Eliminating hum

01-03-15: Grannyring
If a power cord would have a short to the chassis would'nt that trip a breaker in the home?

Hi Bill,

The purpose of the AC safety ground connection (which is what a cheater plug defeats) is to make that happen. Since equipment having 3-prong power plugs connects AC safety ground (the cyclindrical third prong) to chassis, a short between AC "hot" and chassis will cause a very large current to flow through the circuit breaker, resulting in the circuit breaker tripping.

If the AC safety ground connection is defeated with a cheater and that kind of a fault occurs, what is likely to happen is that the 120 volts will cause a current to flow through the shield and/or the return conductor of an interconnect cable to another component, and through that component's AC safety ground back to the service panel. However, depending on the design of the grounding configuration within each of the components a significant impedance, perhaps 10 or 20 ohms or more, will often exist (and should exist, if the equipment is well designed) between the circuit ground and the chassis ground/AC safety ground of each of the components. Chances are good that that resistance will be high enough to reduce the current flow to an amount that is too small to cause the breaker to trip.

In that situation what is likely to happen is that the interconnect cable will get very hot, creating the possibility that it might either go up in flames or ignite whatever it may be in contact with, such as a carpet. And of course the chassis would present a shock hazard to anyone who touched it.

Al, most amps are grounded at the first reservoir cap in the power supply etc...

That doesn't change what I have said, as that involves the amp's circuit ground, not AC safety ground.

BTW, I should add to my earlier mention of ways in which AC "hot" can become shorted to chassis the possibility of a short developing in the power transformer of a component.

... it will at least help you understand if it is a ground loop issue.

Agreed. Using a cheater plug temporarily as a diagnostic tool can be useful, and entail negligible risk.

It is also reasonable to say many things we do every day are far more risky. Things like riding a bike, taking a walk, ice fishing, etc...

Agreed. But it is each individual's call. My point, though, is that a statement that using a cheater is "completely safe" is misleading and incorrect.

Best regards,
-- Al

Thanks Al. I did just read this.....seems we have some cases of electrocution on record. We Aphiles should be aware.

" In the professional audio and video fields, the cheater plug has been identified as a serious safety problem. Its casual use as a method for avoiding ground loops in analog audio and video signals (to eliminate hums and buzzes) is dangerous.[4] Bill Whitlock, president of Jensen Transformers, writes, "never, ever use devices such as 3 to 2-prong AC plug adapters, a.k.a. 'ground lifters', to solve a noise problem!"[4] Whitlock relates how an electrical fault in one device that is connected to its electricity source through an ungrounded cheater plug will result in dangerous, high current flowing through audio or video cables. Whitlock notes that in 1997, consumer audio and video equipment electrocuted nine people.[4]

In reference to amps with a resistor between the circuit ground and chassis ground (usually with a parallel film cap to AC ground the circuit to the chassis), that resistor can be replaced with a inrush thermistor. That way the desired DC impedance between the chassis and circuit ground (it's there to monkey wrench ground currents) is maintained unless there is a fault. If that were to happen resistance drops allowing the breaker to trip.