Helium - a true micromonitor

[davidroberts]

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?

[mga2009]

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

[prxprx]

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.

[Chris Roemer]

(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).

[scottsehlin]

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?

[davidroberts]

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

[Jake]

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.

[prxprx]

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

[JimHRB]

I have a pair of old speakers with really nice walnut veneered cabinets. Size is 4.5" x 8.5" x 8.5" so about double the volume of the PE boxes but the same baffle width. How would the Heliums work in a larger box like that? What tuning do people suggest for the woofer?

Note these would be used in a vacation condo driven by a Pioneer CD/DVD receiver with about 15 watts per channel and located on a shelf above the TV. Are these full BSC and would that be a problem? If so, is there any easy way to modify the crossover to reduce the BSC?.

[bsl100]

I'm planning on building a sealed micro monitor (~2L) with the ND105-8 and ND20FB-4 after reading post #17. The original plan was to go with the Heliums, but after reading the post, I also got interested in the ND105-8 project.

I have a few questions:
1. The WinISD curve I get with the ND105-8 gives me an F3 of around 80-82hz with the peak around 125-150hz (I modelled it with a 500uF cap). With the ND91-4 (vented 25mm x 115mm, tuning around 74hz), the graph is more flat upto around 80-85hz (peak) and then drops down to 65hz F3. In the SPL graph, the ND108 stays above and deeper than the ND91, except in the 60-100hz section. The Helium MT shows an F3 of 90hz, but has a tuning frequency of 56hz (reached around F6-F7). It has a peak around 250-300hz but digs deeper F4 and lower. The question is which would better and offer more bass, the flatter curve of the ND91 (vented 25mm x 115mm) or the sloping one of the Helium and sealed ND108? Will the Helium and ND108 midbass hump colour the sound or is that accounted for in the Xsim crossover design? Also, will there be excursion issues with ND91 (25mm x 115mm) at higher SPL. Is the Helium excursion protected because of the lower tuning?

2. The other question is a general one. I am just about learning to play with Xsim. I wanted to understand how BSC is incorporated in this. Does Xsim take into consideration BSC and the graph rise from around the 700-900hz region once the drivers are installed in the given enclosure? Or do we need to incorporate that in the woofer crossover and ensure a 5-6db dip around the BS region which is continued all the way through to 20Khz.

3. Why do we need such high resistor values for the tweeters in the Helium models. Can the same padding down not be achieved with lower values.

Would appreciate some feedback on this. (Sorry for the long post and edits)

[Chris Roemer]

I've not used the ND91s, but have used the 105-4 (my Nano Neos), and the ND90-8/AURA NS3s (in my Piconeo). If you look at ISD's "max power" curves, you can see that your 105 (2L sealed) can take its full-rated 30w at basically any freq. (mostly due to it being in a somewhat undersized box - w/a "Q" of near 1.0). The vented 91 OTOH can only take full-power down to about 65Hz (its F3) before overexcursion (due to it unloading below its tuning freq.).

AFA bass is concerned, IMO the vented box will have noticeably more lower bass: while about the same output at 100Hz, it's up +5dB @ 80 (that's quite a bit), and still up +3 @ 60 (not so much - but probably noticeable). The diff. in "lift" before rolloff (between 100-200) is only about 1dB different - not really noticeable.
Unless you "prep" your woofer files (to incorporate baffle step), you CAN achieve about the same thing by using a target curve that's not strictly "flat", but instead has slow -6dB drop (or whatever your pref. is) rising between approx. 100 to 1000 Hz (you can model YOUR situation by using Tolvan's "EDGE" software - as a guide).
Note* - I don't regularly use ALL the different sim softwares out there - so if you have to enter your baffle dims (w/driver layout) into "Xsim", then I believe it is trying to incorporate BS and diffraction into the model "on the fly".

Most of the time (in an XO) once resistors get much above 25-30n(ohms) you may as well leave them out (high resistance, basically like an "open").
I model similar results by just upping the SR from 14 to 17n and dropping the parallel component (<1dB change over just a narrow band).
You CAN get a very similar "transfer fn" by using a 9/8 "L" (SR=9n, PR=8n) AND changing the cap up to a 4uF and the coil down to a 0.10mH, but what you end up with is a pass-band "load" of only 2.5 ohms as opposed to Scott's at 4.5 ohms. (Dropping the PR and using an SR of 17n - while keeping the 2uF/0.20mH filter, will give about the same load and xfer fn.)

[bsl100]

Chris.... Super educational stuff. Thanks. Would like to know where baffle size is entered in Xsim. I have version 1.2.00 on my computer. Is there another version of Xsim?

By adding a 500uF cap on the ND105-8, the difference in the 80hz output is reduced to 4db in the TFM graph. But, in the SPL graph the difference is around 2db at 80hz, 1db at 60hz and the graphs cross each other at around 55hz. The ND105-8 continues to play lower below that. It would also be able to handle the full 30W of power whereas the ND91-4 will start struggling from 65hz and below.

I wonder whether the lower port tuning of the Helium MT enables it to play loud without over-excursion.

[Chris Roemer]

Your box is nearly twice the size of the Devono 0.04cf'er. Your box should have noticeably more output at 80Hz Not only that, but "Xmax" is where distortion begins. Xmech (sometimes Xlim) is basically where the suspension bottoms out (sometimes w/lots of bad sounds). While your box sim might Xmax below 65Hz at near full power, I THINK Scott spec'd the Xmech at 25mm "peak-to-peak" (which would stll be over 12mm one way). You don't reach THAT territory until nearly 40Hz, which is below most "normal" music. Not to worry (much).

[scottsehlin]

I have a pair of old speakers with really nice walnut veneered cabinets. Size is 4.5" x 8.5" x 8.5" so about double the volume of the PE boxes but the same baffle width. How would the Heliums work in a larger box like that? What tuning do people suggest for the woofer?

Note these would be used in a vacation condo driven by a Pioneer CD/DVD receiver with about 15 watts per channel and located on a shelf above the TV. Are these full BSC and would that be a problem? If so, is there any easy way to modify the crossover to reduce the BSC?.

In this larger box, the 8 ohm ND-91 would work better. It can be tuned to 60 Hz using a 1" diameter by 6.5" long port, which gives an f3 of about 56 Hz. For the 8 ohm ND-91, the best option is to use the crossover for the Helium Soundbar, which you can get from the Sehlin Sound Solutions link at the bottom of this post.

[JimHRB]

Thanks Scott and Chris. Very helpful.