Hum and Buzz
Posted: Mon May 24, 2010 7:41 amHums and buzzes are caused by radiated interference from nearby equipment and wiring, or by AC currents flowing through cable shields that connect one chassis to another. If you're experiencing problems with humming and buzzing, you can use these basic facts to troubleshoot the problem. Proceed systematically using the suggestions below until you find the culprit.
On the modules
* Transformer leads should be as short as possible.
* Component leads should be trimmed properly.
Inside the chassis
*Make sure all power supply mounting screws are snug.
*Front panel power switch leads should be neatly dressed with wire ties and routed along the bottom of the chassis.
*Front panel bolts should be tightened firmly.
*Rotary switch lock nuts should be snug (but not too tight!)
*Top cover should be attached with all of the screws.
*Verify good grounding by measurement. The resistance between any exposed metal and the power ground pin at the rear of the chassis should be less than one ohm. Make sure you know how to make low resistance measurements with your meter.
Outside the chassis
*Remove the preamp chassis from the equipment rack.
*Keep any equipment that may generate a strong magnetic field at a distance, such as guitar amps, power amps, power conditioners, video monitors, computers, motors, fans, etc. Better yet, turn them off while troubleshooting.
*Turn off fluorescent lights and light dimmers.
Cables from chassis to chassis
*Terminate the preamp inputs! You can't just crank up the gain and expect to get a valid measurement. The inputs must be terminated with a low impedance source. The easiest thing to do is wire a jumper between pins 2 and 3 of a spare XLR plug. Leave pin 1 open.
*Make the signal path as short as possible. Connect the preamp outputs directly to a headphone or power amp.
*Disconnect everything else! Eliminate patchbays, line mixers, monitor controllors, etc.
*Plug the preamp chassis and headphone amp into the same outlet strip.
On the modules
* Transformer leads should be as short as possible.
* Component leads should be trimmed properly.
Inside the chassis
*Make sure all power supply mounting screws are snug.
*Front panel power switch leads should be neatly dressed with wire ties and routed along the bottom of the chassis.
*Front panel bolts should be tightened firmly.
*Rotary switch lock nuts should be snug (but not too tight!)
*Top cover should be attached with all of the screws.
*Verify good grounding by measurement. The resistance between any exposed metal and the power ground pin at the rear of the chassis should be less than one ohm. Make sure you know how to make low resistance measurements with your meter.
Outside the chassis
*Remove the preamp chassis from the equipment rack.
*Keep any equipment that may generate a strong magnetic field at a distance, such as guitar amps, power amps, power conditioners, video monitors, computers, motors, fans, etc. Better yet, turn them off while troubleshooting.
*Turn off fluorescent lights and light dimmers.
Cables from chassis to chassis
*Terminate the preamp inputs! You can't just crank up the gain and expect to get a valid measurement. The inputs must be terminated with a low impedance source. The easiest thing to do is wire a jumper between pins 2 and 3 of a spare XLR plug. Leave pin 1 open.
*Make the signal path as short as possible. Connect the preamp outputs directly to a headphone or power amp.
*Disconnect everything else! Eliminate patchbays, line mixers, monitor controllors, etc.
*Plug the preamp chassis and headphone amp into the same outlet strip.