This low-powered amplifier circuit is enclosed in a large gherkin jar. It has a 1/4″ input jack, power switch and controls for volume and gain. The speaker is mounted in the lid, which can be removed to access the 9-volt battery. The volume and gain controls can be adjusted from nice and clean through to dirty distortion.

The original circuit schematic is from Vol. 09 of Make Magazine.

http://makezine.com/09/crackerboxamp/