DACs come in many forms. The DAC will replace the function of your computer's sound card, and almost always do a better job. (Less noise, clearer sound, etc. Depends on the DAC.) Many of the ones available these days can plug into a USB port. Your MacBook's headphone jack doubles as an optical TOSLink output. So you can use a fiber optic cable to connect to some DACs too.
The HD650s definitely need a proper amp to sound their best. Schiit currently has the Asgard and Valhalla amps available. I have not tried either one with my 650, but others have and you can probably find a few reviews or impressions around here, such as this thread. These amps still need to be hooked up to a DAC. Either that, or connect the amp to your MacBook's hadphone/line-out jack. I recommend getting a DAC if you can.
Some DACs come with a built in headphone amp. These are often referred to as "DAC/amp". The uDAC is one such example. You can use the HD650s straight out of those, or pair it with a separate amp. A dedicated desktop amp will usually power the relatively demanding headphones better than just the DAC/amp by itself.
In your case, since you already have a pair of Grados (according to your sig), I suggest getting a DAC/amp first. You will hear the benefit with your Grados, and over your A5s as well. Then get the HD650. And then get a dedicated amp. That way you'll get to enjoy the improvements each step of the way.

Just following up on this thread... (Funny, I spent the last two days looking for IEMs and now instead I'm gonna drop a grand on a desktop headphone rig instead.)
I want the 650s. It seems like having an amp is required, so I should go buy some Schiit. But what is this uDac thing? Does that obviate the need for an amp? How exactly does a DAC work? My source is lose less or V0 mp3s from a MacBook. Does the DAC plug into the adio port? Or is it USB?
Do I need some Schiit or should I get a DAC for the 650?













. Tube rolling looks like so much fun as well.


It is $249 and about $260 after shipping. The packaging was sufficient and clean. No peanuts, hooray. It comes with a 3.5mm to stereo RCA cable, standard issue power cable, 3.5mm to 1/4" converter, and little plastic/rubber feet you can stick on the amp. I'm relatively new to Head-fi and headphone equipment in general, so if you don't want to read my inexperienced/limited musings, you don't have to. My setup before purchasing the Asgard was Foobar WASAPI -> Realtek ALC889 TOSLINK -> Yamaha RX-V365 -> Sennheiser HD650. That is all I have. If you want to make a suggestion for another pair of headphones you should

