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  • MCM 15" speaker defect

    I don't know how common it is, but in my case I've had a 75% success rate with the build quality of the MCM 4ohm 200W 15" speaker some of you may be familiar with. The defect it easy enough to notice if you look for it, so that's why I'm mentioning it here.

    I don't think the 8 ohm version does, but the 4ohm has little holes in the frame just below the spider that let you barely see the voice coil. If you see about 12turns of wire, then it's probably normal, but on one of mine I saw none. The front of the coil was lined up with the front of the magnet causing very uneven excursion.

    Any ideas of what to do with it? Try fixing it, or get a useful distortion out of it? As of now it's stronger pushing air than pulling.
    Allen 530A

  • #2
    The best choice is probably to return it for replacement/refund.

    If the cone is very heavy (and a low resonant frequency is an indication of cone weight if it isn't specified) and it has been stored on its back or face too long the cone may have drifted from center position. Storing on the other side may return it to the center position if it is cone drift.

    You could also try a break-in test, typically 20 Hz at very low excursion for several hours to flex everything, but do that only if you don't intend on returning it.

    But it would be best to get it replaced.

    Comment


    • #3
      That's a bad deal and I hate that happened to you. Rare though. In all the years I've used MCM speakers, I haven't had more than two or three defective ones out of the several hundred I must have ordered. But I'd suggest that you try to call them and have them send a new one. It's a real pain to have to return a bad one, and may cost you more than just ordering another speaker.

      The two or three times I got a bad one, it would be in an order of a dozen or more, and I just tossed them in the trash, as the time to re-box one, take it to the UPS store four miles away, and all the rest of the hassle, was simply not worth it to me. I've made a lot of money using MCM drivers in organ jobs, since they are generally very inexpensive and 99% of the time they are good, so I just chalked it up to the cost of doing business. That may not work for you though.

      If you're very careful you might be able to repair it, and you might even enjoy the process, learning something about the way these drivers are assembled.
      John
      ----------
      *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

      https://www.facebook.com/pages/Birds...97551893588434

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      • #4
        I got a credit, I could have gotten a 100% free replacement if I took the time to register instead of doing a guest checkout. They were all in the original box of 4 from the manufacturer, so it probably didn't hold an off center position. I'll still check though.
        Allen 530A

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        • #5
          So, they are rated 200W RMS, and since they are sitting around as I'm not quite ready to use them, I decided to work on breaking them in. When I first looked at the voice coils I guestimated based on the wire gauge used that 5A would probably generate a good bit of heat making me feel safer not exceeding 100W RMS just to be on the safe side. 100W is a good bit of power anyway. After 5-10min @ 50W RMS (which should peak about 100W? which uses 5A) I made my temperature checking rounds out of habit, and noticed the dust caps were actually warm: 115F. The coils would probably be a good bit hotter, but I think enamel coated wire can take nearly 400 degrees, and some plastics around 300F.

          Anyway just a little fun fact in how they perform, I've never had warm speakers before. It's too easy to assume little power is being used when I almost hear nothing. A weird noise startled me until i realized the power supply fan kicked in (I was using 150W). I later remembered I could have run them at a fraction of the power at their resonance frequency (27hz), but at the time had chosen a lower frequency peak in excursion.

          I'm going to try some MCM $1 piezo tweeters too. They say they can take 100W. I don't know much about piezo speakers, but at least on others in the past I can't measure their DC resistance, and I've read some of those can handle maybe 200-300v if I remember correctly. I just don't see them drawing 5W especially considering the low amount of power needed for high frequencies, not like I'd even try to drive them up to 5W. I guess if you want them controlled by a crossover, a standard value, such as 8ohm, resistor needs to be used in parallel with it?
          Allen 530A

          Comment


          • #6
            Interesting that the speakers got hot. Not surprised though, as the test you did is far more grueling than any kind of real-world musical application. I've monitored the power going into speakers on an organ a few times, just to see what was happening. About 99% of the time, even with LOUD music, the amps are typically putting out only a watt or two. Yes, 1 or 2 watts! That's with normal fairly efficient speakers such as HC-15s reproducing wide-range music. When a low pedal note is played on an organ and held down for several seconds, as might happen in a real musical piece, the amp might be putting out 5 to 10 watts if the room is large, but I can't imagine an amp having to supply 50 watts for several minutes without a break! Even a massive subwoofer such as the Allen SR-1 would probably get hot with that kind of power applied.

            The only reason organs are equipped with "100 watt" amps is so they are unlikely to ever clip, even on transients that might last a fraction of second and actually draw closer to that 100 watts, which could theoretically happen with a huge tutti chord with a percussive attack transient. But only for a fraction of a second. So your running 50 watts into those coils for 5 to 10 minutes is extreme torture for a speaker, compared to the reproduction of actual music.

