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PA385S build

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  • PA385S build

    A buddy and I are piecing together a PA setup. He picked up a pair of Mackie SRM450's. I'm DIY'ing some subs. A pair of PA385S are on the way. Power is a Crown DSi 1000. I was planning on 1 sub. When I saw only two left on closeout, I bought both. Haven't decided if I'm building 1 or 2 boxes. Do I need two to keep up with the SRM450 mains?

    What do you think of these three arrangements? All are 750watts in a 4 cuf box. Curve's attached.
    A. 45hz tune, green
    B. 37hz tune, orange
    C. 32hz tune, blue

    Since I dont know how low I need it to play, I'm toying with the idea of plugging one port to change tuning. The attached box with triangle ports should do 37hz with 4 ports or 32hz with one port stuffed/blocked off. I think I can arrange a box with 6 ports, 4" diameter allowing 45hz, 37hz or 32hz with 6, 4 or 3 ports open.

    1st sub build so I'm all ears. You wont hurt my feelings.

  • #2
    I think all your box models are valid (not that you asked).
    The idea of 6 4"id x 22"? long ports that you can open or close is GREAT, but ...
    6 ports of that size (IN-side your 4cf box) will consume over a cu.ft., leaving you w/approx. a 2.9cf box.
    2.9 cf seems a little small.
    I think your target Fb should be somewhere between 40-50 Hz.


    • #3
      Chris is right, you usually don't need anything below 40Hz in a PA system. It's a waste of power in most situations. It just eats up the usable excursion on woofers and sucks amplifier power very quickly. Electronic music (DJs) can go really low, but sound companies know to lock down the high-pass filters to avoid driver failure if they are providing support for those types of acts. I don't know as much about box tuning - others here are certainly more knowledgeable in that area. I think there were folks using this driver in a tapped horn with some success. Search diyaudio forums - I think it was the THAM design.

      Never heard the Mackies, so I can't comment on that. But more subs is nearly always a good in live sound! Since your woofers are 8 ohms, you are only going to see 275 w/ch from that Crown amp. You could run just one sub & bridge the amp. But 2 subs would be better, right? If run 2 subs in parallel, you are at 4 ohms. Well...even though Crown says that amp is capable of running 4 ohms bridged, it generates more heat and distortion. Most "seasoned" sound guys avoid running 2 ohm loads, since excessive internal temps can mean a higher failure rate in electronic devices. I now my QSC PLX amps become rack mounted hair dryers trying to run subs at 2 ohms! Not what you want when someone is paying you to play music.
      Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.


      • #4
        Thanks for the info.

        Triangle ports are not recommended and round ports got too busy so I've settled on an approximately 37hz slot port in a 4.5cf box. To keep port velocity down the port grew until it wouldn't fit in the 4 c.f. box. 4.5 cf and 37hz or higher was enough to fit a straight port which hopefully stays under 17 m/s.

        The slot port is in addition to the 4.5 c.f. space. Tuning to 45hz gets another 3~4db but I want solid 40hz capacity for hip hop, etc. A single 4x8 sheet should build the box. If I don't like it, I can cut the port shorter to retune higher. Worst case I burn it and its a learning experience.

        Originally I was planning for one sub bridged 8ohm mono from the amp = no problem. I saw only 2 left in stock, so I bought both. I'm hoping the amp can handle 4 ohm mono. I've received my two and the website still shows two available.... go figure.

        I've never done PA stuff, nor do I plan on ever getting paid. This is a low key "culdisac party" setup.


        • #5
          1st box is built. Not the prettiest. Can't wait to see if it holds together.


          • #6
            I modeled this best I knew how with a 21.5" port which shows 37hz in winISD. Built with a 21" port and measured max port output at 32 hz. Hacked the port down to 17" and it now measures 34hz. I expected alot more of a change so I'm second guessing my measurements/design/process/etc.

            I assume the port sharing walls with the bottom and sides of the box extends its tuning. Is there a way to model this or do I just keep hacking off port 1" at a time until desired tuning is reached?

            *Measurements were taken with both ports open. Picture taken when playing around seeing how low it could go.


            • #7
              Saturday morning testing produced text messages from neighbors so something is working right. A karaoke party and a random evening jam session in a local warehouse gives the impression the sub is tuned too low. It belts the low notes, but could use more kick in the chest. I think I'll build a 3cf box tuned to low 40's and compare. If its better, I'll modify the first box to match.

              Stinks to raise the tune since the low notes work well. I guess I can always plug ports on lower volume gigs and 3cf boxes will be nicer to store/carry/etc.


              • #8
                Do yourself a favour and measure the t/s parameters of those drivers (and the semi-inductance parameters too) before committing to another build. And consider modelling the box as a folded TL (see link below for how I did it with one design) to achieve more accurate recommendations for vent length.


                Brian Steele


                • #9
                  That and do impedance sweeps, much easier to see what the tuned frequency is.
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