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Helium - a true micromonitor

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  • #76
    Re: Helium - a true micromonitor

    It's like early Christmas around here.

    For the past few days I've been listening to the Helium tuning and crossovers in the ND91-4 boombox project that I started last year. It has very similar dimensions for the flat pack enclosure, but is slightly bigger- about 0.055 cu ft. HWD in inches it's 5.5x3.5x5, and a strip of 3/4x1/2x5.5 inch fir along one edge. Crossover is external, so it doesn't eat into the enclosure volume at all.

    First off, props to Scott for the work to figure this one out and post it here. Very nice results, and well documented.

    Adding the NF16 tweeter to my project made a huge improvement. After almost a year listening to just the ND91-4, I'm excited again about this project, not disappointed like before. Plus the port tuning change removed the muddiness I was hearing.

    BTW, tf you see recommendations on the web for the ND91-4 in a 0.035 cu ft. (1 litre) enclosure and a 1 x 4 inch port, I'd approach that as optimistic. You should probably count on the port length being much longer, and the box a little bigger for practical reasons.

    +1 on the 40ohm resistor across the tweeter terminals that Scott suggested. I might make that switchable, but I enjoyed to balance on vocals a lot more with the resistor in place.

    Port tuning I did by subjective listening and fiddling with a tone generator. Best port dimension, using 3/4" PVC is: 4-1/4" for the long straight section, then an elbow, then a short, 1-1/4" section. Pipe ends were flared. So total length ended up as about 7 inches, depending on how one accounts for the flare and elbow. I made adjustments in 1-inch increments.

    My woofer coil was the Jantzen 20g 1mH from PE; caps were Metal Poly from PE also. Not that they needed to be- this isn't a wispy nuance application so more budget components will go into the next build.

    Overall impressions are very good. It's a warm balance on vocals- not husky, just nicely warm, then a smooth, even bass response. No one will be pounded by the raw bass output of this boombox, but the impression is of a full sound. I've heard larger speakers that didn't do this well. Midrange is sweet and the top end smooth. Sounds like a cliche at this point, but the voicing works really well. It's no smiley face sound, just robust, enjoyable performance. My guess is that it would work well for home theatre, so long as you have more modest listening levels in mind.

    I'm thinking of making a set of these for the living room.

    Thanks again, Scott. And Happy Holidays to you and yours.

    Comment


    • #77
      Re: Helium - a true micromonitor

      Originally posted by Naught1 View Post
      Port tuning I did by subjective listening and fiddling with a tone generator. Best port dimension, using 3/4" PVC is: 4-1/4" for the long straight section, then an elbow, then a short, 1-1/4" section. Pipe ends were flared. So total length ended up as about 7 inches, depending on how one accounts for the flare and elbow. I made adjustments in 1-inch increments.
      That's the same dimensions called for in the plans! Any dampening material?

      Comment


      • #78
        Re: Helium - a true micromonitor

        Oh crap. I just realized I've been cutting and fitting 1" PVC. My notes say 3/4" but for some reason I got carried away when I found a piece of 1" in my scrap pile. I think I can salvage the baffles, though, because fortunately I didn't drill the holes all the way through. Epoxy or some such adhesive/filler please rescue me!

        Comment


        • #79
          Re: Helium - a true micromonitor

          Originally posted by Herman Trivilino View Post
          Oh crap. I just realized I've been cutting and fitting 1" PVC. My notes say 3/4" but for some reason I got carried away when I found a piece of 1" in my scrap pile. I think I can salvage the baffles, though, because fortunately I didn't drill the holes all the way through. Epoxy or some such adhesive/filler please rescue me!
          That sounds close enough to recover from. I would have gone with a 1" port diameter if I could have - but it would have been about 12" long and I just didn't see a good way to get that in the box. :eek:
          Keep an open mind, but don't let your brain fall out.

          Sehlin Sound Solutions

          Comment


          • #80
            Re: Helium - a true micromonitor

            Originally posted by Naught1 View Post
            It's like early Christmas around here.

