2. amps-preamps
  3. 126866-10110

Cheater plug safety

So I recently decided to swap out my subwoofer amp for another one I had to see if it worked better with the driver. It did, but I have run into a slight problem. My old sub amplifier had a captive two prong cord. The new one has a 3 prong cord.

The subwoofer amp resides on a different AC circuit than the rest of the system. (Can’t be avoided)

Although the new amp is an upgrade for the sub, there seems to be a ground issue that is hazing up my transparency and sound stage. The system looses some air when the sub amps ground isn't lifted. There is no hum problem through any speaker per se, but just an overall sonic degradation when the ground is left in place. Using a cheater plug just on the sub amp brings back The transparency and it sounds absolutely beautiful.

I read that this can be dangerous, though. (Then why are these created plugs made?) anyway, I also read that as long as the amplifier with lifted ground is connected to the rest of the system via interconnect , and the other components are grounded, then the amp will use the ground from the other circuit that the other equipment is connected to and you are "safe", and only some very unlikely happenings, such as an internal ac wire becoming loose and touching the chassis and standing in a puddle of water should cause a concern. I even had a parasound 750 amp in my college dorm that even suggested using a cheater plug if needed for hum, sooooooo.........

At any rate I am addicted to the sound at the moment and going back without the cheater plug just sounds dull and fuzzy. The system is resolving and shows just about anything you do to it. Mind you, it’s not a night and day difference that would say that there is something wrong with any of the equipment, just a subtle but very tangible improvement that I definitely like and desire. The equipment is working 100% properly.....

It sounds so much better than the old amp and the transparency and air I’m getting right now are rediculous, but I don’t want to get shocked, but the chances seem to be very slim. Can anyone advise?
audiolover718

Showing 10 responses by erik_squires

erik_squires
16,590 posts
 
The issue with the safety ground vs interconnects can be summed up this way: 12 gauge vs. 22 gauge.

Safety grounds must carry the full line current in order to trip the breakers safely. This could melt an interconnect ground, possibly explosively, causing a fire, or leave lethal voltages on the outside. Some subs (like Hsu) are double insulated though, so they safely avoid the ground pin and associated problems.

So NEVER remove the third prong, and don't buy the Pangea crap which allows you to either.

What you could/should try though is a balanced interconnect with the ground lifted at the sub end. If you can do that you'll be golden. Also look for other ground sources such as cable TV or PC's which could in fact be the root cause. Disconnect them to see if the problem goes away, if so add an appropriate isolator.

Best,

Erik 
erik_squires
16,590 posts
 
There is a little confusion in what "lifting a ground" means.

It’s perfectly safe to lift the signal ground. That is, the ground associated with RCA or XLR cables and plugs. Pro gear or devices with XLR inputs often provide this feature. It’s a very good thing and sometimes the most important reason to use XLR inputs.

The AC safety ground is not the same thing. It starts at the round middle pin on the AC plug and terminates at the metal chassis. Lifting, or removing the AC safety ground is NEVER a safe thing. Not all equipment requires it, but if they do, it should not be removed when present.

Cheater plugs originally had a good purpose. They were designed so you could screw the ground to the face plate screw on wall sockets without having to replace the entire receptacle. However, many of these receptacles weren’t grounded to begin with, so the screw was pointless, and no one uses it correctly anyway. :) So, effectively, @bpoletti is right. Leave them at the store. If you have 2 prong wall outlets then it is time to upgrade them anyway, as your wiring is probably 40-50 years old.

Best,


Erik
erik_squires
16,590 posts
 
kijanki,

The Ebtech Hum X is the only possibly safe cheater plug I know of. It lifts the ground unless the ground gets to a few volts, then it closes it, restoring the function of the safety ground:

http://amzn.to/2fIvyM7

The signal shield should NEVER be used as a substitute.

