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Helium - a true micromonitor

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  • Dunk_c
    replied
    Interesting topic!
    They do sound excellent with or without a small sub (Rel T-Zero is what I am using, I posted an amateurish freq response in post 429, and the sub is only doing duty up to 40Hz).
    So as long as the amp is not clipping when sending bass signals to the HMM that it cannot reproduce, all should be OK and bass blocking filters might not have a significant effect on sound quality, especially as I am only listening at <75dB at 4 meters.

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  • scottsehlin
    replied
    I think bass blocking has the potential to help the midrange/midbass sound cleaner. How much difference it will make depends on how loud they are playing. It will be hard to kill them, even running unfiltered, so you can try it out as-is and decide if you want to add a filter later.

    I once had a setup with a pair of bookshelf speakers with an F3 around 50 Hz that I ran wide open, then brought a sub in below that just to add extension. I felt it added something without changing the tonality of the main speakers - so was worth doing. That was a small system in a small to medium sized room mostly for personal listening, so I really wasn't straining the woofers too much.

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  • Dunk_c
    replied
    Thanks for that Guys. Scott, what are your thoughts on HMM integration with a sub bass speaker for fuller range sound reproduction, as opposed to speaker protection implications of a bass blocking filter?

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  • scottsehlin
    replied
    This is some nice work and very helpful to anyone contemplating further low frequency protection to the Heliums.

    The original question by Dunk_c did ask about whether it was necessary and that is an interesting question in itself.

    I would definitely agree that keeping extremely low frequencies out of the Helium's would result in cleaner midrange and likely better tonal balance particularly when playing loud.

    One thing the modeling above does show is that even unfiltered, the Helium's playing at 98 dB, which is quite loud are still well below Dayton's published xlim of 25 mm (which is pretty insane for a 3.5" driver). This correlates pretty well to what I have observed at various events in larger venues where we have tried to challenge the Helium's deep and loud and it has met the challenge surprisingly well every time. Those original Helium's are still alive and kicking today. Being above xmax, but below xlim isn't ideal from a distortion perspective, but is likely survivable.

    The ND-91 is a remarkable driver and the T/S parameters drive it to a tuning that does not promote trying to play too low in too large a box, so this contributes greatly to power handling. This might be one of those rare cases where you have to worry about the voice coil giving out before you have to worry about over excursion taking out the woofer.

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  • LOUT
    replied
    Here are a couple VituixCAD sims of the ND91 in a 1.2L ported box:

    First, unprotected.
    Click image for larger version

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    Then with a 2nd order HP using a 500uF cap and 4mH coil (with total 3-4ohm resistance on the coil).
    Click image for larger version

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    Then a simpler 1st order series HP using only a 200uF capacitor with an 8ohm resistor parallel with the cap.
    Click image for larger version

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    These are kind of aiming to keep the little speakers playing as close to 70-80hz F3 as possible while still protecting them a fair amount...kind of going on the assumption the subwoofer will be placed somewhere it sounds nice but isn't necessarily near the speakers and crossed low because of it.
    If the sub will be somewhat between the speakers where it can be crossed pretty high (closer to 200hz) then you could probably just use a 50-80uF capacitor by itself and the sub would help fill in the wide valley around 150hz that the plain cap cuts out from the ND91. The plain capacitor kinda cuts a wide path of destruction all the way from 75hz to 400hz, so the subwoofer would need to fill in on low-mids as well in that situation.

    The plain 80uF capacitor models like this:
    Click image for larger version

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    Adding the resistor parallel with the larger cap instead (shown in the 3rd cluster of images) helps to keep most of that valley around 3-4db louder, though it's still around 3-4db down compared to the 2nd order HP or the unprotected model.

