Hum and Buzz in Audio Systems, Home Theaters and Studios

What effect does 'hum' and 'buzz' have?

'Hum' and 'buzz' can cause major noise issues with our high performance audiovisual systems.

• Hum is a superposition of the 60Hz mains AC frequency to the audio or video signal.
• Buzz  is a superposition of the harmonics of the 60Hz mains AC frequency to the audio or video signal. Harmonics are multiples of the base frequency - i.e. 120Hz, 180Hz, 240Hz, etc. The more distorted the AC waveform the more harmonics there are.

Experts think that 120dB of signal to noise ratio might be needed for ultimate fidelity in audio systems and 50dB in video. Even minor hum and buzz, such as those caused by a leakage current of 1mA across a 10ft #22 gauge cable connecting two pieces of equipment can cause noise levels of -60dB relative to a full scale unbalanced 'consumer level' signal! Major hum and buzz issues may manifest at levels of -6dB, which is extremely loud!

The hum and buzz issue is especially important due to the fact that the AC frequency (60Hz) and its major harmonics (120Hz, 180Hz, extending to 3000Hz for the 50th harmonic) coincide exactly with the the frequencies and harmonics of nearly all musical instruments. From this analysis it is clear that using good engineering practices to minimize and eliminate the root causes of hum and buzz is a necessity in maximizing the performance of any audiovisual system.

How is hum and buzz caused?

Hum and buzz can be created by interchassis currents between pieces of equipment with OR without a saftey ground as a result of 'stray' capacitance in the power supply:

"The most common source of hum and buzz is the stray capacitance between the A.C. power transformer primary and secondary windings [capacitance is also intentionally added across line and neutral by RFI filters]. 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 capacitivly coupled power line signal will usually contain significant harmonics out to 1MHz or more [due to harmonic distortion of the AC waveform]. These signals will cause currents to flow in the cable shields of unbalanced systems, adding this noise directly to the audio signal [since the cable shield has a non-zero shield resistance which causes a noise voltage to occur when a current is passed through it]. These signals may cause large voltage differences between pieces of equipment in balanced systems, over-whelming the capability of equipment input stages to reject them." Bill Whitlock, Jensen Transformers FAQ page (italics added by Acoustic Frontiers).

The magnitude of leakage currents and hence the magnitude of the potential hum and buzz issues are limited in Underwriter's Lab (UL) certified equipment to prevent electric shock:: 

• For approved equipment without a safety ground (i.e. the power plug has two prongs) this current is limited to 0.75 mA but is frequently much less. Although the voltage present between equipment ground and the earth ground can reach as high as 50v the current, not the voltage, is the limiting factor in the noise voltage created when two pieces of equipment are connected.
• In approved equipment with a safety ground (i.e. the power plug has three prongs) this current is limited to 3.0mA.  Typically the noise voltage created by by leakage current in interconnected equipment with safety grounds is much less than that caused by voltage differences (see below) between safety grounds.

Solutions to this particular issue are to :

• Make the interchassis current flow as low as possible by using short lengths of interconnect cable with low shield resistance.
• Break the interchassis current path by placing an isolation transformer in the signal path where the noise coupling occurs. Make sure you select a high quality transformer to prevent phase shift, disortion and other signal degradation. Jensen Transformers are our favorites.

Hum and buzz can also be created by differences in the voltage of the safety ground wire between interconnected equipment (aka ground loops):

"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, [control system - especially unbalanced RS232] 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." Bill Whitlock, Jensen Transformers FAQ page (italics added by Acoustic Frontiers).

The voltage differences can be up to 1V in equipment linked by short safety ground connections and up to 5V in the large loops created by cable TV connected systems.

Solutions to cable TV, satellite or control system related ground loops:

• Break up ground loops by using isolation transformer in the signal path where the noise coupling occurs.  Choose a high quality transformer to prevent phase shift, disortion and other signal degradation such as the Jensen Transformers VRD-1FF. Telebyte makes suitable RS232 isolators.
• Connect cables coming from cable TV, satellite or control systems to a saftety ground close to the AV equipment - power conditioners sometimes provide these interfaces.

Solutions to interconnected equpment related groud loops:

• People often use 'ground cheater' plugs to 'fix' ground loop issues. Don’t! The safety ground is there to protect you!
• Keep equipment grounds as short as possible - power strips or conditioners local to the system create a low impedance common ground point reducing the potential differences between equipment safety grounds. If there are multiple electrical outlets feeding your system it is best to take them all back to a common 'star' ground point at a dedicated sub panel or isolation transformer.
• Use isolation transformers in the signal path where the noise coupling occurs. In high quality systems this transformer must be carefully selected to minimize impact on the audio and video signals. Jensen Transformers are our favorites.

A final cause of hum and buzz is electromagnetic coupling of AC and signal cables. This occurs only when signal and power cables are run close together and in parallel and is much more of an issue in unbalanced than balanced systems.

Solutions to electromagnetic interference issues:

• Space signal and power cables away from each other and if crossing them, do so at 90 degrees since this reduces the coupling effect.
• Keep the loop area formed by interconnecting cables as small as possible - for example in a stereo setup run the left and right channel cables as close together as possible.

References and further reading:

Understanding, finding and eliminating AV ground loops, Bill Whitlock, Jensen Transformers.

Balanced lines in audio systems: fact, fiction and transformers, Bill Whitlock, Journal of the Audio Engineering Society, Vol 43, June 1995

Hum and buzz in unbalanced interconnect systems, Bill Whitlock, Jensen Transformers application note AN-004.

Isomax troubleshooting guide, Bill Whitlock, Jensen Transformers