KDE said:
I'd look at Drip Electronics. His DIY stuff is no compromise mastering grade stuff. Specifically I'd go for Drip Fairchild 670 ;D
I recommend also the DRIP projects quality wise, boards are amazing well built and designed, the documentation acompaning the Boards is an artwork by itself, what I really wouldn't advise for mastering is the Fairchild 670 project. It's too expensive, around 6000 to 10000 dollars, and also not the best compressor limiter for mastering.
Of course it has a lot of personality and it sounds amazing but with that price you can DIY all the basic equipment you need for a nice mastering setup.
I should also say that the Fairchild is not really used in any of the big and well know mastering studios in the world.
For Fun also, You can search on the internet for Bob Olhshon, this gentleman was the Mastering engineer at Motown for many years. The Fairchild in Motown studio, went from the mastering studio to the recording studio and started to be used in recording sessions, because Mastering engineers in Motown found it unusable for mastering.
Of course Im not saying anything bad about the Fairchild, just pointing something about its use in mastering.
I love it and I would love to have one or two. I used them quite a few times and even had the privileged of asking 2 fairchild 660 in Abbey Road to use in my 2 kick drum microphones of a Rock drum Kit, asked for 2 more for the the room mics. These are the places I like them the most.
I don't have one and will not buy one for sure, so I will stick to the Bombfactory Fairchild plugin that its good enough for the uses I call it for.
And Now answering your Question in a simple way without getting away from the subject:
DIY projects at the forum that can be useful for mastering I think it will be the EQ's and COMPs
Compressors
- SSL Bus Compressor , try some sort of the clean and more modern versions that appeared here on the forum, not the Gyraf first version of the project, but true bypass boards with relays, clean power supply
Use external power supplies, no toroids inside the box
- theres also the Neve 33609 projects here, different variations
1176, La2A, etc I would not advise for mastering, but thats me
EQ (more choices here)
Night EQ (nice air)
Sontec EQ (precision EQ)
Pultec EQ (yes its nice and useful to have also a not surgical EQ as a tool for mastering, large Q's can be usefull for tone shapping a track)
I would have this 3 EQ's for sure.
Actually I have them and use them for mastering depending on what the songs needs
Some sort of M/S encoder and decoder would be nice, and some other different compressor but I'm not sure what is around at the moment,
this forum looks like a supermarket nowadays so everyday theres new equipment to build.
Really important,
I would DIY a High Quality and Clean Monitor Controler and "A/B compare and routing matrix", with a simple and efficient design using only Top Quality components.
Hope this helps
Tiago