The mono-ear bud designed for crystal radios with high impedance feature a 3.5mm plug, making them compatible with all radios. With the brand name USB, these earbuds provide a convenient and efficient way to listen to your favorite radio stations. Perfect for collectors or enthusiasts of vintage radios, these earbuds allow you to enjoy your crystal radio in a unique and easy-to-use manner. Just one earphone. Crystal types are hard to get right now and these will do and are not expensive. Bob