For low frequencies, the voltage across CB follows the head signal, and no differential amplification occurs. Thus, CB acts as a high-pass filter. Read Noise Electronic noise in disk drive systems is a critical concern. There are several sources of noise or interference, including radiated noise picked up by the head leads, transition noise on the magnetic poles in the media, ground or voltage supply noise in the disk drive, and electrical noise in the head and preamp. It is the latter that the preamp is designed to minimize.
Shot noise is associated with the direct flow of current in diodes and bipolar devices.
Since the input devices in the preamp are bipolar, they mainly contribute shot noise. Thermal noise is associated with the random motion of electrons in resistors. Since the MR head is resistive, its noise contribution is mainly thermal. These individual noise contributions, referenced at the preamp inputs, are shown in Figure 6. The heating increases the resistance of the MR, causing a large voltage signal at the amplifier outputs, which can be several times the amplitude of a normal readback signal.