Technically speaking, an amplifier is any device that changes, usually increases, the amplitude of a signal. When choosing an audio amplifier, you need to consider the continuous wattage required to power your speakers and the impedance rating of your speakers in Ohms. Power amplifiers and integrated amplifiers are available in mono, stereo, and multi-channel configurations as well as vacuum tube and solid state models. Adcom manufactures the GFA series of high current stereo and five or seven channel amplifiers. Atoll, based in France, manufactures high-end integrated amplifiers. Conrad-Johnson is well known for their high-end vacuum tube and solid state amplifiers. Magnum-Dynalab integrated amplifiers and stereo receivers are critically acclaimed among audiophiles for extra-ordinary quality and performance. McCormack Audio introduces the new DNA-250 and DNA-750 stereo and mono block solid state power amplifiers. PS Audio manufactures the Trio series of integrated and power amplifiers. Speakercraft builds the Big Bang series of stereo and 8 or 12 channel amplifiers with plenty of power and custom-installer friendly features, as well as their MZC series of keypad-controlled receivers.