https://www.diyaudio.com/
The pre-eminent forum for DIY Audio
http://sound-au.com/index.html
Elliot Sound Projects is probably the most comprehensive resource for DIY articles and projects. Also sells project boards.
https://virtualzeroaudio.com/
Canadian based supplier of amplifier boards, kits, and modules for DIY builds.
https://tangentsoft.net/audio/
Information, resources and projects mostly related to headphone amplifiers with the beginner in mind.
http://www.decdun.me.uk/
Information, resources and projects mostly related to power amplifiers with the beginner in mind.
http://phonoclone.com/ind.html
Website of RJM Audio that has projects, kits and boards available. I have built the Sapphire 4 headphone amplifier and highly recommend it.
http://www.readresearch.co.uk/index.php
UK based Audio projects, articles and utilities with some components for sale.
http://nihtila.com/
UK based site that has some in-depth articles about projects and audio fundamentals. Also sells PCBs and populated boards at a very reasonable price.
http://www.pavouk.org/hw/en_index.html
Quite an eclectic site with lots of audio projects available (amongst others) including Gerber files.
https://electro-dan.co.uk/Electronics/
Part of a wider electronics and computing site but a fair amount of audio here. The build guides are extremely well detailed, have a look at the authors LM3886 for instance. Also has a very good article on wiring transformers, excellent for novices.
http://buildaudioamps.com/projects-description/
Projects with detailed build guides.
http://diyaudioprojects.com/
Exactly as the name suggests.
https://www.tnt-audio.com/clinica/tweaks.html
Collection of articles and guides.
https://headwizememorial.wordpress.com/library/diy-projects/
Tribute site to Pow Chu Moy, founder of HeadWize and the original CMoy headphone amplifier. Plenty of projects saved from the original site.
https://www.learnabout-electronics.org/Amplifiers/amplifiers10.php
An interesting site of electrical theory with lots of information for audio enthusiasts including amplifiers and powers supplies. There is a nice step by step section on designing your own Class A amplifier.