substituted for the loudspeaker when defining
    the available electric power of the source shall be specified by the manufacturer. The lowest value of the modulus of the impedance in the rated frequency range shall be not
    less than 80 % of the rated impedance. If the impedance at any frequency outside this range (including d.c. ) is less than this value, this shali be stated in the specifications.

    That said any modern made, in the last 50 years amp, should be able to handle a 4 ohm load. Even if it's not rated down to 4 ohms it's very doubtful it would have a problem driving the tweeters. The power distribution for music content drops as the frequency increases, so the top 3 octaves have relatively low power needs.

    I personally would be more concerned about potential lobing issues between the two tweeters.

    Comment


    • #4
      Yeah, you'll make a bunch of lobes doing that. You may get lucky though, and have better treble at one spot in space.
      Francis

      Comment


      • #5
        Originally posted by devnull View Post
        I personally would be more concerned about potential lobing issues between the two tweeters.
        +1. If you can't get the dispersion you need from one tweeter you widen the field by cross-firing or spiral arraying, not splaying. Splaying leads to lobing and comb filtering.

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

        Comment


        • #6
          Thank you all for the replies. You gave me more to think about.

          I had previously associated lobing with low and low-mid frequencies more than the top two octaves or so produced by a supertweeter with a high crossover point.

          I assume my mention of a 1st order crossover added to your concerns about lobing and comb filtering.

          I have a Dayton reference mic that I plan to use to during the design and tweaking of the project.

          Bill I'm not familiar with "cross-firing" of tweeters. I could not find anything in a quick search of the net. Any tips?

          Is there agreement that using two 8 ohm tweeters in parallel along with 8 ohm woofer should not be problematic for most amps rated at 8 ohms? This is assuming a high-pass crossover will be used.

          Comment


          • #7
            Originally posted by Joegtech View Post
            Thank you all for the replies. You gave me more to think about.

            I had previously associated lobing with low and low-mid frequencies more than the top two octaves or so produced by a supertweeter with a high crossover point.

            I assume my mention of a 1st order crossover MCDVOICE added to your concerns about lobing and comb filtering.

            I have a Dayton reference mic that I plan to use to during the design and tweaking of the project.

            Bill I'm not familiar with "cross-firing" of tweeters. I could not find anything in a quick search of the net. Any tips?

            Is there agreement that using two 8 ohm tweeters in parallel along with 8 ohm woofer should not be problematic for most amps rated at 8 ohms? This is assuming a high-pass crossover will be used.
            Adding another tweeter to cover a wider area likely means that new tweeter and the old one will both be playing many of the same frequencies so there is some potential for the impedance to drop too low if it's already fairly low before the new tweeter is added. It'll be important to know what your current impedance is at the crossover and above...AKA, what tweeter and midwoofer you're currently using and what the crossover parts are.

            Comment


            • #8
              Joeal, I can understand your question. I have a couple of scenarios in mind, one where I won't have the specs of the original two-way speaker. Remember I'm not trying to duplicate the frequency range of the existing tweeter. That tweeter will distribute 3-6khz over a wider area than 8-12khz. I'm looking to fill-in the latter over a wider area. The crossover point for the second tweeter--what I'm calling the "supertweeter"--will be 1.5 octaves higher than the crossover in the two-way.

              The second scenario is in a church where there are plenty of mids bouncing around. 1.7khz or so tends to feed back in that poorly treated area. The choir members in one area behind the speakers need more upper mids and highs, but the area is too wide for just one tweeter. It is a very low budget situation to I'm trying to make due with what I already have available. That includes an old mono 8 ohm amp.

              Purchasing three new KSN-1025 piezos should work for my second scenario. I've used them in the past with good results for stage monitors where wide distribution of sound was not a factor. however I do not want to spend that much. The KSN1025 knock-offs seem to be junk based on frequency response. The crossover for the piezo would include an appropriate parallel resistor in order to be compatible with the amp as described here.
              http://www.frugal-phile.com/piezo-XO.html
              https://audiokarma.org/forums/index....-2#post-854586

              I'll be receiving some of the surplus Onkyo supertweeters soon that may not be capable enough for this area. They are 6ohm units so I suppose I'll have to use a combo of serial and parallel wiring and high pass filter/crossover with an 8 ohm amp. I've read about serial wiring many times over the years but have never done it.
              https://www.parts-express.com/200-on...ProductDetails

              Comment


              • #9
                I recently picked up a discarded set of JBL J350 floor speakers--10" passive radiator (blown surrounds), 10" woofer, 4" or so for the upper mids, supertweeter crossing over at around 7k.

