2. amps-preamps
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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
erik_squires
15,788 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
jea48
4,303 posts
 

cleeds said:

The NEC is updated every three years and the source you previously cited is 14 years old. Sorry, but using a 15A device on a 20A circuit is not compliant with current code. I don’t doubt, however, that "it is done all the time."

Yes, I know the NEC is updated every three years.

//

From my first response to you when you questioned what I posted to the OP.

Cleeds,

Sorry, you are wrong. It is NEC Code compliant.

2014 NEC 210.21(B)(3) table. Receptacle Ratings for Various Size Circuits.


Page 70-59


That’s right out of my copy of the 2014 NEC edition.

//

As for the link I provided you that is 14 years old, it is exactly the same information as found in the current 2014 NEC. I provided you the link because I thought it would help a layman understand the subject better. I though the author did a good job.


All 15 amp rated duplex receptacles have a pass through rating of 20 amp. In most cases spec grade or better duplex receptacles use 20 amp contacts. The only difference between a manufacture’s 15 and 20 amp duplex receptacle, (for the same style/series number), is the face plate. Example, the 15 amp receptacle will not accept a 20 amp rated plug. It won’t fit.... Just use a flash light and look inside the face plate of a 15 amp duplex receptacle, (neutral side), of a spec grade or better outlet. Good chance you will see the "T" 20 amp contact.

//

As for your Bob Vila link. The only guy that knew what he was talking about is Billhart.

.

jetter
2,959 posts
 
Ralph, you mentioned above regarding the Dynaco ST70....
"No- not saying you can't do it, its just that its more complex than that. The amp employs some chassis grounds that would have to be modified. This is the sort of project that should be done by someone that understands Class 1 grounding and also understands the significance of ground loops."

I have not thought much about shock hazards before, except common sense when operating the units, and the person who works on my equipment (when necessary) has never brought up modifying it to prevent same.  Does the chassis ground you mention above lessen the chance of a shock occurring.  I guess I am asking is there anything really to worry about leaving it well enough alone?
jetter
2,959 posts
 
Hi again Ralph,
Sorry to hear about your auto accident.  A little less than two years ago, on my way home from work, I took 1/2 second to turn down the radio in my car.  Looked up and there were headlights one foot in front of my car, in my lane, coming right at me.  A 19 year old girl said she hit "black ice", here in Vermont.  Hang in there, time takes time.
George  
atmasphere's avatar
atmasphere
11,615 posts
 
Thanks!

I guess I am asking is there anything really to worry about leaving it well enough alone?
That amp has been around since the late 1950s. I would not worry about it.

The concern is usually about what happens if the power switch takes a whack and gets shorted to the chassis. The fuse should blow. But the kind of switch on a Stereo 70 is really really hard to damage in that sort of a way!
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