The cMoy Headphone Amplifier is a popular, well-documented DIY project, which makes it great for beginners to take on. That’s exactly the reason I chose to do it. I followed the instructions and suggestions from the TangentSoft cMoy Tutorial written by Warren Young, and it came out great, especially for my first attempt at building a headphone amplifier.
I won’t get into how I built mine (the tutorial at TagentSoft is where you should go for that) but read on if you’re interested in seeing what parts I used and some of the problems I ran into.
- Enclosure – Serpac H65 9v. An obvious choice if you read the TangentSoft tutorial. Using an Altoids tin seems to be the most popular option for the cMoy, but I liked the idea of having something more “finished” for this build. The built-in 9v battery bay is also a plus, and the price is good. I got the translucent gray version of the case so I can show off my circuit. This also allows the LED to be visible without having to drill a hole in the case.
- Board – Radio Shack #276-150. I only used one half of the board per the tutorial, but as you can see in the photo to the right, the full board will just barely fit inside the enclosure.
- OpAmp – OPA2132A. Obvious choice for a beginner, since it’s not as finicky as other OpAmps.
- Resistors – I went with Vishay-Dale precision resistors. The value of using them over standard resistors in this circuit is debatable, but I’m not mass-producing it, so the extra cost wasn't an issue.
- Gain Resistor – I went with a value of 2.49KΩ for resistor R3, for a gain of approximately 5. The tutorial recommends using a gain of 11 to start (for minimum problems) but I wanted something that would work with my low impedance headphones. I chose 5 since it was in the middle. It’s still too much gain! Depending on the source, I can barely turn the volume knob on with my Grados. I think I’d prefer a gain of about 3 instead. Then again, from what I’ve read this isn’t the best amp for low-impedance (current hungry) headphones anyways, so I don’t expect I’ll be using it with my Grados.
- Input Capacitors – The cMoy circuit calls for .1µf input capacitors, which according to Young are a bit small. He suggests going as high as 1µf, and that’s exactly what I did. I ordered a couple 1µf polypropylene capacitors (Digikey PN 495-4209-ND) seen in the photo to the right. He recommends polypropylene as the best choice, but it results in very large capacitors. I had a feeling they may not fit, so I also ordered some 1µf metallized propylene capacitors (Digikey PN PF2105-ND) and those are the ones I ended up using as there was no way I could fit the first set of caps in the H65 enclosure. Using the H67 enclosure would resolve this, but then I lose the translucent case, the 9v battery compartment and end up with a physically larger amp.
- Power Supply Capacitors – Based on Young’s suggestions, I used 470µf caps instead of 220µf. Specifically, I used Nichicon Fine Gold 470µf 35v caps. They’re big, and I had to mount them horizontally. Are they worth the trouble? I don’t know. Some people claim that audio grade caps in the power supply make a difference, and others claim it’s waste of money and space. I’m inclined to side with the latter. But hey, they look cool. Especially with the transparent case.
- Potentiometer – Alps RK097. Another obvious choice. This is the one sold on Tangent’s website which has the built-in SPST switch. No need for a toggle switch, which keeps the panel clean and simplifies construction.
Problems/Concerns with my build
This was the first headphone amp (and one of the first electronic circuits period) that I’ve built, and I deviated from the “stock” build in several areas, so I had to expect some issues would arise. And they did! It’s not too bad, and the amp works, but these are some things I’d probably try to improve in a second build:
- The input caps are too big (physically) – Even with the smaller of the two input caps I purchased installed, I was still barely able to cram the circuit inside the H65 case. The layout isn’t the greatest either. The input caps are sitting right on top of the the output resistors, because there is nowhere else for them to go. I’ve seen some cMoy circuits with big input caps that have them mounted so they use the space to the left and right of where the power comes in, but I can’t do that because…
- The power supply caps are too big – I knew this of course, but hey, I wanted to use those cool looking Fine Gold caps!. And they do look cool, and they fit inside the case just fine, though it was a bit close. But they take up lots of space that I might have used for the input caps.
- The layout is messy – The input caps sitting on top of the output resistors is not a good design. It doesn’t seem to cause any problems (at least with the OPA2132A), but if I building this “professionally” I wouldn’t leave it like this. If I built this again and still wanted to the use the big caps, I’d use more than half the RS board and spread out the component a bit more so there’s room for everything.
- It didn’t work the first time! – I had a really high DC offset on one side, and the amp would not work right. I looked it over, and tried to think of everything I could do to fix it. I resoldered a few joints. Nothing helped. Finally, I started looking at the troubleshooting page on TangentSoft.net. And what’s the first step? Clean the board. Oh yea, that. I cleaned it and voila, working cMoy! Glad it was something so simple.
Despite (or in spite of) the few issues I had, the finished amp looks and sounds great. I like the simple panel with just input, out and a volume/power knob. The translucent case allows my hard work to be seen. And the amp works great. To be honest though, I've never used it very much. Most of my headphones are low impedance, and this amp doesn't serve them well. But building it was a good experience which helped me build my skills designing and assembling electronic circuits and projects.
Originally posted June 2, 2013 on my old site; posted here April
17, 2018 with minor changes.