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Old 12th July 2008, 12:53 AM   #1
mnr102 is offline mnr102  United States
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Default chassis grounding question

Hello,
I'd like opinions on why/how I should ground my current project. Here's what I planned to do, and why... Please let me know if I've got something wrong:

The project is the cmoy headphone amp. I will, however, have 2 seperate chassis, one containing a linear regulated power supply, and the other containing the op-amp circuit.

The linear regulated power supply puts out +/-15V and "common" (which I believe is not grounded to the unit's chassis). I'll be using a 3 prong'ed AC cord, and it's ground leg will go to the power supply chassis.

1) Should I tie the power-supply's "common" terminal to the chassis that holds it, thereby combining earth ground & "virtual" ground? Or should I leave them seperate, or should I connect them w/ R-C filter between them?

The 3 conductors (+15V, -15V, and common) will be delivered to the op-amp chassis by 3 conductor wire, which may have a shield.

2) If this wire has a shield, should I tie both ends of the shield to the chassis it connects to? (Note, the power supply is the only chassis connected to earth ground. Yamaha Sound Reinforcement Handbook suggests not doing this if both chassis have AC power cords for fear of ground loops, but that's not the case here).

My RCA input jacks for the op-amp chassis aren't uninsulated. That implies that the audio signals would be relative to a chassis that's "common", however, tangentsoft's great pages on the CMoy amp suggest that chassis are often tied to -V, not ground/common.

Thanks for any comments. I've been struggling to get my head around these nuances for years.

mnr102
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Old 12th July 2008, 06:18 AM   #2
fotios is offline fotios  Greece
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Default Re: chassis grounding question

Quote:
Originally posted by mnr102
Hello,
I'd like opinions on why/how I should ground my current project. Here's what I planned to do, and why... Please let me know if I've got something wrong:

The project is the cmoy headphone amp. I will, however, have 2 seperate chassis, one containing a linear regulated power supply, and the other containing the op-amp circuit.

The linear regulated power supply puts out +/-15V and "common" (which I believe is not grounded to the unit's chassis). I'll be using a 3 prong'ed AC cord, and it's ground leg will go to the power supply chassis.

1) Should I tie the power-supply's "common" terminal to the chassis that holds it, thereby combining earth ground & "virtual" ground? Or should I leave them seperate, or should I connect them w/ R-C filter between them?

The 3 conductors (+15V, -15V, and common) will be delivered to the op-amp chassis by 3 conductor wire, which may have a shield.

2) If this wire has a shield, should I tie both ends of the shield to the chassis it connects to? (Note, the power supply is the only chassis connected to earth ground. Yamaha Sound Reinforcement Handbook suggests not doing this if both chassis have AC power cords for fear of ground loops, but that's not the case here).

My RCA input jacks for the op-amp chassis aren't uninsulated. That implies that the audio signals would be relative to a chassis that's "common", however, tangentsoft's great pages on the CMoy amp suggest that chassis are often tied to -V, not ground/common.

Thanks for any comments. I've been struggling to get my head around these nuances for years.

mnr102
Normally the mains earth and the common (that you say virtual gnd) of power supply should be seperated. The same also the shield of input/output RCA sockets because they are tied to com of power supply. To avoid any gnd loop and to obtain a high crosstalk the way is to use seperate power supplies for each channel, that means the com of each channel should be seperated from the com of the other channel.
The chassis should be earthed to mains. But only this which contains the power supply. The chassis which contains the amplifiers should be earthed via the shield of the supply cable between the two units. To this, a screened cable of 6 conductors it is the appropriate solution. As you understand, the screen of cable must be electrically earthed only in the power supply chassis. Then, the screen must be tied in the chassis of amplifier. After this, you should be careful the two chassis not touch the one to other. If you place the two units stacked, use ruber feets.

Fotios
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Old 12th July 2008, 09:14 AM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Looking at just the safety side of your question.

All exposed conductive parts must be connected to Safety Earth.

You propose to bolt the incoming Earth wire to the PSU chassis. OK.
What about the other exposed conductive parts?

Now a failure senario:
the Live comes loose and touches the zero volts PSU output trace.

The PSU output cable has Live on the zero volt conductor.

A young child comes along and puts the exposed end in his/her mouth!!!

Let's go further.
The faulty PSU is plugged into the amplifier box. It is not Safety Earthed.
The zero volts input sits at Live potential.
The wife comes in and thinks the stereo has gone silent and goes behind the speakers to fiddle with the connections. Touches the speaker ground wire and .........!


You must address the safety issues before even thinking about the audio quality/performance side.
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regards Andrew T.
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