I have just learnt a big lesson in the effect of LF EM waves. Trying to improve the power supply of a Swissonic ADC (seems very well designed), I was getting some very shrill sounds when I added decoupling capacitors to the analogue and digital rails. Replacing the wall plug with a substantial toroid made matters worse.
By accident, I had switched on a LF EMC detector and found moderate readings close to my ADC and DAC which had their seperate transformers placed some 10cm from the edge of their boxes.
Moving these away until I couldn't get a reading restored sonics to normality and I am most annoyed as I now need to spend several days evaluating the PS again!
The whole situation is reproducible
Does anyone else have the same experinece? It seems that my transformers need to be at least 20-30 cm away from the boxes!