Physical construction of star ground - diyAudio
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Old 17th May 2012, 10:19 AM   #1
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Default Physical construction of star ground

Hey guys,
I've built a power amp based on the diyaudio designed AB amp (by Ostripper I think) and am at the point of putting it into the chassis. There are quite a few individual PCBs which I have hand etched (2 power amp PCBs, preamp, main HT and low voltage regulated PS's, protection and fan control etc) with numerous ground wires separating signal, decoupling cap and high current grounds.
I have planned my grounding topology using a star ground and have read about 'tophats' or the use of an arc of heavy gauge house wiring to physically construct the star ground point. I've never seen these tophats, so I'll probably go with the house wire method, but I can't think of a good way to firmly attach it the the metal chassis without electrically connecting it the the chassis.
The chassis is sheet metal and the only way I've come up with is screwing a small piece of MDF to the chassis and using that as a nonconductive point to attach the house wiring with a screw.
Any other suggestions?
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Old 17th May 2012, 12:28 PM   #2
DF96 is offline DF96  England
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Use a terminal or tag strip with just one isolated terminal/tag. Or one with a few tags, but just use one of them. Keep PSU circulating currents well away from the star ground - many people seem to get this wrong.
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Old 17th May 2012, 01:01 PM   #3
AndrewT is offline AndrewT  Scotland
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add a solder tag to the end of each grounding wire.
Bolt all the solder tags together.
measure the hum and buzz and noise at the amp output.
Swap around the tags on that bolt. you will find that the order of the tags on the bolt will change the measured results.

Finally, add a further wire+tag to the bolt and use it to connect the "star" to the chassis.
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Old 17th May 2012, 01:43 PM   #4
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Thanks for the suggestions, tag strip sounds easy because it'll allow me to attach it to the chassis with a bolt while keeping it isolated. I like the solder tag idea too, that's what I was originally thinking, but how would you keep that isolated from the earthed chassis? I want to earth the electronics at the input connectors, not at the star ground.
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Old 17th May 2012, 01:52 PM   #5
DF96 is offline DF96  England
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Unless you are handling RF, it may be better to isolate the input connectors where they pass through the case and ground elsewhere. You don't want ground currents going down the input screens/returns inducing a voltage on the input signal conductor.
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Old 17th May 2012, 01:57 PM   #6
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Yeah the location of the connection of the earth to the electronics for minimum hum is one thing I'm not certain about. Self recommends earthing at the input connectors (my connectors are isolated), but others point out that is there is a serious fault and the electronics become live fault currents have to pass through the small signal grounding tracks and screens etc, which isn't ideal.
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Old 17th May 2012, 02:01 PM   #7
AndrewT is offline AndrewT  Scotland
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You do not "earth" the electronics at the "inputs".

The electronics must be connected to chassis. That chassis must be connected to protective earth.
Those two routes must be capable of passing "fault" current back to the electricity distribution board.
That "fault" current can be a few thousand amperes.
Most signal wiring will burn out with 1kA to 2kA !!!!
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Last edited by AndrewT; 17th May 2012 at 02:04 PM.
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Old 17th May 2012, 02:01 PM   #8
DF96 is offline DF96  England
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Think about where the currents go, and regard each ground connection as a resistor (it is a resistor!). Regard each wire as a generator of electric and magnetic fields, with the former dominating for high voltage and the latter for high current. Screen electric fields with grounded conductors; cancel out magnetic fields with opposite currents twisted together. Don't blindly follow anyone else's grounding scheme.
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Old 17th May 2012, 02:03 PM   #9
AndrewT is offline AndrewT  Scotland
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Self does not recommend "earthing" in the safety sense at the inputs. He recommends that the ground reference for the signal be at the inputs. This is the same as Leach offers as an alternative if connecting at the PCB is not successful.
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Old 17th May 2012, 02:13 PM   #10
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Quoting Self, "A better approach is to put the audio-chassis ground connection at the input connector". Check out fig 18.1 on page 485 if you have the book handy; he suggests heavy gauge connection from power connector safety earth to the chassis, and a connection from that point to the input connectors. You're correct when you say he recommends the ground reference for the signal be at the inputs, that's what I meant.
So when I have my isolated connectors and my safety earthed chassis, I take it that the best method going on Self's advice is to make a ground referencing connection to signal ground from safety earth to the input connector?
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