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KEF C80 crossover redesign

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  • #16
    Originally posted by fpitas View Post

    Some amps get unhappy with very low impedances, especially if they are reactive. Newer amps tend to be more tolerant.
    Those speakers aren’t that old, and very stabile amps have been around a long time. Maybe British amps though. Huh.

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    • #17
      Amps are designed for a certain range of impedance. They may put up with impedances outside that range, but they'll work best used where the designer intended.
      Francis

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      • #18
        Originally posted by fpitas View Post

        Some amps get unhappy with very low impedances, especially if they are reactive. Newer amps tend to be more tolerant.
        I've found the opposite to be true. Newer amps seem to be less tolerant, especially home theatre amps. I restore and upgrade a lot of vintage amps (see my threads on AudioKarma), and I always burn them in on 4 ohm loads.

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        • #19
          OK. I'm not familiar with home theater amps.
          Francis

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          • #20
            Originally posted by philthien View Post

            Those speakers aren’t that old, and very stabile amps have been around a long time. Maybe British amps though. Huh.
            British amps? Really?

            I've seen and worked on Japanese and American amps that choke on low impedance loads. I've also read reviews from 30-40 years ago where they have tested amps rated at 100w, which fall on their face when asked to drive a reactive and/or low impedance load.

            If the NAD 3020 that I do a lot of work on is British by design, it has no problem with low impedance and reactive loads. At the HiFi shows, they used to connect four 8 ohm loads in parallel, and it had no problem driving them. It would also drive the Linn Isobarik, which was a difficult load.

            There are many fine British amps : Meridian, Mission, Naim, to name but a few. There are also many fine American and Japanese amps. Please keep any anti-British comments to yourself.

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            • #21
              MY 8-el XO (vs the orig. YELLOW xo - w/22? parts) will create VERY similar pass filters, EXCEPT my mid ends up about +1.5dB hot. (see final line -v)

              The LOWer (3-el) section (at the bottom left of the yellow XO) shunting to gnd looks just like (and acts like) an LCR used to flatten the Zmax at a tweeter's Fs.
              In this case, it drops the Zmax (at the mid/tweet Fc above 2kHz) down from about 32n(ohms) to <=6n. (You can add this out in front of my entire 3-way XO to achieve the same result.)
              I THINK the UPper (3-el) section (which looks and acts like a typical BSC filter) is used primarily to add about ONE OHM to the mid & tweet sections to "scaffold" them up above 5n.
              A side effect of it is that it cuts the M/T sections outputs by about 1.5dB.
              With both those sections out front, it holds my XO load between 5-7 ohms.

              My final XO was simmed to match the KEF's xfer fns w/OUT those 2 Z-comping sections. Add them to mine and the results should match pretty well.
              W/OUT them, my Z-plot looks more like a "normal" 3-way. (A "solid" 6n design: LP holds > 4.5n below 100Hz, HP tops out just above 4.0.)
              You can cut the mid back a little by ADDING about 2ohms out in front of the Band-Pass. (Just using a #20 1.2mH coil could be nearly as effective.)

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              • #22
                Originally posted by leesonic View Post

                British amps? Really?

                I've seen and worked on Japanese and American amps that choke on low impedance loads. I've also read reviews from 30-40 years ago where they have tested amps rated at 100w, which fall on their face when asked to drive a reactive and/or low impedance load.

                If the NAD 3020 that I do a lot of work on is British by design, it has no problem with low impedance and reactive loads. At the HiFi shows, they used to connect four 8 ohm loads in parallel, and it had no problem driving them. It would also drive the Linn Isobarik, which was a difficult load.

                There are many fine British amps : Meridian, Mission, Naim, to name but a few. There are also many fine American and Japanese amps. Please keep any anti-British comments to yourself.
                OMG relax, I wasn't indicting British amps, just questioning whether a pair of speakers designed and produced in Britain might be designed to work with British amplifiers (you forgot Quad) that I might not be familiar with.

                This place is going downhill fast.

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                • #23
                  I've been looking everywhere to find the T/S parameters of the TS1212 version of these KEF B139 woofers. It seems all the other designs using the B139 have a much larger cabinet. The Constructor Series CS7 used a 70 liter (2.47 cubic feet) sealed enclosure, and the Concerto used a 62 liter (2.19 cubic feet) ported enclosure.

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                  • #24
                    Did you check Falcon Acoustics in England?

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                    • #25
                      I would start with both on-axis and off-axis FR measurements and also harmonic distortion measurements at various power levels. First see if the impedance optimised crossover is actually sacrificing any of the previously stated to warrant a redesign.

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                      • #26
                        Sadly, I don't have any measuring equipment apart from my ears. I was just fishing for ideas of what to do with the two pairs of C80s I have left. One pair has really bad cabinets, the other pair so-so. I could make another pair of C80s, but where's the fun in that. I have built cabinets in the past, perhaps making a larger cabinet to give the B139 more volume to work with would be a good idea. Of course, another option would be to make a pair of MTM speakers using the C80 cabinets with two B160s and a T33 per cabinet, and not using the B139s at all...

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                        • #27
                          I really think a crossover design starting ''with ears' is folly .at worst you blow up your amp due to an impedance short.

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