Keyboard Stand & Folded Horn Passive Amp

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Introduction: Keyboard Stand & Folded Horn Passive Amp

Last Christmas, I gave my sister an inexpensive electric piano, a Yamaha P-45. She loved it, but then wanted a stand.

At first, I thought I'd build a simple table. Then I noticed that the piano's speakers fired straight down from the bottom. There was one small, 4-1/2 inch speaker at each end. Hm!

Later, an idea came to me out of the blue: could a set of folded horns serve both as a stand, and as passive sound reinforcement?

I'm no sound engineer, and this design is far from mathematically optimal: no actual logarithms were calculated. I just kind of sketched out some baffles on the inside of one of the side panels, with the aim of limiting all cuts to 30-, 60-, or 90-degrees. Still, the baffles make a huge difference. The sound is fuller, more robust, and more directional, similar to an upright piano.

I thought about connecting the horns with a back brace, but finally left them separate. That way, they can be turned to aim the sound in the direction desired. This also makes them easier to transport and carry -- the sound input holes double as handles.

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Step 1: Get Your Supplies and Gather Your Tools

Materials

  • A little less than 1 sheet of 1/2-inch MDF or other suitable sheet stock such as plywood. Or, you could use real wood boards and pocket-hole screws. Up to you!
  • Wood glue such as Elmer's or Franklin Tite-Bond
  • About 100 1-1/4 inch screws with coarse threads and large, flat heads -- I used Kreg pocket-hole screws

Optional "piano" finish

  • Variety of sandpaper up to 600 grit or ideally 1200
  • Wood filler or putty
  • Spackle
  • Two cans spray primer -- best you can find
  • Two cans black spray lacquer
  • One can clear spray lacquer
  • Sanding block
  • Tub of water

Tools

  • Table saw
  • Compound miter saw
  • Electric drill with bit same diameter as your screws
  • HAND screwdriver
  • Electric sander
  • Corner clamp or square
  • Circle cutter -- I used a brace and bit with adjustable bit -- shown in picture

Step 2: Make Your Cuts

NOTE: I made my horns 9 inches square, with the sound input hole exactly in the center. If your piano is some other size, or its speakers are in a different place, you may have to adjust accordingly.

NOTE 2: I made my horns 24.5 inches tall. Combined with the P-45's height of 5.5 inches, this puts the keyboard height at 30 inches. This is a bit higher than most grand pianos, and a bit lower than some uprights. Ask your player what height they prefer and adjust accordingly.

TABLE SAW RIPS

Some lumber stores will rip stock for you, but you'll still probably want a table saw of your own to make the 60-degree miters. If your saw is too small for a full sheet, ask them to chop your sheet into two 4x4 sections. You'll have more waste, but there's more than plenty in a single sheet to make this project.

  1. Set your tablesaw fence to 9 inches, and rip two lengths from one of your sheet's long edges.
  2. Flip your sheet over, set your fence to 8 inches, and rip two lengths from your sheet's other long edge.

MITER SAW CUTS -- 9-inch stock

  1. Cut four 24-inch-long pieces. Write "side" on each one.
  2. Cut two 9-inch pieces. Write "top" on each one.

MITER SAW CUTS -- 8-inch stock

  1. Cut two 24-inch long pieces. Write "back" on each one.
  2. Cut two 8-inch pieces. Write "bottom" on each one.
  3. Cut two 15-inch pieces. Write "front" on each one.
  4. Cut two 19-inch pieces. Write "19"" on each.
  5. Cut two 7-3/4-inch pieces. Write "7-3/4" on each.
  6. Cut two 7-1/4-inch pieces. Write "7-1/4" on each.
  7. Cut two 6-inch pieces. Write "6"" on each.
  8. Cut two 3-inch pieces. Write "3"" on each.
  9. Cut two 1-inch pieces. Write "1"" on each.

ANGLE CUTS -- MITER SAW

Tilt your miter saw blade to 30-degrees off of vertical. Then, miter one end of each of your 3-, 6-, and 7-1/4 pieces. When you're done, each piece should still be the same length on its longest side, and your scrap pieces should have three sides, not four.

ANGLE CUTS -- TABLE SAW

Tilt your table saw blade to 30 degrees off of vertical. You'll be running your stock through vertically, so the effective angle will be 60 degrees.

Clamp a piece of scrap board to your fence if you don't want saw marks on it. Now, position your fence (or scrap board) as close to the blade as possible without hitting it.

Run each of your 3-, 6-, and 7-1/4 pieces through vertically to make your 60-degree miters. When you're done, the pieces should still be the same length on their longest side; otherwise, you cut the wrong corner. Conserve your scrap pieces to use as the top, front baffle corners.

