AC Comb Filter for VLF Reception

While experimenting with digital comb filters on my computer, it dawned on me that I could use a simple, cheap device to accomplish the same effect. The device I discovered is really simple; a reverb. The reverb reduces or eliminates AC hum by cancelling out multiples or harmonics of the 60 cycle hum. Most any reverb will work, however it must be designed to run on battery power. The reverb I found was an old Realistic Reverb from Radio Shack (model 32-1110A). It is battery powered (six AA cells) and it has mic level input and output - Perfect for VLF receivers!

I have not tested the reverb in the field as of yet, however, test results on the bench are pretty amazing. It does not completely filter the entire multitude of harmonics present on a recent recording, however it does have a dramatic effect on them. The sample below shows the effect of the filter. The delay control sets the filter frequency. It is very difficult to adjust, a fine control for the delay would come in very useful. The reason is because I am trying to adjust the phase of the hum to overlap perfectly on top of itself thereby producing a nulling effect.

Here are the results:     


Audio Sample of above:  reverb.wav (650 Kb)




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