This is the MXR dynacomp, one of the original compressors that has stood the test of time. It's still a strong seller for MXR, but it has a slight problem: the overall EQ. The Hi-fi mod for the Dynacomp yields a clear high end and more bass. These pedals are also known for sounding great as the battery's voltage starts to drop. The compression gets a bit softer and more natural. I install a small trim pot to adjust the voltage from 9V to about 3V. The decay rate switch allows the compression to kick in quicker or fast, depending where the switch is set.