            For the past few days I've been listening to the Helium tuning and crossovers in the ND91-4 boombox project that I started last year. It has very similar dimensions for the flat pack enclosure, but is slightly bigger- about 0.055 cu ft. HWD in inches it's 5.5x3.5x5, and a strip of 3/4x1/2x5.5 inch fir along one edge. Crossover is external, so it doesn't eat into the enclosure volume at all.

            First off, props to Scott for the work to figure this one out and post it here. Very nice results, and well documented.

            Adding the NF16 tweeter to my project made a huge improvement. After almost a year listening to just the ND91-4, I'm excited again about this project, not disappointed like before. Plus the port tuning change removed the muddiness I was hearing.

            BTW, tf you see recommendations on the web for the ND91-4 in a 0.035 cu ft. (1 litre) enclosure and a 1 x 4 inch port, I'd approach that as optimistic. You should probably count on the port length being much longer, and the box a little bigger for practical reasons.

            +1 on the 40ohm resistor across the tweeter terminals that Scott suggested. I might make that switchable, but I enjoyed to balance on vocals a lot more with the resistor in place.

            Port tuning I did by subjective listening and fiddling with a tone generator. Best port dimension, using 3/4" PVC is: 4-1/4" for the long straight section, then an elbow, then a short, 1-1/4" section. Pipe ends were flared. So total length ended up as about 7 inches, depending on how one accounts for the flare and elbow. I made adjustments in 1-inch increments.

            My woofer coil was the Jantzen 20g 1mH from PE; caps were Metal Poly from PE also. Not that they needed to be- this isn't a wispy nuance application so more budget components will go into the next build.

            Overall impressions are very good. It's a warm balance on vocals- not husky, just nicely warm, then a smooth, even bass response. No one will be pounded by the raw bass output of this boombox, but the impression is of a full sound. I've heard larger speakers that didn't do this well. Midrange is sweet and the top end smooth. Sounds like a cliche at this point, but the voicing works really well. It's no smiley face sound, just robust, enjoyable performance. My guess is that it would work well for home theatre, so long as you have more modest listening levels in mind.

            I'm thinking of making a set of these for the living room.

            Thanks again, Scott. And Happy Holidays to you and yours.
            Thank you for the kind words. Happy holidays to you also.

            The 40 ohm resistor has an interesting back story. I struggled with voicing these a little due to several factors probably related to my haste in getting these ready for DIY Iowa. First, one of the tweeters was much lower in level than the other, which I found was due to a bad connection between the crossover and tweeter. Then, when I fixed the wiring problem, I wired the tweeter with reverse polarity (and actually wound up taking it to the show that way). The lack of energy centered around 5kHz kept a similar tonal balance as when the tweeter had a bad connection, so I really didn't detect an issue before or during the show. When I got them back and did a quick measurement, I found the polarity issue. After I fixed that, they were now a little bright on some content. After simming a couple of solutions, I settled on the 40 ohm resistor in parallel with the tweeter. It attenuates everything above the crossover point down by about 1 dB. I tested it by running a couple of test leads out through the port tubes so that I could connect and disconnect the resistor, and in my listening room it brought me back to the tonal balance I was looking for.

            These will handle a surprising amount of power, but the challenge in running them for a larger home theater is that they are very inefficient. It will take a good amp to drive them to significant volume in a large room.
            Keep an open mind, but don't let your brain fall out.

            Sehlin Sound Solutions

            Comment


            • #81
              Re: Helium - a true micromonitor

              Wow, I tested this joinery method and it worked! Cut the end of the PVC pipe square and glue it to the back of the baffle. I used the orange CPVC glue and applied the primer first. Clamped it up for a couple hours and it formed a rock solid joint.

              Comment


              • #82
                Re: Helium - a true micromonitor

                Originally posted by scottsehlin View Post
                I would have gone with a 1" port diameter if I could have - but it would have been about 12" long and I just didn't see a good way to get that in the box. :eek:
                Ha ha. It would have a volume of 9.4 in3 in a 77 in3 box. That's 12% of its volume!

