The Pigtronix Fat Drive creates a pure analogue overdrive by using multiple gain stages that more closely resemble the way tubes operate, thus producing a more tube-like sound.
The Pigtronix Fat Drive pedal is designed to have a tube-like overdrive. It creates a modern, 100% analog style overdrive by using multiple gain stages that round and compress a signal in each stage, as it begins to clip. As a result, it more closely resembles the way that tubes operate, thus giving this pedal a more tube-like sound.
The Tone Control is a variable low pass filter and turning it all the way clockwise removes it from the circuit completely, leaving uncolored, pure tone. Turning the filter to the left rounds off the high frequencies while leaving the mids and lows largely intact. The center-mounted More switch toggles between a low amount of overdrive, and a huge boost to the amount of overdrive added to the signal.
The Pigtronix Fat Drive features true bypass switching, with a blue LED indicator. The inside of the pedal is too packed with circuitry to allow room for a 9-volt battery. The pedal is, therefore, powered with an 18VDC 300mA power supply that they include. Pigtronix states that you can use your own 9 volt power supply to increase the amount of gain delivered by the pedal, but at the cost of losing headroom and signal transparency.