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I'm melting coil bobbins. Maybe.

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  • I'm melting coil bobbins. Maybe.

    This is long.

    When I was experimenting with subwoofer solutions I decided a passive sub based on a DVC driver seemed like a good idea to try.

    I purchased a (used) Canton DVC 12" woofer and crossovers on eBay, thinking I'd build an enclosure and have some fun.

    A few weeks later, before I had a chance to get started, someone listed a complete Canton Plus-C subwoofer for cheap, like $75 or $80 including shipping, so I ordered that.

    Overall I felt the output was good enough but I wanted discrete subs, one under each satellite, because there were times where localization of the bass was a problem.

    And I've been thinking which way I want to go crossover-wise, passive or active with another amp.

    BUT TODAY, while pondering my way forward, I felt it would be an interesting time to open the Canton sub and add stuffing. For a long time I was pretty sure I'd open it and find some white pillow-like stuff and not nearly enough of it. This 12" driver was used in a sealed box that was woefully undersized.

    I told you this was long.

    So I pull the sub out from under the table and remove the driver and sure enough, I find not nearly enough pillow-type stuffing (the cabinet is about 1/3 filled). BUT I also spy that two choke bobbins are melting/separating.

    These would be the smallish air-core chokes in the high-frequency circuits. There is a 200-uf cap in series and the coil is a shunt to ground after the cap.

    Now, I should have measured the D of the wire, it is very small compared to chokes I've used in the past. I thought I'd have a chance to measure the wire later because I have the spare crossovers.

    But now that I had a chance to examine the spare crossovers, I see they're made kind of funny, the bobbin has three "ends," one of these ends is just 1/16" or so from the other, and there is a single wrap of wire in that part of the bobbin that is the lead connected to the circuit board.

    I'm wondering whether the thinner wire has enough resistance that the coil is heating enough to cause problems. I think it very literally melted right through the plastic bobbin's center on one of the chokes and the coil of wire is kind of dangling there.

    I don't know if I caused this or if a previous owner had connected something incorrectly and caused it.

    I do listen at higher SPL at times but my satellite drivers are only 3FE22 4-Ohm drivers, so I'd think if I was melting plastic I'd be taking-out my satellite's voice coil.

    Any thoughts on this? I actually put fiberglass back into the sub and put it all back together so I at least have something to which to listen while I'm building my new subs w/ the GRS drivers.

    The first picture shows the two actual xovers installed in the cabinet. The next two photos show the spare crossovers and hopefully you'll be able to see the construction detail of the bobbin for the choke in the HF section.

  • #2
    It's only the skinny wire that would get hot. Test the cap- if it is passing more than it should, more will go through the coil than should.
    You can map out your cicuit and calculate the power dissipation of the coils.
    If it's used, the previous user may have abused it- it may be fine with you operating it.

    Comment


    • #3
      Thanks David.

      The amp I'm using is an SMSL amp that while rated at 160-WPC, comes with a 180-watt AC adapter. So I have to imagine that while I have listened at higher SPL, that I haven't exceeded the 70-WPC rating of the Canton sub (which should cover the crossover).

      I do wonder if the reason it was sold was that the previous owner fried the satellites and decided to just sell-off the still working subwoofer.

      Now I have to decide if I want to continue debugging this thing or work on the new ones. I think work on the new ones.

      Comment


      • #4
        First let me say that power ratings should be taken with a cattle-sized block of salt. Second, while I cannot comment on the circuit design, those do not look like very well constructed crossovers, with layout that encourages inductive coupling(which changes with frequency and level). I'd say you would be better off making your own, and your drivers would be better off as well. Like fixing an old house- it's far easier to build from scratch. Safer too.

        Have fun!

        Comment


        • #5
          Originally posted by davidB View Post
          First let me say that power ratings should be taken with a cattle-sized block of salt. Second, while I cannot comment on the circuit design, those do not look like very well constructed crossovers, with layout that encourages inductive coupling(which changes with frequency and level). I'd say you would be better off making your own, and your drivers would be better off as well. Like fixing an old house- it's far easier to build from scratch. Safer too.

          Have fun!
          The (physically) larger coil is entirely encased in ferrite. Has anyone seen that before?

          I'd think that would prevent any coupling?

          It is a strange beast of an xover.

          Comment


          • #6
            If you can find the parts list it might have the values for the part numbers shown (if you can't measure). If you get the values and the wiring diagram it would be easy enough to put in Xsim and calculate the power dissipated by each component. Not sure how a 5 watt resistor survived.
            John H

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

            Comment


            • #7
              Did that experiment in jr. high. The same mass of iron inside a coil vs outside. Outside coupled a little less, same order of magnitude. Can't remember exact details, it was 1970 or so, and, well, you know.

              Comment


              • #8
                Originally posted by davidB View Post
                Did that experiment in jr. high. The same mass of iron inside a coil vs outside. Outside coupled a little less, same order of magnitude. Can't remember exact details, it was 1970 or so, and, well, you know.
                When I have time to come back to this project I might play with that iron-covered coil and see how it reacts.

                Comment


                • #9
                  Originally posted by jhollander View Post
                  If you can find the parts list it might have the values for the part numbers shown (if you can't measure). If you get the values and the wiring diagram it would be easy enough to put in Xsim and calculate the power dissipated by each component. Not sure how a 5 watt resistor survived.
                  I'm definitely going to come back to this, I'll get the new subs built and then open this Canton again and desolder those coils and measure them (I have a Woofer Tester 2). I can measure the caps, too.

                  Then if you guys have the time and the willingness you can tell me what I need to do to sim the power across the melted coil.

                  That 5W resister is 1-Ohm. So the coil is in series with the sub driver, and the 1-Ohm resister and cap form a shunt to ground.

                  What is the purpose, then, of that 1-Ohm coil? Anyone know?

                  Comment


                  • #10
                    The resistor helps shape the knee and roll off of the low pass filter. While the value is nominal it could help with phase alignment between the woofer and mid. It's not clear to me how many circuits are on the x-o board.
                    John H

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

                    Comment


                    • #11
                      Originally posted by davidB View Post
                      First let me say that power ratings should be taken with a cattle-sized block of salt ...

                      Have fun!
                      +1. With the 36V PS you'll get <80 W rms into 8 ohm speakers.

                      Comment


                      • #12
                        Originally posted by Millstonemike View Post
                        +1. With the 36V PS you'll get <80 W rms into 8 ohm speakers.
                        What about 4-ohm?

                        Comment


                        • #13
                          Originally posted by philthien View Post

                          What about 4-ohm?
                          Double that for 4 ohm drivers to 160 W - hence the amp's rating. But it will also depend on the PS's amp capability.

                          Sorry, I should have re-read your original post specifying 4 ohm drivers.

                          Comment


                          • #14
                            Originally posted by Millstonemike View Post
                            Double that for 4 ohm drivers to 160 W - hence the amp's rating. But it will also depend on the PS's amp capability.

                            Sorry, I should have re-read your original post specifying 4 ohm drivers.
                            Boy I sure hope my post didn't come off as mean-spirited, I was actually hoping to confirm these small digital amps conformed to the double power into half the resistance rule.

                            There was no need to apologize, I'm the one that should be apologizing to you guys for the avalanche of dumb questions I tend to post in clusters.

                            Comment


                            • #15
                              Having taken apart a pair of Canton speakers before, this doesn't surprise me. They use incredibly cheap parts in the crossovers. Basically the cheapest possible.
                              -Kerry

                              www.pursuitofperfectsound.com

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