Lifting a ground connection is not in itself unsafe (many amps like the Parasound have ground lift switches). But I would be sure your sub has an input transformer. Many new designs do not. If not, there could be a safety issue, especially if that third ground prong is connected to a metal part of the chassis. BTW, the Parasound amps do have input transformers. So without modifying other parts of the circuitry, you may end up making a choice between safety and sound quality.
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?
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?
45 responses Add your response
@kijanki is absolutely correct. If a device has a plug with a safety ground, it's a Class 1 appliance and defeating the ground poses a safety risk. If you have a noisy ground, lifting the ground isn't the solution. Star grounding can help, as can devices such as the Granite Audio Ground Zero. |
jetter ... does older equipment such as a Dynaco ST70 that has its original 2 prong plug have a greater shock potential? Yes, it actually does have a greater shock potential, because the ST70 was obviously a Class II device. This is why the three-prong, separate safety ground standard was adopted. The Dynaco is sort of like driving a '63 Chevy. It's safe to drive if it's in good repair, but it lacks seat belts, ABS and air bags. |
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 |
Probably more so if the amp’s old AC wiring and power transformer are original. What are the chances of a user receiving an electrical shock if an internal hot to chassis fault were to occur? It all depends on the circumstances of where the amp is located and associated equipment it is connected to. It also depends if the amp and all other associated equipment is plugged into the same wall receptacle outlet. Examples where a hot to chassis fault has occurred: First for the user to receive an electrical shock he must place his body, or some part of, in series with the hot chassis and an earth grounded object. In this case the user completes a ground fault circuit path from the Hot chassis through the part of his body that is in contact with the earth grounded object, back to the source the electrical panel grounded service neutral conductor. Current will pass through the user’s body and he will feel the shock. Grounded objects? Is the floor bare concrete? As in a basement? In this case if the user was in his bare feet and touched any metal part of the amp the user would receive an electrical shock. On the other hand if the amp was on an insulated floor like wood he would not. His body would be insulated/isolated from a grounded object. Like a bird perched on an overhead high voltage power line. Back in the day the Dynaco ST70 was first designed what other associated equipment that might be connected to the amp by ICs used a safety equipment ground? Was there even a 3 wire grounding type wall receptacle back then? What other grounded objects, within arms length or other parts of the body, could the user come into contact if he touched the hot chassis of the amp? Say the user has a preamp that uses the wall receptacle safety equipment ground. The outer case/enclosure of the preamp is metal. Maybe the knobs are metal. There is now a grounded object in close proximity of the user for the possibility of the user to come into contact at the same time while touching the metal chassis of the amp. But wait. The signal ground of the ICs will ground the equipment right? Maybe, maybe not.... First I would never rely on the small signal ground wire of an IC to carry the high current of a dead short ground fault back to the source. For a typical 20 amp branch circuit the initial ground fault current can be over 100 amps before the breaker can react and hopefully trips open breaking the ground fault circuit. What if the user buys a new pair of ICs. What happens when he disconnects the second IC from the first RCA jack? Is there a chance his fingers could come into contact with the outer metal shell of the IC and the metal case/enclosure/chassis at the same time? So what if the preamp has a ground lift switch that lifts the signal ground from the safety equipment grounded chassis? The false feeling the IC will protect the user from an electrical shock is gone. Bottom line, just be careful if you are using old vintage 2 wire cord and plug equipment. Especially if the plug is non polarized. When changing ICs unplug the vintage equipment power cord from the wall receptacle outlet first. . |
Thank you for the explanation jea48. Actually I have been zapped once by the unit, but it was my error. I was setting the bias, which of course you do with the unit running, and touched the wrong part of the amp (i.e. the exposed parts around the 7199 driver tubes). Its one of those things you only want to do once. |
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, +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. |
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 |
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 |
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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audiolover, Hopefully the responses posted on this thread answered your questions in your original posted message. Just a guess you will continue to use the ground cheater on the sub amp because it eliminates the ground loop hum. You are not the first to experience a ground loop hum problem with a subwoofer amp, that uses a 3 wire cord and plug, and you won’t be the last. Jensen transformer does make an isolator you could install between the preamp and the sub woofer amp that should/will break the ground loop circuit, thus eliminating the ground loop hum. If you are worried about it degrading the SQ of the sub I doubt with the Jensen isolator you would hear any difference. http://www.jensen-transformers.com/product/sub-1rr/ I would recommend the Jensen transformer isolator over the ground cheater. But if you must use the ground cheater a GFI receptacle in place of your existing wall duplex receptacle would be a safer option. You could add a level of personal protection from any risk of a life threating electrical shock by changing out the existing wall duplex receptacle outlet to a GFI. A GFI receptacle does not need an equipment ground to function/operate as designed. Just buy a good GFI duplex receptacle outlet, not one of those really cheap ones. I have seen the cheap ones fail and not trip open when called upon. Leviton makes a pretty good GFI duplex receptacle. If you do decide to change out the existing duplex receptacle to a GFI replace the old one with a GFI of the same ampere rating. So if the existing is a 15 amp, more than likely the branch circuit is a 15 amp, you must use a 15 amp rated GFI duplex receptacle. You cannot install a 20 amp receptacle on a 15 amp branch circuit. A 20 amp receptacle can only be installed on a 20 amp branch circuit. You can install two or more 15 amp receptacles, (a duplex is two), On a 20 amp circuit though. . |
jea4811-07-2016 10:00am You can install two or more 15 amp receptacles, (a duplex is two), On a 20 amp circuit though.You can install a 15 amp receptacle on a 20 amp circuit, but you shouldn't and it violates NEC. A 15A device isn't rated to carry the 20A of current that a 20A branch circuit can deliver. |
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?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. |
@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. |
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 mmm |
erik_squires1m ... One 20A circuit may feed multpile 15A receptacles ...Strictly speaking, it is not safe. It is against code. Consider an improperly functioning 15A appliance that is now trying to pull 20A of current through the 15A receptacle. The 20A breaker will not trip, and the 15A receptacle will overheat. |
jea481mPerhaps you should read the code that you referenced: " A single receptacle must have an ampere rating of not less than the overcurrent device protecting the branch circuit." You cannot use 15A devices on a 20A circuit and be compliant with NEC. |
Perhaps you should read the code that you referenced: Cleeds, Please go back and reread my post. I said two or more 15 amp receptacles can be installed on a 20 amp branch circuit. A duplex is two receptacles. Trust me it does meet NEC code. It is done all the time in commercial office buildings. Jim |
jea48 Trust me it does meet NEC code 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." Perhaps you'll trust Bob Vila more than me. If so, read this. |
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 |
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.
