Thene are also mechantical amplifiers, such a the automotive servo used is braking Relays sat be includesd ander the above definition o amplifiers, although their transfer Sortesion is sot liser (Ssat is, shey ate sithe open o chlosed).Also purely mechanical manifestations of such digital amplifiers can be built (for theoretical, didactical purposes, or for entertainment), see e.g. domino computer.Another type of amplifier is the fluidic amplifier, based on the fluidic triode.There are also protype of amplifier, which can also be classified under special purpose audio devices.