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Helium - a true micromonitor

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  • scottsehlin
    started a topic Helium - a true micromonitor

    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

  • Dunk_c
    replied
    Scott, I honestly cannot say. I didn’t do any A/B type testing with the other channel before I rewired. I think this is the only way I could tell. I don’t believe I have much ability to hear subtle differences. I think it is something that needs practice and maybe training.

    Leave a comment:


  • scottsehlin
    replied
    Have you noticed a difference in listening with the tweeter polarity corrected?

    Leave a comment:


  • Dunk_c
    replied
    Tweeter polarity corrected and retested.

    Leave a comment:


  • Dunk_c
    replied
    The one speaker I tested did indeed have a reverse wired tweeter, the other was OK. I might redo the sweep and repost results on weekend.
    Thanks

    Leave a comment:


  • scottsehlin
    replied
    If I recall correctly, I measured at 0.5 meters (or about 19 inches) at tweeter height, but with the mic pointed at the center of the baffle. Since the dip is right at the crossover point, and I haven't seen it in several measurements that I have done over the years, I would at least double-check the tweeter polarity.

    Leave a comment:


  • Dunk_c
    replied
    I forgot to say that I put the mic closer, within 6” of the tweeter, not just aligned. Scrappy testing I know!

    Leave a comment:


  • Wolf
    replied
    The tweeter is the typical measurement axis. The polarity is likely just fine.
    Wolf

    Leave a comment:


  • Dunk_c
    replied
    OK, I will check that out pronto!
    When I placed the mic directly in front of the tweeter, the dip was absent.
    Thanks

    Leave a comment:


  • tomzarbo
    replied
    Originally posted by Dunk_c View Post
    Hi
    Just did some crude tesing, thought people might find it interesting. I am no audio engineer.

    I built a set of Heliums a while ago and they sounded good to me.
    I recently bought a pair of BIC DV32 and these have less base and sound harsh and bright to me, probably because they have low output below 100hz
    Play the Heliums with the BICs and I feel that the sound is better. I can now hear more of the high frequencies and still get the bass. For my ears the Heliums are lacking some trebble.
    I have some hearing damage and cannot hear much above 8khz.

    I used an EMM-6 calibrated mic to do some crude frequency response measurements with REW (please don’t give me grief about axis scaling) the speakers were outside on a stand and the mic was about 2ft in front, pointed at the drivers.

    First thing I noticed for the Heliums, good output at low frequencies, even response across the range, but with a dip around 5khz. Did I build the crossovers correctly?

    I don’t know if the 5khz dip is contributing to my listening experience, but the BIC DV32 are flat in this region. Perhaps that is why I am more satisfied with the listening experience when I play them together.

    Any thoughts?
    If I recall, the crossover for the Heliums was pretty high; somewhere around 5k...
    That looks like a reverse null when the tweeter is wired reverse of what was spec'd. In other words, you may have the tweeter wired backwards.

    TomZ

    Leave a comment:


  • garychelette
    replied
    Amazing! First post of these little guys was, Helium - a true micromonitor

    10-29-2014, 09:26 PM

    We are still talking about them! I have built both 4 and 8 ohm versions and have been very happy with them!
    I am currently building a set for a neighbor. He couldn't believe the sound and had to have a pair!
    Maybe PE will make a kit to sell one day!!!

    Gary

    Leave a comment:


  • Dunk_c
    replied
    Hi
    Just did some crude tesing, thought people might find it interesting. I am no audio engineer.

    I built a set of Heliums a while ago and they sounded good to me.
    I recently bought a pair of BIC DV32 and these have less base and sound harsh and bright to me, probably because they have low output below 100hz
    Play the Heliums with the BICs and I feel that the sound is better. I can now hear more of the high frequencies and still get the bass. For my ears the Heliums are lacking some trebble.
    I have some hearing damage and cannot hear much above 8khz.

    I used an EMM-6 calibrated mic to do some crude frequency response measurements with REW (please don’t give me grief about axis scaling) the speakers were outside on a stand and the mic was about 2ft in front, pointed at the drivers.

    First thing I noticed for the Heliums, good output at low frequencies, even response across the range, but with a dip around 5khz. Did I build the crossovers correctly?

    I don’t know if the 5khz dip is contributing to my listening experience, but the BIC DV32 are flat in this region. Perhaps that is why I am more satisfied with the listening experience when I play them together.

    Any thoughts?
    Attached Files
    Last edited by Dunk_c; 06-01-2019, 11:07 PM.

    Leave a comment:


  • scottsehlin
    replied
    I've used the sound bar as a portable music system at a couple of small gatherings, and it has held up well. The internal volume isn't that much larger than the original Helium, but enough to get another 5-10 Hz on the low end.

    Leave a comment:


  • mga2009
    replied
    Originally posted by Chris Roemer View Post
    How big ARE those (internal). Tuning? These are ND91-8s?
    Yes, I have ND91-8. The design is based on the Helium Soundbar (same crossover and same internal volume).

    In post #351 Scott Sehlin said the 8 ohm version required an slightly larger optimal cabinet, and as a result can play deeper.

    Also said the soundbar has isolated chambers for the left and right channel that would be 4" wide by 4.75" deep by 9" tall as stand-alone speakers, and that I could flip that around and make them 4" deep by 4.75" wide by 9" tall. Material is 12mm MDF, no bracing. net internal volume is around 1.5 liters I think.

    Regarding the tuning it's the same as the soundbar: "3/4" schedule 40 PVC, 4 inches long" with the slight difference the tube I have is 20mm (vs 19,50mm)

    Originally posted by Jake View Post

    Have you tested the nd91-8 in that larger volume? You may not be happy.
    Why is that? Any suggestions?

    Leave a comment:


  • Chris Roemer
    replied
    How big ARE those (internal). Tuning? These are ND91-8s?

    Leave a comment:

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