In-wall wiring, like IEC power cables MUST use a ground of equal guage to the power conductors. For instance, you may not use a 22 guage ground with a 12 guage power cables. It’s not allowed because it’s completely unsafe. You must provide low enough impedance AND heat resistance to be able to trip the wall breakers without melting the conductors. That’s a 15-20 A rating, but in the milliseconds between the short occurring and the breaker tripping you could have momentary current of around 100A. Try that through an RCA/XLR socket or cable and you would literally have explosive results while, at the same time, failing to trip the breaker, leaving a lethal voltage at the case, and possibly starting a fire. A complete safety mess.

Over the years manufacturers have used a variety of schemes to tie signal and safety ground together, often through a high value resistor, or capacitor. Some even use an floating signal ground, which is perfectly fine too and often the least noisy. However it should never be treated as the same by end-users, and one cannot substitute the signal ground for the AC ground.

Again, the usual culprits are outside antennas/cable TV and PC’s. Fix those and 95% of ground loop issues disappear.


Best,


Erik
erik_squires
16,590 posts
 
I've been meaning to put together a blog post about this, please find my latest here, which discusses a range of problems and inexpensive solutions!

http://pqltd.blogspot.com/2016/11/quick-safe-and-easy-fixes-for-ground.html
erik_squires
16,590 posts
 
Hi Jetter!

I would be a little careful retrofitting. Designers tried a lot of things before 3 prong outlets became standard and added all sorts of odd solutions to attach the case to the AC line.

I would probably leave yours alone. I'm not an expert, but I can just envision a series of problems. :) I'd talk to other owners or techs who have experience with your particular model.

Possible problems may be that your input grounds are in fact connected to the case ground. This should be disconnected before attempting the retrofit, and if there is any connection it would be star grounded at the transformer's center tap.

Best,

Erik
erik_squires
16,590 posts
 
I think I see the issue. :)

Nominally 15A receptacles can do 20A... I know it’s weird. :) The issue is the socket vs. the current capacity.

The straight pin sockets are not designed to accept 20A plugs, but ARE designed to be connected to a 20A circuit. That is, a circuit with a 20A breaker and 12 ga. wire. Weird, right? :)

This is designed in because using a 20A circuit is super convenient. You can use it to feed a pair or more 15A sockets so that for instance, you can power up to 2 x 10A devices without tripping.

Best,


Erik
erik_squires
16,590 posts
 
@cleeds

I in this case I believe you are mistaken. :) One 20A circuit may feed multpile 15A receptacles.

For those who don’t know, the 20 A receptacles look just like a 15A but with one leg T shaped, allowing for either 15A (normal) or 20A plugs.

More here:

http://diy.stackexchange.com/questions/12763/why-is-it-safe-to-use-15-a-receptacles-on-a-20-a-circui...

It’s perfectly safe to put a 15 A receptacle on a 20 A circuit, but it won’t have the T socket for 20A devices. In residential use these are actually pretty rare.
erik_squires
16,590 posts
 
Only a qualified tech should attempt this, but it appears possible that the signal uses a floating ground and that the chassis and power supply ground are not bonded together. If this is true, then I would recommend:

  • Use standard 3 pin plug
  • Use shielded IEC cable. Most IEC cable is 3 conductor but not shielded. Connect shield at plug, not amp. Connect ground at both ends, of course.
  • Move switch and fuse to the same "hot" side of plug.
  • Bond chassis to AC safety ground.
You don’t want any voltage inside if either the switch or fuse is open. Having them on opposite sides of the incoming AC allows for the small possibility of one or the other opening, but still having 120VAC inside.

Best,


Erik
erik_squires
16,590 posts
 
GB,

You haven’t had a problem because fortunately equipment is reliable, but you are defeating a built in fail-safe. Please don’t do it and don’t suggest it to others.

It's like the guy who never wears a seat belt, because hes' never had a problem without it. :)

There are much better ways to do this.

Best,


Erik
erik_squires
16,590 posts
 
Arcticdeth,

One benefit MIGHT be if you got a shielded power cable. Even if you don't connect the ground at the amp side, the shield will reduce RFI noise.

AND yes, use a good surge protector. :)

Best,


Erik
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