Have you heard about O2+ODAC by NwAvGuy? After some evaluation of products in the market at around 2012 he started his own DIY project for a headphone amplifier (the O2) and later retrofitted a DAC into the design (ODAC, both together make up O2+ODAC). It's a USB Audio Class I device and therefore runs on a lot of platforms out of the box that support USB audio unlike some other devices that require USB Audio Class II, which is not supported in Windows without special drivers.
JDS Labs is one of the companies that offers DIY kits for O2 if you want to build the amplifier yourself but don't want to look deeper into the bill of materials (BOM). They also made a revision with compatibility improvments to the original design based on their customers experience and they have a few other interesting articles on their blog.
If you are looking for something more portable or something bigger with even more power they offer Amp+DAC products like C5D and The Element in their shop. I was uncertain when my EL-8 arrived this year about which product to buy without spending insane amounts of money and was happy to find and read through NwAvGuy's blog.
- If it has to be within $30 read his review of the Behringer UCA202.
- If you want to have a really good but portable device consider the JDS Labs C5D or the O2+ODAC.
- If you don't mind carrying a power "brick" adapter but want to use planar magnetic headphones get The Element. It's only a few CD jewel cases high and very nice.
(I finally bought The Element on Black Friday in addition to the C5D I settled for first. I have no affiliation with any of the companies mentioned here.)