If you are in an analog plant:A Fox and Hound. This is a device for checking and listening to signals, identifying cables, etc.The low-rent version of this is the small battery operated speaker/amp that is sold by radio shack.These are troubleshooting tools (not for doing calibrations or proofs). If you are an an AES (digital plant):There are small battery operated D/A convertors and test generators that can quickly used for troubleshooting (there is one made by Ward-Beck that comes to mind - model number escapes me).Regards,Joe