The pitfalls of avoiding Ground Loops...

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So. I've finally managed to finish my preamp build, which was in the making for several years now.

After considering each and every possible way of switching inputs, I've opted for the "simple" and "proven" way of using small signal relays. I figured I could double up on the relay count and use a double pole relay for each input channel and thus not only switch the signal line, but the ground line, too. I thought that if I switch each ground connection, I could avoid some possible ground loops due to all the sources' grounds being always connected at the preamp side.

Here's what happens. Don't laugh. I didn't see this coming...
The next stage after the relay input selector is comprised of a PGA2311. Most of the time (not always, that is) when I select another input, the volume is gone. I have to change the volume by a click (up or down, doesn't matter) and it's back.
First thought was: The input selector (digital part) is messing with the PGA. So I triple-checked each and every digital line with a Logic Analyzer and a scope, but to no avail. Everything is nice and clean and works as it should. Checked the power and ground lines also, but no glitches to be seen. The relays are powered from the +15V lane, and not from the +5V for the PGA and uC, by the way.
But it gets worse. The more often I switch inputs around, the more crazy the PGA goes. It starts to make awful crackling noises, pops when I change the volume and eventually ceases to react to any changes in volume. After cycling the power, everything is fine again. Until I start switching...

The good thing is: I found a solution to the problem. The bad thing is: I don't like it :(. When I connect all the input ground leads together with some alligator clips, the problem is gone!
What's going on here? In my particular case I was able to measure 85VAC RMS (sic!) between the grounds of two sources, both equipped with a proper three-prong mains connection and both connected to the same outlet. Seems like this potential difference is strong enough to cause the PGA some headache...

What should I do now? Connect all the grounds together and don't switch them? Connect them all together through some resistors? Connect each input ground to PSU ground through an individual resistor?
This all works, hooked up with alligator clips, but what's preferred?

Any hints and opinions are greatly appreciated!
 
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The fix could be as simple as adding a relatively high value resistor to link the grounds and kill the residual 'floating' voltage.

Ground loops form because of low impedance paths, and even adding 1 ohm can substantially reduce true ground loop problems so experiment :)

See what value you need to bring the 85 volts down to say 1 volt or 100mv and then see if that value of resistor fixes the volume issue. Even a 1k may work, perhaps even a 10k.
 
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What resistance reading do you get between the two chassis ?

I'm unable to measure any resistance there due to the prevalent voltage.

Seems like the culprit is the Sony DVP-S9000ES though. There's basically no connection between mains ground and the whole case, including the RCA jacks.
I'm shocked :eek:! Literally, not electrically :skull:...
I'll have to investigate that one further.
 
I'm unable to measure any resistance there due to the prevalent voltage.

Seems like the culprit is the Sony DVP-S9000ES though. There's basically no connection between mains ground and the whole case, including the RCA jacks.
I'm shocked :eek:! Literally, not electrically :skull:...
I'll have to investigate that one further.

That's normal for Class II double insulated stuff. Have a look on the back for a little square within a square symbol.
 
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I know, but this thing is not Class II...

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(Image borrowed from Google, without kind permission)
 

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I can't quite read it but is there a class designation in the printed label at lower right centre of back panel? looks like it may say 'Class ?' but my eyes could be playing tricks on me.

It says "Class B Digital Apparatus". FCC stuff.

Even the service manual shows a three-prong power connector and has a part number for the screw to connect the earth wire to the case. Not sure why there's not even a third wire soldered to the connector in my unit. Anyhow, I added one and now the problem is fixed. No more AC between the RCA connectors and no more hiccups when switching sources.

Well thank you then, Sony :mad:. Dog Eat Dog were right: "when you least expect it, expect the unexpected"... :rolleyes:



Now on to enjoy some music :cool:. Thank you guys for pointing me in the right direction!
 

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