Close Box with LT circuit design, help needed

Discussion in 'Archived Threads 2001-2004' started by Dan M~, Jan 3, 2003.

  1. Dan M~

    Dan M~ Second Unit

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    I've been working on my Linkwitz Transform circuit/Closed box Tempest sub system and would like someone to double check me.

    Putting a Tempest in a 130 liter closed box and applying an LT circuit to produce an F(p)=15 Hz and Q(p)= .70, the LT spread sheet from True Audio's web site shows a boost of 15 dB's will be produced.

    Will the system really be flat (well -3dB) to 15 Hz? Should I worry about a rumble filter?

    Assuming that I have a PE 250 watt amp, what type of performance (SPL) should I expect?

    If I decide to go with another amp, at what power level will "bottoming" become a problem with the Tempest?

    Anything I need to watchout for with this design?

    Comments welcomed...
     
  2. Pete Mazz

    Pete Mazz Supporting Actor

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    That's asking a lot from a Tempest. As long as you don't push it very hard, really very little, it'll be OK. Max output is only ~100 dB @ 1 meter.

    I would shoot for more like 20 Hz and .6 Q. This'll get you ~106 dB/1 meter and only 5 dB down at 20 Hz. Room gain should make up for at least this much.

    Pete
     
  3. Dan M~

    Dan M~ Second Unit

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    Hi Pete,
    100 dB at 15 Hz sounds great considering I have been getting zilch below 30 Hz from my previous sub.[​IMG]
    I'm not looking for extreme SPL levels, the house could not take it. (I own the house, so it matters!) If I can get a clean, flat response curve down to 20 Hz (+/- 0 dB) at 85dB I'd be happy! Is 250 watts enough to achieve this with some headroom?
    I have some music that has notes in the 16 Hz range, I'd like to experience those. [​IMG]
     
  4. Mark_E_Smith

    Mark_E_Smith Second Unit

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    With those parameters you should look at the PE 15" DVC, it needs a smaller box and can make 105 dB @20 Hz with 6 dB room rise. I will be building one with one of the LTs Anthony Gomez did in October. A 22" cube. I to dont need to shake the house, just tight clean bass! [​IMG]
     
  5. Pete Mazz

    Pete Mazz Supporting Actor

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    Don't forget....100 dB/1 meter. That's 94 dB/2 meter, 88 dB/4 meter....depends on listening distance from sub. Of course room gain will add to these numbers, along with boundary loading if you have it in a corner.

    In your example box, the Tempest is excursion limited before the AVA 250 clips. Most of your available power will only be used at the lowest freqs, of course.

    Pete
     
  6. Dan M~

    Dan M~ Second Unit

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    Mark_E-Smith-

    The problem with a smaller box is the need for more power to achieve the same output levels. It's a "Catch 22" situation, and you must compromise as your personal requirements dictate. I (personal opinion) feel the Tempest is a better driver then the PE DVC (a good driver in its own right). My box size is not a problem (at around 4 ft^3). With this volume, the power requirements are reduced, but driver excursion is greater. For a 2 ft^3 box (and the Tempest)I would need an LT that gave me over 19 dB's of gain to achieve the same output!!! (At less total excursion.) The PE DVC would likely need more power, but I have not run the numbers. My concern is how well the driver is protected, the Tempest has an Xmax of 1.2 inches p-p, so I have some room.

    "Subwoofer design is one big trade-off after another."

    Pete Mazz,

    Thanks again.

    Anyone seen Anthony Gomez, I'd like to hear his comments on my set-up?
     
  7. a 110L box (22" cubed with 1.5" walls) looks to be your best bet with 250 watts. THere is nothing wrong with 130L, but you don't really gain much over 110L. 250Watts sets you up really nice excursion wise till 20hz. Since you want the lows for music, but not earth shattering, I would shoot for a Q=0.6 with an Fb around 20hz as already mentioned by Pete. It gives you good extension with room boundaries without sounding over bloated down low, or without over taxing the driver (with your level requirements).
    you DO NOT want to use any rumble filter with an LT. The group delay shoots way up with a second order filter. You really only need the buffer stage (1st order way down low to block DC). By adding a rubmle filter that overlays onto your low Q curve right around the knee, you kind of are just raising the Q. I would see about dissabling the rumble filter or moding it to have an Fc of 12-14hz and a Q of 0.707.
    Hope this helps.
    Sorry for not chiming in early. I like to read other's opinions on LT's before I chime in. [​IMG]
     
  8. Janne Ahonen

    Janne Ahonen Extra

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    Dan,

    If I found correct plate amp type (PE 300-793), then I believe that there is a peaky second order subsonic filter built right into that plate amp. It looks very similar than what is sold here in Finland with different brand. I believe they are identical.

    If that is true, then LT won't help much, unless you modify that plate amp and either remove subsonic filter or change it's fc radically lower. Original peaking is about 4.4 dB at 31.3 Hz (computed from nominal component values). -3 dB point of that subsonic filter is about 19.4 Hz.

    And, it should be noted that box size doesn't actually change maximum excursion limited SPL, it only sets power needed to reach that particular maximum, at any given frequency. That is logical, since the only thing which will produce sound in closed box is the driver.

    With Best Regards,
    Janne
     
  9. Dan M~

    Dan M~ Second Unit

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    Anthony and Janne,

    Thanks for the input. I was using the PE 250 as a reference point. I really do not like the built-in plate amps, they make it difficult to "upgrade" later. I'm building a phase switch into the LT to allow other choices amp wise. What would be my power limits for a 130 Liter box (already in the works) with a Fp=20 and a Qp=.6? I've been looking in the 300 to 400 watt range (For dynamic headroom).

    You all are a great help.
     
  10. I LIKE to have +3db of headroom assuming that at 0db, the driver is at the linear limits. This usually keeps the amp distortion to a reasonable minumum.
    If you were to go by this, then at 250watts, you reach the linear limits at 20hz, so I would LIKE to have 500watts. As you indicated, this is just headroom to keep the distortion levels down and not expected to pump 500watts into the Tempest.
    Any opinions on my opinion ? [​IMG]
    As for plate amps, these are nice, albeit expensive.
    http://www.artofsoundonline.com
     
  11. Dan M~

    Dan M~ Second Unit

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    Anthony-

    Thanks.

    While I'd like to stay away from plate amps, those AOS look nice. Do you have any first hand knowledge of how they actually perform?
     

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