MINT-TIN AMP Pocket Amplifier Punches up Headphones
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D.I.Y. MUSIC MINT-TIN AMP Pocket amplifier punches up headphones. By Warren Young Headphone amps make portable listening good Prepare the Protoboard and loud. Commercial audiophile models can cost Start with a small prototyping board such as $200+, or you can build a great-sounding amp RadioShack’s model #276-150 — anything that inside a mint tin for around $30, following Chu has at least 12 rows of holes and fits into your Moy’s popular design. Powered by a 9-volt battery, case. Larger protoboards can be cut down to size this amp drives high-impedance headphones to with an X-Acto knife. Then solder nine jumper thunderous volumes from even weak sources. wires as shown in Fig. 1 on page 133. To make one, you need an op-amp chip (like The jumpers lower down along the edges are the TI/Burr-Brown OPA132), capacitors, resistors, what I call “M-jumpers.” They tie three two-hole an LED, and a small prototyping board, plus pads together to form a single pad with three free optional knobs, switches, and other bits, all holes. You can make these by taking a one-inch easily obtained. And, of course, you also need piece of stiff wire, folding it in half, pinching the a pocket-sized box, like a Penguin or Altoids kink tight with pliers, tin. See the website listed at the end for a full and then bowing the two Your MP3 player’s music parts list, along with layout diagrams and more ends over. You may also will sound minty fresh when played through detailed instructions. need pliers to stuff the this li’l headphone amp. Photography by Warren Young Warren by Photography thick middle bit into the Make: 131 D1/RLED Batt.+ Batt.– + – R1 R1 D1/RLED C1 C1 – + Batt.+ Batt.– + – D.I.Y. MUSIC R1 R1 C1 C1 MINI-CMOY POCKET AMP WIRING– DIAGRAM + hole. Some people make the same connection by caps; use two rows if your caps are exceptionally threading “S-jumpers” through to the other side, large. Insert the op-amp into the socket. While D1/RLED D1/RLED + – but this makes the jump less visible. placing the other components, refer to the pinout Batt.+ Batt.– Batt.+ Batt.– diagram on the op-amp’s datasheet. Build the Power Supply Next, add all the resistors (see Fig. 2). Notice Solder in the capacitors, and use them to orient in the photo on page 135 that I used jumpers in + – + – R1 R1 R1 R1 everything else. Make sure you put the legs in place of R5. I almost never install these resistors; C1 C1 C1 C1 the right holes — the jumpers you placed previ- they’re there to quiet the low-level hiss that you R.in – + – + ously dictate the path of current flow, so there’s hear with some low-impedance headphones. If L.in V+ V- no room for “creativity” now. All the electrolytic you hear a low hiss at normal volume with the (underside) C2 R2 capacitors are polarized, so you must orient them audio source disconnected, you can try adding R4 R3 Dual R2 C2 op-amp properly. If one leg is shorter than the other, the two 47 to 100Ω resistors in the R5 positions. But R5 R3 R4 short one is negative, and there should also be don’t do this otherwise because it will raise the R5 a stripe or other mark on the negative side. amp’s output impedance, inhibiting control over Now solder the LED and the current-limiting the headphones. resistor. As with the capacitors, the short/nega- For the remaining resistors, I add them in R.out tive leg of the LED should lead to the negative matched pairs: for each resistor in the left chan- L.out side of the power supply, with resistors in series nel, I use my ohmmeter to find another that in between. If you’re mounting the LED on the measures as close to identical as possible for Fig. 1 Add power supply. Fig. 2 Add amplifier section. lid of the mint tin, solder the resistor close to the right channel. I haven’t scientifically studied the board and cover the joint with heat-shrink whether this really helps, but it’s easy and quick, D1/RLED + – tubing. That way, its legs won’t flex and snap so why not? It’s one less thing to blame if the D1/RLEBatt.+ D Batt.– + – from repeated openings of the case (see Fig. 1). final product’s sound has flaws. Fig. 3 Add panel components, Switch Batt– On the underside of the board, solder two 2- Now add the input capacitors (C2). These aren’t enclose, and enjoy! inch hookup wires to each side of the third row, polar like the electrolyticBatt.