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Why tie input ground to earth ground?

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M

Miasma

Member
Joined 2002
#1
I've got a tubelab sse that I built a few years ago. I'm very happy with it. With the inputs disconnected, there is almost no noise/hum at all. I'm trying to troubleshoot why I'm getting some once I connect the inputs.

My main question, though, is why do we tie the input ground to the power/earth ground?

It seems like we should certainly tie chassis (and all metal external surfaces) to earth ground for safety. So I'm onboard there.

What purpose does tying the input ground to earth ground serve though? The circuit would still have a "common" tied to the input "ground". It seems like pulling the input ground down to earth ground would cause ground loops though. Let the circuit ground exist at the same potential as the source (by "grounding through" the input connector) and let the earth ground exist purely for safety.

What am I missing?
 
#2
Imagine a chassis at earth ground, while the rest of the circuit is star tied to the insulated grounds of the input connector - or an insulated standoff. Now imagine that there's a small 60Hz voltage difference between (the chassis) and (the reference of the rest of the circuit enclosed by the chassis). Do you think that voltage difference wouldnt leak into the amplifiers output signal as well?

I think the only way to do what you'd really like is to shed the safety ground. Who in their right mind would sell something these days w/o a safety ground? "Double-insulated" tube amplifier?
 
#3
jjasniew said:
Do you think that voltage difference wouldnt leak into the amplifiers output signal as well?
Click to expand...

I don't know, how would it get there? That's exactly the question I'm asking.

It seems that by tying them together we're providing more opportunity for ground loops since we now have multiple paths to ground (through the input connector and through earth ground).

Also, in no way am I suggesting removing the safety earth ground.

Thanks for replying! 🙂
 
#5
Miasma said:
It seems that by tying them together we're providing more opportunity for ground loops
since we now have multiple paths to ground (through the input connector and through earth ground).
Click to expand...

Often an impedance is connected between the audio common and the safety ground, such as a resistor
and/or a capacitor and/or a diode rectifier block. The first priority is safety, so that if a fault happens
in the audio circuit, there will be a path to the safety ground which will open the breaker or fuse.

Ground loops are a lower priority to deal with, and there are ways to do so.
 
#6
rayma said:
Often an impedance is connected between the audio common and the safety ground, such as a resistor
and/or a capacitor and/or a diode rectifier block. The first priority is safety, so that if a fault happens
in the audio circuit, there will be a path to the safety ground which will open the breaker or fuse.
Click to expand...

Thanks, thinking of the voltages within the tubes, I certainly see why we need a path to safety ground from there.

Isolating the input reference from safety ground still seems like a good idea. Using something like a 1:1 transformer should do the job (but also affect phase). Thoughts?
 
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