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Old 14th April 2012, 01:54 PM   #1
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Default ground in project box.

hello there : )

I am making a preamp in a project box (steel), that is powered by an external psu.

A TL071 is powered from a 12v regulator inside the project box.

and I also have a line balance driver (that needs dual rail power) so to send the audio out the box on a balanced chassis XLR.

So there are two different power needs for the two audio stages inside the same box.

I wonder what would be the best way to create the dual rail power? I see there are regulators with a minus - output, and + output, would I need two regulators? or perhaps use a floating ground design to cater for the balancing line driver?

i believe I need to have the XLR's ground connect to the project box chassis, i just wonder about the 12v regulator ground and the dual rail ground, are they all allowed to connect together? : ?

thanks for any help,
really appreciated : )
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Old 14th April 2012, 02:17 PM   #2
AndrewT is offline AndrewT  Scotland
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Is the external PSU double insulated? Indicated with the concentric squares symbol.
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Old 14th April 2012, 02:36 PM   #3
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hello Andrew, I am not sure ?

it is an old psu i had from a midi keyboard.

however, i could buy a new psu that would make this project work. : )
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Old 14th April 2012, 05:36 PM   #4
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Quote:
Originally Posted by AndrewT View Post
Is the external PSU double insulated? Indicated with the concentric squares symbol.
hi andrew i just found another psu that has a sticker on it, it has a square inside a square symbol, sino american model a41208B, it says 12v , with no load ,on my meter it reads 17v,

would this be of use? : )
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Old 14th April 2012, 05:40 PM   #5
AndrewT is offline AndrewT  Scotland
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The double insulated PSU allows you to build your mains powered project without the need to connect it to Protective Earth..
Without that Safety Earth, it it usually found you have fewer problems with Hum.
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Old 15th April 2012, 06:59 PM   #6
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i always thought that the earth connection was used to draw noise away from the audio system. that the shield in the cables and ground planes of a circuit boards were connected to earth. : /
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Old 15th April 2012, 08:01 PM   #7
AndrewT is offline AndrewT  Scotland
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NO and no again.
The audio side needs it's own reference point from which all other parts of the audio make their reference to. This is usually Ground and I now refer to it as Main Audio Ground MAG.
you will see this variously referred to as Ground, Earth, Return, Power Zero Volts.

The Safety side is a completely separate issue. eg, a battery powered audio appliance works well and safely without a connection to the mains supply system.

The Safety side becomes an issue when your project is powered by the Mains electricity supply.
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Old 15th April 2012, 09:04 PM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by qweetz
i always thought that the earth connection was used to draw noise away from the audio system. that the shield in the cables and ground planes of a circuit boards were connected to earth.
No, its not like draining a swamp. An external ground can sometimes help establish a reference potential, but it is just as likely to make things worse.
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Old 16th April 2012, 06:57 AM   #9
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Quote:
Originally Posted by AndrewT View Post
NO and no again.
The audio side needs it's own reference point from which all other parts of the audio make their reference to. This is usually Ground and I now refer to it as Main Audio Ground MAG.
you will see this variously referred to as Ground, Earth, Return, Power Zero Volts.

The Safety side is a completely separate issue. eg, a battery powered audio appliance works well and safely without a connection to the mains supply system.

The Safety side becomes an issue when your project is powered by the Mains electricity supply.
Andrew T,

Why, for power amps, is the center-tap, & GND, connected to chasis (mains earth)?? Where as, for example, a power supply used for a pre-amp, discrete op-amp, mic-pre, etc,, do NOT have its center-tap & GND connected to Chassis/Mains Earth (even with metal enclosure)??

Thanks
CM
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Old 16th April 2012, 09:18 AM   #10
AndrewT is offline AndrewT  Scotland
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Sorry for the late reply and unfinished post7.

Mains powered electrical equipment comes in a few distinct forms.
We generally use ClassI and ClassII equipment.
We can only build ClassI equipment.
The mains outlet socket for shavers in the bathroom is to yet another class.

ClassII is commonly referred to as "double insulated". The standards and testing and quality control required for ClassII equipment are rigorous. Very stringent.
We amateurs cannot replicate that. I do not know how to start a ClassII design, never mind determining what tests I would need to implement the standards.

ClassI must use the Protective Earth (PE) to help ensure that when a fault develops the fuse on the Hot blows quickly and to help ensure that the residual voltage on the external exposed conductive parts cannot seriously injure a user.

That PE must be permanently connected to Chassis. I call it Safety Earth, to differentiate it from the generally misused term "Ground".

There is a second requirement
All exposed conductive parts must be connected to Safety Earth.
This is why the Main Audio Ground (MAG) is connected to Chassis.
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Last edited by AndrewT; 16th April 2012 at 09:22 AM.
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