[sts9fan]

I just threw together an amp6 Basic kit fro 41hz.  I have on a open aluminum chassis and was wondering how you add a safety gnd to this.  the power comes from a 12v wall wort.  Do I not worry about grounding the chassis?   

[JoshK]

The wall wart power I am presuming is a round dc plug?  Which means it has hot and neutral only?

In which case, you are suppose to use double insulation.  Google the meaning of double insulation.  You need to make sure your circuit is well insulated from the chassis with non conductive standoffs, etc.

That is all I know. 

[WerTicus]

DC 12v converted at the wall in a plug pack typically will only have two prongs not three, grounding isnt required as no 'live' ac voltage is seen by the appliance.

nothing should be coming into contact with your chassis at all.  Adding an earth to your chassis will do nothing for safety or anything else if this is the case.

your probably going to want to be very sure its that type of setup!

[sts9fan]

hmmm.  I guess i will get some plastic standoffs but if the heatsink is external and the chip is attached to the heatsink.......
How dangerous is 12volts and 1200mA?  
With something like the BYOB amp and that DAC that has the board out in the air.  How is that safe?  

[Haoleb]

12v is not enough to shock you. Everybody is different in their skin makeup and what voltage it will break down and shock you but its over 30v for most people. Unless you touch it on your toungue your fine. There shouldnt be any voltage on the heatsink, I am not familiar with the amplifier you are building but i dont know of any transistors that carry a voltage on their outer case.

the wallart doesnt put out "hot" and "neutral" it simply has 12v positive and negative. Unless you were touching traces on the board or whatever your using that shouldnt be touching anything I dont see any way you could get shocked. Not that 12v is enough to shock you anyway.

[andyr]

I just threw together an amp6 Basic kit from 41hz.  I have on a open aluminum chassis and was wondering how you add a safety gnd to this.  the power comes from a 12v wall wort.  Do I worry about grounding the chassis?

In a "conventional" metal cased component, with a 3-wire PC, the case is connected to the earth tag on the IEC socket (or the earth wire on a captive mains cord).  This earths the case ... making an RFI shield out of it, to protect the PCBs and signal wiring inside. 

If your component is powered by a wallwart which delivers DC then you don't have a 3rd wire with which to earth the chassis (unless you mod the wallwart - which is what I did for my active XOs, before I replaced it with a "decent" power supply!  ).  Then again, if you have an "open chassis", RFI is presumably not a problem! 

Regards,

Andy

[sts9fan]

This is what I figured and what I got from DIYaudio.com.  Don't worry about it.  One guy said you can run a wire from the chassis to the ground plug in the wall but I am not worried. 
here is a pic sans feet and mute button

[ghpicard]

12v is not enough to shock you. Everybody is different in their skin makeup and what voltage it will break down and shock you but its over 30v for most people. Unless you touch it on your toungue your fine. There shouldnt be any voltage on the heatsink, I am not familiar with the amplifier you are building but i dont know of any transistors that carry a voltage on their outer case.

the wallart doesnt put out "hot" and "neutral" it simply has 12v positive and negative. Unless you were touching traces on the board or whatever your using that shouldnt be touching anything I dont see any way you could get shocked. Not that 12v is enough to shock you anyway.

Safety ground is there not to protect us from 12 or 30V... it's there for the case that the ideally perfect isolation between the "live" 120 or 220V and the low voltage sections get shorted.

Whether one has low voltage on a heatsink is up to tastes. I wouldn't let any metal part that is exposed to the touch to anything but ground potential.

The wall wart in itself might have double isolation, but unless all the rest of the equipment is doubly insulated as well (no metallic parts exposed, any metallic part away from accidental touch, etc), there *must* be a safety ground.
Actually any audio thing that has connectors exposes all the chain to shock risk, and if the double insulated wall wart shorts to 110V and the rest of the audio chain also lacks of the safety ground (i.e. to avoid ground loops), the probability of having a shocking experience is quite high.

Adding a safety ground could be seen as a hassle. Safety latches in car doors and gun triggers are too, but I wouldn't use things without one without being very conscious that I'm running a risk with a high impact should it come to reality. I'm not saying I never do such things, but when I do, I follow the safety rules related to working with "hot" chassis.

[sts9fan]

So how would you do it with a two prong wall wart and a round DC connector?

[Occam]

If your wallwart is 2 prong (and it is marked UL/CSA or other ETL) it is by definition Class II -
http://en.wikipedia.org/wiki/Appliance_classes
If it feeds a component with no other input from the mains with a low enough voltage (which it does), it is a Class III component (see url above) requiring no mains grounding, as the power supply qualifies as SELV (Separate Extra Low Voltage) -
http://en.wikipedia.org/wiki/Extra_low_voltage

Feel free to add a mains ground to your amp, iff'n you want to spend your time tracking down that ground loop.

