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Old 10th Dec 2018, 6:09 pm   #1
bluepilot
Heptode
 
Join Date: Feb 2010
Location: Duffort, Gers, France
Posts: 714
Default Strange interference phenomenom

I have been digitizing a load of old 78s using a home-built preamp which enables me to adjust the equalization. I went to a lot of effort to screen it against all sorts of hum and noise. However hard I tried there were always two interfering signals, one around 18kHz and one just less that 30kHz, that I couldn't get rid of completely. I thought of nearby switching power supplies but that doesn't seem to be the answer. At present that's not really a problem as the 30kHz disappears when downsampled to put the music on a CD and given the frequency response of 78s, 18kHz doesn't really matter either. However I would love to know where they come from.

A couple of days ago some people turned up to replace the roof. (They were supposed to come next summer but that's another story). They have erected metal scaffolding all around the house. Today I found that the 18kHz signal has disappeared and the 30kHz signal has been reduced by several dB. It may be coincidence but the obvious explanation is that they are coming from something external to the house and have been screened by the scaffolding. The nearest neighbours are several hundred yards away. I can only think that it's some sort of signal which comes in via the overhead power lines supplying the house. They run along the front of the house where my workshop is, just above where the scaffolding now is. Does that sound plausible? Would a smart meter use such signals? How can I screen them? Like I said it's not a problem at present but the next project is to try to digitalize some Quadradiscs and the sub-carrier of those is 30kHz. Given that the carrier is quite strong it probably won't matter much but it would be nice to get rid of the interference.
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