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Dinkin' Around With Star Grounding
Tech tips and other unsolicited advice.
Article By Dan Schmalle From VALVE Volume 2 Number 3 March 1995

  Got a couple of questions about star grounding this month, along the lines of 'what's a star ground?'

OK, here's a description that the more technical among you can shred at your convenience.

The basic concept of star grounding is to bring all components needing to be grounded, or brought to the lowest magnitude of electric potential, together atone point inside the shield of the chassis. The reason for this is to avoid a current being setup across chassis or ground buss (a buss is just a long piece of wire routed through the chassis to which all components needing grounding are connected) which may be spaced apart enough that there is a small resistance through the ground material. More than one of these current paths can be created, resulting in a ground loop, which induces AC hum from external sources in the circuit, much as happens when a shielded cable connects two pieces of equipment and the shield is connected to the equipment at both ends. Yes, your typical audio coble makes ground loops all over the place. But that's another topic worthy of discussion at a future date.

Anyway, the point here is that by connecting every grounded component to a single point, ground loops are avoided, and the equipment is nice and quiet.

The cheap and easy way to ground things is by connecting them to the chassis right where they sit. Then you have all sorts of slight differences in potential between these grounds which are begging for a current to run between them, and all sorts of separate points where EMI can be induced on the circuit.

The hard way of grounding stuff is a star ground scheme. You do this by connect ing a piece of wire to every component needing to be grounded and running these wires to the ground point, which can sort of kinda resembles star pattern on a schematic of you close one eye and poke yourself in the other. Now this is great in theory, but once you start snaking ground wires around a chassis, you find out that you can't get everything where you need it to go, and soldering 20 wires to a single point takes an arc welder.

As a compromise, I usually create a short buss wire by soldering a piece of 12 gauge bare wire to a terminal strip. That makes a long enough thing to connect all the ground wires to. I solder ONE end of the buss wire right to the chassis. The ideal location for this connection is near the input jack. Connect grounds from the low level circuits to the end of the buss wire that's soldered to the chassis, and put the power supply grounds toward the other end of the buss wire. This keeps the most hum sensitive paths the shortest, increasing their immunity to hum from the power supply ground buss. To totally avoid loops remember to insulate the input jack ground from the hole it goes through, and put any filter cans on insulators, instead of directly twisting the mounting tabs into the chassis mounting holes. This means trimming out the holes in the chassis so the tabs don't touch anymore.

Of course if you have a classic piece and it doesn't pick up hum, don't hack it. The last cool touch is to use shielded audio cable for the wire running from the input jack positive lead to the input of the first grid. Ground the shield to the star ground at one end only. This shields the lowest level signal that exists under the chassis.

I've used these techniques on the last three amps I've built and they don't pick up hum, period. Try it.

 

-- Dan