Grounding Question

Absolutely not.

The issue is the amount of current the AC cable is rated for vs. interconnects.

The AC ground is sized so that if a short to the chassis occurs it can safely blow the fuse/breaker in it or at the electrical panel.

The interconnects, and all the little printed circuit board traces are designed for nearly zero current.  A ground fault with only the ICs for grounding is likely to start a fire at the IC cable, or melt it and leave the high voltage at the chassis.

I need to clarify a little.  In many cases, the chasis and signal ground are connected, so if you used a continuity tester or Ohmmeter you would see little to no reistance from the chassis to the outer shield.  Not always though, many either put a floating ground or a resistor to keep the two from being exactly the same.

But generally speaking, yes, there is a current path from the chassis to signal ground.  No, it is not a safe substitute for the AC cable ground.

@chayro Lifting the AC ground is a thing that people do to eliminate ground loop buzz. Its an indication that the grounding scheme of one or more components in the system isn't well thought out.

You can test for this sort of thing using a Digital Voltmeter set to the Ohms scale. The test applies to equipment having a metal chassis.

You connect one probe to the RCA ground connection and the other to the chassis. If the reading is the same as when you connect the two probes to each other then the equipment you are testing has a grounding problem.

Older equipment like the Dynaco ST70 didn't have a 3-prong AC cord. So on account of being vintage it gets a Murphy. But for newer equipment with a 3-prong AC connection there really isn't any excuse.

How does new equipment designed without a ground in the power cord play into this?

@atmasphere  can you elaborate on your last statement bout two pronged older equipment? I have an amp when you turn it on you get a ground loop hum hd it goes away when the preamp fully turns on. 

Thank You @erik_squires!
Finally, someone with a little snippet of Electronics Knowledge.
You just brought up one of my Pet Peeves. Just about every one of the Commerciale Power dist. box manufacturers will try to convince you that they can carry sufficient current for your equipment and yet they almost all exclusively use just a tiny copper trace on a circuit board to deliver all of the power.
On another point I agree that you should NEVER depend on your inner connects to carry the ground. Though most of the time the transients that they do carry can safely handle, it is definitely insufficient to carry a serious load. BUT Most equipment that doesn't haver a dedicated ground wire/pin do utilize the Neutral side of the power cord as a pseudo ground. There is lots of debate over how much is enough or too much grounding. You can go too far and actually create ground loops which will actually DESTROY your sound.

Modern electric devices without a ground prong on the plug are double insulated. They're constructed in such a way that they're safe without one. Audio aside, this is my preference for any power tool.

BUT Most equipment that doesn’t haver a dedicated ground wire/pin do utilize the Neutral side of the power cord as a pseudo ground.

@esarhaddon 

Not true now. Today if you have double insulated equipment there’s no legal/safety need to have an AC ground. The neutral and signal grounds in those pieces of gear have NO relationship.

This USED to be true with older gear and appliances like dryers, etc, however even those appliances now must be 4 prongs. My quite modern (electrically) sub has 2 prongs, but a wooden case so no chance for a short to the chassis.

 can you elaborate on your last statement bout two pronged older equipment? I have an amp when you turn it on you get a ground loop hum hd it goes away when the preamp fully turns on. 

@dinov That doesn't sound like a ground loop at all.

Normally the procedure for turning on any audio system is to power up the preamp (or in pro audio systems, the mixer) first. After it has stabilized then you power up the amplifier. In this way you avoid buzzes, thumps and the like as the preamp turns on.

When the preamp is off, it can act like an antenna for buzz and hum. When its turned on, its a much lower impedance so the antenna thing goes away.

erik, I'd bet that your sub has more to it than merely a wooden case. The electrical portion is likely isolated on plastic. Wood isn't a suitable insulator.

If equipment is designed with 3 prong plug (Class I Equipment) case is grounded protecting user from dangerous voltages.  Grounding the case thru interconnect to another case is not only unsafe because of the limited current interconnect can carry, but also for the simple fact that interconnect can be disconnected making device unsafe.  Two prong plug devices (Class II Equipment) offer user protection by double insulation.  This insulation has to withstand 3000VAC.  Wood alone is likely not suitable insulation since it can absorb moisture.

Ground loop hum would not go away as you describe. You don't have ground loop hum. You have another problem. And your question about the interconnect is complicated.

