Electric shock from power supply (and how to fix it?)

[seadi123]

Whenever im touching the strings on my guitar, and im touching the ground bare foot, i feel a small zap. I only feel it if i slightly touch the strings, bridge, or any part thats connected to ground. When i grab the neck and strings with me whole hand i dont realy feel it. I know the problem is the adapter, I have another adapter that works correctly but it is extremely noisy and unusable. Also, the current faulty adapter makes a lot of noise if im not touching the strings (or any ground point, like the pedal enclosure), and it goes away when im touching them and my feet are on the ground. I've already orderen another power supply but it is going to take a while to ship, i was wondering if there is anything i can do to fix my current adapter? I've already gone through 3 batteries and its not convenient like this. Thanks!

[R.G.]

You're describing a minor amount of AC line leakage. A major amount of AC line leakage would kill you when you do that.

Whether you can fix it or not depends on the nature of the supply and your technical skills. If it's a glued-together plastic box and has only two prongs on the case for plugging into the wall socket, chances are very dim. Even if you do crack it open to work on it, and even if you can dope out what to change to eliminate the AC leakage, chances are poor for gluing it back together.  If it's a power supply you made out of bits and pieces, you have a better chance of fixing it, and we'll start asking questions about the internal schematic and packaging.

There is one relatively sure cure: an isolation transformer. You're getting the leakage because one side of the AC line is "neutral" and connected to earth ground at your breaker box and out on the service pole. The "line" or "hot" wire then swings +/-176V peak, 120Vrms around the neutral. Any leakage from the line to the rest of the circuit is what you're feeling, because you're the human conductor between the "hot" wire and real, no-fooling planet earth ground. An isolation transformer breaks the connection of the neutral to planet earth, so the AC line to the adapter no longer has a path to the planet with you in it. Of course, you could just wear shoes and be careful about it until the right/good/safe adapter comes.

In any case don't go fooling around in there unless you have more knowledge of AC power lines, leakage, etc. We really would rather that you stay alive and/or don't burn down your house.

[seadi123]

I was just about to say that i measured 9.6v DC on my multimeter, but when i switched to AC it also measured 19 on the same spot. I'm guessing you are right about the ac leakage, Is it possible that this would have done any damage to the pedals?

[GibsonGM]

Yes, it's possible that a pedal got damaged...I think you'd know by ear if that happened though.  You will have to go thru them with your batteries installed and see if they perform OK.   

Don't bother to try to 'fix' the faulty adapter...you are more likely to make things worse, and perhaps shock yourself dangerously (fatally?).  We'd rather you wait until your new adapter arrives!!   Use batteries until then   

[amptramp]

You can get some protection at the guitar itself by breaking the direct connection from the strings and bridge to ground and insert a resistor of about 220K in parallel with a capacitor (rated for line voltage divided by a derating factor).  This provides DC isolation and some AC impedance to ensure the bridge and strings stay close to AC ground.

This does not solve the power supply problem and the noise generation when you are not touching anything; only the new supply can do that.

[Rob Strand]

I know the problem is the adapter

If there is a fault in the device I wouldn't use it.   It can actually be dangerous.

How many mains plug pins does the adaptor have?

The other side of the coin is the adaptor isn't fault, it's just like that.  That means you need to buy
a different adaptor.

If you want to continue with the device ...
The best interim solution is to connect the ground on the amp or effects to a mains ground point.
It will divert the leakage away.  Sometimes this can be done using an alligator clip from a 6.5mm jack to the metal case of a device that has a three pin plug.

What about adding a filter to the noisy adaptor you have instead?

[Rob Strand]

If it's a two pin mains plug:  You could build the little circuit in section 6.4 and test according to section 6.6:
http://www.ebme.co.uk/articles/electrical-safety/335-electrical-safety-tests

[PRR]

Please wear shoes!!

Working with electrics on concrete or dirt is extra dangerous. Concrete and dirt are not great conductors but conduct a lot better than your skin. YOU get most of the voltage leakage.

