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Re: [Amps] Amp causing RFI

Subject: Re: [Amps] Amp causing RFI
From: Jim Brown <>
Date: Tue, 4 Feb 2020 20:13:22 -0800
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On 2/4/2020 7:23 PM, MU 4CX250B wrote:
There’s no such thing as a ground loop?That is not correct, Jim! Of course, ground loops exist, along with substantial documentation on how to minimize them. I first encountered ground loops in grad school at Cornell, when part of my dissertation involved measuring nanovolt DC voltages in ultra pure metals at liquid helium temperatures. I’m sure you know that ground loops are a major and common source of hum in low level audio signals.

What you're describing are instrumentation systems, not radio systems.

A few years ago I wrote up some notes on ground loops for builders of my StationPro controller kits. Here’s a link to my writeup which explains what they are, what causes them, and how to minimize them. They can be nasty little buggers.


What you're calling ground loops in this context is part of trying to fix failures caused by improper bonding. BTW -- I spent much of my professional life in pro audio, which regularly works with 30-40 microphones where 120 dB or more of dynamic range is required, and where mics produce a signal that's often in the low nanovolt range. To appreciate how difficult this can be, any noise that is coherent (for example, hum, buzz, RFI that's the same on each mic line) will sum as the square of the number of those coherent signals, while signal, which is different at each mic, sum in proportion to the number of mics (6 dB/doubling). This means that signal to noise degrades in proportion to the number of mics in use (3 dB/doubling).

I suggest that you take a look at my analysisof the problem, which is really caused by leakage currents in the power system. And I first saw this analysis by Bill Whitlock, who by analyzing noise in a balanced circuit as a balanced Wheatstone Bridge, caused the IEC Standard for CMRR to be revised. Bill, Neil Muncy, and I were elected Fellows of the AES, in part on the basis of this work.

The false concept of of a "ground loop" has caused thousands of hams and high futility nuts to do the absolute wrong thing to kill noise that is the result of power line leakage currents.

A detailed analysis is beyond the scope of an email reflector, but it's in that link I posted.

73, Jim K9YC
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