PartTimeProjects CMoy Headphone Amplifier

A lot of people are crazy about headphones. (check out HeadWize.com and Head-Fi.org.) Yet, the quality of the amplifiers used to drive headphones in portable devices and even some mainstream components can be very low. Sometimes this is due to cost savings, other times it is due to the need for portable devices to use a very low amout of power. So, if you want quality sound from your headphones, often it is necessary to get or build a headphone amplifier.

The "CMoy" headphone amplifier is one of the most popular designs. Its name comes from the person who published it- Chu Moy. The amplifier uses a dual opamp to drive headphones, it is very simple, and it can be made very small. Here are some links to get further information on this project:

• Chu Moy's "CMoy" amplifier on HeadWize.com describing the amplifier.
• Dan's Chu Moy Headphone amplifier, describing the CMoy.
• CMoy Tutorial at Tangentsoft.net describing how to build a CMoy. The instructions are written for a novice and are laid out step by step with parts identified.
• Wikipedia CMoy Entry. Hey, its in the Wikipedia!
• If you still don't think these are popular, check out a current search of EBay for CMoy Auctions which shows people selling these for between $30 and $80.
• If you still need a reason to try the CMoy, consider that the Grado RA1 headphone amplifier sells for $350 and has received glowing reviews. However, this comparison between the Grado RA1 and a CMoy reveals that it is essentially the same. While this comparison is critical of the quality of components in the RA1. Note that the RA1 uses a JR4456 op amp (70 mA maximum output current) while the standard CMoy uses an OPA2134 (35 mA maximum output current), and Grado does not use a virtual ground chip so does not limit return current. The Grado approach appears to have sacrificed DC response (may have some offset) for current delivery.

I don't consider myself a "headphone fanatic" but look at this- I've actually got quite a few pairs of heaphones. I use headphones to prevent hearing loss while traveling on noisy airplanes. I grew up in the '80's and need all the help I can get! How good is the CMoy with these- lets see.

Headphones	Image	Headphones
Grado SR60, 32 ohms, 98 db/mW. These are fantastic headphones. Sound good with the CMoy and also without the CMoy. The earpieces broke off during one road trip and I sent them back to Grado. The came back in a week fixed, free of charge. Now that's customer service! AKG K240 Monitor, 600 ohms, 88 db/mW. I bought these in the '80's when they were the standard used in studios across the country. I never could get them to sound good because as it turns out, they are inefficient and present a very high 600 ohm impedance. This means a headphone amplifier should be able to generate high voltages (not current) to make them sound their best. These really come alive with the CMoy, airy highs and a solid image, with deep bass. Very comfortable on your head as well. Sure E2C In-ear, 16 ohms, 105 dB/mW, 20 db isolation. These are sensitive and noise-blocking. I have used these continuously for years of travel but stressed out the right angle 1/8" miniplug jack, and its now become intermittent so I guess I'll cut it off and install a new one. The sound is not as good as the ER-4P but the comfort is higher. Not sure they benefited from the separate amp. Shure SE110 in-ear headphones, 27 ohms, 113 dB/mW, 18 db isolation, sensitive and sound great, good noise isolation. Not sure they benefited from the separate amp. Seem to be comfortable so I'm going to try in travel mode.		Etymotic ER-4P in-ear, 27 ohms, 100 dB/mW (108 db SPL for .79V), 20-25 dB isolation. Sensitive and very good sounding, could not really tell if they benefited from a separate headphone amplifier, so perhaps not. Very detailed headphones and good low end, but not so comfortable wearing for over 3 hours (a long flight). These and the Grado's provide the best sound, in my opinion. Audio Technica ATH-ANC3, in ear noise canceling, 90 ohms, 104 dB/mW, 20dB isolation. An impulse purchase at Narita Airport when the E2C's stopped being reliable. I use these when traveling now because they are more comfortable than the Etymotic's and have noise blocking and noise cancellation. These actually don't sound as good as any of the other headphones but I'm still using them the most. Pretty bad eh? I'm going to try using the SE-110's a little more frequently to see if they are better for travel. Bose Triport (around-ear) TP-1, 32 ohms 97 db/mW. These are actually not that bad, they are not noise canceling but are very lightweight and comfortable. They actully sound better with the CMoy though they are efficient. Not sure why.

What the heck, I thought I'd build one. But only using parts I had around already. I did have to get the input/output connectors but had everything else laying around, including the case which I had gotten at the surplus store long ago. So my CMoy is built just like the basic one with some modification to the cirucit values as well as having a lot of flexibility and features.

• RCA inputs, in parallel with
• 1/8" miniplug input;
• RCA outputs, in parallel with
• 1/4" phone plug output.
• Switchable battery or wall power supply.
• Virtual ground implemented using the TI TLE2426 chip
• mounted in a small case found at the surplus store.

Here is the schematic. This only includes one channel. The other is the same and powered by the same power supply.

CMoy Schematic

Notes: You see here that I used some values different than the traditional CMoy. The goal was to make this out of parts that I already had on hand and not worry about the small things. For example, I did not have a 220uF capacitor to put between the 9 volt battery outputs, but I did have 2 small capacitors that were 10uF. Not as good some would say, but with a battery you don't really need capacitors here anyway. The resistor values may seem a little odd, but they are the measured values, not the values you would specify when purchasing. I did this by hand on a perfboard- old school style. I had to buy the TLE2426 chip, the RCA pcb connectors, and the headphone connectors, and the rest of the parts I scrounged up from things that I already had.

Building

CMoy In Construction (top)	CMoy In Construction (back)	CMoy In construction (bottom)

Notes: You see that i used an existing case and its PCB, which I removed all the parts from except the input DC connector and green LED. I mounted the connectors to the rear of the PCB upside-down and enlarged the rear panel cutouts somewhat. Once again, I'm tight on space. I only had red wire so used it for essentially all the connections. I hope this doesn't break, cause all the wires look the same. I mounted the input/output rear panel connectors upside down on a perfboard because I wanted to use the existing cut-out in the case, and this was the only way to have them fit. More importantly, I wanted to be able to take off the back panel and slide the entire amp out for potential repair and modification. This is why i used PCB mounted connectors instead of panel mount connectors (on the back). This amp disassembles easily, just unscrew the back panel, remove panel, and slide cover off.

Case and Connectors

CMoy Assembly	CMoy Front	CMoy Back

Notes: Once again, I'm short on extra space in my "Trunk Interface II," now a CMoy headphone amplifier. Cannot believe I fit all that stuff in there. The power switch in the "up" position will run the CMoy on battery power. In the middle position the amplifier is turned off. In the "down" position, the amplifier is run on the wall-wart connection (or off if the wall-wart is not connected. Note to self- the middle is + and outer is -). There are no fuses in here- I didn't have room; I may try to add one later.

20 Hz/ 10 ohms 0.2 v/div	1 KHz, 10 ohms, 0.2 v/div

Notes: The first oscilloscope screen shows a punishing 10 ohm load at 20 Hz. Set to 0.2 v/div you see that here the CMOY is putting out more than 3 div, a little over 0.6 volts, which equates to 60 mA. At an easier frequency, 1 KHz, the CMOY is putting out .8 volts into 10 ohms, which is 80 mA. This easily puts out over 10V into high impedances like 400 ohms.

I thought that the virtual ground would limit me to about 32 mA of current but I'm getting over 60, which is pretty good, though I'm not sure why that is!

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