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by G. E. "Buck" Rogers
Sr; K4ABT
Solving hum problems in PC soundcards and Ham Transceivers
"Ground loops will cause horrendous hum, buzzes, and other noises, especially when connected to computerized gear or lighting equipment. The hum and noise are caused by differences in voltage potential from one end of the cable to the other. One remedy, of course, is to use audio isolation transformers in the interface devices similar to those we use in our RASCAL GLX. As Broadcast and design engineers, we understand ground loops. In fact, a major design consideration in broadcast (Radio and TV) stations is to prevent ground loops. When done correct, there are no ground loops and no hum or other noises caused when using audio isolation transformers.
BUT that is only part of the "ounce of prevention"
Designers of personal computers are not usually audio engineers and evidently have never heard of ground loops. In fact, given the noise of the fans, CD ROM spinning, and blaring noise made by disk drives in the PC, it is a wonder they can hear at all. All that noise forces some PC users to keep the computer in another room. But the primary source of ground loop problems in PC audio is the PC soundcard
The typical ground loop hum problem often occurs when you connect your computer (PC) sound card to a transceiver which is connected to an AC outlet that is not connected to the same bond or ground as the PC.
Finding ground loops is not always easy; Once found, the solution may be effortless. Solving ground loop problems in typical HAM radio and PC sound card applications may be as simple as applying practical logic. If you have your transceiver and PC connected to different power outlet/circuits, you may be closer to the solution than you think.
Try connecting both the transceiver and PC to one "grounded" multi-outlet with a "third-wire ground" (bonded outlet), and then plug this multi-outlet into a grounded (bonded), wall AC outlet.
This usually solves eighty-five percent (85%) of all hum (ground-loop) problems, simply because you've created a star-point grounding scheme for both the PC and Amateur Radio transceiver. If this scheme does not cure the ills of the hum-loop, it could be that no bond or third-wire ground exists in your AC (utility) wall outlet.
Other Computer Soundcard Problems:
When, and if, the computer is connected to a separate power outlet which may or may not be connected to a common (same) bond as the transceiver supply, it can result in a potential difference between the PC and the transceiver. This is where many ground loops are formed, causing hum to be superimposed on the transmitted audio.
If the sound card audio ground is indeed connected to the AC (utility) ground, and a high pitched sound or squeal is heard on other receivers, then look for RF feedback (base rectification) at the input or output of the PC sound card. Often a ferrite bead placed around the lead, or a 47 ohm resistor in series with the audio line will act as a block to ingress or stray RF.
In some cases where the remedy mentioned above fails, the addition of a by-pass capacitor may be required. The value of the by-pass cap may be approximately .001 ufd to .01 uf. Under no circumstances will the by-pass cap be larger than .02 uf. A greater value than .02 uf mayroll off in-band audio frequencies, and result in loss of audio output level.
If the by-pass cap is to be used, install it from the transmit audio line to the shield/ground. This is sometimes easiest achieved at the transceiver input connector (or jack).
As an adjunct to all the above, we can also employ the use of a line IN and/or line OUT isolator.
