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Old 1st July 2004, 06:40 AM   #1
lazyfly is offline lazyfly  Australia
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Default A wee shock from a chassis.

Hi forum.

I was over at a friends place with my sc480 (mostly kit amp) trying to convince him it's not a waste of money and is quite fun, too!

Anyway, as he was plugging his amp back up to his gear - more importantly a cd player - he said it gave him a little tingle as he held the rca's in one hand and touched the chassis in the other! It didn't hurt apparently but was certainly felt.

I looked at his source (CD) and saw it was on standby as opposed to unplugging it. I don't know squat, really, and put my amp together as I was instructed to so while I was seeing how it was earthed and thought I understood it - I guess I don't. The reason I say this is he stood there dumbfounded as I said I have no advice to offer after being told 'you should know'!

So my questions are: Is this little shock normal?
Does his amp have a grounding problem?
Is there any advice I should give him (throw it out, perhaps?)
I understood that any current would run through the chassis to the earthing of the amp and safely to ground!?

The amp is an old Akai job.

Thanks in advance?
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Old 1st July 2004, 07:54 AM   #2
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Hi lazyfly,

My advice is not to operate this amp while standing in a puddle of water or a bath.

Always wear rubber gloves and rubber boots while operating this amp.
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Old 1st July 2004, 11:22 AM   #3
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The chassis of your amp SHOULD be solidly connected to the earth pin of your power lead. As most CD players are not directly earthed (supposedly double insulated), I would suspect the tingle was more due to a slight earth leakage on the CD player.

Just to be sure, double check continuity between your amp's power cord earth pin and chassis.

Cheers
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Old 1st July 2004, 11:40 AM   #4
SY is offline SY  United States
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There are a lot of possible sources for the tingle- transformer leakage in one of the components and different ground points because of CATV or satellite are two possibilities. It's a textbook illustration of the power of ground currents and the necessity for solid, single-point grounding. Or you can do the effective cheat of using transformers to break up the loops.

If there's a CATV/satellite connected, disconnect it and see if the tingle goes away. If not, the problem is most likely a cheesy transformer in the CD player as Centauri pointed out.
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Old 1st July 2004, 12:14 PM   #5
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Even if the transformer insulation is perfect, there will always be a small amount of stray capacitance from the primary winding to chassis. If the CD player is not plugged into any other earthed equipment, some amount of voltage will appear on exposed metal parts but will drop to practically zero when you touch it. If you drag the back of your hand on the CD player you may get a fuzzy kind of feeling.
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Old 1st July 2004, 12:30 PM   #6
lazyfly is offline lazyfly  Australia
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Hi folks.

Firstly, thanks for replying

Secondly, Centauri, it isn't my amp but I'm glad you wrote what you did as it reminded me why I was confident my kit amp was grounded correctly in the first place - the continuity test. I've only done it once so it was easily forgotten (but never again.)

I went back shortly after reading your post and checked it out with the dmm and it tested out okay where the tingle happened but there was a little resistence on the front panel :\

Your comment on the earth leakage prompted me to read up on it and I found a Touch current/Leakage current article which helped me understand things a little better but the front panel is still a little worrying. There was approx. 500 Ohms resistence but being a beginner I still don't know if that's much or not!

Thanks again for helping everyone
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Old 1st July 2004, 12:36 PM   #7
SY is offline SY  United States
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Don't rely on resistance readings- you'd be better off measuring differential ground voltages with a high impedance AC voltmeter. If you haven't seen Bill Whitlock's papers on grounding, leakage, and their relationship to hum, noise, and shock, I'd go dig them up and read them- they can be found on Jensen Transformers' web site.
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Old 1st July 2004, 12:41 PM   #8
lazyfly is offline lazyfly  Australia
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SY.

Thanks, I'll read up on what you've said.

I'm so damn naive on this topic that the end result will surely be
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Old 1st July 2004, 02:18 PM   #9
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Quote:
Originally posted by lazyfly
.. front panel is still a little worrying. There was approx. 500 Ohms resistence but being a beginner I still don't know if that's much or not!

With current electrical safety / test and tag regulations, there must be less than 1 ohm resistance btween the earth pin of the plug and any exposed metal (unless device is double insulated), and this is usually measured at a higher current than possible with an ordinary multimeter.

In the case of the front panel, if you have a mains power switch on it and the switch shorts out to the metal panel, any resistance between the panel and main chassis could present lethal voltages. My suggestion would be to make sure there is clean metal-to-metal contact between the front panel and main chassis, and use star washers under the securing screws to bite into the metal.

cheers
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