Step 3: Assemble Top and Bottom Baffles

If you have a good supply of clamps, by all means, use them. I have found, though, that MDF glues well with screws, which can later be removed if you wish. Flat-headed 1-1/4-inch pocket-hole screws with coarse threads are ideal. To avoid splitting the MDF, you must pre-drill all screw holes. Do not use power tools when tightening the screws, either, because MDF strips very easily. Instead, use a hand driver and do not over-tighten.

When gluing, squeeze glue onto both surfaces, and spread it with a finger or piece of cardboard. A little bit (but less than 5 minutes) of drying time prior to assembly won't hurt and will probably help.

Keep a wet rag handy, and wipe drips and squeeze-outs as soon as you can. Once glue soaks into MDF, an even finish is difficult to obtain.

  1. TOP BAFFLE: Dry-fit the 19-, 7-1/4, 7-3/4, and 1-inch pieces. As a sanity check, do it on top of one of your side pieces. If all looks good, glue it up, screwing as you go.
  2. BOTTOM BAFFLE, PART 1: Next, dry-fit the front, bottom, and back on top of one of your side pieces, then glue and screw. If you have one, a picture-frame clamp (see picture) helps ensure a right angle. Otherwise, use a framing square. Let dry briefly.
  3. BOTTOM BAFFLE, PART 2: Add your 6-inch and 3-inch corner pieces. You can't really use screws on these parts. Press them in gently, or else you may inadvertently spread your front and back pieces. Set the pieces upright, and let gravity do the rest.

Step 4: Assemble and Finish Your Speakers

Once your top and bottom baffles are dry enough to handle (an hour or two), place them one side, and trace around them. Repeat with the other side. This will show you where to spread the glue.

Next, glue the bottom baffle to one side, screwing as you go. Then, glue the top baffle. Immediately turn the whole thing over, and glue and screw the other side.

Repeat with the other horn.

While waiting for your glue to dry, cut circles into the top pieces. Then, glue and screw them on.

FINISH

Once your glue has dried, remove all exterior screws and fill with wood putty or paste.

Next, use a power sander as needed so that all edges and joints are flush and even.

Seal all MDF edges with spackling. Otherwise, they will absorb an infinite amount of paint.

Finally, apply your finish. If you choose to attempt a piano finish, as I did, you will likely find that it takes about three times longer than the entire rest of the project put together, since it involves about 10 coats of spray primer, spray lacquer, and spray clear coat, with wet-sanding between each coat. Instructions are widely available elsewhere.

I was unsure about whether to make a seal between the bottom of the piano and the sound hole. I bought a thick rubber pipe stent in the plumbing section of my local hardware store, figuring I could cut about 3/8-inch from either end to create seals. I have not tried it, though, since I feared it could limit high frequency dispersion too much (not sure?), and since the sound -- such as it is -- is already pleasing enough to me.

CONCLUSION

There is no doubt that modern, electric pianos have many advantages. They don't go out of tune, have built-in metronomes, many different patches or voices, midi outputs, and they can even be tuned or modulated up or down to accommodate different singing ranges.

One thing they lack, though, is furniture-like solidity and the timbre and richness of sound that come with that. Hopefully, designs such as this could fix that. I hope those with true audio expertise may post comments below on how to improve the design, and that this concept is something others may enjoy. Who knows -- maybe someone will even make something like it available commercially someday.

Anyway, thank you to anyone to read through this whole thing. I hope you enjoyed my first instructable as much as I enjoyed making it.

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1 Person Made This Project!

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11 Discussions

0
ander
ander

2 months ago

[NOTE: If you haven't read my original comment, below this one, please read it first.]

Hi again everybody,

Thought you'd get rid of me that easily, did you? (Buh ha ha!) Just wanted to let you know, I've added two more refinements to my build:

• I happened to use the keyboard on a folding stand, and noticed I heard more treble (though the mids and bass weren't as good as they were on the baffles). So I used a couple of hole saws to add treble ports (on the player's sides only) where sound enters the baffles. It was a nice improvement! I didn't size the holes scientifically; I just used saws I had that worked well in those spaces. (Feel free to correct me—but I think these smaller holes vent higher sounds because their waveforms are shorter.)

• I used a Forstner bit, which makes flat-bottomed holes, to add 1/8"-deep recesses where my keyboard's feet go. (To find the hole positions, I placed my keyboard face down, centered a square piece of sticky tape on each foot, then carefully lowered each top into place, centering its hole on the speaker, till the tape stuck. I'm sure there are other ways to do that sort of thing though, huh?) The recesses make it easy to place the baffles where the speakers are, and help keep the baffles from moving.

I took a couple of photos (before repainting, so you could see the changes more clearly), but for some reason Instructables won't let me include them—so I've posted them here:

https://i.postimg.cc/BZNpjT3h/baffle-sides.jpg
https://i.postimg.cc/tJrN8jD0/baffle-top.jpg

Cheers!

