Introduced in 1982 the Lambda Pro was among the first Stax Earspeakers to use the new 5 pin pro-bias socket which delivered 580V instead of the normal 230V, and was for a while the top-of-the-line offering. Sold into the 1990s, the Lambda series was reincarnated, and it's frame is still found today in the SR-202, SR-303 and SR-404 range. The Lambda Pro's feature a U shaped sound signature, with more prominent highs and lows, with a slightly recessed mids section. While slightly more present, the bass remains light enough to allow a transparency into the music not afforded by the newer more bassy Stax earspeakers. The mids, despite being slightly "sucked out" also remain the best characteristic of the headphone. While not extremely rare or sought after, the Lambda Pro remains one of the most popular vintage Stax, bought and sold often on the online headphone community Head-fi.