Signal earth lift explanation

[Riggers]

Hi all
I tried to find a thread in answer to my question but couldn’t so here goes. Why is the (what I understand to be an) signal earth lift 10R resistor in parallel with 0.1 uF cap (installed from negative speaker post to chassis earth post) fitted to only one side/one channel of the amp (WD88VA). I naturally thought what was good for the right channel would be good for the left as well
Thanks in advance
Phil.

[bob orbell]

Speaker Zobel / Impedance Equalization Network. An Impedance Equalization Circuit, also know as a Zobel circuit, can be used to counteract the rising impedance of a voice coil caused by inductive reactance. The cause of this impedance rise is due to the speaker's voice coil inductance. I think this is what its for Phil, but some one better than me can explain. Bob

[John Caswell]

Not quite!
This network is a "signal lift" to help eliminate ground loops which can be the devil to find. It works by "lifting" ie semi isolating the signal ground from mains ground.
Usually you break ground loops by lifting the earth/ground cable in the preamplifier (if you have one) so that all earthing/grounding is done at one point the main amplifier earth/ground.
You would not probably need it if you have an integrated amplifier, but there are still many items out there in the real world that still have all their metalwork earthed/grounded to the power line earth/ground.
Personally I think it is good practice and the 0.1µF cap is to bypass any RF to earth/ground.
It normally only needs one per system unit but I have had several cases where I have had to fit this network in both L & R preamp output feeds.

John

[Richard]

Hi Riggers, all,

Checking the schematic, there should be one “signal lift” and 2 “zobels” fitted.

C15 0.1uF in parallel with R27 10R is the “signal lift” described by John and shown fitted between signal ground and mains earth on the psu schematic.

C11 0.047uF in series with R24 10R is a “Zobel” described by Bob fitted across the speaker terminals on both channels as per the circuit page of the schematic.

[Riggers]

Hi all and thanks for replies.

The bit I don’t get is why the signal ground lift is only implemented on one channel (in the case of my WD88VA the right channel). Would that not mean that the left channel signal ground would be at a different potential than the right? Or is that the whole idea?

I hope I’m not trying your patience too much but keen to understand.

Phil.

[John Caswell]

Hi Phil,
Well no, the signal earth/ground is common to both channels, or should be, so you will only need one earth/ground lift. Sometimes I think our patience is limitless:-)

John

[bob orbell]

Phil, if you look at lay out page you should see the wires coming from the output transformers, left and right and two more black wires from each neg speaker post to the 0 volt connection at the two large PSU capacitors C12-13, therefor, the earth lift is connected to both left and right neg speaker posts. I hope this helps you understand. Bob

[Richard]

Many schematics (not just WAD and WD) are not clear exactly if and how an earth lift has been fitted.

In the case of WD88VA there would be no earth lift if the circuit was constructed as per the circuit drawings although the details may be in the text. Correct me if I’m wrong but,

On the Circuit Diagram, the 2 earth symbols should be labelled Signal Ground 0V and the symbols changed to empty triangles pointing downwards.

On the PSU Diagram, the earth symbol should be labelled Mains and Chassis Earth. A Signal Ground 0V connection should then be drawn down from the meeting point of R29/R29/C12/C13 and labelled as such to show the connection to those on the circuit.

[bob orbell]

You are correct Richard, but there is a page that shows components and wire connections in block form. Bob

[Riggers]

Hi All
Thanks for taking the time to explain and being patient. It is a real feature of this forum that the experienced members genuinely try to help without the "who is this idiot" type replies.

The penny finally dropped. Somehow in my minds eye I overlooked those sneaky black wires running from each black speaker post to the star earth bus bar on the power supply caps. Of course the black speaker posts of BOTH channels have a 10R resistance to chassis earth post!

Personally I blame inexperience in reading schematics and COVID fatigue

Anyway as always your help is much appreciated.

Phil.