Your *amplifier* should be GROUNDED. It should have a 3-pin wall-plug into a properly wired 3-pin outlet. If you have this, and get signal through, then the guitar cord ground will divert leakage from un-grounded pedal power packs to ground rather than through you to concrete/dirt. Amplifier makers have really cleaned-up their act in recent decades: they hate shocked customers (and survivors' lawsuits). The amp should solidly ground your whole guitar chain. But can't, if you have busted-off the ground pin, used a 3/2 adaptor, or if the house wiring is incorrect.

[seadi123]

The adapter has 2 pins like most 230v adapters; i live in EU. I dont have an actual high voltage amplifier; i use a small lm386 Ruby variation. I have tried filtering the other noisy adapter with that Beavis Audio circuit but it doesnt do anything. I went to the local market today and i thought i scored this 9.6V used adapter for 2 bucks, then i came home, tested it and it measures 14v!

During the time i had the failty adapter i have built two pedals which i have only used with the failty one, so i cant tell if they are damaged because i dont know exactly how they are supposed to sound like. My basic understandin is that all passive components like resistors, caps, diodes and such should be ok. The only thing that could be damaged are transistors and op amps?

[deadastronaut]

you can get decent effects power supplies for around 20 quid which will

come with 6-10 way chain leads...

i use mine for testing, playing, breadboarding, never had a problem.

worth it for the peace of mind, and above all safety.

http://www.effectspedalpowersupplies.co.uk

no affiliation, other than being a customer...

you can get power supplies with 9v and 18v options too for around the same price on ebay etc...

be safe. 

[bool]

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   Electric shock from power supply (and how to fix it?)
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That would be a nice title to a AC/DC tribute band's mixtape!

[PRR]

> ...i cant tell if they are damaged

Never mind the pedals. It is YOU we are worried about.

Powered by what?? Another 2-pin supply?

You NEED a 3-pin somewhere. Your gear (and exposed metal on it) MUST return to wall-outlet Ground.

I worry especially about $2 thrift-shop warts, which may have way-excessive leakage due to age or abuse.

[anotherjim]

There should be a data label on the power supply.
For 2pin, you should have the "double insulated" square-within-a-square symbol. Double Insulated reduces the chance of the output getting into contact with the AC supply due to an internal fault. Generally, this is very safe, but if the psu has been involved in a serious voltage surge (lightning strike in your area?), the insulation can still be "punched through" and then you can have leakage from the AC input to the output.

If it does not say "regulated" it isn't. Pedals really want regulated. Amplifiers can usually get by with just the #2 type in the following list.

DC supplies can be either...
#1: Transformer + rectifier & nothing else.
#2: As 1 but with a smoothing capacitor on the output.
#3: Linear regulated. The ideal type.
#4: Switchmode (SMPS) regulated. Lightweight. May say "only for office/IT use" in which case it is too noisy/spiky for audio.

For #1, it is possible to measure both AC and DC. The AC is the ripple voltage.
For #2 it should measure very little AC off-load, but will show some as load increases as the smoothing capacitor can't fully charge up.
For both #1 and #2, voltage off-load can measure higher than the rated voltage. Rated will only be true at the rated current load. Plus or minus the variation in AC supply voltage.

To check for stray voltage on exposed metalwork, you need...
A known AC supply earth point. The metal of something with a 3pin AC plug plugged into an outlet (doesn't have to be switched on) can serve. Continuity test that it is connected to the earth prong of its AC plug to prove this earth point is true.
Connect a 100k to 150k as a "shunt" resistor across the voltage test terminals of your multimeter. If you don't add a shunt resistance, the high sensitivity of the meter input will pick up harmless stray voltage that will only confuse things.
With one meter probe on your earth point, test for AC & DC voltages with the other probe on the metalwork of your gear.

Handling potentially dangerous things just to find out if they are dangerous, can earn you a Darwin Award! So take precautions - insulated footwear, insulated tools, don't touch directly etc...

