How Does "Hum and Buzz" Get Into My System?
p>Hum and buzz (50Hz/60Hz and it's harmonics) occur in unbalanced systems when currents flow in the cable shield connections between different pieces of equipment. Hum and buzz occur in balanced systems when the ground voltage differences between different pieces of equipment are so large that the Common Mode Rejection of the input stage of the receiving device is insufficient to reduce the noise signal to in-audible levels.
The cable shield currents and ground voltage differences are caused by several mechanisms.
The most common source is the capacitance between the A.C. power transformer primary and secondary windings. Since the secondary winding which provides the power to the device must at some point connect to the circuit ground, a portion of the A.C. line voltage will ALWAYS be capacitively coupled directly to this same circuit ground. This capacitively coupled power line signal will usually contain significant harmonics out to 1MHz or more. These signals will cause currents to flow in the cable shields of unbalanced systems, adding this noise directly to the audio signal. These signals will cause large voltage differences between pieces of equipment in balanced systems, over-whelming the capability of equipment input stages to reject them.
The second most common source of hum and buzz is the voltage difference between two safety grounds separated by a large distance or the voltage difference between a safety ground and an "Earth" ground (such as a grounded satellite dish or cable TV source). This source of hum and buzz is more of a problem in unbalanced systems, since the low impedance nature of these sources can cause very large currents to flow from one device to another through the cable shields.
Hum and buzz can also be magnetically induced or capacitively induced directly into signal cables when A.C. power cables are run right next to them. This tends not to be a problem in any but the most unusual situations where common sense cable routing is not possible for some reason.
Transformers eliminate hum and buzz in unbalanced systems by preventing the flow of ground currents in the cable shields, and in balanced systems by greatly increasing the Common Mode Rejection capabilities of equipment input circuits.
For a more detailed explanation of these topics, see article HUM & BUZZ IN UNBALANCED INTERCONNECT SYSTEMS (AN-004).
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