Complex sentence: Standard headphones used in telephone work had a low impedance, often 75 Ω, and required more current than a crystal radio could supply, so the type used with radios was wound with more turns of finer wire and had an impedance of 2000-8000 Ω.
Split into simpler sentences: Standard headphones used in telephone work had a low impedance, often 75 Ω, and required more current than a crystal radio could supply. Therefore the type used with crystal set radios was wound with more turns of finer wire and so had a high impedance of 2000-8000 Ω.