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Build - Polytope

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  • xenzmann
    replied
    Re: Build - Polytope

    I got a chance to do a few measurements on the speakers. I tried three different amps (Old Dayton T-Amp, AudioSource AMP 100, Peavey IPR DSP 1600) and two speaker cables (12 AWG and 14 AWG) with HOLMImpulse to double check that I was measuring the speakers, not what's in between. The mic is calibrated and comes into the PC through a Xenyx 302/USB so the measurements are reasonably accurate. I settled on using the Peavey as the amp for most uses as it has sufficient overhead to cleanly drive the speakers to full volume. I'm also hoping to make use of the built in parametric EQ to further flatten out the response. Not worth doing too much on that while they are in the garage.

    The HOLM Impulse response measurements are shown below. Given the space I'm testing in, it will be a bit tough separating the room modes out and determining what areas to focus on. Any suggestions will be appreciated.

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    Tests of White noise give similar results to the frequency sweep performed in HOLMImpulse. Here are left and right speakers with solid green showing the averaged output and dashed green showing the measured/averaged response. Levels were adjusted so the two graphs were close to superimposed. Repeating those tests with pink noise gave comparable results.

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    Pointing the mic between the speakers and averaging the spectrum for a hour or two gave pretty reasonable looking results. The first image is a variety of Beethoven, the second is a variety of STS9. The 30-60 Hz bump in the latter is pretty typical of that kind of music, and quite expected.

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  • xenzmann
    replied
    Re: Build - Polytope

    Part 6 - Tuning and Crossover Design

    There are many great tools out there for this part of speaker design. My favorites are: Speaker Workshop, Passive Crossover Design, HOLMImpulse, and the LinearTeam calculators. While fiddling with the crossover design actually started before any of the cutting, actually evaluating them didn't happen till near the end.

    The nominal volume is 2.5 ft^3, and after a bit of playing, tuning to low 20's gives a decent rolloff, with only a little bump between 48 and 100. Down 3 db at 28 and down 6 db at 23. Two ports with 1.5" diameter, 6" long.

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    A quick on the LinearTeam website for the Acoustic Power calculator and Vent Calculator give these:

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    Looks like the vent mach # will be ok. Would prefer a little lower, but it will do.

    With much fussing and twiddling of values in PCD and Speaker Workshop, the crossover looked like this:

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    The LCR in there is to tame a bump in the midrange. After putting in the offsets for the three drivers, both SpeakerWorkshop and PCD were pretty close in their predictions. PCD looked like this:

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    The prediction is a frequency response that is within a few db over the majority of of the spectrum. The impedance is a bit squirrly, but I'm not going to drive it with tubes so really not a problem. Just need a 4 ohm safe amp.

    Tangent - While I think tubes have a nice sound, particularly as they near their limits, I'm a bit biased against them. As a kid, when the TV failed, we would have to: unscrew the back, take out the dozen or so tubes, take them down to Radio Shack, test each one individually on the tube tester machine, buy a replacement for the failed one, bring them all back and put into TV, screw back onto TV. What a pain. Just give me solid state with 3-4 times the rated power of the tube amp and it will sound fine...

    Note that this is still a paper exercise. Speaker placement and room modes will result is much larger variances. Additionally, I've noticed that both SW and PCD seem to assume point sources and really don't cleanly predict the response for tweeters on waveguides. I've seen this in a couple of builds now.

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  • xenzmann
    replied
    Re: Build - Polytope

    Part 5 - Wiring

    My goal for these is to be able to run the speakers in simple mode where a crossover does the filtering, but when I want, turn them into either bi or tri-amp configuration.

    Being able to work w/o special amps is important, because these will be used in the short term for casual listening and driven from a commercial receiver. Given how sound can change in different rooms as well as tastes of the person listening, I knew that level adjustments would useful. Also, I do plan in the future to put more sophisticated electronics behind them so I needed a way to be able to select modes.

    The end result is: two Neutrix SpeakON connectors, two switches, and two adjustable L-Pads. Each of the SpeakON connectors are four pole and are wired like this:
    • Connector 1: 1+/1- to crossover, 2+/2- to woofer
    • Connector 2: 1+/1- to midrange, 2+/2- to tweeter


    One switch is a DPDT, one pair of inputs comes from the woofer part of the crossover, the other comes from 2+/2- of SpeakON number 1. The output of the DPDT switch goes to the woofer. The second switch is a 4PDT, one set of inputs come from the mid+tweet section of the crossover, the other set of inputs come direction from 1+/1-/2+/2- of the second SpeakON. The output of the 4PDT go to the midrange and tweeter. The result is four possible configurations, three of which are useful:
    • DPDT on, 4PDT on: All speakers go through the crossover and are fed by a single input
    • DPDT off, 4PDT on: Midrange and Tweeter go through crossover and woofer is fed directly. A reasonable biamp mode.
    • DPDT on, 4PDT off: Woofer through crossover, midrange and tweeter fed separately through second SpeakON. Probably not useful mode
    • DPDT off, 4PDT off: Woofer fed directly by 2+/2- of one connector. Midrange fed directly by 1+/1- of second connector. Tweeter fed directly by 2+/2- of second connector. Triamp mode w/ no filtering on any driver.