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  • Dunk_c
    replied
    OK, not a trivial mod then with that much impact on SPL due to uneven impedance across the freq range.
    Can I ask a basic question. Even though the speaker gets effectively worse at producing sound below a threshold, it does still receive the full signal at the lower end, what impact does this have on the sound produced within its freq range? Crossovers (eg 120Hz) set in HT AV receivers are supposed to relieve the small speakers from bass duty allowing them to do better without the bass-burden. Without a DSP that is all I was hoping to achieve with a simple inline cap (I guess those car audio boys aren’t worried about HiFi reproduction so much when they use simple bass blocking caps).
    Last edited by Dunk_c; 06-13-2020, 07:49 PM.

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  • Chris Roemer
    replied
    By itself, a 100uF series cap will cut the bottom end by about -4dB from 300-200Hz, but only -1@125Hz. Below that you'll be -6 near 100Hz, -12 from 50-80Hz, only -5dB @ 40Hz, but dropping below that. (The effective of a 1st-order filter (just a series cap) will give an output that mimics the appearance of the vented box's impedance curve: dual peaks w/a "valley" in-between near 70Hz.)

    If you also run Dayton's 10mH iron-core in parallel (so, a 2nd order high-pass), you'll be -3 from 300-200, up to +2dB@125, then dropping below that: -6dB@100, -18dB near 60Hz, and down.

    An 80uF series cap (and 10mH shunt coil) puts you at -4dB from 300-200, -1 near 130Hz, -6@110, -12 near 90, -18 near 70, and -24dB @ 45Hz.

    I think if you just plugged the port (and used no additional filtering) your bottom end should be down (compared to vented) -1dB@200Hz, -4@100, -5@80, -6@60, and -4dB(again)@50Hz.

    These ND drivers CAN take quite a bit of abuse, so...
    I'd try them "as is" (and see if you have issues).
    Then try blocking the port (your sub integration might be OK just like that).
    If you still have excursion/distortion trouble (@100Hz and below), I'd try the 2nd order w/the 80uF cap and 10mH (iron) coil. This will be the most expensive option.

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  • Dunk_c
    replied
    How about a simple inline first order filter.
    So, for 100hz cutoff. The plot on page 1 shows 15ohm impedance at 100Hz, Using an online calculator, 100 uF cap required. (For 70Hz, 150uF). I guess 60V woukdbe plenty. Is this about right?

    Leave a comment:


  • LOUT
    replied
    Originally posted by Dunk_c View Post
    Is it possible (or even necessary) to make a passive high pass filter that is inline with the speaker cables to reduce low frequency signals (below those that these speakers can faithfully reproduce) to these speakers where they have been teamed up with a separate sub base speaker that has no pass through filter? Wondering if this is easy, effective and worthwhile? The sub has a low pass filter but the Heliums still get full range signals from the amp.

    I found these but they are signal level not speaker level
    Harrison Labs FMOD Inline Crossover Pair 100 Hz High Pass RCA on PE
    Yes, I think it should be possible because it should act just like a series high-pass at the start of both crossover parts where it'll have little/no effect on the tweeter section because that'll already have crossed much higher and it'll taper down the woofer's lows about how you'd expect such a high-pass to do. You'll still want to test or model it to tune how the highpass behaves around the woofer's impedance peak/s.
    I'm assuming it's easier to build externally rather than inside in this case?

    It should be pretty easy and effective for an amp that's passing full-range, but expect the cap/coil to be fairly large and expensive.
    It looks like a standard 2nd order low-pass using a ~500uF-750uF capacitor followed by a 4mH inductor to ground (with a resistor added to the inductor to increase total resistance to around 3-4ohms) might work.
    Last edited by LOUT; 06-14-2020, 08:52 AM.

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  • Dunk_c
    replied
    Hi
    Is it possible (or even necessary) to make a passive high pass filter that is inline with the speaker cables to reduce low frequency signals (below those that these speakers can faithfully reproduce) to these speakers where they have been teamed up with a separate sub base speaker that has no pass through filter? Wondering if this is easy, effective and worthwhile? The sub has a low pass filter but the Heliums still get full range signals from the amp.

