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Helium - a true micromonitor

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  • bsl100
    replied
    Newbie questions with regard to post #320 and #322 and the overall design in general.

    1. What is the load that the amp will see if the drivers in the Helium are 4 ohms and 6 ohms and they are connected in parallel.
    2. Are the resistors across the tweeter changing the load on the amp or the work as an L-PAD.
    3. What are the F3 and port tuning frequencies.

    Thanks in advance.

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  • Dunk_c
    replied
    Thanks.
    Taking another look at my requirements for a variety of listening setups, using online calculators and info (CrownAudio), here are some calculated amp requirements (assuming that the Heliums are 75dB at 1M and 1W; with 3dB headroom to reduce chance clipping, and probably many other assumptions):

    SPL , Distance , Amp Watts/ch into 4ohms with low THD
    75dB 1M 2W (desk)
    75dB 2M 8W (my small workshop)
    75dB 3M 18W (medium sized room, perhaps)
    80dB 1M 6W
    80dB 2M 25W
    80dB 3M 57W
    85dB 1M 20W
    85dB 2M 80W
    85dB 3M 180W
    90dB 1M 63W (added just for interest)

    Another web calculator determined that for 75dB speakers, with max power handling of 30W, that the maximum output of the speaker would be 90dB. Not sure how this fits in with the data above?

    From this I could conclude that when used as nearfiled speakers, hardly need any power at all. I am mucking around with chinese D-class amps and I get plenty of volume (80-85dB, measured with Radioshack SPLM C weighted) from 10-20W (as listed) at 1-2M listening distance in a very small room. This seems to make sense.

    If I found an amp with 50-60W clean power that would cover quite a few options.

    How about a DTA-120 ? But it only has
    Last edited by Dunk_c; 03-20-2018, 06:21 AM.

    Leave a comment:


  • scottsehlin
    replied
    That's what the math says if the 25 mm xlim is real. It couldn't take a sine wave at that power level, but theoretically could take occasional peaks. It is an inefficient 4 ohm speaker, so that doesn't translate into extreme SPL's. They also roll off high enough to provide some protection for the woofer, as opposed to designs that try to extend down into the 40-50 Hz range with a 3-4 inch driver. In reality, I doubt that the suspension can support an inch of travel - so one would hear distress at some point and eventually damage the surround and/or spider.

    I haven't done any conclusive testing where I was monitoring the voltage input level, but they have survived some rough treatment at DIY events...

    I do think the biggest enemy to the voice coil is clipping, so having clean power is the key to trying to play them closer to their limits. Clean watts can be a challenge to figure out in this era of overstated specs. If you have an amp that can do 50 wpc with less than 0.1% THD from 20 Hz to 20 kHz and is 4 ohm stable, you should be fine. If the power rating is 100 wpc at 10% THD at 1 kHz (which you will see far too often), you really don't have any idea what you have - but it probably isn't very good.

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  • Wolf
    replied
    I really doubt the Heliums can handle a full 75W before some kind of mechanical/thermal issues occur. That's a lot of power for a 3" driver to take.
    Later,
    Wolf

    Leave a comment:


  • Dunk_c
    replied
    So delivering 75W of power into each Helium will give 95dB of audio output and the drivers willl be comfortable. I think this means that the speakers can deliver 77dB at 1.2W which is close enough to the standard reference point, as far as I know. What is the rule that determines how sound level changes with number of speaker units? How many dB will be delivered when 2 speakers are being driven?

    Can you roughly speculate on amplifier power requirements in order to get 75W of clean power into Heliums without clipping? I belive that knowing this number is more about protecting the tweeter than preventing audible distortion (could be wrong, just learning). I am guessing 100-150W nominal.

    I don’t expect I will ever run them at high outputs - just curious about the specs. In fact, while they sounded good to my ears when played in a large room using a cheap 15W D-class amp, they were obviously no match for the floor standers being used as “speaker stands” so I am now enjoying them as personal speakers in my workshop. This is not a criticism at all, I think I could use them through the house and may make another two pairs for my teenagers when they want their own hifi system (using cheap D-class amplifier boards, because I enjoy the low cost DIY).

    Leave a comment:


  • scottsehlin
    replied
    One of the great things about the Xsim program is that it can directly answer this question. I set a simulated amp to output 30W into 4 ohms into a simulation of the Helium design. This is the resulting power dissipation for each of the drivers as a function of frequency (S1 is the ND91, S2 is the ND16 tweeter). The tweeter sees very little power in this design.

