Electro-Harmonix BIG MUFF π Russia Purchased about 20 years ago and modified to true bypass. If I remember correctly, I made the change because the effect sound mixed with the bypass sound due to the high gain. Since the original has a 6p switch, the input signal is not isolated. Please refrain from purchasing if you prioritize the original. Operation has been confirmed, but it is old, so there is some scratchiness in the pots, but I think it will disappear with use.