- Posted by Cut-Throat (A) on June 25, 2009 at 12:11:52
In Reply to: RE: Why ?.............................. posted by twystd on June 25, 2009 at 11:32:40
The hum may not bug some folks, but it bugs me. Read the following. Written when most audio was 2 prong plugs.
Figure 1. Ground loop involving line cords.
Both the preamp and power amp are equipped with 3 wire line cords and 3 prong plugs. (Note: this is almost unheard of in home audio equipment. This example is given for illustration purposes only). The green wire in each line cord is connected to the chassis of each unit. The ground prong of each plug connects to the local power ground. The two chassis' are also connected together by the shields of the audio cable. The audio cables are much longer than indicated in the diagram.
The shaded area shows the ground loop. As you can see there is a continuous loop from the preamp chassis to the power amp through the audio cable shields, from the power amp through the grounding wire of the line cord to the wall socket, and from the wall socket to the preamp chassis.
The two shields of the stereo audio cable are held so close together for most of their length that they rarely contribute any significant interference.
What can be done?
In the case of Figure 1.
You might try grounding the two chassis' together with very heavy wire in an attempt to short out the induced voltage. The added wire will just create another loop unless you place it physically very close to the audio cable. Even if you do that all you will succeed in doing will be to reduce the volume of the hum but it will still be audible.
You might also try bringing the line cords close together and close to the audio cable but most likely all you will accomplish will be to increase the hum. You will place AC carrying conductors into close proximity with the audio cables thus inducing more hum than you are getting rid of.
The only sure way is to break the loop. Cutting the shields on the audio cable rarely works with unbalanced inputs. There will always be a potential difference between the two chassis' even though they are grounded to the power line ground. The reason is that the grounding wire gets a current induced in it because it is in such close proximity to the current carrying conductors of the line cord. If this were a real situation, the only solution would be to somehow disconnect the power line ground from one of the chassis'. This is not approved by electrical inspectors and fortunately doesn't need to be done in home stereo equipment.Cut-Throat
