Isolation of signal earths

[bumpy]

Having introduced two amps (one for bass frequencies and one for top frequencies), I now have an earthing hum.

I have looked at all the normal things, like all components earthing back to a single point etc, but it looks like it pops up when I connect the second amp to the spare pre amp via phono cables. My mind has turned to the possibility of 'breaking' the earth at this point. There are gizmos available via amazon for less than £10, but they get a few bad reviews ranging from increased hum to drop in sound quality.

https://www.amazon.co.uk/AV-Link-Ground-Loop-Isolator/dp/B000NVWB9O

Also of interest to solve the hum is the Russ Andrews RF router

https://theaudiophileman.com/rf-router-mkii-review-russ-andrews/

Does anyone have any hints on what will work?

[Barry]

Hum due to earth loops can be difficult to cure - there are several ways an earth loop can be formed and the first step is to work out how. If your equipment is unbalanced, that is the signal return or ground is connected to the mains safety ground, then that offers several possibilities for a ground loop to be created.

Have alook to see if and where the signal return (the outer part of the RCA connectors) connects with the metalwork of the amplifiers.

I have tried transformer breaks (similar to the one you show) and have found them pretty ineffectual in curing earth loops.

[bumpy]

Thanks Barry

Yes it is an unbalanced system.

"Have a look to see if and where the signal return (the outer part of the RCA connectors) connects with the metalwork of the amplifiers."

1. Is it just a question of making sure the RCA earth is connected to the casing at each component?

2. Does the same rule apply to passive components like volume controls?

3. Is there any value in creating a star earthing system for all components?

[Firebottle]

Thanks Barry

Yes it is an unbalanced system.

"Have a look to see if and where the signal return (the outer part of the RCA connectors) connects with the metalwork of the amplifiers."

1. Is it just a question of making sure the RCA earth is connected to the casing at each component?

2. Does the same rule apply to passive components like volume controls?

3. Is there any value in creating a star earthing system for all components?

1. No, you are looking to see if the RCA earth is NOT connected to the case. The earth loop comes about because the signal ground (also known as common or zero volts depending on circuit viewpoint) of both amplifiers is connected to the mains earth/case of both, such that when the mains lead is plugged in a circle of connections is made.

2. The RCA earth/common isn't necessarily the only point at which a connection is made to the case, but most unlikely to be via a volume control pot. There may be a pad on the pcb that connects to the earth through the securing screw, this is quite possible with a power amp when the designer has decided to earth the signal circuitry.

3. Unlikely in this instance.

[bumpy]

1. No, you are looking to see if the RCA earth is NOT connected to the case. The earth loop comes about because the signal ground (also known as common or zero volts depending on circuit viewpoint) of both amplifiers is connected to the mains earth/case of both, such that when the mains lead is plugged in a circle of connections is made.

This is making no sense to me.

I have tested 3 bits of my kit so far. I have disconnected everything, but in every case there is continuity between the outer part of the RCA connector (signal ground) and the mains earth terminal. What's happening?

[Firebottle]

Herein lies the problem.
It seems that all 3 of your bits of kit have the signal ground connected to the mains ground. This leads to a hum loop being created when
1. the kit is plugged into the mains (all are connected through the mains earth wire), and
2. you connect another link that connects both amplifiers, this being from the preamp that is driving both.

Do a little experiment, will be easier with just one channel.
Connect one amplifier up as normal, then with the second amplifier only push the RCA in far enough to connect the centre pin but not far enough to connect the outer part. This should work without a hum loop being created, once you push the RCA fully home I expect you to get the hum back.

The only real solution is for one of the amplifiers to have its signal ground separated from the chassis/mains ground, or an 'earth lift' network fitted.

Caution. Do not disconnect the mains earth (PE Protective Earth) from the casework as that is there for safety.

[Firebottle]

PM sent.