QuestionI own a vintage solid-state electronic organ that I have completely rebuilt & restored over the last five years. The organ has 4 separate audio channels, and so is equipped with 4 audio power-amplifiers inside in the power supply chassis of the instrument located at the bottom of the organ. The audio sources feeding these amplifiers are located in the upper top portion of the instrument, the outputs of which are then feed down into the amplifiers by shielded cables equipped with foil shielding & a drain wire. My question/problem is that there is a 60 Hz hum in the organ speakers that is much more pronounced in 2 of the four channels. The audio sources for the two channels with the lowest amount of hum are located on the same circuit board assembly, with the remaining two sources in a different location in the organ. When the 4 audio inputs are disconnected from the amps by unplugging them from the amplifier chassis, the hum disappears completely in all channels. When the audio cables are disconnected at the audio sources (in the upper portion of the organ) the hum also disappears. However, if the audio cables are connected to the amp at the bottom chassis, and then intentionally shorted at the sources located in the upper portion of the organ, even when the cables are NOT CONNECTED to the actual audio outputs themselves, the hum reappears in the speakers.
Does this mean that the hum is being produced through magnetic coupling in the input cables?
In other words, if a hum is produced in ANY amplifier by merely attaching a length of audio cable to its input that is not connected to any audio source, but that has been shorted at the farthest end from the amp, any noise or hum then coming from that amp could only be produced from magnetic influence on the attached cable? Correct?
AnswerI found your question in the pool so let me start by stating this is definetly out of my expertise but I'd like to try and help. When you stated:
"When the 4 audio inputs are disconnected from the amps by unplugging them from the amplifier chassis, the hum disappears completely in all channels. When the audio cables are disconnected at the audio sources (in the upper portion of the organ) the hum also disappears. "
This leads me to believe its your source that's causing the problem. The hum is happening before it reaches the amp. In both scenarios you're unplugging the source, just at two separate points. To confirm this try to plug "source A" into "amp B" and "source B" into "amp A". The hum should follow the source.