    The terminal plates started with holes for two SpeakON and two holes for 1/4" connectors. After widening the 1/4" holes a bit, the switches went in there. On the right side of the plate I drilled holes for mounting the adjustable L-Pads.

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    The size of the midrange L-Pad made the fit difficult. I decided on a 100 Watt for the mid because the next lower option was only 15 Watts. With the size of the knobs to factor in plus the width of the back, they just barely fit. Front/back views below.

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    Only short wires are necessary for the direct connections between SpeakON and the switches. In the image below you see the inputs wired up and short pieces of wire for the outputs. Labelling everything is a good idea if, like me, you use all white wire. Note that two of the legs were cut off with a coping saw so the L-Pads would fit. The other four will be used later for the fixed resistor part of the L-Pad.

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    I used spade terminals on the ends of the wires that go to the switches. Crimped on, then soldered. This makes the connection much easier - you really don't want to try to connect stranded wire to all those little screws. Below is all the wires that go to/from the switches, with soldered connections to the adjustable L-Pads. It doesn't fit through the hole in the cabinet with L-Pads attached as they stick out the sides too much, so they have to be ratcheted in place after.

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  • johnnyrichards
    replied
    Re: Build - Polytope

    Awesome build! ...and I totally get the "happy guy" impression - when I had Emoticon in raw mdf cabinet form, my wife noted that the back looked like the text version of this: :eek: So was born the name.

    Again, love this build. Well done.

    Leave a comment:


  • xenzmann
    replied
    Re: Build - Polytope

    I wanted to widen the port holes a bit, without getting dust and bits of paint all over the inside of the cabinet. To do this I ended up lining up a shop vacuum with each port hole from one side:

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    Now as I applied the sanding drums to the port holes, all the bits of crud that didn't stick to the sandpaper went right into the vac.

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    The speakers were now at a point where the filters and wiring could be added and tested.

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    Ok, a garage is not the best placce to do audio testing. However, the way that all the bits of wire and things tend to spread around, doing this anywhere else wasn't an option.

    Under each speaker is one of the round pieces that you get when you cut out driver holes, covered by a nice clean cloth. I haven't decided what sort of feet to attach to the base yet.

    Happy day, the new terminal plates arrived. Time to do surgery on the cabinet. First mask off any place the jig saw might touch:

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    After cutting, the speaker definitely looked like a happy guy.

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  • xenzmann
    replied
    Re: Build - Polytope

    Part 4 - Assembly

    With a nice coat of paint on, and a few days to let the paint harden a bit, it is time to assemble the major pieces. First was a test fit of the midrange (Eminence Beta 10-CBMRA) and the waveguide for the tweeter (Selemium D220 Ti, not attached in photo). At this point I was drilling the mounting holes and pulling the T-Nuts tight. Note - remember to do all that before putting the cabinet together. I was pretty tense, knowing that if I shredded the baffle, repair would be very difficult.

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    Kudos to Parts Express here. The mounting kits they sell (like this one: http://www.parts-express.com/pe/show...number=260-776) are far cheaper than purchasing the bolts and T-Nuts from any hardware store near me. The nuts and bolts all run between 0.50 and 1.00 each so the $3.50 from PE for a set of 8 each is a great deal, a half to quarter of what I would otherwise pay.

    After getting the mounting hardware in for the mid and high, I did the same for the woofer. Below are images of the woofer mounted in place, before and after the speaker base is screwed on.

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    Next to add are the SpeakON connectors and ports. During testing I'll be running separate wires to each driver, so 2+/2- on one connector goes to the woofer, 1+/1- on the second to the mid, and 2+/2- to the tweeter. In the end, 1+/1- on the first connector will go to the xover. A bit of blue tape is attached to each wire so I can tell what it is for while wiring.

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    The shiny tubes next to the cabinet are the ports. I use metal drain flanges - they come in various lengths and I prefer the look to the typical plastic tubes. It takes a Dremel or hacksaw to adjust the length. The biggest drawback of them is that the rounded outer part is quite narrow - you have very little tolerance for error when cutting the hole or your mistakes will show. You also need to round the outside of the port hole a bit to accomodate the bend from the straight part to the flange.

    Here was another gotcha. The port holes I drilled out with the Forstner bit were exactly the right size. Before painting. They don't fit any more :(

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  • xenzmann
    replied
    Re: Build - Polytope

    Originally posted by bkeane1259 View Post
    Very nice and it seems like you knocked them out in no time flat. I too am interested in how you are running them. Looks like they are bi-amped. How are the switches and variable L-pads incorporated?? How do you like the sound?
    I hope it doesn't seem like this went too quickly. The project started about two and a half months ago and is just now nearing completion. It's only been in the last week that I started organizing the pictures I took along the way to write up this thread.

    Stay tuned for xovers and wiring options.

    Leave a comment:


  • xenzmann
    replied
    Re: Build - Polytope

    Originally posted by OlderMongrel View Post
    Very nice. Is that the Duplicolor Mirage 3-step paint?

    What sort of room are they in, and how are they located in it?

