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Helium - a true micromonitor

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  • Helium - a true micromonitor

    Objective:

    When I designed the Lithium, the goal was to create something small enough to go almost anywhere, without giving up much in the way of sound quality (even bass). One of the things I have learned from the feedback (particularly non-audio enthusiasts) is that what I or other folks who frequent this forum may consider small isn't really small in the eyes of the larger population.

    The laws of Physics won't really let you go much smaller than designs like the Lithium or Overnight Sensation and remain nearly full range (for music anyway), so I had to set my expectations accordingly. For the Helium, the goal was to create something the size of a typical computer speaker or HTIB satellite that could either provide a reasonable music experience on a desktop or legitimately reach down to a desirable subwoofer crossover point in a small room home theater application (80 Hz or so).

    Enclosure:

    The Parts Express 0.04 cu. ft. flat pack (Part Number 300-7060) has external dimensions of 6.5" x 4.5" x 5", which is just about perfect for what I have in mind. While (as my better half would attest), I could easily find enough scrap laying around the garage to put together something like this, the flat pack is a real time and sawdust saver, and an easy choice at the current price of $19/pair.

    Driver Selection:

    The Dayton ND91-4 (290-224) is a pretty impressive little driver with 4.6 mm xmax and a claimed 25 mm of peak excursion. According to simulations, this driver will be quite happy in 0.03 to 0.04 cu. ft., reaching an f3 of around 70 Hz in the selected enclosure. It's not the most economical small driver in the world, but my experience with the ND105 along with the specs gives me confidence that the ND91 can take the beating that trying to reach below 70 Hz as a 3" driver will dish out.

    Once the ND91 is in that box, there isn't enough room on the baffle for a lot of tweeters. Fortunately, the ND91 should be able to play pretty high in frequency, which should open the door to a small tweeter. The Dayton ND16 (275-025) has been used to good effect in a number of designs and is very small and under $10, so it is the obvious choice.

    Crossover Design:

    The ND91 does pretty well up to 8-10 kHz, while the ND16 can extend down to 3.5 kHz, so there is a fairly broad window to work with. I decided to target something around 5 kHz, so I would have some flexibility with respect to crossover slope. One consideration is that there is not a lot of room in the box for a crossover (the floor of the box is only 4" x 3.5" and some of that space will be constrained by the woofer frame.) This means that to some extent, the crossover point and slope needs to be determined by finding a point where the drivers will integrate with a small number of components. After a number of iterations, I settled on a 6 component crossover network that yielded approximately 4th order acoustic slopes with a 5 kHz crossover point.

    The final crossover requires a 1 mH inductor, which will definitely be a challenge to fit into this box. Recently a thread on PETT regarding small, inexpensive crossover components included the suggestion of dynavox.com. Sure enough, they have a 1 mH laminate core inductor that is about the size of a sand cast resistor (for $0.84) - so I used it. You can also get the 4 uF capacitor and a terminal cup from Dynavox. You might even get away with their 2.2 uF capacitor for the specified 2 uF. For the 0.2 mH inductor, I used a surplus inductor from some old Klipsch crossovers I bought from Ken McCullough. The closest match I have been able to find is a 23 ga Visaton air core sold by MCM Electronics. The 20 ga Jantzen air core from Parts Express will also work fine. Since there is a 2 ohm resistor in series with that inductor, the small change in DCR of the inductor will have no significant impact. The 2 uF capacitor I used was a surplus mylar from Madisound (still available at $0.35), but the Dayton met poly (027-414) will work.

    Impressions:

    I can attest that the original design goals were met. Those who were at the evening hotel session Saturday night at DIY Iowa 2014 may have more to offer from a review perspective, as I am probably too biased to provide anything useful.
    Attached Files
    Last edited by scottsehlin; 05-14-2015, 08:47 PM. Reason: Added 39 ohm resistor to schematic
    Keep an open mind, but don't let your brain fall out.

    Argon | Helium | Lithium | LithMTM | Hafnium
    Mercury | Matrix | Shrubbery

  • #2
    Re: Helium - a true micromonitor

    {reserved for additional design and build information}
    Keep an open mind, but don't let your brain fall out.

    Argon | Helium | Lithium | LithMTM | Hafnium
    Mercury | Matrix | Shrubbery

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    • #3
      Re: Helium - a true micromonitor

      {reserved for shameless self-promotion and blather}
      Keep an open mind, but don't let your brain fall out.

      Argon | Helium | Lithium | LithMTM | Hafnium
      Mercury | Matrix | Shrubbery

      Comment


      • #4
        Re: Helium - a true micromonitor

        These little speakers were really impressive, they were taking every bass-heavy track I could throw at them and were pretty impressively loud doing so, especially for their size. I'm actually putting together the parts in my cart right now, so I can build them asap!

        रेतुर्न तो थे स्रोत
        return to the source
        leviathan system thread
        deadhorse thread
        shockwave build thread

        instagram :: greywarden_13

        in war, victory . . . in peace, vigilance . . . in death, sacrifice.

