Grounding the amp - where to do it

[A.B87]

I'm quite confused about the grounding of music equipment. I've always grounded my decks on the mixer because there's a built in ground wire on the deck. When I attach the Left and Right phono cables to the inputs on the mixer, I always attach the ground wire to the srewy thing.

But what has got me confused is that I got my amp out for the first time in ages and noticed that there's also a srewy thing for grounding at the inputs. I've never grounded there before, only attached the amp to the mixer with a phono lead (no ground wire attached) and carried on.

So I looked into this and read that there are three types of grounding for electronics each type having a different symbol.
1. earth ground - which is in the mains plug - I know the symbol for this one
2. signal ground - to prevent a humming
3. chassis ground - to ground the metal casing of the equipment

So I looked the the symbol by the srew thing on my mixer and it's the earth ground symbol. There is no symbol on the amp. So basically I'm very confused as to what this grounding on the mixer is for - my guess is that it's the chassis grounding.

Am I supposed to ground everything metal to the screw on the mixer as I also have a soundcard wired up to the mixer?

Just to simplify my questions
1. Do I need to get a phono cable with ground wire and to take it from the mixer to the amp?
2. Do I need to do the same from the soundcard? There's no grounding facility on the soundcard
3. Where is signal grounding done?


Any information at all would be much appreciated,

 

[Sergeauckland]

To answer the specific questions:-
1) No, a standard screened phono cable is fine. It doesn't need an extra ground wire.
2) No, as per (1)
3) This rather depends:-

Traditionally, equipment has a mains safety earth which attaches to the metal chassis of the equipment. This MUST NEVER EVER be removed. Double Insulated equipment, which is designated by the double-square symbol has only a 2 core mains cable and doesn't need a safety earth as the whole point of double insulation is that under fault conditions, no metal part can ever become live.

How audio ground is handled depends on the equipment. Some Pro equipment will have a ground-lift switch which connects or disconnects the audio ground from the safety ground. THE SAFETY GROUND ALWAYS STAYS CONNECTED. Lifting the audio ground means that there is no connection between the audio ground, (the outer screen of the RCA phono socket) and the metal frame. This avoids ground loops that can cause hum.

In the case of a turntable, the correct procedure is that the turntable's metalwork is safety grounded to mains earth, (or is Double-Insulated) but the arm metalwork is connected to the flying lead, separate from the phono cable from the cartridge. This allows the arm to be grounded to the amp's audio ground and/or safety ground depending on the amp. Which is best depends on which creates least hum. The idea is that the arm metalwork should act as a screen for the arm wiring as the arm wiring necessarily is very thin to allow arm movement, and is almost never screened as screened cable is too stiff.

The take-away from all this is that safety earth where present must always be connected to mains earth, audio earth can be separated, ideally with a switch, and one sets up the equipment for lowest hum.

S.

 

[chris_w]

Google star earthing. It is designed to prevent hum caused by ground loops. then apply those principles to your system. Dont forget that most RCA/DIN interconnects provide a ground connection at each end. It is usually better than the lore of grounding at the pre-amp from a noise floor prospective.