    Is this an active/passive hybrid, or just passive, or...? What's the XO, assuming any of it is passive? Can you do any measurements?

    John
    The room they probably will go in is a living room w/ carpeted floors, about 15x25x7. Speakers will be close to corners and between 6" to a foot from the walls. Not ideal, but that's how it will be. There are considerations beyond pure sound

    Not the Duplicolor, it is Rustoleum's "ColorShift". Unlike Duplicolor, you can buy just cans of the speciality paint and use your own black and clear. Still a three step process, but cheaper in the end because you aren't paying the markup on the black/clear parts. Plus, my local Lowes had it on sale (which is why I tried it) for USD $11 a can.

    I'll describe the xover & wiring options in later parts of the thread, just haven't written it up yet. In simple terms though, through switches you can select between: pure xover, biamp, or triamp.

    Measurements upcoming as well.

    Leave a comment:


  • bkeane1259
    replied
    Re: Build - Polytope

    Very nice and it seems like you knocked them out in no time flat. I too am interested in how you are running them. Looks like they are bi-amped. How are the switches and variable L-pads incorporated?? How do you like the sound?

    Leave a comment:


  • OlderMongrel
    replied
    Re: Build - Polytope

    Very nice. Is that the Duplicolor Mirage 3-step paint?

    What sort of room are they in, and how are they located in it?

    Is this an active/passive hybrid, or just passive, or...? What's the XO, assuming any of it is passive? Can you do any measurements?

    John

    Leave a comment:


  • hillbillydeluxe
    replied
    Re: Build - Polytope

    hats off to you.looks like a PITA to build but the end result justified the work:applause:

    Leave a comment:


  • xenzmann
    replied
    Re: Build - Polytope

    Skipping ahead a bit. The sun came out for the first time in a couple of weeks, so I took the opportunity to take photos.

    Here's what it looks like from in front:

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    A view with the light coming from behind:

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    And a shot of the back with: SpeakONs, xover/biamp/triamp switches, and mid/high L-Pads.

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    Some of the smudges are just that, from picking up and moving them and not wiping the bottom faces. Some are unfortunate reflections and glare.

    Yes, they sound fine and are the first speakers I've built that can properly play "Space Time" by Delta Heavy w/o a separate subwoofer. The song that I like when checking vocal clarity is "Wuthering Heights" by Kate Bush it should be clean and clear without hurting your ears.

    Xander

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  • xenzmann
    replied
    Re: Build - Polytope

    Originally posted by ReissM View Post
    Way cool. Just........ way cool. Thanks for taking all of us on your journey, step by step. Very neat! D@mn that's a lot of work when compared to a basic rectangular box. (And I thought my Baby Eidolons were a mathematical nightmare... not even close when compared to yours!) Keep up the great work.
    Thanks - I hope people get something out of it. I don't post a lot, but I really enjoy reading the build threads and discussions of the tech bits that go into good audio. Since this project is a little different, I figured I'd post what happened along the way so others like myself will either pick up something or see something to avoid.

    One thing I learned was the joy of biscuit joining. There is a tool that is totally worth the $'s. I'll never go back to simple b**t joins, , even on standard 90 degree angles. (why does that word get filtered when it is a standard carpentry term?) The biscuit provides so much extra stability and strength for so little effort.

    Xander

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  • ReissM
    replied
    Re: Build - Polytope

    Way cool. Just........ way cool. Thanks for taking all of us on your journey, step by step. Very neat! D@mn that's a lot of work when compared to a basic rectangular box. (And I thought my Baby Eidolons were a mathematical nightmare... not even close when compared to yours!) Keep up the great work.

    Leave a comment:


  • xenzmann
    replied
    Re: Build - Polytope

    With the cabinet covered with a garbage bag, I flipped the cabinet over and masked off everything that already had ColorShift on it. I also masked off the woofer opening. I figured that unlike earlier when there was nothing to protect, if paint went in through now it could go out the mid or tweeter opening and get on the good paint.

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    After several passes of base color + sanding and a few layers of ColorShift, the bottoms were done. Maybe I should have just left them black - after all, who is going to be down on the floor looking up at that part?

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    This where the next big "Oh S**t" moment happened. Remember that soft cloth I put on the top? Well, I hadn't waited long enough before putting weight on the surface and the paint on the top was hosed.

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    So, as before, mask off everything. Yes, garbage bag again, just the opposite direction. Sand down to base color, build up base, add layers of ColorShift, and finish with layers of clear. Fortunately in this orientation, I could put strips of wood under the bottom at the points where the base will attach, so any marring of the paint will not be seen because they will be between the main chamber and the top part(s) of the base.

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    Once all that was fixed, it was followed by four coats of clear. At this point, the conventional wisdom is that I should wait a month or so before performing any sort of sealing/waxing on the paint. Apparently the paint outgasses for weeks and sealing them can cause blotching and bubbles. Overall the whole painting thing was a very frustrating process. In addition, when I added up the cost of the materials ($'s per can of black, ColorShift, and clear) I realized that I spent more on the paint job for these speakers that I had for the entire last build I did.

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    Next up will be installing the drivers and various other assembly tasks.

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