Frequently-Asked Questions
Who should build this amp?
This is a somewhat technical project. You won't need to understand how the amp or its components work, but you will have to be able to follow some pretty exacting directions. If you build it incorrectly, it can take hours or even days of troubleshooting to find the problem.
Yet, the CMoy is a good choice for beginners because this amp and variants of it have probably been built by thousands of people over the years. Most of these people are members of the HeadWize and Head-Fi web forums. These forums' DIY archives are treasure troves of troubleshooting tips: someone, somewhere, has probably had just about every problem with a CMoy that can be had, and had the problem diagnosed and resolved on the forums. Failing that, many of these people are still active on the forums, so you can ask a question there and usually get an answer.
What if I don't understand the circuit?
Most people build these amps without understanding what everything means. You can build this amp by following the assembly instructions blindly.
If you are diligent, you learn a little with each circuit you build. You do not go into your first project knowing everything about the circuit. Even this simple circuit has subtleties that you will not find documented elsewhere in this tutorial or in Chu Moy's original articles. It's simply not important most of the time to understand all of these subtleties.
However, if you're the sort of person who has to understand a circuit before you build it, I can recommend some good books and online resources. The best way to learn analog electronics is by reading The Art of Electronics by Horowitz and Hill. For the purposes of understanding this circuit, you really only need to read chapters 1 and 4. For a more in-depth treatment of op-amps than Horowitz and Hill, read Op Amps for Everyone by Mancini et al. This book is available in a free PDF form, as well as in paper form, but you really should have some basic electronics knowledge before diving into it. Finally, I have collected links to several free resources that you may find helpful.
How can I modify the circuit?
Many popular modifications are discussed on the Tweaks page of this tutorial.
Is it a speaker amplifier?
No, it's a headphone amplifier.
It's a question of power: the CMoy pocket amp can drive small speakers if you only need them to be loud when your ear is right next to them. Kind of like headphones. Go figure.
There is a popular power amp circuit with similar complexity to the CMoy pocket amp, called a "Gainclone". Click the link to see the current Google results.
Is it a guitar amplifier?
No, it's a headphone amplifier.
It's a question of gain: you could increase the amp's gain to around 100 to make it loud enough, but the noise floor will be unacceptably high. You need an amp design made for low noise with high gain.
If you want a guitar monitor amp, I can't really help you, as I know next to nothing about guitars. Perhaps the mighty Google can help you find a suitable circuit.
Is it a microphone amplifier?
No, it's a headphone amplifier.
Partly it's a question of gain, even more so than with a guitar amp: you might need a gain of more like 1000 with some microphones. But also, some types of microphones require what's called "phantom power", so you really should build a circuit designed to be a microphone amplifier.
Can I make it drive multiple sets of headphones?
Not really.
You might think you could just hook up additional output jacks in parallel, but this doesn't work very well, for a couple of reasons:
- The first problem is that different headphones require
different amounts of voltage to reach a given volume level.
The only way this works is if you use identical headphones
and everyone wants to listen at the same volume level, or
you arrange things so that different people can listen at
different volumes by picking a suitable set of headphones
based on how loud it gets in combination with the others
being used.
- Even if you can solve the volume problem through headphone selection, this still doesn't work very well. The problem is that this puts the phones in parallel, increasing the amp's output power requirements. The CMoy isn't the strongest headphone amp in the world; it's got about as much work as it can handle driving just one set of phones. Adding a second set is likely to drive the op-amp into clipping. You could try different op-amps until you found one strong enough to drive multiple sets of headphones at once, but then you still have the first problem.
What you really want is a distribution amp. This is essentially a separate headphone amp for each headphone output, with a single power supply and a single set of inputs common to all of the channels. This gives each set of headphones its own volume control and its own driver circuit, fixing both of the problems detailed above.
Unfortunately, the CMoy topology isn't suitable for this, because it has a resistive input: you'd be putting the volume pots in parallel, so that a change to the volume of one channel would also change the volume in the other channels. Also, the weaknesses in the CMoy's virtual ground circuit only get worse when you do this. By the time you fix both of these problems, you no longer have anything resembling a CMoy.
Bottom line, if you want a distribution amp, build a distribution amp, don't hack a CMoy.
Does DIY save money?
You shouldn't build this amp to save money, unless you are confident enough in your skills that you think you'll get it right the first time and you don't buy extra components for tweaking it. If you mess up your first amp or decide you want to improve the amp after building it, you can surpass the cost of an equivalent commercial amp fairly quickly. This is especially true if you build one of the more complex variants, which can easily require $50 worth of parts; my most expensive CMoy variant has about a hundred bucks worth of parts in it, and one guy I know built an überCMoy costing $300.
The primary purpose of DIY is control, creativity, and craft. It's also a great way to teach yourself electronics: many of the gurus on the forums started out completely clueless, and built a CMoy amp as their introduction to electronics.
The biggest mistake people make when considering cost is trying to make their first amp their last. I don't believe it's possible to make the perfect amp your first time. I think it's a far better plan to build something simple, like the CMoy, as your first project. This will teach you what you need to know to choose your next project.
Can I get someone to build it for me?
The easiest way to find pre-built headphone amps is in the Headwize and Head-Fi marketplace forums. At any one time, there are a number of forum members who are willing to build amps for others, and they often advertise in these forums. (The list of active builders changes frequently, so don't email me asking about this.) If you want your amp to be built by a forum member but none of the advertised build services appeals to you, you can post a want-to-buy message instead. And finally, regular DIYers frequently sell their previous creations in these forums for cheap, usually so they can afford to go build a better amp. :)
Auction sites can also be a reasonable choice. I find that you often end up paying more at them, however.
Please don't email me asking if I will build one for you. I am extremely choosy about who I build amps for these days. My primary purpose in putting up these pages is to teach people how to build their own electronics.
How long will this take to build?
It depends on your skill. The fastest I've built a basic CMoy so far is about 3 hours, including casework. My first CMoy amp took about 8 hours to assemble, and another 8 to debug. I've heard horror stories of people spending their weekend free time for a month getting their first amp built. The key to keeping debugging time under control is to test the amp frequently while building it. The following assembly instructions point out good times during the process to test.
The assembly guide has a "par" for each section of the process, adding up to 6 hours. This is like golf: if you can build this amp in 6 hours, you're pretty good. Newbies aren't expected to build the amp this quickly.
What are the differences between this tutorial and the original article?
There are several significant differences:
Chu Moy used two separate schematics in his article, one for the power supply and one for the amplifier proper. He reused some component names between the two, which has been found to be a source of confusion. I've remedied that in my new schematic. The two circuits are identical, however.
Chu used two single-channel op-amp chips (e.g. OPA134) to get stereo in his amp, while I use a single dual-channel op-amp (e.g. OPA2134) because I think it's simpler to set up.
The original CMoy pocket amplifier didn't have a volume control, but instead a "level" switch, giving two fixed volume levels. A modification to replace this switch with a volume control is discussed later in his article as an upgrade. In my article, I assume that you will either want full volume control or no volume control within the amp at all; the idea of two fixed volume levels doesn't appeal to me, nor I think to most other people.
This space intentionally left blank. :)