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Helium - a true micromonitor

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  • Scott,

    Throughout the thread there has been mention of a MTM version and I wanted to see if you had given that any more consideration. I saw the post about using two ND91-4's in series but would it be easier to use to ND91-8's in parallel and adjust the padding on the tweeter?

    I am very interested in a MTM version but haven't quite mastered the measurement process to the point I feel secure in making my own design. I have a set of the originals in a spare room but the room is a little large for them and I think an MTM version may fill the room a little better. My thoughts are to pair them with the 8" DSP subwoofer kit for a 2.1 system.

    Would you be willing to share your measurement files for others to tinker with?

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    • hi, i am trying to model the box size and port length for the 8ohm version. somewhere in the thread scott said the 8ohm goes a little bit lower but needs a bigger box... should i aim for a 70hz tuning? or maybe a little higher? i can make the internal volume around 1.5 / 1.7 litters, or more if needed. same on the port lenght, i can go a little bit longer, as i am trying to keep the aor speed on the vent under 40 m/s. Sent from my ONEPLUS A3000 using Tapatalk

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      • Originally posted by davidroberts View Post
        Scott,

        Throughout the thread there has been mention of a MTM version and I wanted to see if you had given that any more consideration. I saw the post about using two ND91-4's in series but would it be easier to use to ND91-8's in parallel and adjust the padding on the tweeter?

        I am very interested in a MTM version but haven't quite mastered the measurement process to the point I feel secure in making my own design. I have a set of the originals in a spare room but the room is a little large for them and I think an MTM version may fill the room a little better. My thoughts are to pair them with the 8" DSP subwoofer kit for a 2.1 system.

        Would you be willing to share your measurement files for others to tinker with?
        You are not the only one! Just registered on here in the hope of finding an MTM version one day. I've been a long time lurker around the forums, so I figured I had to register at some point. ;)
        Scott's LithMTM is also a beautiful design, but still too large for me. An HelMTM version would be awesome.

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        • (WART Scott), for a pair of 91-4s in series, doubling the coil (to 2.0mH) and resistor (to 4ohms), and halving the cap (to 2uF) should get you pretty close.
          The HP on the tweeter shouldn't change.

          For a 4ohm MTM (w/two 91-8s in parallel), use the orig. XO since the LP section is still 4ohms.
          The pair of -8s (in theory) SHOULD be 2-3dB louder than the -4s. You can "unpad" the tweeter by droppping the parallel resistor (or making it really large - effectively "opening" it) and reducing the series resistor to about 11ohms (for +2dB) or around 7ohms (for a +3dB change).

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          • It sounds like I should move the HeliMTM up on the priority list. It would help to have some feedback on the two primary options. If the design uses two 4 ohm ND91's, then it will be a very compact MTM with an 8 ohm nominal impedance and very good power handling, but won't have significantly different bass extension or voltage sensitivity than the Helium. If two 8 ohm ND91's are used in parallel, the box will get somewhat bigger and the impedance will be 4 ohms (like the Helium), but in exchange for that, one would get an extra 5-10 Hz of bass extension and improved sensitivity.

            Thoughts?
            Last edited by scottsehlin; 09-24-2018, 11:08 AM. Reason: typed dB, but meant Hz...
            Keep an open mind, but don't let your brain fall out.

            Sehlin Sound Solutions

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            • I would like to see the 4 ohm version with the extra extension and sensitivity.

              Comment


              • Originally posted by scottsehlin View Post
                It sounds like I should move the HeliMTM up on the priority list. It would help to have some feedback on the two primary options. If the design uses two 4 ohm ND91's, then it will be a very compact MTM with an 8 ohm nominal impedance and very good power handling, but won't have significantly different bass extension or voltage sensitivity than the Helium. If two 8 ohm ND91's are used in parallel, the box will get somewhat bigger and the impedance will be 4 ohms (like the Helium), but in exchange for that, one would get an extra 5-10 dB of bass extension and improved sensitivity.

                Thoughts?
                Def go with the two 8ohm. More bass and sensitivity.

                Comment


                • Jep, two 8ohm in parallel would be great!
                  As long at it's smaller than LithMTM ;-)

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