We had some issues with a UHF PMR base station being "heard" 50KHz away, due to
residual ripple from a switch-mode. No time for experiments with it so it just
got it's power source shifted to a linear supply.
Undoubtedly the switch-mode supply has much going for it as a power source for
heaters, built in soft start (usually) and insensitivity to widely varying line
voltage.
Obviously the degree of filtering you will have to do to keep the spurs
manageable is up for debate... with respect to mechanical resonances of the
filament structure I suspect these would be an issue if the filament was driven
from AC at some frequencies, I suspect a little residual ripple of perhaps a
few mV superimposed on a dc supply would have minimal effect.
Sadly I have in the past done mechanical resonance experiments on lamp
filaments, at the time I worked for GEC Osram, a very conventional strip type
lamp suffered an inexplicable short life in many applications. Running one on a
vibrating table at swept frequency showed a filament resonance at a low
harmonic of our mains frequency, It seems that the filament buzzed itself to
death in the earths magnetic field when powered at 50Hz. I suspect that a
tube/valve heater being more robust and operated at a much lower temperature
could have a mechanical resonance somewhere in the audio spectrum as suggested
by another contributor, I doubt it would be up at the typical switch-mode
frequency though.
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