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Refurbishing "Vintage" Polk Bookshelf Speakers

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  • Refurbishing "Vintage" Polk Bookshelf Speakers

    Hi Everyone,
    Last weekend I was given a pair of Polk RT55s with fried XOs (the resistors were in many pieces and XO scorched) and am going to rebuild them for one of my kids. I'm thinking about using two FaitalPRO 6FE100 and a Peerless BC25TG15-04. I have a few questions, as this is my first attempt at refurbishing speakers.

    The front baffle on the Polk was designed for 155mm wide truncated woofers, which is about 12mm smaller than the Faitals. I assume Polk did this on purpose. So, I think I can make a new baffle to replace the plastic one that is there now and affix it to the old one. What do you all think will be the ultimate effect on the sound if I do this? Will it cause horrible resonances? Will it just sound weird? My plan is to sand and then affix with Titebond III the pieces.

    Next, how does this new XO look? (I'm using VituixCAD for modeling.) I realize it's simple and I didn't update the wattage on the resistors, but am planning on using 20W since they aren't that expensive.

    Thanks in advance.

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  • #2
    I don't understand the Lpad on the woofers to ground - looks like the reason the original resisters burned up?


    • #3
      If the drivers are in good shape why not just redo just the crossovers with beefier components?


      • M@tt
        [email protected] commented
        Editing a comment
        Hi DGJ, The tweeters were shot, 2 parts fell out when I remove the front baffles, and knowing my brother the woofers are too, but honestly I have no way of testing them other than checking the ohms, which were between 1.9 and 2.1 each.

    • #4
      If not, why put an L-pad on the woofer?? You're cutting its output by about -5dB. Dump the "L" and you can (probably) unpad the tweeter by 5dB ! (unless you don't have any baffle-step, which I'm guessin' you don't).
      If anything (extra), put a "Zobel" across the woofer pair (and probably a notch/tank inline w/them).


      • M@tt
        [email protected] commented
        Editing a comment
        Sure, I'm quite new at this, obviously, and was trying to pull down the impedance on the woofer. I'll give those a shot, see how it looks, and post some more graphs then. This seems like it will be a fun project and I hope to learn a bit along the way. Thanks for the feedback.

    • #5
      Originally posted by [email protected] View Post
      Sure, I'm quite new at this, obviously, and was trying to pull down the impedance on the woofer. I'll give those a shot, see how it looks, and post some more graphs then. This seems like it will be a fun project and I hope to learn a bit along the way. Thanks for the feedback.
      There's no reason to lower the impedance on the woofer (unless maybe if you're connecting to a tube amplifier that only has 4ohm outputs). The amp you'll be connecting these to may run a little cleaner with the woofers at their naturally higher impedance, AND lowering the woofer impedance with XO parts typically just means those XO parts will waste a lot of amplifier wattage as won't make the speakers louder or more sensitive (like using a 4ohm version of a woofer instead of an 8ohm version of the same woofer can).

      You might also want to play around with the Vituix "Diffraction" option in the Tools section at the top. Measure your box's height and width (in millimeters) and your woofer's diameter at the middle of its rubber surround, then move the woofer into position. On the bottom/right you can select the 0-axis FRD file for the woofer just like you're using in the XO simulation itself...then check the box for "Full Space" to apply the woofer's FRD file to that box's baffle diffraction effect/shape.
      Then click Export (make sure the change the file type to FRD) and save it somewhere you can easily find use this newly created FRD file for your XO simulation.
      You'll notice there's a lot less bass output (and less lower-mids, and more upper-mids in a small hill). This is what your box's baffle is doing to the speaker's sound...roughly....approximately. :D
      My first 2way build


      • #6
        Thanks Chris, LOUT, I took both of your suggestions and ran them through the simulator and came up with the following

        I used an online calculator for the calculations.
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        And no calculators needed on this one:
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        To my eye, granted I didn't use the diffraction calculator yet, these two are very similar, save for the impedance and only slightly different from my original where the DB is lower but the simulated peaks and valleys are not as pronounced. If the notch filter calculation is correct and it really needs 770uF of capacitance to attenuate the impedance, that's going to be quite a bit more expensive than both option A and C.

        I won't be running these on a tube-amp, I have a spare Aiyima A07 on the shelf for these. (Aside, I'm using the A07 with a pair of Classix-II, it seems to be a nice cheap amp)

        In you-all's opinion, what would the difference ultimately be sound wise? Is the more complex XO worth the added cost? ~93db versus ~90db? Would less variance between the decibels along the range from 70 - 20K be "better"?

        Thanks! I am really starting to enjoy this hobby and if these work out I should be able to convince my wife to make some floor-standing 2ways. :-}
        Last edited by [email protected]; 10-01-2021, 12:22 PM.


        • #7
          I don't think it looks like either of those extra bits (that particular LCR and the C+R) are adding anything good (at least with the current values)...not saying a tweaked version couldn't help, but I don't think those particular ones are.

          You miiiight want to add a tiny capacitor+~8ohm resistor (cap and resistor are series with eachother) in parallel along-side the woofers' 1.2mH inductor. That can knock down the woofers' frequency spike around 4khz IF you want to do that.
          This is probably the notch tank filter that Chris Roemer mentioned earlier.