0
ander
ander

6 months ago

I've made my stand! My keyboard's speakers are at the rear of its lower surface, so I was able to modify my interior plan to allow a more direct path for the sound. It seemed important for the sound's path to keep expanding (as in the author's original design), so I made sure to keep my center partition angled to achieve that. My other "mods":

• I made my holes as large as my speakers (which are also 4.5"). Since the holes are concealed by the keyboard, I didn't feel obligated to be particularly tidy with them... I just drilled some small corner holes, then used my jigsaw to cut out the rest.

• I placed my lower baffle pieces at 45° angles, which seems to reflect the sound along the path as well as possible.

• Rather than leaving the screws visible, decided to use conical-head wood screws so I could countersink them (see photo), fill the holes with wood filler, and paint over them.

• Rather than regular MDF, which is coarser and contains various materials, I used higher-quality "ranger board" made from fine softwood sawdust. It allowed me to get a nice even finish with just a couple light coats of brush-on flat black paint. (In case anyone's considering spray paint, I highly recommend brush-on paint instead. On materials this absorbent, brush-on paint covers much better, and isn't nearly as messy, or as potentially toxic, as spray paint.)

• The author's glossy finish is beautiful, but I decided to paint my stands a simple flat black to match my keyboard's flat finish. My local paint shop mixed a quart of flat black for me quite inexpensively.

In my profile diagram, I've shown the distances between the center baffle piece and the front and back of the box, in case that helps. When I cut the top baffle piece, I cut its lower end at 20.5° (the upper piece's angle, minus the lower piece's) so I could cut both ends of the center piece straight, rather than trying to deal with two odd, connecting angles.

Rather than painting all the surfaces of my enclosures, I painted only the parts that would be visible once they were assembled. I sealed the remaining interior surfaces with a hard, clear finish, to do two things:

• Seal in the odor of untreated MDF (which can be unpleasant).

• Make a harder, smoother surface for the sound to reflect on.

For my interior, I used spar varnish, made for wooden surfaces on boats. It's inexpensive and dries to a very hard, durable finish.

If you decide to do this, it's much easier to finish your interiors before you assemble them. Be sure to trace around the areas where you'll be gluing, so you can avoid applying finish there and inhibiting the glue. MDF products are very absorbent, so I suggest at least two coats of interior finish to seal it well.

My keyboard sounds not only fuller and more resonant, but clearer, too, since the sound now comes from the front rather than the bottom. Thanks again to nerdyH for this great idea!

open_box_01.jpgopen_box_02.jpgcountersunk_screws.jpgunpainted_stands_00.jpgunpainted_stands_01.jpgkeyboard_stand.jpgenclosure_profile.pngpainted_stand.jpg
0
nerdyH
nerdyH

Reply 6 months ago

How cool, congratulations, and I'm excited that someone actually took a shot at this! I'm glad you are pleased with the results.

Since posting this, I took one for analysis by Zu Audio, in Utah. Zu uses folded horns in audiophile speakers typically priced upwards of $5K per pair. Sean Casey, one of the mad geniuses behind Zu, immediately picked one up and started singing into it at various pitches. His voice did sound weird at some pitches, similar to singing into a culvert. Casey said this showed the design had a lot of room for improvement. Eliminating reverb is obviously essential for clean passive amplification.

I told Sean the design was loosely inspired by the square Fostex full-range folded horn kits widely available from Madisound, Parts Express, and similar companies. "Those are crap," Casey commented with a laugh.

By the end of the session, Sean and one of his engineer/draftsmen had sketched out what they thought might be a superior design, which also benefited from being much simpler and presumably easier to have manufactured, since the baffles and reflectors would be easily sourced. Being speaker designers, they also thought including a driver and possibly even amplification would be better, making the design more universal regardless of keyboard make and model.

Then, we talked about how much it might cost to develop the product, the size of the potential market, and how much people might be willing to pay for a keyboard amplifier. It was a pretty interesting talk! But definitely one that left me with a healthy respect for people who actually take the risks and do the work of bringing a product to market.

Keep building!

-Henry

0
ander
ander

Reply 6 months ago

nerdyH> By the end of the session, Sean and one of his engineer/draftsmen had
sketched out what they thought might be a superior design...

That's exciting! Would you mind posting a diagram of your enhanced idea? Or is it confidential?

0
ander
ander

Reply 2 months ago

Guess it was confidential... I'm "baffled" (LOL).

0
ander
ander

6 months ago on Step 4

What an awesome idea! I was going to make a normal wood stand, but this is much better. Can't wait to try it!

0
nerdyH
nerdyH

Reply 6 months ago

Thanks! Good luck with your build, Ander!

0
Swansong
Swansong

2 years ago

That was a really good idea to add that feature into the stand :)

0
nerdyH
nerdyH

Reply 2 years ago

Thanks, Swansong. Your username makes me think if Led Zeppelin

0
Swansong
Swansong

Reply 2 years ago

Lol, I do like that song. My dad was a big Zepplin fan. ^.^

0
Flenters
Flenters

2 years ago

Brilliant