Designed in 1951 by Eugene Shenk, this program equalizer is still revered today for its performance and quality. Totally passive with push-pull tube stage amplifiers. Two units, previously refurbished, have been sampled. This equalizer is renowned for its unique ability to boost and cut the same frequencies simultaneously, thereby creating a resonant shelf. The wide range of equalization curves provided makes it possible to boost the very low or very high frequency notes without “muddying up" the middle register instruments, this made the unit useful for mixing and mastering use. It has also always been famous for making audio program sound better just by passing through it even in "bypass" mode.