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Some fun with FST...

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  • Some fun with FST...

    For anyone generally curious -

    For the better part of 18 years I've admired the engineers at Bowers & Wilkins for their methods in driver development, specifically the FST midranges. The way they controlled the amount of resin in select strands of the Kevlar to control break-up modes, cancel odd order harmonics, the use of FEA etc. has always fascinated me. Zaph measured the older Kevlar variant several years ago and it really appeared to be a stellar performer, besting even the top of the food chain (available to us DIYers, at least) with distortion numbers and efficiency in particular. I got a wild hare a few weeks ago and snagged a pair of newer OEM FST Continuum mids that are normally used in the 704 S2 towers.

    Build quality is second to none. Everything here was designed with a purpose and they feel solid as a rock. The fit and finish is gorgeous. The aluminum trim ring, the stout aluminum frame, massive neo ring and thick top plates, everything about these screams quality.

    Click image for larger version

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    I assumed these mounted with an M6 bolt from the rear like the older models, but these have an interesting party piece. The dust cap is a thick foam plug that pops out, providing access to the top of the pole piece. This is how you remove the original brass nut to replace the driver in the 704's according to the service manual I found (they even make a special tool kit for it, because of course they do).

    Click image for larger version

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    Kapton formers, copper shorting rings, and the top of the former edge has a plastic ring around the perimeter which holds the dust cap in place firmly.

    I cut a test baffle for these and stuck them in our chamber here at the office.

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    Unsmoothed 0-60 degrees, note the sensitivity.

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    THD is below 0.2% from 500-3k, averaging around 0.16%. Third order is down more than 60 dB in this range.

    They appear to be as good as I hoped, even out of a "lesser" 700 series model. I plan to use these in a future 3/3.5-way tower with dual Scan Speak M22WSR-46 woofers and TL N26CR2-A tweeters, but that probably wont be happening for a few months. I'll try to do a build thread when that happens.
    Your results may vary.

  • #2
    How did you manage to acquire these drivers?

    They look amazing . . .

    Comment


    • jhollander
      jhollander commented
      Editing a comment
      Found them on Ebay

  • #3
    Yes I want some too...What's the nominal size? Any TS parms to go with the graphs?
    John H

    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

    Comment


    • #4
      Those and the response look quite nice. That style of mud is and has been the star of the B&W show. Curious what is different / better in the higher end models.

      Your build should be quite nice, please do a build thread if you can, be cool to see something higher end like that.

      Comment


      • #5
        Originally posted by Steve Lee View Post
        How did you manage to acquire these drivers?
        They look amazing . . .
        Steve/John,

        I got these brand new at encompassparts.com. Genuine UK built drivers. I did a Google search for "bowers and wilkins replacement FST midrange" and it's one of the first options that come up. These happen to be the LF27170 model. You have to select the base speaker model and scroll to find the parts you want. I had a heck of a time trying to find them directly on their site, so I recommend using the Googles. They have a lot of model options available, some require separate trim rings that are also usually available if you can decode the model numbers.

        Originally posted by jhollander View Post
        Yes I want some too...What's the nominal size? Any TS parms to go with the graphs?
        John,

        Overall frame diameter is 5-13/16". I do have free air TS parameters I'll share when I'm back in the office tomorrow. IIRC, Fs is ~250, Qm/Qe/Qt are all a bit odd due to the surround design but the impedance curve is well under control. Le @ 1k is too low to register on DATS but measured 0.021 mH @ 10kHz.

        These appear to be underhung.
        Last edited by mattp; 01-06-2023, 06:51 PM.
        Your results may vary.

        Comment


        • #6
          Thanks for the heads-up and sharing your data, Matt.


          Comment


          • #7
            I don't like all that off axis bunching around 1800 Hz, especially for a midrange driver.
            Craig

            I drive way too fast to worry about cholesterol.

            Comment


            • Steve Lee
              Steve Lee commented
              Editing a comment
              Care to explain your concerns for our understanding, PWR RYD?

          • #8
            Here are the TS parameters.

            Click image for larger version

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            Your results may vary.

            Comment


            • #9
              Originally posted by PWR RYD View Post
              I don't like all that off axis bunching around 1800 Hz, especially for a midrange driver.
              Craig,

              The 12" test baffle I cut didn't have the recess quite deep enough in the wall which could be causing some of this issue. The driver was flush, but the baffle stuck out about 1/4" from the test wall when mounted. Not perfect but good enough for what I wanted.

              I'm going to have these Klippel near field scanned for crossover work once the cabs are finished so I'll know more then.
              Your results may vary.

              Comment


              • #10
                Just remember that the surroundless FST midrange was highpassed with a 4th order electrical filter in the 800 series. I agree that these mids are the shining gems that make B&W worth it to some degree. The 7" woven FST was present back in 2000 when Rick, Jeff, and I went to that hifi shop way back then. Mids were great, but treble was a bit bright and sibilant, and the 15" woofers while well defined just did not dig all that deeply.

                Wolf
                "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                *InDIYana event website*

                Photobucket pages:
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                My blog/writeups/thoughts here at PE:
                http://techtalk.parts-express.com/blog.php?u=4102

                Comment


                • #11
                  Originally posted by Wolf View Post
                  Just remember that the surroundless FST midrange was highpassed with a 4th order electrical filter in the 800 series. I agree that these mids are the shining gems that make B&W worth it to some degree. The 7" woven FST was present back in 2000 when Rick, Jeff, and I went to that hifi shop way back then. Mids were great, but treble was a bit bright and sibilant, and the 15" woofers while well defined just did not dig all that deeply.

                  Wolf
                  Good to know, Ben. I feel like there have been major improvements with these Continuum variants since those days. They do have very mild suspension travel. The foam around the cone periphery is probably 1/4" thick to the frame. During testing the cone moved in unison a good 2mm P-P. I'm not pulling any punches with this next design so I'll do what I need to with filters. I'll share the NFS files once I have them so everyone can experiment as they want to.
                  Your results may vary.

                  Comment

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