Required Parts

These are the parts required to build just the amplifier circuit. Enclosures, optional components, and enhancement components are in the next section.

I've been happiest with the parts from Digi-Key and Newark. Mouser sells cheap parts, in both senses of the word, so you do have to be careful when buying from them. Radio Shack's part quality varies all over the map, but the worst thing is that their overhead inflates their prices dramatically relative to the distributors. If you do get some of your parts from Radio Shack, beware that the part numbers I give below are the highest quality ones they offer of each type; I recommend you don't pick cheaper parts, as the cheaper ones always perform worse.

For those not in the US, I've added RS Components, who will ship almost anywhere in the world. All the parts numbers are simply a result of me searching through their online catalog. Many times I was able to find the same parts I recommend from the other distributors, but occasionally I had to just guess at a replacement, hoping that it is suitable.

Optional Parts

Here are some parts that you may need, or that are "preference" things: I can't call them required, since some may disagree.

Precision Resistors

All resistors spec'd above are generic 1% metal film resistors. Generic resistors work fine, but many people swear by premium resistors because of their higher inherent accuracy and higher thermal stability. (The latter means that as the resistor heats up due to the power it's dissipating, it changes value less than generic resistors. All resistors change value as they change temperture; premium resistors simply change value to a lesser degree.)

Are 1/8W Resistors Sufficient?

I spec 1/4W resistors for the generics above simply because they're the most readily available sort, not because the CMoy amp really dissipates 1/4W anywhere in the circuit. The resistors in a CMoy amp that see the highest load are the power supply splitter resistors. With the highest supply voltage you're likely to use — 30 V — these resistors only dissipate about 50 mW. Therefore, even 1/8W resistors are adequate for all aspects of the CMoy design.

The Radio Shack Metal Film Resistor Assortment

Radio Shack's part number 271-0309 is a package containing 50 1/4W assorted metal film resistors, including all values necessary to complete this project. One package is sufficient for two CMoy amps, with the exception of the 10 KΩ resistor: it only has five of these, and you need three for each amp.

Circuit Board Alternatives

This tutorial is centered on the Radio Shack 276-0150 patterned perfboard. The Radio Shack brand is US-only, but those in other parts of the world do still have options here.

Choosing an Op-Amp

The op-amp (operational amplifier) is the chip that does the actual amplification in the CMoy circuit. It has the single biggest effect on sound and power draw of any component, so it behooves you to pick this part carefully. It's also the part most likely to fail if your implementation is imperfect; a tolerant op-amp will reduce the chances of failure.

If you're a raw newbie, I recommend that you try one of the Burr-Brown OPA132/134 series op-amps first because they have low voltage requirements, they don't oscillate easily (see below), the lowest grade is adequate for this project and is quite reasonably priced, and they're very popular so a lot of your fellow DIYers are familiar with them and so can help you more easily. I won't say they're easy to find; you'll probably have to mail-order one. Take my word: these are very good chips for the price. Any replacement you try and source locally will likely not perform as well.

("Oscillation" is a condition where the chip develops periodic or constant noise due to problems in the surrounding circuit. If you keep at this hobby, you will eventually develop the necessary knowledge and experience to avoid oscillation in your circuits. Until then, stick with tolerant op-amps.)

The specific chip I recommend for beginners is the OPA2132PA. Under ideal conditions, the cheaper OPA2134PA performs just as well, but your first DIY headphone amp probably won't be flawless. I've had circuits where the OPA2134PA would distort or oscillate, but popping in a 2132PA or 2132P would fix the problem. As a beginner, you don't need problems like this. You'll have enough difficulties just assembling and testing the thing without the op-amp acting up as well! If you later want to build another amp, go ahead and try the 2134PA. If you run into problems, you can pull the 2132 out of your first amp temporarily and try that in the new amp to see if it fixes the problem.

As for the OPA2132P, it's about twice the price of the 2132PA, and I can't hear a difference relative to the 2132PA. This isn't surprising, since the only specs that are different between the two chips are the DC specs, and there is no DC in audio.

If you think you might want to try different op-amps in your headphone amp, it's all but mandatory that you use DIP sockets in your design so you don't have to desolder the chip to try a new one. Sockets are useful even if you never change the chip since they prevent damage to the sensitive op-amp chip during soldering. However, you should only use high-quality machined sockets. If you can only find cheap sockets, you're better off just soldering the op-amp to the board, because a cheap socket will result in weak connections, which is a serious risk factor for bad sound.

Choosing an Enclosure

There are two main kinds of enclosures used for pocket amps: plastic enclosures designed to house electronics, and various types of boxes designed for other uses and converted to hold an amp. The latter includes the popular mint tins, plus other found objects.

Notes on Audio Jacks

All of the recommended jacks above will work, but there are differences among them. Here are some things I've noticed that aren't obvious from the information in the table above or the distributor's web site:

• The Mouser 161-* RCA jacks include insulators. The other RCA jacks mentioned don't. (This is useful if you're using a metal case that has some other voltage tied to it, such as with an external DC power supply, which will usually tie V- to the case.)

• The recommended Radio Shack 1/8" jack may be the same as the Switchcraft 1/8" jack from Newark. I say "may" because some stores carry a similar-looking jack that isn't as good as the Switchcraft. The Switchcraft jack has solid lugs, while the cheaper Radio Shack one has small holes in the end of the lugs.