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  • Dunk_c
    replied
    Thanks for the tech details. Excuse my ignorance, but where does the power handling capability of the tiny 10W tweeter come into the story?

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  • scottsehlin
    replied
    30W is the thermal power handling. It means that running a sine wave through these at 30W or more continuous RMS power will risk melting the voice coil. Musical content would be much less stressful on the voice coil (amplifier clipping can be an exception).

    Usually, small speakers are limited more by excursion than thermal considerations. The ND91 woofer used in the Helium is really quite special. Linear excursion is rated at 4.6 mm, but the woofer reportedly can travel 25 mm one-way (one inch!) before reaching it's mechanical limits. At 30W input, the ND91-4 will output approximately 96 dB at 1 meter. The Helium's enclosure and tuning helps control the woofer's excursion. With that 30W input, excursion hits a local peak of right around 4.5 mm between 90 and 100 Hz, then it drops to a minimum around the port tuning frequency, back to 4.5 mm at 55 Hz, then increasing up to 15 mm at 30 Hz. At 30 Hz, the box and tuning results in the response 25 dB below that 96 dB level at higher frequencies, so the distortion due to the woofer being exercised that much won't be very prominent - and the extremely large amount of mechanical travel before bottoming out means that this speaker will handle the full 30W pretty gracefully - even without a high pass filter. With a high pass filter, even at 80 Hz, this speaker will handle considerably higher peaks. At 75W input, the ND91 in the Helium's enclosure will be outputting about 100 dB and will still be just below 25mm of excursion at 30 Hz, so 75W peaks are about all the Helium would want to see without a high pass filter of some sort for music with deep bass content.

    What that all means is that your amp is less than about 75W, they could be played louder if you had the juice. Whether you would want to play them louder depends on the size of your room, how far away your listening position is, background noise, etc... In practice, I find that if I design something that can handle being played at 95 dB, I don't really ever have trouble with over-excursion while listening to music at non-painful levels, even in venues such as DIY events. The Heliums have passed that test on a few occasions.

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  • Dunk_c
    replied
    So are these speakers rated at 30W RMS power handling when finished? If so, do we need about 60W of clean power per channel from the amp to drive them fully?

    Leave a comment:


  • scottsehlin
    replied
    The order of those two parts doesn't matter. The convention usually is to place the capacitor before the resistor - so that's what I did. In practice, I sometimes switch them if it helps improve the layout of the crossover board.

    Leave a comment:


  • mmu7
    replied
    Just got the crossover parts. Looking at the two schematics the 2 ohm resistor and 4 uF capacitor are switched, does the order of them matter or is there a typo in one of the diagrams?

    Leave a comment:


  • scottsehlin
    replied
    If you already have the parts to do 14 and 40 ohms, I would stick with those. The 15 and 47 ohm values were chosen to avoid the need to go with multiple parts or parts larger than they need to be. PE used to carry a 5W 39 ohm resistor when the design was first published.

    Leave a comment:


  • mmu7
    replied
    Originally posted by scottsehlin View Post
    Check out the first page of the thread. I have an updated parts list. It was long overdue. Thanks for bringing that to my attention, Matt.

    Thanks, so you changed from the 40 Ohm to the 47 Ohm across the driver and are now using a 15 Ohm resistor instead of 14 Ohms?

    I ask because I placed an order for the parts a couple of days ago and I went with 2 x Dayton Audio DNR-7.0 7 Ohm 10W Precision Audio Grade Resistor so I can wire them in series to get 14 Ohm and Dayton Audio DNR-40 40 Ohm 10W Precision Audio Grade Resistor across the drivers. Should I go and grab the updated design with the 15Ohm and 47Ohm resistors?

    BTW I ended up getting the 10 Watt versions of the resistors and the 18AWG inductors, not sure if that matters or not. Hopefully I can fit it all in the cabinets.

    Will post pictures when I get all the parts and assemble.

    Leave a comment:


  • Matt Clara
    replied
    Originally posted by scottsehlin View Post
    Check out the first page of the thread. I have an updated parts list. It was long overdue. Thanks for bringing that to my attention, Matt.

    Thanks Scott! Whoa--47 Ohm instead of 39? I'm calling it the new and improved BOM!

    Leave a comment:


  • scottsehlin
    replied
    Check out the first page of the thread. I have an updated parts list. It was long overdue. Thanks for bringing that to my attention, Matt.

    Leave a comment:

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