How to Build the CMoy Pocket Headphone Amplifier for $20Okay, so you were intrigued by my claim that you can build a CMoy pocket amp for US$20, but then disappointed when you saw that the parts can easily crest $40. What's the deal? It's pretty simple: in the main part of this article, I'm in a position to recommend parts, which means I have to like the parts and I have to think you'll like them, too. That leads me to be conservative. However, if you really wanted to skimp, here's how you could get the cost down to around $20:
This ignores the $5 charge you'll get hit with if you buy less than $25 worth of parts from one of the mail-order houses. I think it's smarter to order some extra parts, partly for safety, and partly to avoid this charge. You can always make a second amp later. Or, you can order some different kinds of op-amps, for tweaking. One interesting and inexpensive alternative is the OPA2227PA. Another way to get the order up to $25 is to order extra R3 resistors, for gain tweaking, or perhaps some extra LED current-limiting resistors, for brightness tweaking. I've ignored tax and shipping costs. I've ignored the cost of basic supplies like wire, shrink tubing, and electrical tape. Notice that there is no volume control (or it associated knob). Instead there's the "level" switch, which is the true form of the original CMoy pocket amp. If you take the level switch and its resistors out and put the volume control and its knob in, that adds another $2-3, if you go cheap on the parts. Another way to cut costs would be to use a mint tin, or some other box you already have. That will cut $3-6 out of the total cost. The resistor cost is stated as $1.21, because in small quantities, metal film resistors are around 11 cents, and you need 11 of them for a CMoy, if you leave out the R5s. If you get them at Radio Shack, you have to buy the $5 metal film resistor assortment, which gets you 50 resistors. If you get them at Digi-Key, you end up buying four 5-packs at 54 cents apiece, or $2.16. So, you end up with more variety in the Radio Shack assortment, but a more useful selection from Digi-Key, and the initial price is lower at Digi-Key. Other mail-order houses often make you buy 10 or more of everything, which is why I'm focusing on Digi-Key. All told, though, the resistors you actually end up using cost around $1.21 no matter how you slice it. If you use OPA2134s instead of 2132s, it's false economy in my opinion, because a 2134 won't sound as good at lower voltages as a 2132 will, so you end up spending more money on batteries. Notice that there are no sockets included. For a first time DIYer, I think you should definitely use sockets. |
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