JDS Labs provided me with this kit to build and review. Big thanks to John for helping to support the DIY ecosystem and make the review possible!
The CMoy amplifier, first published in 2001 by its namesake Chu Moy, is a mainstay of DIY headphone amplification. This humble amp has sailed a thousand DIY ships, including my own, and today is still the most frequently recommended starting place for any builder that wants to try making their own gear. And that’s for good reason. There are few parts needed, no mains wiring required, enough tweak-ability to keep things interesting, and a very modest parts cost. Although the amp was originally published online for free use by others, several companies offer all-in-one kits with or without their own circuit tweaks. For this article, I’ll be taking a look at JDS Lab’s CMOYBB (BB for ‘bass boost’). Specifically, my kit came with the rechargeable option, but the construction and audio performance is the same as the non-rechargeable version.
The CMoy and its variants use a dual opamp to power two channels of audio. Opamps (operational amplifiers) are intrinsically differential devices with high efficiency, a high common mode rejection ratio, and, given the right conditions, very low distortion. The output stages of opamps are typically Class B. If you could shrink yourself down and walk around under the black monolithic plastic case of an opamp, you’d find an arrangement of transistors that really is not all that different from high power, high feedback, discrete amplifiers (see work by Douglas Self, Kevin Gilmore, or datasheets from TI). That is to say, opamps are not magic or all that different from other types of amplifiers. They are just miniaturized.
Despite their similarity to higher power amplifiers, opamps can be finicky when given a low impedance load, like a pair of headphones. If asking for too much power at too low an impedance, distortion rises due to (primarily) crossover distortion in the Class B output stages. This often is worse at higher frequencies, leading to sibilance or harsh treble experiences. Good circuit design alleviates this and, luckily, JDS Labs has tweaked their version of the CMoy to give it the best fighting chance feeding low impedance loads at reasonable listening levels.
JDS Labs has also included a bass boost feature (hence the “BB” in the name) which is enabled via a small slider switch on the board. This EQ boost is done within the feedback loop (which also sets the opamp gain) so it requires minimal parts and keeps the EQ components out of the direct signal path. The standard voltage gain of the kit is about 6, making it a good match for medium and high impedance headphones, but it can easily be modified for high sensitivity or high impedance headphones by swapping just a couple of resistors.
The JDS Labs CMOYBB kit includes everything you need, save for a tin and a battery. Because this is a design often recommended for the DIY newbie, a complete kit is a huge advantage. Knowing where and how to source parts is an essential skill of DIY, but not necessarily something that needs to be a part of your first project. Starting out, the myriad of capacitor types, transistor and opamp packages, resistor ratings, etc available from suppliers like Mouser or Digikey is overwhelming. Kits eliminate the potential analysis paralysis and get you building quickly and easily.
The parts come in an attractive gift box and JDS packs and labels everything individually (or in pairs) to make sorting and finding the next part quick and easy. Sensitive parts (the opamp, rail splitter, and LM317 for the rechargeable version) arrive in electrostatic pouches to protect them on their journey. The kit includes a socket for the opamp, a nice touch, so that you can swap opamps to sample different flavors. Warren Young has an excellent resource on the types available that work in a 9V CMoy amplifier here.
The kit’s PCB is good quality and parts locations are clearly labeled. During my build, I never found a need to consult the schematic as I matched parts to where they go. This definitely qualifies as a solder by numbers kit, another point in favor of this as an introduction to DIY. Solder pads are reasonably sized (though actually smaller in some cases than the SMD Pocket Class A I also reviewed). That said, the parts are through-hole and the lead sticking out of the bottom of the board makes an easy target for your iron tip. Speaking of, no special soldering work is needed here; just use a clean small tip and you’ll do fine.
I like the layout of the volume control and input and output jacks. The switch for the bass boost is located on the board, so you need to open the tin to activate it and won’t be accidentally activating the boost in your pocket. The switch JDS includes is good quality and the actuator is long enough to be easily flicked back and forth without having to break out tweezers or screwdrivers. The switched volume knob handles on/off duty and, in addition, the output jack is switched so that if your headphones are disconnected, the amp automatically powers off. This is a nice usability touch and, potentially, a battery saver. JDS Labs specs the battery life (one 9V) at 30-50 hours.
JDS Labs publishes an excellent 10 page PDF assembly manual that can be found on their product page. The manual includes everything from tools required for building, to step-by step construction, to troubleshooting, to potential modifications. For such a simple circuit, JDS squeezes out a lot of content to help builders. This speaks volumes about their dedication to the DIY community. The kit isn’t a design with parts thrown in a box; this is a doorway to a new hobby.
