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.
Setup:
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 ( http://www.kynix.com/Parts/3231833/RS232.html , etc.)
Problem:
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?
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