Chu Moy headphone amplifier

Review date: 20 March 2004.
Last modified 03-Dec-2011.


I spend a lot of time wearing headphones. A computer is my principal source of musical entertainment, and my Sennheiser HD 590s are my preferred output transducers. It's been a while since my main computer was even connected to a speaker system.

Headphones need about a thousandth as much power as loudspeakers to achieve similar sound pressure levels at the eardrums. This generally means a few milliwatts per side, at most, for normal listening. A few tens of milliwatts, for loud music or if you're using unusually inefficient 'phones - as with loudspeakers, manufacturers of high-end headphones may sacrifice efficiency for higher fidelity.

You wouldn't think even cheap music hardware would have much trouble delivering this little power. But, actually, even quite expensive home stereo gear frequently doesn't drive headphones very well, especially at high volume. And if you're pumping your tunes out of a mobile multi-faceted transformative device at sufficient volume to cut through the sound of the bus, surf or very upset policeman tied to a chair, you're likely to be asking for rather more power, from an amp of rather lower capability.

Because market research has determined that most consumers are cloth-eared gits non-critical listeners, manufacturers of audio hardware in general and portable players in particular often don't care much whether their products drive headphones very well.

Some portable players (including big-brand units) sound lousy at all volume levels, usually because of a unavoidable equalisation settings where "ROCK", "JAZZ", "POP" and "CLASS" each manage, paradoxically, to sound worse than the other three. Lots of cheap portables also come with ghastly bundled earphones that add their own vacuum-cleaner-tube ambience to everything. Even if you avoid these problems, though, many portables sound OK playing quiet and lousy playing loud.

This is partly because of cost cutting (tiddly little amp chips, running at less than their full rated input voltage), and partly because of battery life maximisation. 50 milliwatts is a vanishingly small amount of power by home hi-fi standards (many modern devices draw at least a couple of watts even when they're "off" and listening for a remote control turn-on signal), but it becomes important when you have to carry your power supply around.

A player with, say, 16 hours of playback time from two quality AA alkalines (not at all uncommon these days) is only drawing around 150 milliamps out of them, on average. The mere 20mA-odd that you save by making the headphones draw 50 milliwatts less (probably a bit more than 20mA, actually, if you take amplifier inefficiency into account) would chop play time down to about 14 hours, which is something that consumers notice a lot more readily than dodgy audio at full volume.

The power-saving issue is a bigger one for players that run even longer than this, of which there are more than a few these days, and for tiny players that run from super-slim rechargeable batteries or a single AAA cell. One AAA alkaline isn't likely to be able to provide much more than a fifth as much energy as two AAs.

And then there are those simpler problems, like when you're watching a DVD on your computer and even after you've turned up every volume slider at your disposal, the audio through your headphones (plugged into the appropriate output on the back of the computer) is still so quiet that you can clearly hear the power supply exhaust fan.

The solution to all of these problems is a headphone amplifier - an amp designed to connect between a headphone socket and/or line level output and a pair of headphones.

Headphone amplifiers are standard equipment for 'phone-loving audiophiles. Basic models don't cost much, but, as usual, the sky's the limit for the amount of money you can spend if you start talking valve-powered zero-tolerance Class-Better-Than-A voodoo-infused gear.

You don't have to spend a fortune for a commercial headphone amp, but it's definitely possible.

Or you can choose not to choose a commercial product. You can choose something else.

Headphone amp

Something like this.

Your eyes do not deceive you. This headphone amplifier is built into a Penguin tin. I think that's pretty darn cool, myself, and so do a lot of other people who've bought amps of this type. Or built them.

The circuit this amplifier uses, you see, was invented by a chap called Chu Moy, who published it here, for anyone to build or improve upon.

Chu created a monster. So-called "CMoy" amps spread like wildfire through the audio-twiddler world. With good reason; they're based on a simple operational amplifier chip, need few other components, run happily from a simple nine volt battery for a long time, and sound really good.

Now, in addition to the three Addendums (so far) that've been added to the original CMoy project page and a couple of related projects also on the Headwize site, you can find CMoy-based circuits and related info in various places on the Web. There are lots of 'em here, for instance. Here's a neat how-I-did-it page. Heck, CMoys have even been on TV now.

If you're handy with a soldering iron and short of cash, a CMoy is just a few hours' soldering away.

I coulda built one if I'd wanted to. Sure I could. But I'm a busy man, so I bought one instead, from a guy called Brandon.

A kid called Brandon, actually; he's still in high school. But the enterprising lad builds a perfectly good little amp, and sells the things on eBay.

Brandon doesn't have any for sale, as I write this. If you just can't wait, try doing a titles-and-descriptions search for "cmoy", "chu moy" and such, and you'll find plenty more, constructed with varying degrees of professionalism. Expect to pay around $US50 to $US100, depending on fanciness, prettiness and the audacity of the seller.