FWIW,
Paul

PS- very nice looking build

[BobM]

Damn, if you added cables to that little thing it would tip over. I think it needs a heavy 1" aluminum case around it to hold it down   

That should quadruple your costs, for sure.

Bob

[Haoleb]

So how would you do it with a two prong wall wart and a round DC connector?

You could run a seperate wire from the chassis to the ground pin in your outlet, i would probably do this by getting a cord and cutting off the two blades and only having the ground pin hooked up. Or you could run a wire from the chassis to the chassis of something that does have a safety ground.

frankly though I really just wouldnt worry about it. A safety ground has its purposes obviously, as stated in one of the posts above, but theres about 30 million other things i would be worrying about before i would worry about this issue.

[ghpicard]

So how would you do it with a two prong wall wart and a round DC connector?

I am from a 220V/all polarized sockets part of the world so I deal with this things from time to time...

The simplest way IMO would be to use a polarized to non-polarized adapter, with an additional cable from the ground leg on the non-polarized side of the adapter (I happen to have one like that, will post the photo as soon as I get home) to the amp "ground" or chassis. For fully pluggable solution, a threaded post (board side) and a spade (cable side) should be good enough.

What about your signal source? Are you using a "floating" one, or one that has safety ground connection? In any case there could be some ground interactions, but they would be different.

EDIT:
Just saw Occam's answer... *To me*, in a world where many times a compliance seal just means a rubber stamp on a paper sheet, I'd play safe first.
Not the first time that I feel that "low current voltage sensation" from wall-warts and certified PS. And no, I don't intend to start a Grounds war thread

[sts9fan]

Thanks everyone.  I think I am gonna take Occam's "its fine and legal" approach.  Also Bob it is actually quite heavy for its size, with the heatsink and 1/8" bottom plate.  I set it up like that so the heatsink will balance out the cables.  This is why I sold the Trends I had been using for "utility" detail around the house.  Stupid thing was always leaning back unless I put a weight on it.
Thanks again
Kris

[sts9fan]

Couldn't help but post a picture of the amp with its new fancypants mute button.  Does nothing for safety but whatever.  Also a am pissed because I wanted it to only light up when on but it is an always lit type jobby. 

[sts9fan]

I would like to revisit this question with high voltage.  Obviously I will not be having an open case with my Akidio pre but I am considering a separate PS.  How is safety ground handled with an umbilical?  I would have my four voltage conductors then do I add a safety?  Will this cause ground loops?
Thanks
Kris

[ghpicard]

IMO you should, exactly as with main power, use separate chassis and GND connections. The chassis should be connected at the nearest points of both sockets / cable inputs, and the GND would be connected depending your grounding technique (star, bus, etc.). The union between chassis and gnd can be made with resistors in the tens of ohms order values and in that way you would make ground loops less feasible.

[sts9fan]

OK so you mean that the safety should have a separate wire from the second chassis?  Why have I never seen this on a comercial product or a DIY effort?

As far as signal GND goes you are saying that all input and output GND wires should go to the same point.  If the amp board has a chassis GND through one standoff I could connect all the signal GNDs there correct?

Also I have seen power GND connected to the chassis with a thermistor.  What does this achieve?

If I ask or say anything that is unsafe that is why I am asking.  I tend to ask a million questions before I do anything.

Thanks for all your help
Kris

[TomS]

Why have I never seen this on a commercial product or a DIY effort?

Actually some do, but as we also know, a lot of commercial products don't get grounding correct.    Whether one chassis or two, the idea is that if some internal problem shunts or leaks lethal voltages to the chassis, the user will not be exposed to said nasty dangerous voltage by touching it.  The safety certification folks care about this aspect, not whether it hums like a bee in your system.  My external Aikido power supply has a chassis ground, but I didn't use a separate chassis ground conductor to the preamp chassis since in my case it is actually all wood and hardwired.  If it were metal, then yes, I would include one to protect the user.  Sometimes with an umbilical and metal shell connectors, this is done through a shield conductor and/or individual conductor.  Not always good if they're painted or anodized though.  Whether either end is also connected to power supply and/or signal ground directly, through resistors, or a thermistor, is up to the designer/builder.  There needs to be one of those paths, though, otherwise your touch could do the job 

Also I have seen power GND connected to the chassis with a thermistor.  What does this achieve?

Nelson Pass explains the use of thermistor from chassis to ground and transformer primaries here:
http://www.diyaudio.com/forums/showthread.php?s=&postid=643023&highlight=#post643023

The first time I used a CL60 from power ground to chassis has worked well in my F4's.  Dead quiet, yet still provides adequate safety path for internal voltages to earth ground.

[sts9fan]

Hi Tom,

  Did you just float the gnd instead of using the jumper through the stand off?  Did you connect your signal gnds all together?  I think one box is gonna be easier and safer.