Cmoy Pocket Amp

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Last Christmas I bought myself a pair of Grado SR-80 headphones to go with my new Minidisk player. The headphones sounded awesome but even though they are pretty efficient, they are low impedance. They require a lot of current to get them to work properly and the tiny little chip in the Minidisk player just Isn't able to supply the kind of current that the headphones want. I did some research on this and found out that many audiophiles build separate amplifiers for their headphones. The one design that popped out at me was a simple Opamp based one designed by Chu Moy. I tinkered around with it and a year later I had What you see above.

This is the final schematic, the one I used in my amp. Note that there are no capacitors in the circuit at all. Many would argue that this is an advantage sonically since capacitors tend to create nastly little filters and distortion, and I generally agree, but be careful since now there is no DC protection. Make sure the offset voltage is low or else you could risk burning out the voice coils in your expensive headphones.

The biggest thing I changed from the original Cmoy design is the power supply. Instead of using a single 9v battery and the virtual ground circuitry, I just used a real dual power supply using two 9v batteries. This setup is simpler and more reliable and I've heard that a higher voltage makes it sound better anyway.

Note: I accidentally omitted the power swich from the schematic, any DPST or DPDT swich will work as long as you wire it to shut off both the positive and negative supplies to the opamp.

I designed my own board for this amp using CadSoft's EAGLE Layout Editor and used the toner transfer method of etching a PCB. You can download the PostScript file of the layout here.

The holes on this board were drilled with a 1/16" steel bit at about 3600 RPM on my drill press with lots of WD-40 used during the drilling. The bit was twice as large as it should have been, but it was all I had at the time.

On the top side of the board you can see the components. The opamp isn't set in the socket yet but all the wires and other components have been soldered. The patch of copper in the upper right corner was due to some oil or something on the board when I was etching it, the acid couldn't get to the copper to dissolve it there. It doesn't interfere here so I left it as is.

It's starting to come together here. The board fits snugly into the can with the corner pinched between the tin and the output connector. On the bottom there's a peice of plastic to insulate the electronics and I later put a peice of packing tape on the lid so it wouldn't short out there either. Notice the long wires coming from the board. You can always cut a wire shorter, but it it's too short making them longer is messy and a chore. ;-)

And here it is all put together. I drilled holes in the side of the case with a drill press, stepping up the bit size until I achived the desired diameter in order to avoid tearing the thin sheet metal. Nothing but the volume pot and the output socket are physically attached to the tin, the parts fit snugly enough to stay put once the cover is closed.

Originally I was going to find a place to mount it on the outside, but for the sake of getting it done when I was putting it together I just shoved it in there. It actually works out very well, you don't need the power switch on the outside anyway and gives me another excuse to open it up and show it off to people.

Even using an opamp that I scrounged out of an old tape deck and metal film resistors liberated from the graphic EQ of an old stereo, this thing sounds great and is dead silent even with nothing on the input. It really does give my headphones some nice kick, and for the invested price of about $10 for random things like the potentiometer and the connectors, it rocks pretty damn well!

And now, random photographs:

5/12/07