Mcm 55-2421

Discussion in 'Home Theater Projects' started by Brad Boydston, Jun 1, 2004.

  1. Brad Boydston

    Brad Boydston Auditioning

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    I've read most of the comments on the MCM High Excursion (55-2421) 8" woofer, and it seemed like most people liked.
    I've put the specifications into Unibox and they didn't look very impressive. So, I decided the jack around with adding four woofers in parallel. Still...not too impressive. The sensitivity boost was great, but that was it. Then I started playing with changing the series resistances (RS) in Unibox.

    4 drive units in parallel with RS(series resistance) of .9 ohms yielded a 20.3Hz @ 107.3db's with 480 watts in a 128L vented box. Wow! I decided to increase the RS to 1.2 Ohms and increase the box volume to the tempest adire alignment of 213L. The four woofers put out just about the same amount of DB's as the tempest with LESS power.

    Am I missing something here? It seems like a viable option when you can buy four woofers for $27.49 each in quantities of four with free shipping until June 15.
     
  2. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Four woofers in parallel at 4 ohms each gives you a 1 ohm impedence. Put a 1.2 ohm resistor in series and it will burn about half of the power.

    Where are you going to get a 1.2 ohm resistor that can take 240 watts?

    How much is that going to cost?

    Do you really want to spend half of your amplifier power heating up a resistor?

    What does the resistor actually do for you anyway? If you're trying to extend the low-end response it would be much easier to just add mass to the woofers.
     
  3. Brad Boydston

    Brad Boydston Auditioning

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    >>Four woofers in parallel at 4 ohms each gives you a 1 ohm impedence. Put a 1.2 ohm resistor in series and it will burn about half of the power.

    I had no idea idea you would need a resistor that had to handle that much power. As far as I can see Unibox doesn't yield the final number of ohms that amp see's. Plugging 4 ohms in to a Parallel calculator, I can now see the amp would see 1 ohm load. Now I know the limitations of series resistance. WHile good for a single mid-woofer, it's not a very viable option on a subwoofer.

    >>Where are you going to get a 1.2 ohm resistor that can take 240 watts?

    Maybe NASA has one? :O

    Things I've learned in this post...

    A series resistor is not a good idea for subwoofers.

    Four 4-ohm drivers in parallel yield a 1 ohm load.

    Adding mass to a woofer extends low end response.
     
  4. Rory Buszka

    Rory Buszka Supporting Actor

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    Adding mass to a woofer also kills sensitivity. The result is you need more power to output a given frequency after mass has been added to a driver.

    The MCM 55-2421s have also been known as a not-that-durable driver, with the spider losing its spidey-ness after a few months, causing a shift in the T/S parameters.
     
  5. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Brad,
    I don't know about Unibox, but I do know that WinISD pro has an easy input for added mass on the woofer. You can use this to see how it changes the performance.

    Also, if you want to wire the 4 speakers to achieve a 4 ohm load then you can wire in parallel/series. That is, you wire woofer 1&2 in parallel with each other and 3&4 in parallel with each other. Then wire the pairs in series to connect them all up.

    Rory,
    Yeah, it certainly will reduce the sensitivity. But I figured that burning half your power in a resistor would sort of kill the sensitivity too. So if Brad was happy with the 1.2 ohms in series with 4 parallel woofers then he'd probably not mind losing a little sensitivity.
     
  6. Geoff L

    Geoff L Screenwriter

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    Unless they have done something with the spider and surround on that 8", it's good for about a month and goes kapot.

    The suspension sags out and ends up being basicaly worthless. Now these where with MCM~8s that where shipping 6 months ago and maybe this has been addressed.

    A google search for the driver # will bring up others (posting in forums and personal web site info) who have experienced the same problems.

    In the end, the TB-8 is the better built choice if your looking at cheaper long throw 8's for small sub design. BUT,,,,they have been said to have their own problem also, with quite wide varying TS-params with each driver...Vas and FS sometimes showing quite a large difference from TB's posted spec's.. YMMV

    Regards
    Geoff
     
  7. GabeT

    GabeT Extra

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    Yea the Tang Band driver is IMO superior to the MCM. It has a better spider and surround, and bigger magnet (for what thats worth) than the MCM. I would use that, it seems more durable... And the differences driver to driver Im sure are VERY miniscule..
     
  8. AllanRW

    AllanRW Second Unit

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    [​IMG] [​IMG] [​IMG] [​IMG] [​IMG]
    Here is the data sheets that came direct from TB when the 740 drivers were ordered.
    Never mind the data sheet that Nuera ever posted wrong data.

    Al
     
  9. Geoff L

    Geoff L Screenwriter

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    Hi Allan

    Didn't you *take an average* for the TB-W8 drivers TS-Params {that where actualy measured}, and use them instead of TB's site data??

    And if so what did the average #'s end up being for FS, Vas, Qts, etc....

    Sorry if you posted this already, as I must have missed it...:b

    Thanks for the jpg data sheets, tho with my junk monitor it's a bitch to see some of the numbers

    Regards
    Geoff
     
  10. AllanRW

    AllanRW Second Unit

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    Yes i did have a average from the 12 speakers that the data sheets were taken from.
    But I do not have that data with me i will post it later.

    Al
     
  11. Cam McFarland

    Cam McFarland Supporting Actor

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    Allan....what is that a picture of.......[​IMG]
     
  12. AllanRW

    AllanRW Second Unit

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    My old test room.
    Things spead out since then.

    Al
     

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