[seadi123]

Ok guys, the other psu arrived and its basically doing the same thing. It maybe the problem of the power outlet, i will test this in another house. Or maybe this adapter is also faulty, it's not a Boss original. It's a Soundstation psu10, it is actually "designed" for pedals (Soundstation is a guitar and amps company)

[seadi123]

Update: tried it at my neighbour's house, same thing

[R.G.]

There are many things that could be contributing to this.

First, there could be similar problems with your house and your friend's house if they were constructed before the ... um... mid 1960s. That was when conversion to three-wire was under way, and the retrofits were not always of good quality, or even done. There is at least some possibility that both houses have a similar problem. Maybe not a big possibility, but possibly.

Second, there are inherent compromises to be made in the design of a two-prong AC mains adapter. My day job includes responsibility for field support and new design of a high volume 9V adapter designed expecially for guitars, and I've run into many of the issues that can happen. In a switching power supply, there is always some leakage. This is because by law the adapter must limit how much switching noise can go back out onto the AC power line, which effectively requires at least one capacitor between the Line and Neutral AC wires. This is to shunt RF noise, but it also lets some AC leak through. Generally this doesn't leak very much into the secondary, but there is capacitive leakage of some amount between every two conductors in the universe. The inverse square law makes this very small except for things that are close together - as they are in a compact AC power adapter. This is layout and construction dependent. In the day job, I have to worry about these kinds of issues frequently, and we sometimes make manufacturing changes specifically to address one or another odd problem in the field. It can be difficult to separate design problems from manufacturing defects from non-fatal component failures from weird real-world conditions.

I make a real effort not to mention our stuff on forums, as I'm in general opposed to using forums for advertising. I do sometimes when I think it's a possible technical solution. Can you PM me?

[PRR]

> before the ... um... mid 1960s. That was when conversion to three-wire was under way

"Mid 1960s" is spot-on for new US work.

Note that seadi123 is "in EU". While the addition of ground sockets happened in most areas at similar times, it happened different in, say, urban Switzerland and rural Portugal.

I was just looking-up and commenting on the US situation for someone. An excerpt:
Our new house in 1961 was 2-pin. 1965-1968 the NEC changed to require 3-pin grounded outlets in nearly all spaces, so most such work will be 3-pin and nominally properly grounded. Existing installations never need to be updated unless there is major rebuilding. All new venues (from taverns to bedrooms) should be 3-pin and the 3rd pin should bond back to supply Neutral and dirt-rods. But musicians are low-income and often found in old buildings, pre-1965 and never updated. This may run half the houses and buildings in many neighborhoods. Maybe more as far as musicians go.

[seadi123]

First of all, i want to thank you all for the help ! My house and my neighbour's house (my uncle), were built in 50's and 70's. I'm pretty sure here in Albania(EU), nobody knew anything about three wires back then. Actually i'm pretty sure electricity was used only for the fridge and lights (Albania used to be under communism back then, we got our first Tv in '95). However, the elctrical stuff were re-done 15-20 years ago, to today's stadards i believe, but it's still probable that both mine and my uncle's house may have some defect or something. I will try in another house as soon as i get the chance, until then i'm wearing slippers whenever i play the guitar. The static noise however is unbearable. How about adding a 7809 regulator?

[anotherjim]

Local house grounding used to depend on metal water pipes going into ground. UK added a ground rod as the primary local earth a long time ago. Renewal with plastic water pipes can remove the connection if there is no "dirt rod".

The neutral should go to ground at the street supply transformer, but if this connection failed it may not be obvious to many customers and multiple properties would be affected.

[Rob Strand]

When you have an ungrounded amplifier (ie. an amplifier where the circuit ground doesn't have a low impedance path to mains earth, you often get buzz problems.   Normally the path to mains earth diverts the noise away from the signal lines but when the mains earth isn't there it finds a path into the signal lines.

If you can connect you amplifier ground to a mains earth then it will help a lot.

Things like PC speakers rely on the PC being connected to mains earth.

(The ground-lift switch on the old amps was to prevent ground *loop*s under assumption that there is a another point in the system connecting to mains earth.  This is a different issue.)