Dear fellow users,
I'm keen to understand star grounding more - specifically asking some practical advice for when using metal (or otherwise conductive - salty ice anyone?) enclosures.
Let's say you are building a stereo preamp with a linear power supply in it. The 'power' ground is connected to the mains earth. If you are using a metal chassis/project case - you would also ground the case (usually with a wire with a lug on the end and a machine screw to the case). This is important for safety reasons (if a live wire should break loose and touch the enclosure it shorts to ground rather than shocking the user). However, I also have my "signal" ground to account for. Most potentiometers/phono jacks have a grounded casing which will make contact with the project enclosure. This means (inconveniently) that there is some internal connection of grounds at the desired star point, but also ground connections all over the place through the casing. How does one avoid this in practice? Use plastic bits (ugly)? Use wood bits (fire hazard)? Put a low resistance between earth and the case (potentially dangerous/fine but illegal, I don't know)?. Something else?
Cheers in advance for suggestions.
I'm keen to understand star grounding more - specifically asking some practical advice for when using metal (or otherwise conductive - salty ice anyone?) enclosures.
Let's say you are building a stereo preamp with a linear power supply in it. The 'power' ground is connected to the mains earth. If you are using a metal chassis/project case - you would also ground the case (usually with a wire with a lug on the end and a machine screw to the case). This is important for safety reasons (if a live wire should break loose and touch the enclosure it shorts to ground rather than shocking the user). However, I also have my "signal" ground to account for. Most potentiometers/phono jacks have a grounded casing which will make contact with the project enclosure. This means (inconveniently) that there is some internal connection of grounds at the desired star point, but also ground connections all over the place through the casing. How does one avoid this in practice? Use plastic bits (ugly)? Use wood bits (fire hazard)? Put a low resistance between earth and the case (potentially dangerous/fine but illegal, I don't know)?. Something else?
Cheers in advance for suggestions.
All potentiometers are isolated from their cases, which serve only to screen.
Your signal ground only needs to meet the chassis at one point, if at all (ground lifter), but
your power supply ground often doubles as signal ground, requiring careful attention to star-grounding layout, and usually requiring signal ground be bonded to the chassis.
Isolated jacks are available, and in the old days often jacks came on insulated sub-assemblies, and pcb-mount jacks are often like this.
Your signal ground only needs to meet the chassis at one point, if at all (ground lifter), but
your power supply ground often doubles as signal ground, requiring careful attention to star-grounding layout, and usually requiring signal ground be bonded to the chassis.
Isolated jacks are available, and in the old days often jacks came on insulated sub-assemblies, and pcb-mount jacks are often like this.
On panels like switchcraft where all the RCA jack grounds are tied together, I separate analog ground from case ground with 1/32" o rings. I get them from industrial supplies. The I run a wire from the RCA ring to the PCB near the input device (op amp) to ground.
On single connections the theoreticians say the least noise is to connect RCA ring to case & safety ground, but with multiple inputs running over 36 square feet I find hum is lower with the rings & shields separately grounded.
Bands with 200 sq ft setups often find using case ground as power supply analog ground leads to hum and other noise. DI units are used to separate case ground (legally required to be connected to building safety ground) from analog ground. Or some of us buy obsolete equipment with 2 pin power cords like my old dynakit & hammond equipment, to avoid that ground loop issue. My power amp is hidden behind a sofa to prevent potential electrical shock from case ground, which wouldn't work with a baby in the house. One benefit of no grandchildren. Differential inputs to use twisted pair to reject hum weren't available in 1966, except as $150 vacuum tube "op amps" drawing 100 watts each from philbrick, etc. Now PA amps come with a $.38 op amp to reject hum first thing.
On single connections the theoreticians say the least noise is to connect RCA ring to case & safety ground, but with multiple inputs running over 36 square feet I find hum is lower with the rings & shields separately grounded.
Bands with 200 sq ft setups often find using case ground as power supply analog ground leads to hum and other noise. DI units are used to separate case ground (legally required to be connected to building safety ground) from analog ground. Or some of us buy obsolete equipment with 2 pin power cords like my old dynakit & hammond equipment, to avoid that ground loop issue. My power amp is hidden behind a sofa to prevent potential electrical shock from case ground, which wouldn't work with a baby in the house. One benefit of no grandchildren. Differential inputs to use twisted pair to reject hum weren't available in 1966, except as $150 vacuum tube "op amps" drawing 100 watts each from philbrick, etc. Now PA amps come with a $.38 op amp to reject hum first thing.
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