understanding star grounding

[AndrewT]

Hi,
cheat and take the easy way out. Build two monoblocks.

[Alabama978]

Firstly I am no expert in audio.

I have built the original Pearl Phono stage and the MC stages from the boards in the group buy thread (Promthieus). I realise this is not the thread but it seems to me that much brain power has been concentrated into the discussions of grounding, so I am hoping that you can help me diagnose where the problem is.

I have a terrible bad hum on both channels without a source connected.

My layout is a separate PSU with XLR terminated cable. I have one toroidal Trans feeding both psu boards.

From there they are connected by the cable to the Phono Stage. I have one common ground in the cable connected to the IEC connector Earth point, and the other end to the Chassis in the phono stage.

All RCA's in the phono stage are insulated from the chassis and meet at a star ground point.

This is where I might be doing something stupid

It seems to me the RCA's will all be grounded to the chassis anyway at the star point? Or should the star ground point also be insulated from the chassis

Anyway the hum is so bad I can barely listen to the stage, which is disappointing because when the sound level is high the detail and imaging is excellent.

Grateful of any help as I have spent many hours building and don't want to abandon the project.

[David Davenport]

To specifically answer your question: The star ground should be connected to the chassis.

You have a choice where the star ground is located. The chassis must be connected to the mains safety ground close to the IEC connector. This point may be the system star ground. Alternately, if the star ground is located elsewhere, perhaps on a circuit board, then a wire should be installed between the star ground and the point where the safety ground connects to the chassis.

Grounding is important between your two chassis.

First, there must be a dedicated wire from the point where the safety ground connects to the power supply chassis through the connecting DC power cable to the chassis containing the phono circuitry. The connector(s) on the cable should be such that this safety ground makes first when the connector is inserted and breaks last when the connector is removed.

Second, each power common (ground) should have a separate wire in the cable, as each voltage should have a separate wire. That is, voltages and grounds for the two channels should be separately wired through the cable.

In the power supply design, be careful about circuit layout and where you pick up the voltage and common.

Be as careful on routing power and ground currents as you are for signal currents.

As this is an MC preamp, shielding and layout are very important as are cable and wire routing.

This is a simple overview, there is a lot more information in my article at http://www.raleighaudio.com/Audio%20...connection.pdf

Dave Davenport

[Alabama978]

Thanks for the detailed reply.

I am going to start at the beginning, always a good place to start

I will check all my wiring in the PSU and then look at the power cable to ensure I have the correct safety and ground connections. Then I will recheck every connection in the Phono Stage.

You may not hear back from me for a while due to work commitments, but I will report back when I have checked it all.

Many thanks for making the requirements easy to understand.

[uzernaam]

When connecting a multi-channel amp to a computer, if you use the 1/8" stereo out, the grounds of the two signal wires are connected.

Does it automatically create a ground loop, no matter how your amplifier is wired?

Or does the noise that shows up when the cable is plugged in come from the computer itself?

I have been experimenting with different grounding schemes for over a week, and can't get rid of the noise that shows up when I plug my amp into my computer.

[AndriyOL]

Originally Posted by uzernaam

When connecting a multi-channel amp to a computer, if you use the 1/8" stereo out, the grounds of the two signal wires are connected.

Does it automatically create a ground loop, no matter how your amplifier is wired?

Or does the noise that shows up when the cable is plugged in come from the computer itself?

I have been experimenting with different grounding schemes for over a week, and can't get rid of the noise that shows up when I plug my amp into my computer.

I solved this problem with my chipamp.

[jamal]

hello, there is a interesting book by Ralph Morrison. title GROUNDING and SHIELDING, CIRCUIT and INTERFERENCE. could anybody do a read up and tell us the story......thanks

[AndrewT]

Originally Posted by AndriyOL

I solved this problem with my chipamp.

be nice, tell us how.
Or post a link to your solution.

[jamal]

yah, be nice to us.

[pacificblue]

Originally Posted by uzernaam

When connecting a multi-channel amp to a computer, if you use the 1/8" stereo out, the grounds of the two signal wires are connected.

Does it automatically create a ground loop, no matter how your amplifier is wired?

No. In the '70s and '80s audio gear from Germany usually came with DIN connectors that also shared the ground for both stereo channels. All manufacturers got such a setup working without audible ground-loops.

Originally Posted by uzernaam

Or does the noise that shows up when the cable is plugged in come from the computer itself?

Maybe. Sometimes the computer's power supply is the culprit. You can check that with headphones. If there is no noise, recheck by connecting a different amplifier.

Originally Posted by uzernaam

I have been experimenting with different grounding schemes for over a week, and can't get rid of the noise that shows up when I plug my amp into my computer.

Try not to see your amplifier as an isolated device, but to include the PC into the grounding scheme.
Use the same outlet for PC and amplifier. That will make the loop through the PE leads smaller.
Check if the noise is reduced, when you turn either of the mains plugs by 180°.
Did you already try to insert a resistor (< 10 Ohm) between signal and power ground?