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Old 2nd November 2009, 02:12 PM   #11
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Quote:
Originally Posted by HiFiNutNut View Post
TV but I don't think (not sure though) it is earthed. The TV connects to a digital set-top box, not an outdoor antenna direct.
But the TV antenna input, or set-top-box antenna input (75R coax) shares a common galvanially coupled ground with the audio wiring.

This 'remote' ground at the antenna gave me all kinds of problems on a configuration of (Antenna -> set-top box -> VCR -> amplifier) which got even worse with an FM tuner in the system - because that has its own outdoor antenna, also with a ground. Solution is to capacitively couple all antenna co-ax cables , centre AND braid

Quote:
Why was there a measured almost constant 35mV across the RCA shield ground of the DVD player and earth?
Most (all?) DVD players and set top boxes have switched mode power supplies. These are very 'leaky' as regards coupling between primary & secondary - they tend to drag the secondary (=low voltage wiring including audio earth) up to a few tens or even a hundred volts above true ground, even though the current availability is very low and it's not an electrocution hazard. It can still destroy semiconductor input stages. This could well be why you blew up your amplifier when making connections.
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Old 2nd November 2009, 03:14 PM   #12
cbdb is offline cbdb  Canada
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Is that 33mv AC? Is there any DC on the DVD output.
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Old 2nd November 2009, 10:36 PM   #13
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Quote:
Originally Posted by Steerpike View Post

These are very 'leaky' as regards coupling between primary & secondary - they tend to drag the secondary (=low voltage wiring including audio earth) up to a few tens or even a hundred volts above true ground, even though the current availability is very low and it's not an electrocution hazard. It can still destroy semiconductor input stages. This could well be why you blew up your amplifier when making connections.

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Originally Posted by cbdb View Post
Is that 33mv AC? Is there any DC on the DVD output.
I first disconnected the DVD from the TV.

When the DVD is turned off, measured 33mV DC and 45V AC between the DVD RCA Shield and earth.

When the DVD is turned on, measured 30mV DC and 30mV AC between the DVD RCA Shield and earth.


I then performed the same tests on the Marnatz SA11 CD player. It was 0V for both DC and AC when turned on and off.

So should I throw away the NAD DVD player?

But it worked with the TV alone without connecting to my HiFi. And it worked before I introduced the ground lift in my preamp and power amp, despite some low level hum.

I have not tried to connect the DVD to the preamp this time even with the TV connection unplugged. First I worred about the risk of burning the amplifier again, secondly, it does not make sense to connect the DVD to the hifi without connecting to a TV.
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Old 2nd November 2009, 11:48 PM   #14
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Both of those voltages (AC & DC) may be what electricians call phantom voltages. That is high impedance leakage voltages. When making your voltage measurements try adding a 100K to 1M resistor in parallel with your probes.
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