In the end, I'm left with the feeling that simply double-insulating the ac wiring and isolating safety ground from chassis could very well be a solution that can be directly applied to most if not all designs without sonic penalty, but the thread would seem to leave at least a little bit of doubt about that.Hi Al . . . actually I've experienced at least as many grounding-related problems with "double-insulated" equipment as those with safety grounds. This is because they still have leakage reactances in their power transformers, and thus AC-related currents will still flow between their chassis -- this will now be flowing wholly through the interconnect shields. And for unbalanced interconnects, the shield resistance then becomes the primary dictator of the attainable hum level.
On top of that, it **compromises** the sound rather than improving it. Another way of looking at that is that audio components really don't like to be grounded on account of noise in the AC ground and also because of ground loops. If, OTOH, the ground *floats* at AC ground potential, the noise issue is gone and so is the ground loop issue.So much of the terminology commonly used with grounding ends up implying that "noise in the AC ground" is somehow external to the audio system, and evilly injected through third-prong grounds. This is exactly the opposite of what's happening - it's the audio gear itself that's producing the noise current. When it flows through a path that's shared with the audio signal, we then hear it. Taking away the draining path away by using a cheater plug simply alters the magnitude and/or path of the offending part of the flow, frequently transferring it from wiring to the operator. That last bit is why it's dangerous.
If you have to use cheaters to get rid of hum, then the problem is that the equipment itself is incorrectly wired. The problem is that the chassis ground and the circuit ground are the same thing, which means the equipment is wired with a ground loop. IMO, you should send the equipment back to Mac and have them fix it,I wholly agree that most hum problems in audio equipment are caused by poor grounding choices inside the gear. The classic expose of this is an AES paper by Neil Muncy from the early 1990s, where he coins the term "the Pin 1 problem".
The "pin 1 problem" refers to the ground pin on an XLR connector, and the idea is that simply flowing AC current between pin 1 of any two (or more) XLR connectors on a piece of audio equipment should NEVER cause any change in the noise floor of any of its outputs . . . and in practice this should include unbalanced connector grounds and AC safety grounds. If I had to guess, I'd say that at least 75% of all consumer and professional audio products suffer from a "pin 1 problem" to a certain degree.
I'd also have to say that from my direct experience, many McIntosh products suffer from "pin 1 problems", including the C39 and MC7150.