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The power output stage often consists of two or two pairs of transistors connected in series between the positive and negative supplies. There will be one set for the left and one for the right in a stereo amplifier. These may be either bipolar or field-effect types. Bipolar transistors can be subject to thermal runaway: overheating causes an increase in the current drawn, which in turn causes the overheating to get worse, and so on until the transistor fails. This often results in an effective short circuit between the positive and negative supplies, causing a fuse to blow, sometimes spectacularly.