All CMoys are not alike, and I dare say some seriously screwed-up ones have been made and possibly even sold. But, in the real world, most CMoys are pretty darn similar.


Inside the conveniently hinged tin, this is pretty much a reference standard home-made CMoy. It's only built on strip-board, not a custom-made circuit board, but the hook-up wires are right length, the wire links on the board are tidy, and the connections are neatly heat-shrunk over non-lumpy soldering. Nothing scratched, nothing dented, nothing jammed in place. Little foam cushion under the battery. Nice.

Because this is a bespoke amplifier, made to order for the discerning nerd, you can order one with extras. I wanted a DC input jack, to save the battery when I was using the amp on my desk...

DC input jack Brandon obligingly included one. It's just wired in parallel with the battery connector, so it's not necessarily a brilliant idea to leave the battery connected while running from DC power, and it's also good to put a bit of tape over the terminals of the battery snap while it's just floating around unconnected inside that metal tin.

But audio products from major international manufacturers have shipped with much worse problems than this.

Hooking up

Controls and connectors

Connecting an amp like this to your computer, portable player or whatever is easy. All you need is an input cable that terminates in a portable/computer standard 1/8th inch stereo plug, and headphones with the same kind of plug. Plug in (no harm will be done if you get the plugs the wrong way around), flick the switch on the side, admire the green LED, and get listening. The volume control in this (common) CMoy layout is between the two plug connectors, which is slightly inconvenient, but also guards the knob from accidental turning if the amp's sitting in your pocket.

I ran the CMoy from the cheap carbon-zinc battery it came with for a while, then decided to dig up a plugpack for it.

In the course of so doing, something happened which I feel duty-bound to mention.

I've got a box full of wall-wart power adapters.


I'm sure it'll turn up the next time I'm looking for the box full of Lian Li case parts.

Since I couldn't find the power adapter box, I rummaged through another box, this one full of very carefully categorised Miscellaneous Junk, in the glum certainty that the only power adapter I'd be likely to find in there would be an 18VAC modem PSU, or one for a Commodore Plus/4, or a round-cornered American-made three volt battery eliminator for a vacuum-fluorescent-display calculator that was last retailed in the year of my birth.

Close to the top of the box, there was a perfect, late-model, switchmode, regulated, one amp rating, nine volt DC plugpack, complete with its perforated rubber strip of interchangeable output plugs.

I just had to tell you all that. Sometimes, fate smiles.

Anyway. The "regulated" part of the above plugpack description is important. You can't necessarily plug any old "9VDC" adaptor into a device that normally runs from nine volts worth of batteries. Any old adapter may work, if the device is made to accept a wide input voltage range, but you shouldn't assume that.

Unregulated plugpacks, you see, only deliver their rated voltage when they're fully loaded. The lighter the load, the closer they get to their open-circuit voltage, which is root-2 times their rated voltage. For a 9V pack, that's 12.7 volts, which might well fry your gadget. Regulated plugpacks always deliver their rated voltage, no matter what load they face.

The current capacity of the power adapter, by the way, doesn't matter when you're running something that draws as little juice as a CMoy. Few plugpacks are rated at less than 300mA, and that should be enough for at least ten CMoys.

Excess current capacity isn't dangerous, though; I could run the CMoy from my 25-amp-capable bench power supply (and in fact did, for the power consumption measurements below) without any danger beyond the chance of entertainingly welding the supply leads together and popping a fuse in my ammeter if I was clumsy.


It irks me when people reviewing audio gear rabbit on about "air" and "space" and "musicality" as if those terms meant anything. The easiest person in the world for you to fool is yourself, and you'll be especially prone to do it when you're trying out some new gadget that's meant to make your music sound different.

It's well established that people normally perceive a small volume boost as being a sound quality improvement, and you can't do a blinded test when you're plugging and unplugging a widget on your desk.

That said, though, I'm very pleased with the sound of the CMoy. I auditioned it with computer audio (I'll get to the mobile device tests shortly - I've got a few little MP3 players on my to-review pile...), and it made differences both obvious and subtle.

The obvious difference was that the CMoy could drive my HD 590s much louder than the computer could by itself.

Awww, yeah. Live fast, go deaf young.

Before I got the amp, I'd grown used to hearing clear buzzy distortion on low bass when I wound the volume up. I assumed it was the poor Sennheiser headphone transducers being pushed past their limits.

Nope, it was the wimpy motherboard sound hardware running out of juice, and clipping.

When powered by the amp, the 'phones remained clear and defined well past this volume level - deep into the unwise-loudness zone.

And, yes, though I wouldn't swear to it in court, computer audio does sound crisper and better defined, to me, through the amp. It's not a night and day difference, but I do think it's there.