In addition to the assembly manual, JDS includes a full bill of materials, schematic, and an enclosure cut-out template. All of the resources are well organized in PDF. I should note that these are publicly accessible as well. If you’re thinking about trying a CMoy, you can easily preview the process and schematic beforehand. This is not always the case with other designs (presumably to protect the IP inherent in the schematic, instructions, etc). A big thumbs up to JDS for their thoroughness here.
The JDS Labs Cmoy can be built by virtually anyone with a soldering iron, likely in just one afternoon. All parts are through-hole and contained on the board. The most challenging part of the process is punching holes in the tin in the right places. Beginners start with training wheels on their bike or water wings in the pool; there is nothing wrong with solder by numbers kits. There’s a reason the CMoy design is often recommended for beginners on forums and in chat groups. JDS Labs’s CMOYBB in a kit spin makes things even easier.
Once you’ve made your couple dozen or so solder connections, what should you expect as far as sound goes? In my experience, the audio is just as fun as the build process. This is a little headphone amp that really likes to rock and after listening to several tracks and genres starting out, rock is ultimately what I found myself enjoying the most. The dynamics, specifically, are what stood out to me the most. Tom-heavy drum fills sound big; bass grooves have real meat on them; a Marshall stack sounds like a Marshall stack should.
“Colossal” by Wolfmother presents a wall of rock chaos that the CMOYBB reproduces without compression or muddy incoherence. Vocals and even reverb effects come across cleanly despite heavy guitar work. The Beyerdynamic DT880-250s are a nice match here with a relatively smooth sound. The sealed Blue Lolas (32 ohms) ratchet up the dynamics and slam a bit but start to encounter some low frequency distortion at high levels.
Supergroup Black Country Communion’s “The Battle for Hadrian’s Wall” is another epic rock track that the CMOYBB is capable of doing justice. The acoustic riff intro has a natural timbre and stays tonally distinct when the electric guitar enters the mix. The bass line is smooth and the big drum fill at 1:25 really shows off the dynamic ability of the amp (that’s Jason Bonham punishing the skins, by the way). Again the higher impedance Beyers are a little smoother and more dynamic than low impedance headphones, but the basic “eager to rock” character of the little opamp stays true.
Finally, “Wheels” by the Foo Fighters is another rock track that ends up on a lot of my playlists. The count-in to start the song is clearly heard and the full band downbeat to start the music is big and beautiful. The kick drum sounds a little more realistic than the cymbals here on both high and low impedance headphones. Fret noise on the bass guitar comes across well. I believe that if Dave Grohl wanted to build a little headphone amp, he’d probably build a CMoy: no nonsense, big dynamics, enough battery life to rock for a couple days straight. The Foo sounds good, too.
Testing the CMOYBB with IEMs (UE900S – 30 ohms) showed a little weakness with the standard design. I encountered some sibilance on the Wolfmother track and some others I listened to and there was audible noise (without music) with the volume knob cranked up. At normal listening levels though, the amp was silent. By and large, the IEMs showed the same basic “fun” character as the others headphones used, but I would caution against going much lower than 30 ohms impedance with the included gain/feedback resistors. When heavily loaded, opamps do have a tendency to increase distortion, especially at high frequencies (which may be the source of the sibilance I encountered). That said, if you’re planning to use IEMs below around 40 ohms exclusively, you can change one pair of resistors to lower the gain of the opamp (JDS also recommends adding a second rail splitter and cap if going this route). This would be a worthwhile tweak if using the amp with low impedance headphones.
All of the above listening was done without the bass-boost engaged. To be honest, I just could not get used to the boosted sound. My hunch is that it is meant to be used with earbuds or other bass-deficient headphones and it could be very useful in that circumstance. With mid-fi headphones that don’t need a boost, it sounded a little too bloated when engaged. It’s a fun feature all the same and can be left “off” if unneeded.
Ok, everyone builds one at some point, so it’s not the most unique project in the world. But it is a rite of passage. You’re not really a dork until you Cmoy’d. The JDS Labs CMOYBB is an example of an amplifier that sounds great for the money. It will not make you regret building a Bottlehead Crack or buying whatever new $500 stack Schitt is hocking, but it will sound excellent for the price and you will have fun building it. If you’re a headphone enthusiast, I can think of very few more interesting or rewarding ways to spend $40 and an afternoon of your time than by building the CMOYBB.
Specifications provided by JDS Labs:
|Frequency Response 20Hz-20kHz||+/- 0.1 dB|
|THD+N 20-20kHz, 150 Ω||0.002%|
|Noise, A-Weighted||-90 dBV|
|Channel Balance||+/- 0.6dB|
|Output Impedance||0.6 ohms|
|Gain||6x (or Custom)|