Speaking of broad, "early" in this context basically means anything prior to the late 1980's, though it depends a lot on the technology [and often the intent] of the work involved. Personally, I only consider electronic music up to and including the late 1970's as "early," but if it's notably experimental in nature or captures a similar sound/aesthetic, I'll make an exception.
My goal is to post a youtube video or sound clip a day... or every other day... or until I get too busy with real life matters. For now, let's start with a few relevant links:
- I just found this article today by virtue of me writing up this very topic: I haven't explored it much but suffice to say it looks quite comprehensive and very interesting. While most of my personal favorite recordings date to maybe the late 1920's at their earliest, it's worth pointing out the roots of electronic music go much further back; the first synthesizer, arguably, dates back to the 19th century!
- Creel Pone is a legendary label in some circles, bootl--I mean reissuing incredibly rare early electronic and avant garde records since 2005 (and even earlier counting the "proto"-CP releases). I've bought from the label many times and can vouch for label operator Keith Fullerton Whitman's prompt customer service.
- If you want to try before you buy, there's plenty of people sharing this sort of material, though one I often find myself frequenting is Electronic Orgy, an active blog with a well-curated collection of high quality and hard-to-find recordings. Recently, they've been re-uploading entries from the notorious Avant-Garde Project as well as some obscure live recordings from the late Klaus Schulze.