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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: RE: Then this device is tuned for audio-band noise. posted by tomsyl1 on June 4, 2008 at 11:36:38:
I'd love to see a spectrum of noise taken from my AC line, but I don't have access to the instruments necessary.
Power transformers do vary in their low-level audio-band frequency response. I have an old Eico signal generator (20 Hz to 200 KHz, with about 20 volts maximum output), and my portable DVM seems to have wide-band response for AC voltage measurements. I've connected various old junker transformers to the signal generator and observed variation in the roll-off frequency from about 5 KHz to above 60 KHz. Interestingly, the widest frequency response I observed was in an E-I core transformer with multiple secondary windings (I have no idea what it came out of). I don't have any junker toroids, but I would expect them to have much wider frequency response. The Paul Speltz ZERO autotransformer has a bandwidth over 1 MHz, I believe.
The frequency response will depend on the details of transformer construction which include parasitic capacitances and inductances as well as core characteristics. Electrostatic shields will reduce capacitive coupling from the primary to the secondary windings. However, connecting the shield to ground may create trouble, as the "ground" is not really at earth potential for all frequencies of interest.
The actual frequency response of the transformer would have to be measured, as general rules of thumb do not seem to work very well. The cleverness in the Noise Harvester design is that noise frequencies above the transformer roll-off will be shunted in the primary winding, and dissipated if it is sufficiently lossy.
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