A type of connection found on many combo guitar and bass amps — which feature a built-in speaker — and used to hook up an external speaker cabinet. Typically these external speaker jacks are use 1/4-inch connectors. In some cases, hooking up a speaker to the external speaker jack will disconnect the internal (built-in) speaker; in other cases, both speakers will work at the same time. It is important to observe proper speaker impedances when using external speakers in order to prevent damage to the amplifier.