          You'll probably end up needing to make some significant XO changes to both the woofers and tweeter after seeing the diffraction results on the sound. You could easily end up dropping the woofers'+tweeter's high-end by 4-6db and dropping a section of the mids by 6-9db compared to what you're seeing now to flatten things back out.
          So don't get too focused on tweaking what you've got right now without the diffraction, because that can change things a lot.

          After diffraction is used instead of the drivers' plain halfspace frequencies you'll probably have to enlarge the woofer inductor and cap, tweak the parallel cap+R notch that's next to the woofer inductor and add an L-pad pair of resistors to the tweeter (or maybe a single R if it happens to give good results).
          My first 2way build


          • #8
            Sometimes pictures are easier than words:
            Notch/filter thingy....I'm not sure exactly what capacitor value will work best for your woofers, but it'll probably be somewhere around there.
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            My first 2way build


            • wogg
              wogg commented
              Editing a comment
              That centers the notch at 3.7kHz, sounds about right but I'm guessing without seeing the response. I just like calculating stuff. F = 1/(2*pi*SQRT(L*C))

          • #9
            The 8 ohm resistor in that bottomless tank circuit looks kinda large. 2 or 3 ohms is probably more than enough.

            I drive way too fast to worry about cholesterol.


            • #10
              Hi Lout, Craig,

              Thanks for that parallel notch filter, indeed 2Ohms has a good effect pulling 4K area that down.

              I've not used the diffraction tool before, but if my hunch is correct the order of operations will be:
              • Choose the driver from the database
              • Input the specs of the Polk enclosure's internal volume & ports
              • Export the resulting FRD
              • Exchange new FRD in XO
              • Tune from there.
              I'll hopefully get some free time later this week to do all the above. :-}


              • #11
                Hi Everyone,

                I had some time today and added the notch filter and baffle step compensation circuit. I think I should note that these speakers will probably be placed near a wall, the room that they'll go in doesn't have enough space to put them on pedestals and move them into free space. With that in mind, I believe I put in a smaller 3Db drop instead of 6.

                For the notch filter, I removed the resistor and compared the results and the CAD software didn't show anything meaningful. For my knowledge, what is the purpose of the resistor and should its effect be evident in the CAD software's charts?

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                Thanks for taking a look at these.


                • #12
                  If your "shunt" cap (20uF, to gnd) was a Zobel-like thing (w/a resistor in-line w/the cap - to gnd), then you'd be okay, BUT ...
                  since it's just a simple 2nd order (cap to ground), the resistor in-line w/the (tiny) 1uF cap (sometimes they're even smaller) just adds some resistance so that high freqs (passed by the notch cap, and THEN the shunt cap) don't have a direct path to ground (= H.F. short). SOME amps would be OK w/that, but some will let the magic smoke out. (As you've noted, it doesn't affect the FR at all - it's just insurance.)


                  • #13
                    Hi Chris,

                    Thanks, I'm a big fan of insurance, a few resistors is going to cost a lot less than my amp. (Well, not much less, it's a pretty cheap amp)
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                    Speaking of amps, is that 3ohm load around 175Hz going to also pose a problem?


                    • #14
                      A 4ohm "nominal" system typically DOES show about a 3ohm minimum somewhere (usually in the valley above Z-max) - "8-ohm" runs near 6.
                      Nothing wrong w/LOUT's orig. 8n(ohm) value on the notch cap. Usually I'll use whatever the driver load is (4ohms - in this case).
                      By using just a 2n resistor there, you've cut your amp insurance in half ...


                      • #15
                        Hi Chris,

                        Thank you, and LOUT, for the advise. It took me a while to retrofit (fancy word for crossfingers and fire up the jigsaw)

                        Here's the final XO I settled on. Since this was a one-off test I didn't go crazy on expensive caps and inductors. I wanted to see what a "budget" XO sounds like first, the final cost was about $25 per board for the hardware and one burnt index finger.

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                        Here's the 95% finished product. I need to go back and make a new baffle since the original was specific to the original drivers.

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                        I'm still breaking them in with many hours of diverse music genre, my initial impression is that they are a tad bright, but voices stand out wonderfully. The bass is tight and pronounced when called for, and not boom'y, it should really come on well when the speakers are in their intended location, currently they're just sitting on the table in the picture 6'+ from any wall. Their efficiency seems to be in the low 90s, they're definitely more than my Classix IIs, but possibly less than the Tekton Mini Lore Monitors in my 2.1 HT setup. I'm using a fairly cheap Aiyima A07 amp and an Echo Dot (Amazon, Spotify, Pandora, etc) as the source.

                        I give these a WAF factor of 0.5. Where a 1.0, for my WAF, are some ESL11A electrostats from Martin Logan and a 0.1 would be anything that looks like a coffin or could wake the dead.

                        When I find the time I'll try an A-B-C-D ( D == C-Notes) comparison.

                        So, thanks again.

                        Like Columbo, I have one more question. What is the functional difference between the LR notch filter and a LCR?

                        Thanks again.