The Penumbra Bass Fuzz is inspired by the Zvex Woolly Mammoth and Mastotron, taking elements from each but with some modifications for better usability.
The Woolly Mammoth is a full-frequency fuzz with no low-end cut, making it great for bass, but of course it works great on guitar as well. It features controls for fuzz level, EQ, pinch (pulse width of the wave form) and output volume. The Mastotron added a few controls but is otherwise identical.
The Penumbra is a combination of the two, including the Subs (Low) toggle but not the “Relax / Push” control. This has been replaced by a transformer-based pickup simulator, originally designed by Jack Orman of AMZ and used by others including Earthquaker Devices in their Erupter pedal.
Like many vintage fuzz circuits, the Woolly Mammoth works best when it’s the first pedal after the guitar. If its input signal is too low-impedance (i.e. coming from a buffer or an active pedal), it can drastically change the character of the effect. The pickup simulator will convert any input impedance into the impedance of a guitar pickup, so the Penumbra will behave as it should no matter where it’s placed in the chain.
This pickup simulator is on a toggle switch, allowing it to be easily bypassed. This way you can leave it off if the Mammoth is the first pedal in the chain. When the pickup simulator is turned off, the Penumbra is sonically identical to the original Woolly Mammoth / Mastotron circuit.