All components even with just two plugs are grounded. It is not a Murphy, it is just the way electrical panels are wired. Black is power, white is neutral, or utility ground. Millions of homes and older appliances wired this way, perfectly safe. The third plug is earth ground, redundant since neutral is already ground.

Vast majority of components every speaker output and RCA in and out, they are all grounded to the same chassis ground. Look inside, all the RCA are wired together. Ditto speaker terminals. Since this is the same on both components then yes connecting them with the interconnects connects the grounds and if one is earth grounded (third prong) then both are.

Where it gets complicated is the whole point of that redundant extra earth ground (third plug) is extra safety, which in order to work it needs to be able to carry a lot of current. Which is why the ground wire is always the same gauge as current. Not so the interconnect. So yes it is grounded, but no it isn't. If you think this correct answer is hard to understand, just wait till you see how thoroughly messed up it gets from here. Electricity questions are just the worst! Makes me want to bang my head into a Max Headroom sign.

@erik_squires

I don’t know where You got your Electricians license or your Electronics Eng Degree, but I took my training at USC! Now, I do admit that I did take my schooling a long time ago but US Electrical Code has changed very little and Electronics has not done anything other than to Improve on a theme. If that confuses you, Well, you can’t be any more confused than I am over your statement. You mixed metaphors so badly you were almost impossible to follow. Talking about 4 prong plugs. I hate to tell you I don’t know of ANY city in the US that uses a 4 prong plug for 110v circuits. SO your 220v and 440v discussion is irrelevant, at least for those of us in the USA. Then talking about a wooden case, HOW does that even enter into the argument. The standard for electrical plugs is the NEMA 1-15 plug.

Now I just did a continuity test on a 2 prong IEC C7 socket to chassis on my not too old SACD player and it OHMed out at near ‘0’! Similar on my computer chassis to motherboard. I will agree that on the computer, they have through the last 20 years played with isolating the motherboard from the chassis, but that is no longer the case. Also I am not saying that it the Neutral side of the IEC C7 plug is meant to function as a full on Ground, for overload purposes. There exists Two standards for the socket for the IEC C7 and one is reversible and the other is keyed for polarity. Even on my BRAND NEW NAD, the IEC 60320 plug doesn’t have an active ground pin so the 3 prong cord is not even active.
None of you statement seems to be germane to the conversation.

Devices fitted with two-prong plugs often have a signal ground available (e.g., a dedicated ground lug on the rear panel, typically used with turntables, in many cases), which should be utilized and should eliminate hum. If not fitted with such a connection, verify chassis (using an ohmmeter) is electrically tied to low side of input/output connections (i.e., very near 0 Ohms) and connect components together using an external wire or braided cable. If there is more than one (common) path to the power line ground for your audio equipment, you will likely have hum and/or perhaps a potential shock hazard. If your system includes devices that employ both power and signal grounding schemes, you'll need to trace out which is which to eliminate hum and/or shock hazards, if they exist.

The low side of RCA (unbalanced) connections are typically tied to ground (power or signal, depending on construction) internally, with balanced connections having a dedicated ground. These grounds are signal connections and shouldn't need to carry excessive current as mentioned previously.

Also keep in mind that for 120 VAC in the US, the neutral (white) is tied to ground at the electrical panel. Many years ago, my folks purchased a cheap, Western Auto stereo (not double insulated) with a two prong plug. When my brother and I used it, the chassis would 'float' at 120 V, shocking us if we were grounded. If the plug was reversed, operation was normal.

Good Luck!

My friend had used a cheater plug for a decade on his RAM-9 amp.  I told him to remove it and also replaced a very old cheap power outlet (I gave him a Black Synergistic Research plug).  The difference in his system is phenomenal.  Never use a cheater plug on tube amps or pre-amps.  Probably never use them because they usually are made of inferior/garbage quality contacts, case, etc.  

If it came with a two prong power cord get an AFCI/GFCI combo receptacle. Sure are a lot of clowns on this site!

I do admit that I did take my schooling a long time ago but US Electrical Code has changed very little

Are you kidding??? 🤣

Well, it changes less often than the weather, lol.

As mentioned, a star ground for all components is recommended. In reality, trial and error may reveal less groud loop hum with some components not connected.  It becomes more magic than science, the scientific reason being discovered after the fact.  It is worth going through a "finished" system with a scope on the output to get a few dB more S/N.