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. . |
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? |
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 |
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! |
Not sure if this is the schematic wiring diagram for your amp. http://diyaudioprojects.com/Schematics/Dynaco-ST70-Tube-Amp-Schematic.htm If it is you will notice the 120Vac power line side of the power transformer does not use a noise filter capacitor from either AC power line connected to the chassis. Given the age, if original not restored, to me that is a good thing from an electrical AC leakage to chassis standpoint. Why? Is the male plug on the end of the power cord polarized? Meaning it will only plug into the wall power receptacle outlet one way. If not you would have a 50/50 chance of having the noise capacitor fed from the HOT conductor from the wall outlet. If the capacitor was bad, leaking more AC to the metal chassis than it normally should, the chassis would be HOT with respect to earth ground. Depending on the condition of the old capacitor could/would have an impact on how much current could pass from the chassis through your body to a grounded object, your body might come into contact with. It only takes a few milliamps for the body to feel the sensation of the current passing through the body. 4ma or 5ma you know you are definitely being shocked. So with no noise capacitor on the AC Line, that eliminates THAT possibility of a HOT chassis. The 3 wire power cord and grounding type plug would be the safest way to go. But like Ralph said, doing so could create noise problems and possible ground loop hum. You would be safe, but your audio system could sound like crap. As I posted in an earlier post, GFCI protection will also protect from a lethal electrical shock. Note I said lethal. A GFCI trips open, (is supposed to), when it senses a hot to ground current flow around 5ma to 6ma. // Speaking of noise... Have you ever measured the AC voltage potential from the metal chassis of the amp to the wall receptacle safety equipment ground contact? If not, you should check it. First the ICs from the preamp need to be disconnected from the amp. The chassis of the amp can not be connected to any earth ground. * Turn on the amp. * Measure the voltage from the amp chassis to the equipment ground contact of the wall receptacle. Note the voltage reading. * Turn off the amp. Reverse the AC power plug in the wall receptacle. * Next. You might want to wait about 5 minutes for the electrolytic caps in the power supply to bleed off before turning the amp back on. Turn on the amp and check the voltage from the chassis of the amp to the wall receptacle equipment ground contact again. * The lower of the two voltage reading is the correct polarity orientation for the plug. Mark the side of the plug that aligns on the same side as the neutral contact/neutral conductor of the wall receptacle. What you are actually doing is connecting the AC power hot and neutral lines to the primary winding of the amp's power transformer for the proper polarity orientation. The lower of the two voltage reading can lower the noise floor of the amp and the overall sound from your audio system. . |
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:
Best, Erik |
but it appears possible that the signal uses a floating ground and that the chassis and power supply ground are not bonded together.Eric, What power supply ground? Equipment today that have a 2 wire power cord and polarized plug, that use double insulated AC power wiring, does not have an earth ground. In the world of this audio equipment the metal chassis and metal enclosure that houses the chassis is it’s ground for the power supply of the secondary and signal ground is all it knows. Am I wrong?
Agree. That could be a possible problem, IF..... Jim . |
my sunfire sig has only the two prong plug, no ground plug. so putting a three prong adapter will essentially do nothing ? correct? also the house wiring from box to outlet is basic thin stuff, commercial wire. adding an expensive power cable will also do nothing. as if theres 300 feet of garbage wire, how do you expect to get better sound by adding another 4 foot of high quality wire? don't make sense, now if you wired your house with all 8 gauge to your dedicated 20 or 30 amp breakers, and used a good power cable, i can see an upgrade. other than that, its a waste of money |
I haven't read all the posts, but I've been lifting grounds for 35 years, and have not had a problem. From what I was taught (in High End audio), the ground should be at the preamp, and you lift the ground at other places. The thing is, all cheater plugs now are made with one side polarized. That didn't start happening 'til the late 90s, I could always find a cheater plug with two small prongs, so I could turn them whichever way I needed. |
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 |
gbmcleod ... I've been lifting grounds for 35 years, and have not had a problem. From what I was taught (in High End audio), the ground should be at the preamp, and you lift the ground at other places.I agree with @eric_squires: you are defeating a built in fail-safe. Please don’t do it and don’t suggest it to others.It's simply hazardous to defeat a safety ground. Period. |
