• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

100R ground lift (chassis to circuit 0v) Why?

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#21
Chris Hornbeck said:
Infety "deed. Why all the strurm und drang about separating signal "ground" from saground"? If there weren't a safety issue, why would they ever have been connected in the first place? Pretty fair question, I'd say
Chris
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I'd say you are misinformed, and quite frankly- out of place.
I have an unrestricted electrical license, 'A Class - can contract for and complete any electrical works' - you?

SM
 
#22
Sam Magrath said:
Chassis is at 0 volts.
Transformer primaries should be fused locally (at the chassis).
We need to make distinction between ground (mains earth) and signal common (which can be floated).

SM
Click to expand...
Sam Magrath said:
Chassis is at 0 volts.
Transformer primaries should be fused locally (at the chassis).
We need to make distinction between ground (mains earth) and signal common (which can be floated).

SM
Click to expand...
Depending on where you live, mains current through a 100R resistor will be at least 1.2 amps (right?). Hardly enough to trip a 10/15 A breaker. The other problem is that the resistor would have to be rated for (in the case of 120 VAC) at least 144 W.

I guess you could say this would be a form of high-resistance grounding but it’s not generally deemed safe in the DIY world.
 
#24
tonescout said:
Thanks everyone,

So my take on this is the safety earth operates as normal from the chassis to earth and will operate conventionally as it is unimpeded.

The circuit ground in the power amp is lifted by 100R from the chassis earth of the power amp, BUT the chassis earth of the preamp is NOT lifted so the easiest path to earth (lowest resistance) is via the phono leads earth back into the preamp.

I have 3 options in my opinion, all of which are safe I think

1. Leave alone - the majority of the return for the Power Amp is via the Preamp due to the 100R
2. Lift the Preamp in a similar manner - But I still don't know WHY I would benefit, the lift is a standard approach for this amplifier manufacturer for both pre and power amps (high end tube)
3. Remove the 100R it creates a 2nd pathway and has the potential to create a subsidiary loop.

Thoughts
Click to expand...

A ground loop works in the same way as a transformer secondary. When you close the loop an AC current will flow around the loop.

Personally, I would go with option 2, modified with 35A bridge rectifiers parallel with 10ohm resistors.
 
#25
454Casull said:
Depending on where you live, mains current through a 100R resistor will be at least 1.2 amps (right?). Hardly enough to trip a 10/15 A breaker. The other problem is that the resistor would have to be rated for (in the case of 120 VAC) at least 144 W.

I guess you could say this would be a form of high-resistance grounding but it’s not generally deemed safe in the DIY world.
Click to expand...


If you have no idea (demonstrated), best to keep quiet.

I am happy that you are not building my amplifier.
I am happy that you are not wiring my house.


🙂
 
#30
454Casull said:
Took damn long enough for this to pop up!
Click to expand...

The number of posts/threads for which the article contains the answers is immense.
It should be a sticky on the home page -

'Welcome to the diyAudio.
If you have any questions regarding signal ground, safety ground, ground loops, shielding, hum, interconnects, noise, cables, amplifier & component building.... read this first!'
 
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#37
tonescout said:
My power amp has this and my preamp does not. So of course the preferred return will be via the preamp input leads ground rather than the 100R to the chassis and earth on the power amp mains plug?
Click to expand...

Make it a 10 Ohm 2W resistor for all the reasons mentioned. Metal casing of DIY gear ALWAYS directly to PE. Audio GND to same point with a 10 Ohm. No ground loop and when L would short to Audio GND the breaker/fuse still will protect humans.

The anglosaxon system with PE directly to Audio GND in all devices (when one has repaired British audio... many have the same issue) makes hunting hum and ground loops an ever returning subject on this forum and every where else. It can be solved once and for all with the 10 Ohm without penalties but as usual some will tell horror stories of injury and possible death. Nonsense. The metal casing must/should be connected to PE, no debate here.

* A different way could be to use the 10 Ohm in all devices except the power amplifier. In the power amplifier PE and Audio ground are then connected directly. Then all Audio GND of source devices will be referenced to PE via the RCA plugs/cables. This works but now suppose you have a cable tuner with coax input and a grounded antenna cable...Again a loop! So use the 10 Ohm 2W system in all devices and have both safety and no hum 🙂

* Another interesting phenomenon is that a DIY linear 5V 3A DC PSU had a + output and - output. Now the metal casing is connected to PE as should be. Leave "-" floating. Now measure from "-" to PE. Repeat this from "+" to PE. Don't be surprised to measure 70 to 90V AC on both "+" and "-" 😀 This explains small sparks when plugging in. Also in this case the 10 Ohm will solve the sparks and the leakage voltage on both "+" and "-".
 
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