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        • #5
          Re: Helium - a true micromonitor

          Those are pretty cool little speakers, Scott, you always seem to be pushing the envelope :D

          Comment


          • #6
            Re: Helium - a true micromonitor

            Nice design and very WAF friendly! :applause:
            Don't worry, if your parachute fails, you have the rest of your life to fix it.

            If we all did the things we are capable of doing, we would literally ASTOUND ourselves - Thomas A. Edison

            Some people collect stamps, Imelda Marcos collected shoes. I collect speakers.:D

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            • #7
              Re: Helium - a true micromonitor

              Getting into my 'shoehorning' territory there, Scottie! Close to a liter per cab creates some interesting dilemmas doesn't it? Parts selection is also kinda fun.

              My Audinum, Xenum, and PC are all in this category, as well as Mark65's 'Dadiels', and cjd's 'Pecorino'; All have interesting solutions to the miniscule at hand.

              Later,
              Wolf
              "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
              "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
              "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
              "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

              *InDIYana event website*

              Photobucket pages:
              http://photobucket.com/Wolf-Speakers_and_more

              My blog/writeups/thoughts here at PE:
              http://techtalk.parts-express.com/blog.php?u=4102

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              • #8
                Re: Helium - a true micromonitor

                These are the best Bose busters I ever heard.
                Bryan K.

                Midwest Audio Club

                Speedster | Sub Attachť | The Wildeman | Sean's NLA Towers | CO‹GAR, COUGAR II and CO‹GAR JR | Triton | Lithium | J-Boom | Trym MLTL | Docere MLTL

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                • #9
                  Re: Helium - a true micromonitor

                  They did sound much larger.
                  " To me, the soundstage presentation is more about phase and distortion and less about size. However, when you talk about bass extension, there's no replacement for displacement". Tyger23. 4.2015

                  Quote Originally Posted by hongrn. Oct 2014
                  Do you realize that being an American is like winning the biggest jackpot ever??

                  http://www.midwestaudioclub.com/spot...owell-simpson/
                  http://s413.photobucket.com/albums/pp216/arlis/

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                  • #10
                    Re: Helium - a true micromonitor

                    seriously? You come out with this as I'm about to wrap up my soundbar with your other design? Great, another build to add to the list.....

                    :D

                    Looks great!! Add the Choti to Wolf's list of micros......
                    Builds - C-Killa - Speedsters - LithMTM - Talking Sticks - Pocket Rockets - Khanspires - Dayton RS Center - RS225/28A - Kairos - Adelphos - SEOS TD12X - Dayton 8 - Needles - 871S - eD6c - Overnight Sensations - Tritrix (ported) - Lineup F4 - Stentorians - The Cheapies - Tub Thumpers - Barbells - Tuba HT - Numerous subwoofers - probably missing a few...... :p

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                    • #11
                      Re: Helium - a true micromonitor

                      Nice little speaker with great imaging. That little BR port was really pumping. I like
                      how one person got up and performed a mechanical front port air velocity "test" as we listened.
                      Plumber's Delight: http://techtalk.parts-express.com/fo...notech-winners
                      Linehopper: http://techtalk.parts-express.com/sh...Esoteric-build
                      Plan-Tanic: http://techtalk.parts-express.com/sh...ght=Plan-Tanic

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                      • #12
                        Re: Helium - a true micromonitor

                        Originally posted by 4thtry View Post
                        Nice little speaker with great imaging. That little BR port was really pumping. I like
                        how one person got up and performed a mechanical front port air velocity "test" as we listened.
                        We were running the Helium at pretty high SPL without any filtering. This speaker, IMO, is a great consideration for a compact HT. Even a stealthy front stage crossed over to a small sub at 100Hz would sound fantastic. I wish Bill would have captured the looks on people's faces when Scott hooked these up at the hotel. HUGE sound and big WAF = winner. I see myself gifting these to a couple family members soon.
                        Bryan K.

                        Midwest Audio Club

                        Speedster | Sub Attachť | The Wildeman | Sean's NLA Towers | CO‹GAR, COUGAR II and CO‹GAR JR | Triton | Lithium | J-Boom | Trym MLTL | Docere MLTL

                        Comment


                        • #13
                          Re: Helium - a true micromonitor

                          Or rather, your face when you realized they were tiny speakers playing haha!

                          रेतुर्न तो थे स्रोत
                          return to the source
                          leviathan system thread
                          deadhorse thread
                          shockwave build thread

                          instagram :: greywarden_13

                          in war, victory . . . in peace, vigilance . . . in death, sacrifice.

                          Comment


                          • #14
                            Re: Helium - a true micromonitor

                            I likey. I mean, I really likey. If someone was to offer this as a kit with precut baffle, I'd buy it in an instant. I need to replace my "Mini-Me" speakers, and these look like they'd fit the bill easily.
                            Brian Steele
                            www.diysubwoofers.org

                            Comment


                            • #15
                              Re: Helium - a true micromonitor

                              Nicely done, as always, Scott. Wish I'd gotten to hear them at Iowa. I'm curious about the choice of ND91. I've used the ND90 lots of times and wonder what the tradeoffs are.
                              Isn't it about time we started answering rhetorical questions?

                              Paul Carmody's DIY Audio Projects
                              Twitter: @undefinition1

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