Speaker Simulator | General Guitar Gadgets Speaker Simulator. Here’s a project that originated from Aron’s Stompbox page. GFR designed a “lite version of the Fender simulator from the LXH site (the site no longer seems to exist). I drew up the layouts and here they are. Please Note: This is a speaker simulator, not a power attenuator. Effects > Marshall Cab Sim : DIY Fever – Building my own Hi! I want to build one of these, but I still lack some knowledge when it comes to op-amps. Most pedals out there use negative tip 9V power supplies, so I experimented in a Circuit simulator with -9V, and my first thought was to just flip all polarities, what goes to Op-Amp In- moved to Op-Amp In+ etc. but that didn’t work as expected.