Just picked up a Canon EOS M and it lacks a headphone jack, but does have an analog A/V port that presumably takes a three pin component cable - you know with the mini jack on one end and a fan of three RCA plugs (yellow, red and black) for the video and audio signals. Can anybody think of a simple adapter with a built in attenuator or something that would let me use this port as a headphone jack?

The phono plugs will come out at domestic line level so you will actually need to amplify that signal to drive some headphones.

The ART My monitor or a similar battery headphone amp with a phono to stereo jack adaptor would do the job or a low cost phono headphone amp from e-bay would boost the signal to drive some headpbones.

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Magic Lantern used on the Canon 550D will give headphone output from the AV port. The port is mechanically similar to a mini USB socket so use the oem cable connected to a twin phono to 3.5mm jack adaptor.
There are posts elswhere on the fora about using EOS M cameras with Magic Lantrthe

Have you tried just plugging your headphones into the jack? There is a good chance that it will just work. The worst that could happen is that you will hear "video buzz" on one side of your headphones depending on how they designated the Tip, Ring1, Ring2, and Sleeve connections.

Many modern headphones are sensitive enough that they will operate on the "line-level" signal out of that connector. I have done this with a few consumer camcorders and it worked surprisingly well.

I don't have a Canon 550D to try it myself.

According to the EOS M manual, the analog AV output jack is also shared with the digital output for connecting with the PC, and requires a special cable with that unique connector on one end and the 3 AV RCA connectors on the other end. The AV cable doesn't come with the camera. The cable is the AVC-DC400ST.

The output isn't a mini-jack like on other Canon cameras where you can switch in the menu whether it outputs headphone or AV signals on a standard 4-conductor 1/8-inch jack.

Thanks. It appears I was wrong about the jack taking a regular 3 pin, 3.5 mm A/V component cable. It would be great if I could use that proprietary cable and a simple adapter to connect to an old Sign Video VU meter I have lying around.

Sign Video VU150 audio level meter (http://www.signvideo.com/products/product_information/vu150_audio-meter.htm)

If you dare. You can hack the USB to A/V cable by cutting it and splicing onto a 3.5mm 4 pole jack. I did with mine and can still use it for my cheap monitor (which I can plug in headphones.) or passing through a Fiio amp. FiiO E05 Headphone Amplifier: Amazon.co.uk: Electronics (http://www.amazon.co.uk/FiiO-E5-BK-E05-Headphone-Amplifier/dp/B0026RJQJ6) Mind you I do have a t3i but I'm assuming it's the same plug.

Just a thought but have you had a look in the menu system?

My canon HF11 has a multi function 3.5mm jack socket and you can select it via the menu system to be either an AV output with the 4 pin jack adaptor or a headphone monitoring output so you can just plug a set of headphones with a 3 pin 3.5mm jack straight in.