    I found these but they are signal level not speaker level
    Harrison Labs FMOD Inline Crossover Pair 100 Hz High Pass RCA on PE

    Leave a comment:


  • mga2009
    replied
    Just leaving a picutre of my Heliums 8ohm Rev. 2.0 WIP. Probably finishing them within this week.

    I don't know how it will turn up, they are tuned at 65hz with a slot port, using the XO of the Helium soundbar.
    Attached Files

    Leave a comment:


  • alandavidhenry
    replied
    Here is a comparison of the two. The 4-ohm seems to be more sensitive and handle more power:
    ND16FA-6 ND16FA-4
    Impedance: 6 ohms 4 ohms
    Power Handling (RMS): 10 Watts 30 Watts
    Frequency Response: 3,500 to 27,000 Hz 4,000 to 20,000 Hz
    Sensitivity: 88 dB 2.83V/1m 93 dB 2.83V/1m
    Fs: 2125 Hz 2246 Hz
    Re: 5.80 ohms 3.17 ohms
    Le: 0.04 mH 0.03 mH
    Qms: 3.45 3.71
    Qes: 6.09 3.46
    Qts: 2.20 1.79
















    On the crossover, I removed the parallel 39 ohm, series from 15 ohm to 22 ohm, capacitor from 2.2uF to 1.8uF and inductor from 220uH to 270uH.

    Look very similar to me:

    16FA-6

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    16FA-4

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  • Chris Roemer
    replied
    Well, that's MOST of it...

    I'd try the 16FA-4 'cause it's the same dia. (similar radiation pattern PROBably, and same size V.C. - should handle similar power).
    I looked at it using Scott's HP XO and it looks like an "almost drop-in". I'd drop the series resistor from 15n to maybe 10n (and you COULD probably omit the 39n parallel resistor - if you wanted). What'd you come up with?

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  • alandavidhenry
    replied
    I'm wanting to build five of these for a HT system but the ND16FA-6 driver isn't in stock anywhere (I'm in the UK and there is a supplier in France but they only have two in stock).

    I can get the ND16FA-4 or ND13FA-4.
    I have modelled all three tweeters in VirtuixCAD but it's the first time I've used it. I modified the crossover component values to produce a similar plots on the SPL graph but I suspect there is more to it than that to make a great sounding crossover?

    Sorry if this is basic to everyone else on here but I'm trying to learn as much as I can whilst I have all this spare time whilst in isolation.

    Click image for larger version

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  • wogg
    replied
    Originally posted by Matt Clara View Post

    What happens if you use too fine a gauge? Asking for a friend...
    What Jeff said...

    Basically, too small a gauge is effectively adding a small resistor in series with your speakers. Heat wouldn't be a concern unless you're running crazy high wattage, but you could simulate the effect in Xsim if you want.

    Use an online wire resistance calculator like this one. Add the resistance as a series element before your crossover in Xsim. for example, a 50 foot run of 18 gauge is 0.3192 ohms each way, so add 0.638 ohms. That will affect frequency response a bit based on impedance, if you've got a 4 ohm speaker wherever it dips in impedance will have more loss in the wire than areas of high impedance. Larger gauge wire reduces that series resistance and that effect on the output.

    Let's say you have a 3 ohm minimum speaker with that 50 foot 18Ga run, you'll make a voltage divider with 3.638 ohms total and 3 ohms for the output = 82.4% of the signal makes it to the speaker. That calculates out to a 1.67dB drop at that frequency. Not huge, but adds up. Swap that 18Ga for 12Ga and your series resistance drops to 0.1588 ohms total. Now you've got 95% of the signal making it to the speaker and a 0.45dB loss.

    If you want to get really crazy, determine the capacitance and inductance of the wire as well and model that. Spoiler alert: the capacitance and inductance is almost negligible.

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