Why do my amps want signal ground and safety earth connected? - diyAudio
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Old 26th February 2010, 04:07 AM   #1
pjanda1 is offline pjanda1  United States
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Default Why do my amps want signal ground and safety earth connected?

The amps I'm working on seem to share a trait with the last amp I built: they make noise (some hum, some higher frequency noise) unless signal ground (a star) is connected to safety earth (not directly, necessarily). I don't like to do that, as it creates potential ground loop problems. On my last amp, I tried using small value resistors bypassed with a cap. It still made noise. When I connect them with a straight wire it is really quiet. I haven't played with the new projects much, and I'll sort them out somehow.

But, my question is, why do my amps want to have signal ground and the safety/earth connected? Shouldn't I be able to leave them separate if I want (or at least at separate potentials)? Is this a symptom of a larger problem with my construction technique? Heck, the stock amp chassis I built on were fairly quiet, and they lacked a safety earth all together! I've read all sorts of talk about grounding schemes, but I haven't seen an explanation for why connecting signal ground to safety earth would result in lower noise. Something to do with currents capacitively induced in the chassis?

Paul
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Old 26th February 2010, 04:24 AM   #2
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Is it noise (hiss) or Hum? Live near a power line, radio station, cell tower? Whatis the chassis made of and do you have any pictures?
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Old 26th February 2010, 04:28 AM   #3
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Quote:
Originally Posted by pjanda1 View Post
The amps I'm working on seem to share a trait with the last amp I built: they make noise (some hum, some higher frequency noise) unless signal ground (a star) is connected to safety earth (not directly, necessarily). I don't like to do that, as it creates potential ground loop problems.
I've always found it's just the opposite, doing so helps prevent them.
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Old 26th February 2010, 05:40 AM   #4
pjanda1 is offline pjanda1  United States
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These amps seem a little less picky than the last one. A resistor between signal ground and safety earth may work. Sgregory, I could list details, but I'm talking about completely different amplifiers, that I've used in different houses, in different states, with similar issues. I'm asking to gain understanding of the issue in general, not to address a specific piece.

HollowState, really? It seems to me like you can't have a ground loop unless you have more than one component that ties signal ground to the safety earth. And, if you have two or more that do (like my monoblocks would, or one amp plus CATV), you have by definition, a ground loop. Seems easier to me to avoid signal ground and safety earth at the same potential in anything (even if it is the only piece in your system) for fear of acquiring a component that does the same.

But, specifics aside, why would the amp be quieter with signal ground connected to safety earth? That is my general question.

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Old 26th February 2010, 06:07 AM   #5
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My answer is that earth ground is the ultimate ground. Zero potential and not just for safety. Assuming the building is wired correctly, and that means a lot. Anything not at earth is considered above ground and subject to pickup.
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Old 26th February 2010, 09:36 PM   #6
m6tt is offline m6tt  United States
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Neutral in the US is the center tap of the "pole pig" transformer, and safety ground is generally either your pipes or a big copper spike driven into the ground somewhere. The differences can be somewhat (or very significant)...I measure around 1v AC difference, which matters a bit at the input stage.

Question, does the CT of either the pole pig or the power transformer have to be exactly at the center for neutral to be 0v, assuming a true sine wave (haha!)? Does loading of each phase affect neutral V vs. earth ground?
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Old 26th February 2010, 11:42 PM   #7
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Hi Paul,

You may find your answer here:

http://www.raleighaudio.com/Audio%20...connection.pdf

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Old 27th February 2010, 03:45 AM   #8
pjanda1 is offline pjanda1  United States
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Wow Dave, I don't know if I've seen that one before. I'll get through it ASAP!

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Old 27th February 2010, 02:56 PM   #9
wicked1 is online now wicked1  United States
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Quote:
Originally Posted by m6tt View Post
Neutral in the US is the center tap of the "pole pig" transformer, and safety ground is generally either your pipes or a big copper spike driven into the ground somewhere. The differences can be somewhat (or very significant)...I measure around 1v AC difference, which matters a bit at the input stage.
One thing I don't get... In the US (or at least the few states I've lived in) neutral and safety ground are tied together at the breaker box. My electricians manual (had to pass a test to do some home electrical work) specifies that this is the correct way to do it.
Doesn't that throw all this neutral vs safety ground stuff out? It's the same thing!
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Old 27th February 2010, 03:27 PM   #10
jjman is offline jjman  United States
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I don't see the neutral to ground connection in my box in my current house built in '73. I've thought about adding one. I know it was there in my previous house built in '80's.
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