                The frequency response of the supertweeters is good to 18khz. I was impressed. I'm sure they were expensive units back in the 1980s when I think they were made.

                However notice that the impedance of the supertweeters is much lower than the other speakers. I wonder if the lower impedance was intended to try to get more amplifier power for the less sensitive tweeters?

                I tested each speaker with DATS. The approximate impedance in the octave or two above the resonant peak (Fs) for each speaker:

                10" Woofer (Fs 75hz): 5.5 ohm

                4" mid (Fs 225hz): 5 ohm

                Supertweeter: (Fs 800hz) : 3.5 ohm

                I was surprised that the super tweeters were quite a few db softer than the upper mid speaker.
                The frequency response of the speakers with the passive radiators removed represented by the dark blue and red curves:

                Click image for larger version  Name:	Capture.JPG Views:	0 Size:	58.4 KB ID:	1440615

                I had no problem getting a good sound out of them after hacking the upper mids and working on the bass with graphic EQ--curve not shown above.



                Originally posted by Joegtech View Post
                How important is the impedance of the tweeter to the stability of an amplifier?

                I understand that if the impedance of the speakers is too low for the amp, for example 4 ohm speaker with 8 ohm amp, the amp can overheat and be damaged.

                Typically the "nominal" impedance of a low-mid speaker will be based on the rating in the low-mids around 300hz.

                I understand that the impedance of the tweeter is important for designing the crossover's components.

                However is the impedance of the tweeter plus its crossover important to the amp's stability?

                ...

                Comment


                • #10
                  I'm surprised that the Fs of a tweeter, let alone a supertweeter, would be so low. That in part would explain their low sensitivity.
                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

                  Comment


                  • #11
                    Originally posted by billfitzmaurice View Post
                    I'm surprised that the Fs of a tweeter, let alone a supertweeter, would be so low. That in part would explain their low sensitivity.
                    800 really isn't that low is it? CAT 308s are around 650 if I remember correctly and Morel can't be the only manufacturer doing it. I just wonder if the reduced high frequency response is due to the ferro fluid gumming up or maybe caps in the crossover starting to fail open. They are how old., maybe 35 year old tweeters...

                    Comment


                    • #12
                      The Fs of a tweeter is typically one octave below the recommended low frequency limit, so in the case of a supertweeter with a 7kHz crossover an Fs of 3.5kHz or thereabouts would be expected. The lower the Fs the higher the Mms, which results in reduced HF extension and sensitivity.
                      www.billfitzmaurice.com
                      www.billfitzmaurice.info/forum

                      Comment


                      • #13
                        Originally posted by billfitzmaurice View Post
                        I'm surprised that the Fs of a tweeter, let alone a supertweeter, would be so low. That in part would explain their low sensitivity.
                        I hear you. The Fs of the other unit was actually a little lower.

                        Click image for larger version

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                        To make it more interesting, check out the frequency response without a crossover. This was at 12" with an EMM-6 mic's calibration file installed in REW.

                        Click image for larger version

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                        • #14
                          That could be part of the reason why the J350 isn't well regarded. You can find some information on them at the Lansing Heritage site. http://www.audioheritage.org/vbullet...c56ec5e3df9641
                          www.billfitzmaurice.com
                          www.billfitzmaurice.info/forum

                          Comment


                          • #15
                            Originally posted by billfitzmaurice View Post
                            That could be part of the reason why the J350 isn't well regarded. You can find some information on them at the Lansing Heritage site. http://www.audioheritage.org/vbullet...c56ec5e3df9641
                            Thanks, Bill; however I did not see anything obvious when I clicked the link. Could that be due to you having a user account for the site?

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