Low-gain overdrive. Works perfectly. I've been using it and liked it, but I haven't been using it much lately, so I'm putting it up for sale. It's a popular Zen Drive, and the Supreme is probably a very rare model. Compared to the previous Zen Drive, it has less compression and a more subdued gain. You can get plenty of gain by stepping on both switches, so you can also produce a pretty intense sound. The two circuits use different op-amp configurations, resulting in different sound nuances, and the combination with the 3-tone switch allows for a wide variety of sound variations. You can also choose between 9V and 18V power, so you can experience the difference in sound depending on the voltage.