Earthing questions

[Black Stuart]

headphone jack chassis socket and volume pots - should they always be earthed?

Is earthing by direct contact with the chassis correct?

[John Caswell]

Hi Stuart et al
The simple answer is yes purely on safety grounds.
Earthing volume controls immediately alleviates the problem of hum from fingers when using the v/c if it has metal knobs.
Because of the general construction of headphones it is unlikely that the body will come into contact with the signal carrying parts, with the exception of the jack plug or whatever connector is used, however it is very good engineering practice to keep all parts that are liable to come in contact with humans at earth potential to avoid anything nasty.
When "live" chassis were prevalent extraordinary measures were taken to avoid being able to touch anything associated with internal workings. This such as slotted back panels of specific slot sizes, wax filled grub screen holes. no direct earth/aerial connections and so on.

Hope that helps

John

[Black Stuart]

Hi John,
thanks for that. I need to clarify your remark 'earthing volume controls immediately' - does that mean that the pot and jack socket being earthed directly to the chassis is correct?.

When I built the Stereo Coffee I did'nt isolate the Bourne pot from the chassis on both sides, which meant the S/C did'nt perform correctly. When I used Teflon washers on both sides of the chassis plate it did.

Yesterday I removed the pot fitted to the D/V and the pot PCB, imagine my surprise when I discovered that the jack socket earth post/signal earth wire and hard wire R/hand channel (after the first resistor from signal valve base) were soldered to no connection at all - what was the point.

This meant the the input RCA PCB had no earth at all, nor did the R channel wiring. Could this be the source of all the hum problems.

Should I (1) make an earth from the R hand channel to the chassis earth I made and (2) make another earth connection from the jack socket, effectively making it a star earth?

[John Caswell]

Hi Stuart
Definition - chassis is metalwork earth directly connected to power line earth
Signal earth - signal earth which could be isolated from chassis via a 10-100R plus a 0.1µF in parallel or perhaps directly connected to chassis
'earthing volume controls immediately alleviates' slightly out of context.
Yes you really should make sure the shaft and case of the v/c are earthed to chassis.
With regard the headphone jack, there are types where the "sleeve" (as in TRS = Tip L/H, Ring R/H, Sleeve Common Signal return ) is earthed directly through the metalwork of the jack socket itself and thence to chassis so may need isolating as above to avoid hum loops, and the plastic variety where it is necessary to make sure the "sleeve" is connected to signal earth.
I would have made sure the shaft of the Bourne pot was earthed to chassis whereas RCA input sockets should/would have all their outer screen connections taken to the signal earth.
I am not sure about how you are wiring various PCB but I would certainly think about a separate signal earth to a "star' point and also take the PSU earth to the same.
I think the best solution is connect everything up signal earth wise then experiment with how you connect them to the chassis.
Obviously make sure the RCA sockets are the isolated type as is the headphone jack socket that makes things so much easier.

John

[Black Stuart]

Thanks John,
as I said so many who have a D/V have hum problems and blame various makes of valves.

The hum I first had was'nt bad but it definitely interfered with the music. Once I disconnected the pot and used the Stereo Coffee as volume it is really minimal. I reckon experimenting as you advise will cure the problem.

Not sure your posts on earthing should be on the FAQ section - clear and concise.

[A Stuart]

Just a thought - reading between lines of the above:
I think what Black Stuart (no relation) in post #3 wanted clarified was where in post #2 I think John meant "immediately alleviates" rather than (immediately meaning directly) "earthing ... immediately".

Shoot me down if the adds mud to the water.

[Black Stuart]

To another Stuart - no shooting allowed on this forum and yes you read me right.

[bliza3017]

ok