            As to piezo tweeters, they of course have nothing inside that actually burns up any power. If you open it up, you'll see a small white disk of ceramic-like material with two electrodes glued on. Electrically, this unit appears to the amplifier as if it were a small capacitor, probably about .35 mfd or so, depending on the model. That means it becomes almost like a short circuit to extremely high frequencies. So I always practice safety by installing a resistor in SERIES with the piezo unit, for the protection of the amplifier.

            The piezo element is probably responding only to the variations in voltage at the amplifier outputs, and not to the amperage or power. And there isn't any way to reliably construct a crossover network, since the piezo is in fact a capacitor, for electrical purposes, and it simply responds to all the frequencies represented in the amplifier output voltage. Because the apparent impedance of the piezo goes up dramatically as the frequency drops, it is not going to respond very much to low frequencies, and that is why they are advertised as needing no crossover.

            Someone posted a link a while back to a special crossover network that had been designed to actually control the frequencies sent to a piezo tweeter. It involved using a capacitor and resistor that served as a high-pass network, and the voltage appearing at the junction of the capacitor and resistor could be tapped and routed into the piezo. That might work to some extent. I know Rodgers used a circuit something like that in their old analog speaker cabinets that had piezo tweeters.

            For my purposes, I seem to get decent results from a piezo by using a resistor of about 20 to 50 ohms in series between the amp and the piezo. Protects the amp from the "dead short" that the piezo is at super-sonic frequencies and also takes away some of the harshness of the tone.
            John
            ----------
            *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

            https://www.facebook.com/pages/Birds...97551893588434

            Comment


            • #7
              Yea, I know I was way overdoing it time wise, and without an enclosure I almost couldn't hear them. Eventually the magnet and frame will gather heat, and help the coil get even hotter, so the 200W continuous rating is likely not intended to last for hours. The 4x100W Sure amp started clipping too at 50W RMS or more, but I realized it was peaking over 100W. I need to check if it is rated for 100W rms or what. It stayed cool, except for 2 little chips that don't have a heatsink. Those ran very hot.

              Running my HC-15s at 10W gives me a headache.
              Allen 530A

              Comment


              • #8
                I got an MCM 12" too to test when I ordered a replacement 15". I noticed these seem to have a lot more coil overhang, and possibly larger wire. I was breaking it in using my phone as the sound generator, and once when I went to check my phone I accidentally hit the volume up button. I didn't hear a bad sound, so I checked the excursion at that level, and it looks like it could have been moving a total 2cm at around 60W (at the resonance of 29hz). I don't know if exceeding the 1cm total Xmax would cause much noticeable distortion, but these, and I think the 15" too, have extremely large Xmech. I think due to the extra coil overhang on the 12", it might still work well beyond 1cm.
                Allen 530A

                Comment


                • #9
                  The power rating on most speakers is the maximum thermal power that the speaker can absorb. There are two other power ratings which can't be determined without knowing the speaker enclosure design and low frequency limit--those ratings provide the maximum electrical power allowed that doesn't exceed the excursion limits of the speaker, and the resulting acoustical power. These are much lower than the thermal power rating.

                  Also, be aware that when an amp is clipping, it is providing much more power to the speaker than the RMS power provided by a sine wave. This is because the clipping provides the maximum power for a greater percentage of the waveform than a sine wave provides.

                  JBL Pro used to publish, and might still do so, a guide that discussed why an underpower amplifier is much more likely to damage a speaker than an amp with greater power. The clipping does nasty things to speaker drivers, and is much more likely when an amp is has a low power rating.

                  The "break in" of a woofer is typically set just so that the cone excurses to its linear limit (X-max), and not by the amount of power you are providing it. X-max is achieved at very low power levels when the speaker is not enclosed, since there is almost no air mass to resist the excursion.

                  Comment


                  • #10
                    That's what I thought about the break in procedure. I ran it at about 8W and around the rated Xmax
                    Allen 530A

                    Comment


                    • #11
                      Another note about the 15" speakers; sometimes they have so much epoxy under the foam around the rim that it is squished out against the surround and makes a tapping sound as the surround hits it. It should be easily cut off with a sharp X-acto knife
                      Allen 530A

                      Comment


                      • #12
                        I used a pair of these to replace the very expensive EV drivers in some Allen HC-18 cabinets recently, and they worked beautifully. Smooth and solid bass and apparently just as efficient as the originals for probably 1/10 the cost! I continue to be impressed with the quality and value of the MCM brand drivers, even though you get a clunker now and then.

                        As you know, Allen did not use the HC-18 design for very long. I suppose it was a transitional cabinet between the HC-15 and the HR-200 which is now the standard full-range unit. But the HC-18 was used as a "universal" cabinet for a while, and some MDS organs were shipped with these units on all channels instead of having 15's on most channels and a 12 on the 32' channel. But the 18's had a fatal flaw -- the foam in the EV woofer -- and they are all failing now, just like the HC-12's.
                        John
                        ----------
                        *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

                        https://www.facebook.com/pages/Birds...97551893588434

                        Comment

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