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Thread: New Project - Kickin' A-- and Taking Names!

  1. #31
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    I glued the faces on and finished the rough sanding for the two cabinets. I'll need to repaint where the material that was removed from the sides/faces (~1/8" all around plus that portion of the box that extended beyond the faces). That's a lot of sanding and my hands and arms haven't been this tired since I was a teenager.


    I also need to stain the cabinets. I am trying to decide if I should do the paint up, or stain first. Probably stain. Opinions?


    We are currently having strong Santa Ana winds, and any painting will have to wait.


    Meanwhile, I think that I will mount the speakers and see if there is anything to be gained by going with open baffles. (Could be an epiphany!) This might be the better way to go with the Heils.


    I can either just use a capacitor to block the bass, or set my speakers as "small" and let the Yamaha roll them off below 200 Hz and run them full range. Could be interesting.



  2. #32
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    Ready for some A/B between open /closed back performance.



  3. #33
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    OK..., got some RTA screen shots showing the open vs closed back with the 2251Js in ~1/2 cu ft cabinets. The only filtering being used is a 33.6 mfd cap inline with the positive terminal and the speaker is set to "large."

    The microphone was placed inline between the two speakers at a distance of 2 meters on a tripod sitting on the couch cushion (legs in). "Close" micing was not performed because this defeats the purpose of the open back. No eq of any type is being used. Of course this does include the effects of the room.

    The intent is to run these from ~250 to ~2,500 Hz.

    Note that with the closed back, the frequency response "falls" as the frequency is reduced such that between 250 and 2,500 Hz there is a span of about 16 dB (i.e., -83 to -67 dB).



    But with the open back, the frequency response remains fairly flat dropping substantially below the crossover point, which is essentially what we want here. The span is reduced to about 9 dB (i.e., -74 to -65). The overall volume is also louder.

    It is interesting that even though the closed back sounds like it has more bass, the open back must by responding with things/reflections in the room causing more deep bass. I am simply switching between the two speakers and recording the results without any provisions to equalize their volume. What you see is what the mic hears.


  4. #34
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    I have found that while no back is better than having a full back..., having some back can produce a flatter bass response than no back. Therefore, I'll cut both backs down to two strips to go along the bottom to hold the terminal cup, and the top to match, leaving about 31.5 sq in in open. This is also roughly the area of the cone so we'll see how that goes.

    Meanwhile, I pulled the back off the other one and have each sitting on an L200/300 cabinet. These are wired with 35 mfd in series with the positive lead to the "B" speaker output though my speaker switcher that allows for volume matching. I actually have these turned down by ~8 dB according to the panel markings.

    I'm loving it! If you look at the RTA plots you will see that the 2251Js have a nice little "bump" that fills in the gap left by the crossover between the 2235 and 2420. Then you get a near perfect natural rolloff to the 2405 with no electrial filter necessary! Vocals and cymbols become so much clearer, or there where they weren't before. I should have done this years ago.

    Actually, I'm betting that the 2251J could easily replace the 2420/horn all together, go much lower, and would be smoother doing it. May just need to turn off the horns and give this a try.

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