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Thread: Project M2 DIY Thread

  1. #31
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    Quote Originally Posted by Hoerninger View Post
    Study the circuit
    ____________
    Peter
    I am not the "study the circuit guy" unfortunately, so maybe it was a stupid question from me
    If it was an answer like, "put two caps with sum equaling 68uf in parallel, and a 9v battery chargeing the caps" or something that simple, then I maybe could have pulled it off

  2. #32
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    Found this simple explanation:
    http://www.enjoythemusic.com/diy/091...crossovers.htm

    But for 68uf I need two very large caps in series...

    Sorry for the OT.

  3. #33
    Member Fitero's Avatar
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    Thank you for the interesting technical information and gentle comments.

    I don't mean to be obtuse, but this is how I see it; the engineers at JBL have already completed the necessary port calculations, and through subsequent trial and error, have fitted ports that they deem the best compromise for this enclosure.

    Port design theory got them close. Listening and modification got them exactly how they wanted it to sound.
    It seems more prudent to defer to their experience and knowledge and build as closely as possible to theirs.
    By blindly copying their dimensions, I can save an enormous amount of time, and avoid making errors.

    Where I can surpass the original design is by making the ports with better flow characteristics. Their method of joining two bells in the center has left a sharp ridge, which for a venturi is anathema and results in disruption of laminar air flow.

    So again, a simple measurement of the bell diameters, port length and centre I.D. is all that is needed to make the copy.

    If that can't be obtained, then you are right in that I would have to rely on available port calculations to get a brute proximity.

  4. #34
    Senior Member srm51555's Avatar
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    Quote Originally Posted by bubbleboy76 View Post
    Anyone having a good link for a description of how to do that?
    (I might be interested in trying it for my 68uf protective cap, just for fun, if it does not change phase behaviour or something like that.)
    Just double the capacitor value (3.3uf to 6.6uf) and then double the amount of capacitors used. Instead of one 3.3uf you will have two 6.6uf caps in series. I'm going old school and biasing via 9V for now.

    Good info here

    Edit: Sorry didn't see post 31, keeping the post for the CC Timbers link

  5. #35
    Senior Member srm51555's Avatar
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    Ports

    I posted this in the M2 thread.

    Using a volume of 5.2 cubic feet, a tuning frequency of 27hz, port diameter of 3” and the number of ports needed set at 2 using, the precision port calculator recommends this port http://www.parts-express.com/precisi...-tube--268-368 (and it's inner mate) to be set with a total length of 8.85.

  6. #36
    Senior Member srm51555's Avatar
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    Quote Originally Posted by 1audiohack View Post
    Well the M2 HF protection network comes with a small surprise.

    The cap values are 3.3µF and 4.7µF.
    Thank you for posting this.

  7. #37
    Senior Member 1audiohack's Avatar
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    Quote Originally Posted by srm51555 View Post
    Thank you for posting this.
    You are welcome!

    So I have a port question: On the other thread post 842 shows the inside of the port and front baffle board and the port major outside diameter is smaller than the front. Surly to go through the baffle hole in one piece so, do we know the cut out diameter and or the diameter of the flare? At a glance it looks like a eponential flare? Do we know the center minor diameter?

    I looked carefully at the ports in my 4365's and they are a plain radius on the front and rear with a piece of tube in the middle. Not as racey as the M2 port. Nothing to learn there.

    It also looks like that part of the M2 front baffle is a laminate of two pieces of MDF. How thick?

    Edit: Thinking about this a bit, it is doubtful that my box, or many, will tune in just like the M2 so working to make identical ports will only be academic. Edit.

    Thank you,
    Barry.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  8. #38
    Senior Member pos's Avatar
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    The geometry of the port baffle should be identical to the upper part of the horn, as the lower part of the grill does fit that part almost exactly (I tried this when I considered reversing the horn...)

  9. #39
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    Quote Originally Posted by Fitero View Post
    Where I can surpass the original design is by making the ports with better flow characteristics. Their method of joining two bells in the center has left a sharp ridge, which for a venturi is anathema and results in disruption of laminar air flow.

    So again, a simple measurement of the bell diameters, port length and centre I.D. is all that is needed to make the copy.

    If that can't be obtained, then you are right in that I would have to rely on available port calculations to get a brute proximity.
    http://koti.kapsi.fi/jahonen/Audio/P..._PortPaper.pdf

  10. #40
    Senior Member ivica's Avatar
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    Quote Originally Posted by 1audiohack View Post
    You are welcome!

    So I have a port question: On the other thread post 842 shows the inside of the port and front baffle board and the port major outside diameter is smaller than the front. Surly to go through the baffle hole in one piece so, do we know the cut out diameter and or the diameter of the flare? At a glance it looks like a eponential flare? Do we know the center minor diameter?

    I looked carefully at the ports in my 4365's and they are a plain radius on the front and rear with a piece of tube in the middle. Not as racey as the M2 port. Nothing to learn there.

    It also looks like that part of the M2 front baffle is a laminate of two pieces of MDF. How thick? If we can get the real geometry I will make the ports for anyone that wants them at material cost.

    Thank you,
    Barry.
    Hi Barry

    I have read somewhere that the suggested port flare would have be about 5 to 7 deg-off-axis, but I have no idea about M2 port off-axis flare, is it conical, hyperbolic or exponential.
    From the:
    http://www.aes.org/e-lib/browse.cfm?elib=11094
    "Maximizing Performance from Loudspeaker Ports" [Alex Salvatti and Doug Button, JBL Professional,Allan Devantier, Infinity systems,Northridge, California]

    I have understood that COSH function would give good results, on the other side seems to me that if the curvature radius equals to the port length would give very low SPL compression. On the other side port diameter vs 'inlet radius' about 5 (Dp/r) would be 'loss-less', so different walls flare can be expected.

    May be some ideas can be find in JBL patent: US7711134
    http://www.google.com/patents/US7711134


    regards
    ivica
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  11. #41
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    For the level of precision that I wish to attain;

    The dimensions that are missing to make an approximate facsimile are the inner bell diameter and the port's minimum inner diameter.

    A more precise measurement of port length could be obtained by placing a straight edge across the port bells and passing a tape measure through the port and obtaining the measurement.

    For the inner minimum diameter, and in the absence of a snap gauge, a piece of paper, gradually folded into smaller dimensions until it fits perfectly in the smallest portion of the port then measured would be quite close.

    This port could be modified to approximate the OEM port dimensions once they are known.
    The curved lower port mount panel appears to be 21mm thick at it's maximum, judging from the JBL's published dimensions and those that have been posted here.
    This is how the port molding pieces are joined and typically mounted to front baffles;
    Last edited by Fitero; 12-13-2015 at 08:25 AM. Reason: clarification

  12. #42
    Senior Member pos's Avatar
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    Nice!
    Where does this port come from?

  13. #43
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    http://www.ebay.com/itm/331700198468...%3AMEBIDX%3AIT

    Almost the exact outer diameter as the OEM ports too.

    Duh! Same thing, but much cheaper here;
    http://www.parts-express.com/flared-...-ends--269-165
    Last edited by Fitero; 12-14-2015 at 05:20 AM. Reason: additional information

  14. #44
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    It looks like it is too long though, and the inner part looks too large...

  15. #45
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    It is, but it can be cut at the junction seam, heated and re-molded into a small diameter, then cut to the correct length and re-joined.

    The inner bell can be expanded to the correct diameter through heating and re-molding as well.

    Once I am able to obtain the correct dimensions of the original ports, then I will buy one of these and give it a go.

    At the very least, it may provide me with a mold to make a fiberglass port with if I choose to go that route.

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