I built a Vibro Champ and all is well except there is a scratchiness on the Intensity control as it is initially increased, from say 1 to 4. The rest of its range is quiet. I thought is might be the pot and replaced it, but with the same results. There is DC feeding the pot (see schematic). Could that be the cause, and is there a way to avoid that on the pot? Thanks for any help.