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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
kijanki's avatar
kijanki
4,475 posts
 
erik_squires, +1, It is important distinction - chassis ground is different from the signal ground (in IBM computers system and chassis grounds is the same causing all sorts of problems).  It was mentioned here that IC might not provide adequate grounding thru other components because of the lower IC gage (they might burn).  Most of the time grounds are separated with small capacitor (0.01-0.1uF) and high value resistor (>100k) between them.  The only case I know where IC might serve as grounding is XLR IC with shield connected on both ends (assuming gear uses chassis ground for the shield). It is still dangerous since it can be disconnected causing possible electrocution.

Erik is right - I feel much safer with three prong wall outlets.
erik_squires
16,841 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
16,841 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
jetter
3,114 posts
 
In order to make my Dynaco st70 safe to use, I would need to have a three prong plug attached?  Easy to have done, but just asking if that is the solution?
erik_squires
16,841 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
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