Ground Question

[Sheldon]

Hello,

I'd like to have a confirmation.
So here is the situation : I build a preamp tube stompbox including a nixie switching PSU.
- The input mains is from a wall wart DC power supply, via a metallic DC jack
- The signal input and output are from Neutrik NMJ4HC-S Mono switched jacks (metallic, but with washer isolation)

In the stompbox, I created 2 different grounds : a power ground for the nixie switching PSU and a signal ground.
I assume that as the DC jack is metallic, as soon as I will plug my wall wart DC power supply into it, the power ground will be connected to the chassis via the DC jack.
I assume if I want the whole circuit to be connected to the chassis in only one point, regardless if the guitar is plugged or not, I have to :
- install the washer on both input and output jacks to isolate them from the chassis
- connect both jack sleeves to my signal ground
- put a wire with a lug from one of the two jacks sleeve to the chassis
- separate the signal ground and the power ground in the circuit
- the power ground will be connected to the signal ground via the chassis as soon as something will be plugged into the DC jack

Is it correct?
thank you

[pupil]

that seems correct to me. i think you could also do away with the chassis lug and connect one of the audio jack sleeves straight to dc jack sleeve via wire. It would achieve the same thing unless I'm missing something.

[Sheldon]

thank you. It makes me think about another simpler option : leave the signal jacks isolated from the chassis, and directly connect the power ground and the signal ground on the pcb!
I will have signal ground and power ground connected in one point : on the PCB, so I'll have only one common ground.
The common ground will be connected to the chassis via the DC jack.

[GGBB]

I assume that as the DC jack is metallic, as soon as I will plug my wall wart DC power supply into it, the power ground will be connected to the chassis via the DC jack.

Maybe not. What is the polarity of the supply? A standard BOSS-style DC adapter is center negative. Using one of those with a non-isolated metal DC jack will connect V+ to the chassis. You will need to make sure your DC supply is center-positive, which in the guitar effects world is non-standard.

[pupil]

^
a very good point

[Sheldon]

yes, I realised that yesterday. In fact I either have to use a center positive power supply, or buy an isolated DC jack and an input jack that conduct between sleeve and chassis

[GGBB]

yes, I realised that yesterday. In fact I either have to use a center positive power supply, or buy an isolated DC jack and an input jack that conduct between sleeve and chassis

That's the reason most pedal builders use an isolated DC jack and a non-isolated input jack to take care of grounding the chassis. If you've got other effects/devices using center-negative power supplies, adding a center-positive one to the mix may very well lead to a momentary lapse of reason resulting in some level of destruction.