                Comment


                • #83
                  Re: Helium - a true micromonitor

                  Originally posted by scottsehlin View Post
                  After simming a couple of solutions, I settled on the 40 ohm resistor in parallel with the tweeter. It attenuates everything above the crossover point down by about 1 dB. I tested it by running a couple of test leads out through the port tubes so that I could connect and disconnect the resistor, and in my listening room it brought me back to the tonal balance I was looking for.
                  I would be very interested in how this effect compares to replacing that 14 ohm inline resistor with 15 ohms. 15 ohm resistors are much easier to include in the build as I've found 14 ohm resistors impossible to find, and using a combination that adds up to 14 means cramming more components into these small enclosures.

                  Comment


                  • #84
                    Re: Helium - a true micromonitor

                    Originally posted by scottsehlin View Post
                    These will handle a surprising amount of power, but the challenge in running them for a larger home theater is that they are very inefficient. It will take a good amp to drive them to significant volume in a large room.
                    Hear that Santa? I'm going to need a new amp.

                    Comment


                    • #85
                      Re: Helium - a true micromonitor

                      Originally posted by Herman Trivilino View Post
                      Any dampening material?
                      No, not yet. I think it would mess with the air flow too much. In my cabinet there's really no room on sides or top for anything, and if I place it on the bottom or rear panel, it risks impeding the air flow near the port, which is already very close to the boundaries. Plus, I haven't thought once that it would benefit from dampening.

                      Comment


                      • #86
                        Re: Helium - a true micromonitor

                        Originally posted by scottsehlin View Post
                        First, one of the tweeters was much lower in level than the other, which I found was due to a bad connection between the crossover and tweeter. Then, when I fixed the wiring problem, I wired the tweeter with reverse polarity (and actually wound up taking it to the show that way). The lack of energy centered around 5kHz kept a similar tonal balance as when the tweeter had a bad connection, so I really didn't detect an issue before or during the show. When I got them back and did a quick measurement, I found the polarity issue. After I fixed that, they were now a little bright on some content.
                        Whhhhaaaat?? A mistake? Shocking! ;)

                        Okay now that the truth is coming out, I'm listening to a 30 ohm resistor, not 40. I don't have any 40's handy. No idea how that missing 10 ohms affects the response, but it's lovely. Do you have any idea what the value the attenuation is with 40 ohms?

                        My $16 measurement mic is on its way to my xmas stocking, so I'll be able to check that myself in a couple of weeks.

                        Comment


                        • #87
                          Re: Helium - a true micromonitor

                          30 ohms will actually drop the tweeter response a little more than 40 ohms, but not too much. Switching the other resistor from 14 to 15 ohms has a different effect. The level at the lower end of the tweeter range doesn't change much, but the top end (particularly above 10 kHz) drops more.
                          Keep an open mind, but don't let your brain fall out.

                          Sehlin Sound Solutions

                          Comment


                          • #88
                            Re: Helium - a true micromonitor

                            Originally posted by scottsehlin View Post
                            30 ohms will actually drop the tweeter response a little more than 40 ohms, but not too much. Switching the other resistor from 14 to 15 ohms has a different effect. The level at the lower end of the tweeter range doesn't change much, but the top end (particularly above 10 kHz) drops more.

                            Scott, can you send me, or tell me what I'd need to know to get, the files I would need to run this sim? I've never done a sim before and I think this is as good a time as any to try and learn.

                            Cause I'm thinking things like if I use the 15 ohm resistor should I also add a resistor in parallel?

                            Comment


                            • #89
                              Re: Helium - a true micromonitor

                              Hooked up the crossover components and drivers for an audition today. This is the mess you get when you decide to not use a crossover board.

                              Click image for larger version

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                              Waiting for rock hard water putty to dry. It was too cold in the shop today. Maybe paint tomorrow.

                              Click image for larger version

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                              • #90
                                Re: Helium - a true micromonitor

                                Here's a couple pictures of the insides.

                                Click image for larger version

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                                Waiting now for glue to dry on backs, then they'll be ready for listening!

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