Passive Smartphone Amplifier

Amp your jams and charge your battery with this two-piece tune booster

I have thousands of songs in my phone, to say nothing of the apps streaming satellite radio, live sports, and more. But when I’m without headphones or a Bluetooth speaker, getting all that entertainment out of the phone and into my ears leaves much to be desired. When I found that lodging the phone in my car’s cup holder amplified and deepened the sound, I started putting it in a Solo cup as I listened in my shop. This proved an effective—if inelegant—solution to the volume problem, but did nothing for the fact the phone’s aging battery played fewer songs on every charge.

This project aims to tackle both these issues. First, the laminated amplifier box adds both depth and volume to your tunes. Second, the phone rest features an inductive Qi charger (see Buyer’s Guide, p. 60) recessed into the back, cleverly concealed beneath the wood but able to charge a phone’s battery right through it. Attach the two units with recessed magnets and slide your smartphone into the slot to enjoy music while charging. Or go totally wireless by disconnecting the two units and upending the amplifier box. Your phone stays upright and the jams stay pumping in a smaller footprint. Suit the unit’s dimensions to your particular phone. I strongly suggest first making a mock-up (see p.41) to determine your phone fit.

More sound, less machine

This two-part assembly consists of a laminated amplifier box and a phone rest that connect via rare-earth magnets. A curved-bottom slot routed with a round-nosed bit intersects the box sound hole, creating a cradle for a smart phone whose speaker projects into the sound hole. The phone rest houses a Qi charger that works wirelessly through the wood. When locating the charger recess, ensure that it will be centered behind the phone. 

Dimensions shown are for iPhone 8. Slot length, width and depth, and phone support rod locations can be varied to suit your smartphone

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Hear the amplifier’s performance and get a closer look at some of the technical aspects of the project in a video tour.

Order of Work

  • Make amplifier
  • Rout slot
  • Make phone rest
  • Bevel bottom
  • Attach support leg
  • Drill charger hole and finish up
  • Get your jam on

Build the amplifier box

Face glue layers of wood to create a block big enough for the amp box—I used cherry and maple with rosewood veneer in between. Cut the block to size and drill the sound hole. Make a simple jig (drawing, below) to guide your plunge router as you cut the phone slot. Then use the same jig to position the box as you drill for the magnets that hold the phone holder in place. I found that by drilling precisely, the magnets can be press fit into place just below the surface where they won’t interfere with sanding or finishing. If yours are loose, secure them with some epoxy.

Keep it down. Grasp the amp box in a handscrew clamp, then fasten that clamp to the drill press table to safely bore the 3" hole, 31⁄4" deep in the laminated box. Go slow and stop often to clear chips.



Birds Mouth Jig

Lay out and make a birds mouth cut (as seen below) near the end of a 16" length of scrap. Pocket-screw a fence parallel to the long side of the birds mouth cut to allow the jig to be clamped to the workpiece while keeping the clamp out of the way. 

Get your groove on. Lay out the centerline of the phone slot as shown in the drawing (p.27). Size the slot length and width to suit your phone. Clamp the box in the birds mouth jig and hold the assembly in a vise. Rout the slot in several light passes, guiding the router along the jig with an edge guide. Leave the shoulders at the bottom of the slot wide enough to support the phone without blocking its speakers. Clean up any rough edges with a file.

Side A. Keep the amp box clamped in the jig. Attach a stopblock to your drill press fence and drill the upper magnet hole where indicated in the drawing on page 27. Then insert a 3" spacer between the jig and the stopblock to locate the second hole. Press the magnets into place. 

Build the phone holder

Edge glue the same sequence of species as you used for the amp box to create the phone holder, making the bottom portion of the holder at least 3/4" wider than specified. Drill one end for the magnets, as shown. (Magnetism won’t affect phone or charger.) Press-fit them in place, orienting them for attraction with the magnets in the amp box, then mark and bevel the bottom of the holder. Use the stopblock and spacer method one last time to drill the holder for the 1/4" aluminum support rods, making sure the top edge of the rods aligns with the bottom of the phone slot in the amp box. Cut the rods to length with a hacksaw. Then chuck each rod in the drill press and use a sanding block to polish. Drill the hole for the Qi charger in the back of the phone rest. After sanding and finishing both parts, press the support rods into their holes and stick the Qi charger in place before rocking out.

Side B. Mark the center of the upper magnet hole 1⁄2" down from the top edge of the holder. Use a stop block and the same 3" spacer you made earlier to position the oversized holder to drill this hole. Then, remove the spacer to drill the second hole.

Come together. With the magnets pressed in place, connect both units. Mark the bevel for the bottom edge of the phone stand, then cut it at the table saw.

Lean on me. Cut the support leg at a 60-degree angle and glue it to the back of the phone holder so the bases of both pieces align. To keep the leg from sliding under clamp pressure, drive a couple of brads into the back of the phone stand where the leg goes, then cut their heads off close to the surface leaving the nubs to grip the mating surface. Use the angled cutoff as shown to help distribute the clamp pressure evenly.

Amp it up. On the back side of the phone holder, locate and drill a recess for the Qi charger. Use the depth stop to make sure the thickness at the bottom of the recess is less than 1⁄4" but stop before the bit’s center spur comes through the holder’s face. The bottom of the charger should sit just below the support rods so the charger is centered on the phone. 

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