Like many just starting out with LF, I'm using an audio amplifier to
I had a bit of a surprise when tinkering about with it.
I had connected a loudspeaker to the output and put a 1kHz signal through as
a confidence check, that the amp was working ok.
Afterwards I reconnected the antenna, before removing the loudspeaker. As I
did so there was a horrible hum - well buzz really, since I have reduced the
size of coupling capacitors to give it less gain at low frequencies.
It didn't take long to realise that there was a big earth loop. The signal
source is referenced to mains earth, and when I connected the aerial (and
earth) the amplifier was also connected to RF earth.
I have cured the problem by introducing a transformer between the signal
source and the amplifier, but I wouldn't have realised that the amplifier
was trying to amplify both 136kHz and huge amounts of 50Hz at the same time.
If I had a (working) oscilloscope, I would probably have spotted the problem
earlier, but it's not something that I expected.
I did make me wonder whether hum had a bearing on the mysterious FET and
fuse blowing problems with the BK amplifiers - but I expect large amounts of
hum would have been quickly noticed when the problem was investigated.