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mr_push_pull 14th September 2012 07:36 PM

Apparently trivial issue gives me the headaches - PLEASE help
 
in short: when connecting audio chain to earthed equipment something goes.

in detail:

I have an audio chain made of laptop (running on mains-powered SMPS), USB DAC (transformer supply), USB HDD (SMPS), amp (SMPS) and, obviously, speakers.

the apartment I live in only has earthed outlets in the kitchen and near the bathroom. old building, different regulations back then.

a while ago I thought it was a good idea to earth my equipment. I made an extension cord running to the kitchen. I was in the kitchen when I connected it to the outlet, only to hear a loud bang. I went shaking to the living room, hoping that it wasn't the speakers ($$$) that went.

upon investigation it turned that it was the amplifier. the input stage (transformer powered) got fried. it's an input switching and volume control front-end, controlled by a MCU. all this is powered by separate transformers, the amplifier modules themselves (UCD180) are powered by a SMPS.
the MCU controlling the front-end essentially burned (part of it basically exploded). everything directly connected to its outputs burned too. the amp has indicator LEDs in series with current limiting resistors and all those resistors burned. strange thing, they looked intact upon inspection, that possibly indicating a very large and short voltage spike.

well, I fixed the amp, all working.

until one day when I thought about connecting the audio chain to the TV to watch a movie. small spark upon connection, TV shut down. TV was OK fortunately, front end of amp partially burned again. not so bad as the first time, I got away with only replacing the MCU controlling the input stage.

needless to say, the audio chain works perfectly if not earthed or not connected to TV.

what those 2 situations had in common was that the chain got earthed. first time through earthed outlet, second time by TV cable ground. I measured 0V from TV cable screen to mains null and 230V to hot.

still, I have no idea WHY this happened. I measured resistance from amp ground to mains hot and cold. nothing measurable. voltage from ground to hot and cold is ~mains/2, likely due to Y caps inside the amp's SMPS. that stands for all the SPMS's in the audio chain, the same mains/2 voltage from ground to hot/cold was measured for each and one of them.

I have no idea what to check next, please help.

DF96 14th September 2012 07:50 PM

Could be one or more of your power sockets is miswired. Also, remember that neutral and ground are not necessarily the same (in the UK - not sure about where you are!) and should not be connected together (except where the power company may have done this deliberately).

mr_push_pull 14th September 2012 08:05 PM

Quote:

Originally Posted by DF96 (Post 3165157)
Could be one or more of your power sockets is miswired. Also, remember that neutral and ground are not necessarily the same (in the UK - not sure about where you are!) and should not be connected together (except where the power company may have done this deliberately).

here hot and cold are not differentiated in any way, the plug is revertible.
still this would not explain the problem I'm having as currently the audio chain (PC, amp, HDD and DAC) is not earthed.

I have never imagined such a thing making me feel helpless.

DF96 14th September 2012 08:22 PM

It sounds like you are connecting live to ground/neutral, so something has either been miswired or has insulation breakdown. One possibility is the caps in a mains filter - in theory these should always fail-safe but these days you can't trust any components.

mr_push_pull 14th September 2012 08:36 PM

you mean insulation breakdown under high voltage only? that would explain some things.

DF96 14th September 2012 08:42 PM

Could be.

mr_push_pull 15th September 2012 03:04 PM

I think I'll make myself a small signal isolation transformer that will run between TV output and amp input. I know I'm curing the effect instead of the cause but I'm stumped.

PlasticIsGood 15th September 2012 05:13 PM

Quote:

what those 2 situations had in common was that the chain got earthed. first time through earthed outlet, second time by TV cable ground. I measured 0V from TV cable screen to mains null and 230V to hot.

still, I have no idea WHY this happened. I measured resistance from amp ground to mains hot and cold. nothing measurable. voltage from ground to hot and cold is ~mains/2, likely due to Y caps inside the amp's SMPS. that stands for all the SPMS's in the audio chain, the same mains/2 voltage from ground to hot/cold was measured for each and one of them.
What ground, in each case? It seems from what you write that one set of equipment has its ground at 125V with respect to the other. Did you measure the voltage between the ground levels of machines on the two different supplies? Sorry if you've already said so, but I got confused.

You refer to hot and cold, but say later that there is no difference. That also suggests some confusion about ground reference.

Whatever, there may be some danger of electrocution if you don't find out what's wrong.

mr_push_pull 15th September 2012 06:01 PM

Quote:

Originally Posted by PlasticIsGood (Post 3166145)
What ground, in each case? It seems from what you write that one set of equipment has its ground at 125V with respect to the other. Did you measure the voltage between the ground levels of machines on the two different supplies? Sorry if you've already said so, but I got confused.

I meant that there is ~120 V (that is mains voltage (230V) divided by 2) between ground of each SMPS-powered device in my chain and earth (or mains cold). that is most likely because of Y caps (small current).

Quote:

Originally Posted by PlasticIsGood (Post 3166145)
You refer to hot and cold, but say later that there is no difference. That also suggests some confusion about ground reference.

your profile shows the UK flag so I'm assuming you live there. AFAIK the UK power outlets are three-pronged and are not revertible. where I live the power outlet is revertible, each of the wires going my gear receives either hot or cold, depending on how I insert the plug.
here's a pic in case you don't know ahat I'm talking about:
http://us.123rf.com/400wm/400/400/co...d-on-white.jpg

Quote:

Originally Posted by PlasticIsGood (Post 3166145)
Whatever, there may be some danger of electrocution if you don't find out what's wrong.

I know, this is why I posted this.

anyway, since there is no measurable current flowing from ground of every piece of gear except the TV (which is earthed by means of the coaxial cable) to mains plug. I don't know where your confusion comes from. I'm pretty confused myself.

PlasticIsGood 15th September 2012 09:16 PM

Loads of appliances here don't use the mains earth. Leads terminating in "figure of eight" sockets don't care which way they're plugged in. Nevertheless, one wire is live, "hot", and the other neutral, "cold". The neutral is earthed at the local distribution transformer, and may be at some low voltage by the time it gets to the consumer. That's not the bit that confuses me.

On the equipment powered by the the two-pin mains, you say there is 1/2 mains voltage between ground and each of the supply wires. Is this ground the signal ground of each machine?

Are the two phases on the two pin system the same as, and connected to, the two non-earth wires in the three pin supply?

Is the TV ground earthed?

Once again, it seems from what you write that one ground is at 1/2 mains potential, and the other is earthed. Connecting the two grounds together means the one at 1/2 mains voltage must be dragged down to earth. That requires a surge of current, depending on several things we don't know.

I could imagine a worst case, where one system is centre-earthed and the other singe ended, and you connect them together and electrocute half of Bulgaria...

Measure the voltage difference between the grounds of the two systems. Current between the two when connected together may be momentary, but large.

If I'm totally barking up the wrong tree, ignore me. I've never measured where grounds in non-earthed machines float to...nowhere special, I rather assumed...

:confused: Consider rewiring your house :(


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