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First custom build attempt, with a little Forum help...

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  • First custom build attempt, with a little Forum help...

    So... I had some drivers laying around from a failed upgrade attempt a few years ago. This forum, plus a few kit builds from Parts Express inspired me to to attempt a more custom build.

    My current Onkyo 7.1 system was one of those “surround sound in a box” packages that served very well for a long time, right up until the receiver smoked itself one morning while listening to “Kind of Blue” in the middle of making pancakes. I’m now running a Denon AVR-S900W. Aside from the Ultimax 15” sub kit, the rest of the speakers are stock. After listening to My TriTrix TL’s and a pair of C-Notes I built for a friend, the deficiencies in the rest if the speakers are now grossly apparent.

    I have two pair of DSA135-8's and two DC28FT-8's. I was going to try and build a set of TriTrix MTMs with the parts (before I really started to read the stickys), but the astute forum members rightly pointed out that using different drivers changes many things about the design.

    Chris Roemer (along with several others) was kind enough to provide some sage advice. Chris even took the time to design a prototype crossover. Thanks, Chris!


    I took his advice and worked up some plans for a set of roided bookshelf speakers that will be used in my main living space as the left and right front channels of my 7.1 setup. Here are the basic specs:


    1) Two DSA138-8s in parallel, with a single DC28FT-8. 4 Ohm design, with Chris estimating there will be dip to 3 Ohms at 200 Hz.

    2) Same basic driver spacing as the TriTrix TL’s, though this baffle is approximately an inch wider (8.5” vs. 7.5”).

    3) 0.5 ft^3 cabinets out of 3/4” MDF.Internal dimensions are 7” W x 8” L x17” H, with a 2” D x 5” L port directly behind the tweeter. I accounted for the driver volume and a single brace, though the brace is probably pretty useless given the size of the cabinets.

    4) Per Chris: the crossovers will be as follows:

    "The HP (high pass filter for tweeter) needs a 0.5ohm series resistor and a 10uF series cap, with a 0.50mH shunt coil (to gnd) across the tweeter. If Dayton has a #20, use it. We need to "un"-pad the tweeter a bit to keep up w/the paralleled woofers.

    The LP for the paralleled woofers uses a 1.0mH series coil, and a 50uF (npe - a 47uF would also be fine) shunt cap. A cheap coil (#20) with a higher DCR will give a little less BSC than a low DCR (iron core) coil."

    The cabinets are constructed, and I plan to work on the crossovers today. Forgive my terrible router work on the tweeters (saw dust inhalation on top of a sinus infection is brutal). I'm thinking of repairing my mistake, plus rounding the outside edges of the baffle to the best of my router's ability. I did read somewhere that the radius of the curve needs to be pretty large, and I'm pretty sure I don't have bits that big.

    With luck, I might get to give them a trial run this weekend. I’m a little worried how these things are going to play with the Denon, but the spec sheet allows it. Let me know what you think so far!


  • #2
    Your build is coming along nicely, well done. If it's the MTM impedance you're worried about with respect to your Denon, here's a quote from the product blurb:

    "Rated at 185 watts per channel maximum power, the AVR-S900W features a high current, discrete power amplifier section that's also compatible with low impedance 4 ohm speakers"

    We run our 4 ohm nominal MTMs through a 20 years old Yamaha RX-596, never had an issue. The speaker designer advises that the average impedance of the speakers - with two RS180P-8s - is closer to 6 ohms.

    Please keep us posted

    Geoff

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    • #3
      One crossover was completed last night. I'll try to get the pictures and wring diagram posted later today. I could probably use a sanity check from the folks who are more electrically gifted.

      Measured resistance of the crossovers and drivers was approximately 3.7 Ohms.

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      • #4
        Crossover #1. Not my finest work, but all the solder joints are functional.

        I stuffed the cabinets, installed the drivers, and gave the speakers a little test run tonight. First impressions are that they are a little too crisp. There seems to be a high-mid bias. Cymbals and string noises come through cleanly. I feel like the mid-range and bass response is a little lacking. I might give them a few days of running to see if they surrounds on the cones soften up.

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        • #5
          I THINK those woofers only model about 50Hz in your cabs (so, not QUITE the full musical scale).
          Your XO only has about half "baffle-step compensation" (-3dB or so).
          For full BSC (6dB) up the tweeter's 0.5 ohm resistor to 2.5ohms (or just try swapping in a 2 or 3 ohm resistor). Roughly, each ohm of added series resistance should cut the tweeter level by about -1dB.

