Impedence in MTM designs ... need clarification regarding parallel or series wiring

Discussion in 'Home Theater Projects' started by Dan Miller, Mar 2, 2004.

  1. Dan Miller

    Dan Miller Extra

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    Hello,

    I wish to build some MTM style of cabinets for HT. All the speakers I have in mind are 8 ohms. This means that I can either wire the mids to show 4 or 16 ohms. My receiver wants to see 8 ohms. I think most crossover schematics, that I have seen, regarding MTM setups, illustrate parallel wiring of the two mids. Does this not make 4 ohms? Do most HT receivers, of today, tollerate this? I plan on buying a Yamaha RX-V740 ... or something close to this.

    Are there any 6 1/2" shielded speakers, suitable for HT use, you guys may have in mind, that come in either 4 or 16 ohms? My price limit, per speaker is about $30.00. I am leaning toward the PE catalog. I figure one phone call and get it all at once.

    Do I just go ahead and follow the D3 plans? Are there enough components, involved in the crossover design, to sort of "fool" my receiver into thinking the MTM setup is close to 8 ohms? My local Yamaha tech rep says "Eight ohms, it's right here in the book".

    I have soldered up some vacuum tube guitar amps in my day but this new solid state stuff makes me wonder. It is no big deal, for me, when a tube amp starts smokin'. This I can deal with. Blowing the top off a few tubes is no real panic. But I have no desire to smoke my new HT setup, because I'm not goin' in the fix it!

    The project I really want to do is the D3 Dayton design that's floating around. This was done by "The Reverend Wayne" ??? The two fronts and the center channel really appeal to me. I also very much like the idea of a fourth driver installed in the center channel cabinet. That extra speaker, in the center channel cabinet, gives me no mental stress.

    My rears will be based on single Audax 50w AT170MO 6 1/2" at 8 ohms. This is simply because I already have a pair that were given to me. These want a 1.00 cu ft box, according to the data published in the PE catalog. I will find some tweeter to go with them. Likely the Dayton 275-075, which is the shielded version, of the 275-070 tweeter, I will be using in the front array.

    My subwoofer will be the default setting of a Dayton Titanic MK 2, in a sealed 54L box, with a PE 250w plate amp.

    I want the satisfaction of the "first project" building process but do not want to struggle over speaker selection or cabinet math. There are many fine examples, of low cost, great sounding speakers, that have already been completed and found to be effective. The D3 project looks just fine to me and the comments made about it back this up.

    Any comments on the above will be met with gratitude.

    Thanks,

    Dan Miller
     
  2. Dan Wesnor

    Dan Wesnor Second Unit

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    The extra sensitivity you get from wiring in parallel may offset the low impedance, but there's no way to be sure your receiver won't overheat except to try it.

    GR has an MTM using 16 Ohm woofer which will give you an 8 Ohm load, so you might want to try it to be safe.
     
  3. Jesse Sharrow

    Jesse Sharrow Supporting Actor

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    I am pretty sure that the yamaha can handle a 4ohm load anyways. Look in your manual, it should tell you.
     
  4. Javier_Huerta

    Javier_Huerta Supporting Actor

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    AFAIK, Yamahas have a switch to select between 4 & 8 ohms.

    At least both of my Yammies (RXV 995 & 992) had it. My Pioneer 859THX has one... and my H/K integrated has it, too! [​IMG]
     
  5. Rory Buszka

    Rory Buszka Supporting Actor

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    A friend of mine bought a bottom-of-the-line Yamaha recently and it has the switch. It's a current-limiting thing, to keep the amp from trying to put out twice as much power into the 4 ohms, because that is what causes the overheating.
     
  6. Allen Ross

    Allen Ross Supporting Actor

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    even if it doesn't have it, it should be fine just as long as you don't plan on hitting reference every chance you get. I have run my Onkyo on a 4 ohm load and it was fine, granted i didn't even attempt to push it to its limits, cause i know it couldn't handle it.

    Think of that double the power for +3db thing, its only when you are getting with in 90% of full out up will it start to over heat and then thermally shutdown, or something like that, my audio guru buddy told me this when i was contemplating MTM designs.
     
  7. Dan Miller

    Dan Miller Extra

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    Guys ..... Thanks for your input. I have a few more questions involving speakers in general.

    Dan Wesnor ...... Thanks for mentioning the GR line of drivers. I will probably go with the M-130/16 for all three front speakers. Maybe GR will help out with some opinion on which of their shielded tweeters to use. The GR-T1 still looks good to me even up against the newer GR tweeter.

    I wish to down size the cost of this project but still enjoy something I built myself. I will ask if GR can supply some average quality crossover components as well as supply some advice on crossover values and cabinet sizes.

    Not knowing beans about HT speaker setups, let me lay out a few ideas and you guys can come back to me with your thoughts.

    My receiver will be about 90 watts per channel. Can a single M-130 8 ohm handle the rear speaker position? Just how much speaker will I need in the rear positions to keep up with the array of M-130's up front? I will not play movies cranked but I want to feel I am there. Somewhere along the line, I will go for a PE 12" sub and 250 watt plate amp. This should give me enough "feeling" to make the viewing experience enjoyable. I just want my system to feel sort of balanced.

    By the way, what does "reference level" mean?

    I have come to this forum because the salespeople in the big local electronics stores know next to zero. On top of this, I only seem to be listening to firewood that squawks, whenever I am in the big box halls of low end offerings.

    The participants frequenting this forum have already answered so many of my questions, just by being active in various threads.

    Thanks,

    Dan Miller
     
  8. Javier_Huerta

    Javier_Huerta Supporting Actor

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    Dan,

    Rear speakers never hit (as a general rule) reference or high power levels. If you define them as "small" on your Yamaha you can be sure you'll never fry them. I'm using a couple of *very* small ProMonitor 80's (4" woofers!) with an 80W RMS THX-Select amp, and they have never bottomed out or did anything wrong. You'll be fine.

    Reference level, at least on my amp, means adjusting the volume level at 0 dB and calibrating every speaker to put out 75 dB's with the internal test tone generator. If you haven't, buy a Radio Shack SPL meter. It'll become an invaluable tool for you. Reference level is too loud for me, though. YMMV.

    Hope this helps!

    javier.
     
  9. Rudi B

    Rudi B Stunt Coordinator

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    Grab the AV2 kit. You'll have more than enough construction fun with it.
     
  10. Yohan Pamudji

    Yohan Pamudji Second Unit

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    This thread came up right on time. I'm trying to "help" a friend (he's doing the work really) with speakers he's putting together as a first time DIY speaker project and he's come across a problem. He says that he doesn't have enough sensitivity out of his woofers and wants to boost the output by running 2 identical 8 ohm woofers in parallel. The problem is that this will make the impedance 4 ohms. A few questions:
    1. His claim about boosting effeciency by putting another woofer on there doesn't seem correct. Wouldn't he be drawing twice as much wattage? That would make it half as efficient, right? Twice as many watts for the same output.
    2. Is there any way to combine a certain number of 8 ohm woofers to maintain an overall resistance of 8 ohms but also boost output without increasing necessary wattage?
    3. He got a pre-made crossover, one that on paper looks good for his original single woof single tweeter design. As long as the resistance remains 8 ohms will he be ok with the crossover he bought, or will he have to find another one for this new design?[/list=1]
     
  11. Joe Ku

    Joe Ku Stunt Coordinator

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    The D3 is a 4 ohm speaker but has a pretty benign impediance curve, your Yamaha will handle it just fine to very loud levels. Don't worry about it.
     

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