+ power supply caps, so + Batt.–– R1 R1 in the holes marked V+ and V- (for voltage) in Fig. you can orient them any way you like. Axial capacC1 - C1 R.in 1. Leave the other ends dangling loose, but don’t itors are easiest to fit into tight spaces, but most– + let them touch while power is applied. caps can be made to work with a bit of creative L.in Batt+ D1+ RLED Now figure out a temporary way to add power lead bending. C2 R2 + – to the board. For example, you can solder a 9V Next, take the hookup wires coming fromR4 theR3 Dual R2 C2 op-amp battery clip’s leads into the holes marked “Batt V+ and V- points in the power supply areaR5R1 andR1 run R3 R4 +” and “Batt -.” This only needs to work until you them to pins 4 and 8 of the op-amp,C1 the chip’s C1 R5 start working on the case. R.in V+ and V- points. I recommend you do this on the – + To test the power, set your meter to DC volts, bottom side of the board, since you’ll be adding L.in apply power to the board, and measure from the more wires to the top later. The moreR.out wires you IN signal ground (“Ground” in the diagram) to the can put on the bottom, the cleaner the top side L.out hookup wires going to the V+ and V- holes. With of the finished amp C2will be, facilitatingR2 any repairs a fresh 9V battery, you should see about +4.5V and tweaks later. R.in DC and -4.5V DC at each capacitor, respectively, Finally, add testR4 points at R.outR3 and L.out,Dual R2 C2 op-amp and their magnitudes should be nearly identical. and also at R.in andR5 L.in if an alligator clip can’t R3 R4 L. R. If you’re off by more than a tenth of a volt, check attach to the input capacitor leads directly (see Ground the wiring and look for solder bridges, wayward Fig. 2). I use half-inch pieces of stiff wire, usually R5 POT Ground drips of solder shorting out to neighboring nodes. clippings from resistor and capacitor legs, bent Fix any problems and unplug the power supply in an upside-down “U.” These are temporary, used R.out L.out before continuing. only for the signal test, the next step. OUT Add the Amplifier Test the Amp Ground Solder the IC socket to the board with the notch Now you’reR.ou readyt to see if you have an amplifier away from the power supply. Leave one or two yet or not! Apply power to the board, and let it L.out L.in rows of holes between it and the power supply sit a bit. Then, carefully touch the op-amp and Young Warren by Illustrations 132 Make: Volume 04 Make: 133 D.I.Y. MUSIC Green wire = Gound Red wire = Right channel White wire = Left channel Testing the circuitry: Plug a stereo mini-to-mini cable the plug’s ring (right channel) goes to R.in, and the Alligator jumpers all hooked up for testing, from music into any audio device, and use alligator clips to connect sleeve goes to ground. Then take some headphones and source (I use a CD player) and headphones. Be sure to the other end to your circuit board. The plug’s tip (left alligator-clip three more connections to L.out, R.out, and use a cheap pair of headphones, because an improperly channel) goes to the L.in test point (see Fig. 2, page 133), Ground. Play some music, and see if it all works. wired circuit can destroy the headphone’s drivers. all the resistors’ bodies to make sure nothing is With the headphones sitting on the table, you don’t have signals or power going to ground the board in the case and the panel components overheating. You shouldn’t feel any heat at all; slowly turn up the volume on the player until you when they shouldn’t. Check all connections with fastened into their holes, but not connected to if you do, unplug the power and find out what’s can hear some sound out of the headphones. Put your meter; sometimes a connection will look the board. This ensures that everything’s going to going wrong. them on now and adjust the volume. If you get right but have high resistance, in which case it fit before you start soldering again. I use a portable CD player for testing since they good sound, you’re done! You might listen for a needs to be re-soldered. have volume controls, useful when you’re testing while longer and try to stress the amp a bit, but If your wiring fully checks out, try adding a Add the Panel Components a circuit without a volume control.