          BTW - your XO looks OK.
          (If I'd suggest anything, it would be to add a wire-tie around ea. coil (ESP. the "upright" one). There's enough mass there that if a cab got dropped, the coil could break free of the silicone on the board - now, if that's epoxy, then nevermind. Also a very simple strain-relief (on the exiting wires) could be had w/a pair of tiny holes and a few wraps of nylon string, or similar.)

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          • #6
            Originally posted by Chris Roemer View Post
            I THINK those woofers only model about 50Hz in your cabs (so, not QUITE the full musical scale).
            Your XO only has about half "baffle-step compensation" (-3dB or so).
            For full BSC (6dB) up the tweeter's 0.5 ohm resistor to 2.5ohms (or just try swapping in a 2 or 3 ohm resistor). Roughly, each ohm of added series resistance should cut the tweeter level by about -1dB.


            Chris, I'd probably say this is more like your crossover since you designed it! Thanks for the advice. I'll probably get some parts on order this week and try it out.



            Originally posted by Chris Roemer View Post
            BTW - your XO looks OK.
            (If I'd suggest anything, it would be to add a wire-tie around ea. coil (ESP. the "upright" one). There's enough mass there that if a cab got dropped, the coil could break free of the silicone on the board - now, if that's epoxy, then nevermind. Also a very simple strain-relief (on the exiting wires) could be had w/a pair of tiny holes and a few wraps of nylon string, or similar.)


            From a previous accident on another set of speakers I built, I know what you're talking about. I'll add a little more security. Good point about using the nylon string. I'll probably use small zip ties. I was using high-temp hot glue. Once I have the sound where I like it, i usually secure the board to the bottom of the cab using industrial Velcro,

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            • #7
              Slightly off topic here....

              I dislike guessing at what components I need to adjust based on my ears. Does anyone have any recommendations for cost-effective speaker characterization? Based on a little research: at minimum, I assume a USB microphone with flat response, spectrum analysis software, and a test CD.

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              • #8
                ARTA is one of the free softwares out there that does FR measurements.
                Add a (calibrated) mic (MINE needed a power supply - I use a "Blue Icicle"), and some odds & ends (cable & stand?), and you're in business. Using "gated" measurements, you can get equivalent to an anechoic chamber down to around 300Hz if you've got a room that's at least an 8 ft. cube.

                (PM'd you)

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                • #9
                  UPDATE:


                  So I was able to swap in a 2 Ohm resistor for the 0.5 Ohm into the tweeter side of the crossover. Significant positive improvement! The balance is far better. I'm going to spend some time with them tonight and see what I think.

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                  • #10
                    After giving them a little more time, I think I'm going to stick with the 2 Ohm adjustment. I've moved on to finishing the cabinets, and I'll try to post a few more pictures. The forum is killing me though. Still having issues posting pictures, and it is constantly signing me out.....

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                    • #11

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                      • #12
                        Nothing "wrong" w/your schemo., but it'd be a little simpler to just have the 2nd woof paralleled right off (next to) the 1st woofer.

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                        • #13
                          Chris,

                          Is there a chance you can post the crossover design assumptions and response curves for posterity? I know it was a long time ago since you worked up the design.

                          Also, I managed to finish up the construction of the cabinets this weekend. I'll see if I can post a few pictures of them this evening.

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                          • #14
                            Even though Dayton's got separate data files for 275-070 ("Silky") and -076 (w/the truncated faceplate), the internal data files are, in fact, the same.
                            Jay ended up liking 2ohms of series resistance (I believe), making a system that runs about 87dB w/4-5dB of baffle-step.
                            Z-min is 3.5ohms near 200Hz, so, a 4ohm nominal design.

                            Click image for larger version

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                            • #15
                              Thanks Chris!

                              I uploaded the finished product. I was really thinking doing something slick with some maple veneer that I had around, but a combination of laziness and impatience led me to roll on a few coats of black Behr Premium exterior satin paint. Boring, I know.

                              I did swap out the 2 Ohm series resistors for 3 Ohm. I found that the treble was still a little overwhelming at higher volumes. If I get around to it, I'd love to try and characterize and tune the crossovers further. That said, they are a significant improvement over the stock speakers in tone and sound stage.

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