Circuitos Electricos

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MUSIC

Pocket amplifier punches up headphones.
By Warren Young
Headphone amps make portable listening good and loud. Commercial audiophile models can cost $200+, or you can build a great-sounding amp inside a mint tin for around $30, following Chu Moy’s popular design. Powered by a 9-volt battery, this amp drives high-impedance headphones to thunderous volumes from even weak sources.To make one, you need an op-amp chip (like the TI/Burr-Brown OPA132), capacitors, resistors, an LED, and a small prototyping board, plus optional knobs, switches, and other bits, all easily obtained. And, of course, you also need a pocket-sized box, like a Penguin or Altoids tin. See the website listed at the end for a full parts list, along with layout diagrams and more detailed instructions.MINT-TIN AMP

Prepare the Protoboard
Start with a small prototyping board such as RadioShack’s model #276-150 — anything that has at least 12 rows of holes and fits into your case. Larger protoboards can be cut down to size with an X-Acto knife. Then solder nine jumper wires as shown in Fig. 1 on page 133. The jumpers lower down along the edges are what I call “M-jumpers.” They tie threetwo-hole pads together to form a single pad with three free holes. You can make these by taking a one-inch piece of stiff wire, folding it in half, pinching the kink tight with pliers, and then bowing the two Your MP3 player’s music will sound minty fresh ends over. You may also when played through need pliers to stuff the this li’l headphone amp. thick middle bit into the
Make:

Photography byWarren Young

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caps; use two rows if your caps are exceptionally large. Insert the op-amp into the socket. While placing the other components, refer to the pinout diagram on the op-amp’s datasheet. Next, add all the resistors (see Fig. 2). Notice in the photo on page 135 that I used jumpers in place of R5. I almost never install these resistors; they’re there to quiet the low-levelhiss that you hear with some low-impedance headphones. If you hear a low hiss at normal volume with the audio source disconnected, you can try adding two 47 to 100Ω resistors in the R5 positions. But don’t do this otherwise because it will raise the amp’s output impedance, inhibiting control over the headphones. For the remaining resistors, I add them in matched pairs: for each resistor in the leftchannel, I use my ohmmeter to find another that measures as close to identical as possible for the right channel. I haven’t scientifically studied whether this really helps, but it’s easy and quick, so why not? It’s one less thing to blame if Batt.+ the D1/RLED final product’s sound has flaws. Now add the input capacitors (C2). These aren’t Batt.+ polar like the electrolytic power supply caps, so +you can orient them any way you like. Axial capacC1 itors are easiest to fit into R.in spaces, but most tight – caps can be made to work with a bit of creative lead bending. R2 C2 + Next, take the hookup wires coming from theR3 R4 R1 run R5 V+ and V- points in the power supply area andR1 C1 the chip’s them to pins 4 and 8 of the op-amp, V+ and V- points. I recommend you do this on the – bottomside of the board, since you’ll be adding more wires to the top later. The more wires you R.out can put on the bottom, the cleaner the top side of the finished amp C2 be, facilitating any repairs will R2 and tweaks later. R4 R3 Finally, add test points at R.out and L.out, Dual op-amp and also at R.in and L.in if an alligator clip can’t R5 attach to the input capacitor leads directly (see Fig. 2). Iuse half-inch pieces of stiff wire, usually clippings from resistor and capacitor legs, bent in an upside-down “U.” These are temporary, used only for the signal test, the next step.

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+
C1

R1 R1

–
C1

– MINI-CMOY POCKET AMP WIRING DIAGRAM +
hole. Some people make the same connection by threading “S-jumpers” through to the other side, but this makes the jump less visible....