my first MTM setup

Discussion in 'Home Theater Projects' started by Matt Dolejsi, May 14, 2003.

  1. Matt Dolejsi

    Matt Dolejsi Agent

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    I'm looking to build my first MTM setup and I was wondering what the typical impendence setup was to get the final speaker to round out to about 8 ohms, My denon AVR1700 will do fine with 6 ohms it says so thats a possibility (does anyone know if it will have a problem with 4 ohms). Anyway, my tweeters have been purchased and they are 8 ohms, what impendece should i look for in my mid drivers. Any input will be appreciated. I'm looking to build sealed, not vented cabinets.

    i guess that'll do for my first post
     
  2. Matt Dolejsi

    Matt Dolejsi Agent

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    seriously, its not that i haven't looked around, and no i'm not a total newbie, i've build alot of HT and car Stuff, but never an MTM. I've been looking around particularly the most on partsexpress and the MTM systems there are mostly 4 ohm nominal, If someone could confirm that a Good denon can handle that, i'll be glad to use that as a framework
     
  3. Aaron_Morris

    Aaron_Morris Stunt Coordinator

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    I am curious about this as well. Sorry I have no real help for you.

    I'd like to know if matching the sensitivities is also advisable.
     
  4. Darren_T

    Darren_T Second Unit

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    I can confirm that a good receiver can handle a 4ohm load. I was powering five 4ohm AV1+ MTM speakers with my Onkyo receiver and it had no problem, it did get a bit warm at higher volumes but with 5 speakers I expected that and placed a small computer fan on top of the case to suck the hot air out. I did that for about 3 months of daily use without any problems before I bought separates. I'd think a good Denon receiver would have no problems.
     
  5. Mark_E_Smith

    Mark_E_Smith Second Unit

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    You have 2 choices, 2 4 ohm speakers in series or 2 16 ohm speakers in parallel. The reason most MTMs are 4 ohm is they use 8 ohm speakers in parallel. I hope your tweeter has a low Fs so that you can cross it at about 2K to 2.5K.
     
  6. Matt Dolejsi

    Matt Dolejsi Agent

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    i can cross the tweeter at 1800hz, possibly lower with some tweaking

    So you guys don't think that i'll have a problem with running a 4 ohm load on my denon, if it does cause a problem it should shut off before problems correct???
     
  7. you can get some 16ohm midwoofers from GR research and wire it at 8ohms. Danny should be getting the new 5.25" 16ohms one in soon. He might be out of the 6.5" 16ohm ones.
     
  8. Darren_T

    Darren_T Second Unit

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    You can test your receiver out with the 4 ohm loads...if it's protection kicks in at any time I'd stop using it for 4 ohm loads. The protection circuitry will only protect the equipment for a short period of time before it eventually damages the receiver, speakers or both. I myself never had any problem but you may want to consult your manual or talk to Denon customer service.
     
  9. Matt Dolejsi

    Matt Dolejsi Agent

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    what about 6 ohm woofers in series, and then paralleled with the tweet, that would yield a little more than 5 ohms and could be easier on the receiver, but if the receiver isn't a problem then i'll be fine, thats my biggest concern, i tried emailing denon and they didn't respond, its been at least a week
     
  10. Patrick Sun

    Patrick Sun Moderator
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    Simplisticly speaking, the parallel hookup of 2 midrange drivers and the tweeter (through the crossover) does not mean the nominal impedance is the paralleled impedance of the 3 drivers. Instead, more likely, the nominal impedance is the lowest of the impedance of the tweeter or the midrange drivers (wired in series or parallel).

    But that's not the full story, as impedance varies with frequency, and if the total interaction of the crossover and the drivers creates low impedance levels at low frequencies, the amp might clip at loud volume levels (due to large current demands on the amp).

    I would avoid wiring midranges up in series if possible (you can run into the problem with the back EMF from the first driver causing not so pleasant audio output for both of the midrange drivers). It's much "cleaner" to deal with paralleled drivers in terms of crossover design.
     
  11. Matt Dolejsi

    Matt Dolejsi Agent

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    thanks for that input on series/parallel wiring, so i should try to have all the drivers run parallel, or as long as the mids are in parallel i can run them in series with the tweet??? This is going to get interesting, i might end up with something like a MTMT or TTMM or MTTM, any input on that, i find i have more wiring flex that way, would it sound terrible????
     
  12. Patrick Sun

    Patrick Sun Moderator
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    I'm not quite sure you are grasping the fundamentals here.

    The crossover for the speaker is composed of filters for each driver.

    For a MTM, that's basically a 2-way speaker. There's a highpass filter connected to the tweeter, and a lowpass filter connected to the midranges (connected in series or parallel). The lowpass and highpass input side is simply parallelled from the amp's positive and negative terminals via speaker wire. Sure, you can bi-wire (but it's effectively connecting the filters in parallel), or you can bi-amp, where there's individual amps for each filter that is connected to its driver(s).

    The typical 2-way crossover network looks like this (just pretend there are paralleled midranges where the single midrange is shown):

    http://www.patcave.com/suntwo/s2-xo.jpg

    In the odd case that a MTM is a 2.5 way speaker (a very non-typical speaker configuration), there's 3 filters, and each one is connected to one of the 3 drivers. One filter is highpass for the tweeter, one filter is lowpass for the upper midrange driver, and the last filter is a lowpass (where the frequency cutoff down low is around the baffle-step frequency region, or below).

    This is what the crossover diagram looks like for a 2.5 way speaker crossover:

    http://www.patcave.com/sunmain/s1-xo.jpg

    I can't think of a good reason to connect the midranges (however you connect them, parallel or series) in series with the tweeter. That just doesn't make sense.

    The integration of the tweeter output and the midranges' output is crucial for optimal sound quality from the speaker. The crossover design portion is what makes or breaks a speaker, no matter how good the drivers used in the speaker. If you plan on using stock crossover networks, you may get lucky and get a speaker that sounds okay, but it'll be a rare occurrence that the speaker will be optimized to take advantage of each driver's strength for the best sound quality possible, if you use stock crossovers.
     
  13. Matt Dolejsi

    Matt Dolejsi Agent

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    Ok, interesting about the impendences

    ************************************************
    Simplisticly speaking, the parallel hookup of 2 midrange drivers and the tweeter (through the crossover) does not mean the nominal impedance is the paralleled impedance of the 3 drivers. Instead, more likely, the nominal impedance is the lowest of the impedance of the tweeter or the midrange drivers (wired in series or parallel).
    **************************************************

    can you expand a little, so if i had an 8 ohm tweeter, and two 16 ohm drivers for the mids, my nominal should be around 8 except at certain peaks, (low hz)

    and i take it i shouldn't try 4 ohm series mids....could i benefit from simply using one 8 inch driver that matched well with the tweeter and make them both 8 ohm???
     
  14. ChrisAK

    ChrisAK Extra

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    The tweeter and the woofers are technically hooked up in parallel. However, the highpass filter takes the woofer(s) out of the circuit with high frequencies, and the low pass takes the tweeter out of the circuit with low frequencies. The problem with some 4ohm speakers is that they do not use impedence compensation. As it has already been stated the impedence for a woofer will vary with frequency. A 4 ohm speaker can dip to
     

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