The amp certainly doesn't seem to add any noticeable character of its own. Amplifiers are meant to be "straight wires with gain"; this one pretty much is.

A headphone amp can't put back what wasn't there in the first place, of course. Many PC audio adapters these days have quite acceptable output quality - their signal to noise ratio may not be great, but if you're not into music with a lot of quiet passages, this makes no difference to anything.

Many digital audio files, however, don't sound too good, and game sound is of highly variable quality too. You mustn't expect a new amp and fancy pair of headphones to make 96 kilobit per second MP3s sound nice.

I've listened to quite a lot of music through the amp now, though, from the cool and cooler, through the retro-wacky and spaciously rhythmic. OK, some of my test material lacked a certain finesse, but much of it didn't.

As I said, superior clarity at modest volume, I can't swear to. Think it's there, don't trust my brain.

But excellent clarity at much higher volume than I could manage before - that's definite. Winding everything up to maximum for the Passacaglia and Fugue in C Minor, sipping my Drambuie, and reflecting on the nature of decadent society, was worth the purchase price by itself.

The CMoy circuit is, by the way, very well behaved. There's no pop on turn-on, and just a brief rasp on turn-off as the caps run out of juice.

When running from the plugpack, I managed to create ghastly buzzes when wiggling the input plug and DC plug in their sockets, as the plugpack and computer engaged in an animated discussion concerning just what earth should be taken to be.

This isn't a big problem, though; it's easy to avoid (it'll never happen at all if you run from battery power), and a better 1/8th inch lead than the cheapie I'm using would probably help.

A plugpack with nasty dirty output might create a permanent buzz. The one I used isn't a special super-quiet audio model, though, and seemed as quiet as the battery, when I wasn't frobbing the connectors.

Power consumption

The CMoy's quiescent power draw, with or without its input plug connected, is 16 milliamps at nine volts.

Listening to something suitably doofy with my Sennheisers pumping about as loud as I'd ever want them to boosted the current consumption to a big 18mA or so. Taking the 'phones well and truly off and winding everything to eleven gave an average draw of a bit more than 20mA, with absolute screaming peaks up around 24mA.

Next, I wound the supply voltage down, to test low-battery performance. Surprisingly, the amp kept playing quite strongly, with only slightly reduced volume, down to not a lot more than four volts. 4.5 volts was definitely A-OK. I didn't do critical listening tests to see whether there was any loss of fidelity, but it sounded pretty much OK to me.

This is impressive. It means this amp should be able to use pretty much all of the capacity of its battery, and it also means you can grab a couple of alligator clip leads and run it from a six volt lantern battery, in a pinch.

More practically, 20mA isn't too ferocious a load even for a weedy little 9V battery. A decent 9V alkaline ought to deliver not much less than 30 hours of service at this load (see this PDF datasheet). A carbon-zinc 9V battery like the one that came with my CMoy (datasheet here) should manage around 20 hours.

If you're really looking for value, then six AA alkalines in a holder would give you rather more than a hundred hours of service, and six humble D-sized carbon-zinc cells would probably last rather more than a fortnight.

A rechargeable 9V battery wouldn't be that great an idea, unless you intend to use the amp a great deal and don't mind charging the battery fairly often. Rechargeable nine volters all have pretty miserable capacity (and often deliver only 7.2 volts, not nine), but a modern one ought to give you around ten hours of play, provided you don't leave your amp on the shelf a lot, whereupon the charge will leak away.


If you use headphones a lot, and often wish they were a bit louder, then you need a headphone amp. Never mind airy-fairy audiophile double-talk; your headphones probably can play painfully loudly without running out of puff, if only the amp driving them can provide enough power.

Still on the subject of prosaic-reasons-to-buy - it's handy to have one simple manual control for computer volume, as people with regular speakers, and/or bells-and-whistles multimedia keyboards, already know. Just run a game, and discovered the menu clicks are inaudible/deafening, never mind the actual game sound? Tweak the volume appropriately without quitting, or tempting fate by switching back to the desktop. Neat.

Now, if your headphones are awful bargain-bin pieces of plastic garbage, your first purchase should be a decent set of cans (man), rather than something to make your current crap smellier. Hie thee to the hi-fi store with some of your favourite CDs, and do some auditioning. You can get a seriously excellent pair of headphones for the price of a quite modest pair of speakers, and it's not too hard to figure out what's likely to suit you.

If you've got some decent 'phones, though, a simple amp like a CMoy can considerably increase your listening pleasure. More volume, and very probably better sound too.

And tons of battery life from a plain old nine-volter.

And the sheer geekly joy of owning a lolly tin with a switch and a light.

Highly recommended.

My CMoy pusher

See also

HeadRoom BitHead

The same, only more so.



Boostaroo circuit board

Here's another headphone amp option.


Give Dan some money!
(and no-one gets hurt)