I have what is surely a classic problem with regard to power supply switching noise and audio but I am unable to sort myth from reality with regard to what I found so far on the topic.
- I have a notebook with an external power suppy and/or battery
- A radio receiver which has its own power supply (i.e. not fed by notebook SMPS)
- the radio receiver feeds an audio signal into line-in of the notebook
- the radio receiver is controlled by the notebook via RS232 (tuning, etc.)
- If I unplug the notebook from its power supply and run it from battery everything works perfectly
- But if I use the notebook SMPS, I hear a tremendous amount of noise in the audio
Can anybody tell me where the problem is likely to lie? There is a lot of talk about ground loops but I have difficulty believing they really exist in such a small-scale installation.
Am I right to assume that it is likely a problem of a varying ground level in the notebook and the fact that the line-in input of the notebook is non-differential? Or is there a more likely explanation?
What is the best solution? Use an opamp to construct a differential input amplifier and feed its output to the line in? What do I use as ground reference for the opamp?
Proposed solutions in the comments and answers
From the answers it would appear that there are two possible problems: 1. ground loops and 2. RF pickup from the external SMPS in the audio wire.
Suggested solutions are:
- Differential amplifier solution. Advantages/Disadvantages?
- Kortuk: Combat RF pickup from the SMPS in the audio link with a grounded shield. Advantage: invisible solution; Disadvantage? Question: does not help with any ground loops?
- Russell McMahon: Audio transformer in the audio-line. Advantage: Simple; Disadvantage: not easy to source, expensive or poor frequency response. Question: does this help with RF pickup in the audio line?
- Russell McMahon: clamp EMC ferrites on the audio line to combat RF pickup. Does not help against ground loops. Question: does this help with noise in the audible range? It was my understanding that ferrites only help to filter very high frequencies.
- David Kessner & Mary: Grounding the notebook. This shunts CM noise to ground. Advantage: cheap, simple; Disadvantage: additional wire to handle. Question: combats both RF pickup (if audio-ground is shunted) and avoids ground loops?
- Mary: ferrite absorber around the DC line to the notebook and RF CM chokes in the audio line and RS232 lines. Disadvantage: high component count & effort with the RF CM chokes. Does not prevent ground loops.