Proper grounding with multiple ins/outs

Hey everyone! I'm tinkering with a box with 6 inputs and 6 outputs, all on XLR, all wired to a PCB with standard twisted pair mic cable.    What is best practice in terms of grounding each input/output for prevention of ground loops?  All inputs and outputs are THAT chips.

Thanks!

balanced input... perhaps search "pin 1 problem"

JR

Yep, read that some time ago, and always seem to forget about it.  For single input devices I'm always connecting the input/output pin 1 straight to the chassis tab.  Wasn't sure if I should be doing that for ALL of 'em though.  Guess it makes sense.  Fun fact: last time I was in a Distressor, I saw that all three pins of both input and output go straight to the PCB.  I've seen this in a surprsign number of commercial products.

Raises another question for me, though: on devices that have 8 balanced channels of I/O on a DB-25 connector, how is grounding handled?

JohnRoberts said:

balanced input... perhaps search "pin 1 problem"

JR

Click to expand...

For all balanced ins and outs, the general rule is ground the screen to the chassis at the connector. The far end of screened cables from this connector should be left unconnected.

So with a DB25 you can connect all the screens together and connect thenmto the chassis at the connector.

Cheers

Ian

ruffrecords said:

For all balanced ins and outs, the general rule is ground the screen to the chassis at the connector. The far end of screened cables from this connector should be left unconnected.

So with a DB25 you can connect all the screens together and connect thenmto the chassis at the connector.

Cheers

Ian

Click to expand...

Awesome!  Thanks, Ian!

- Ian

sircletus said:

Hey everyone! I'm tinkering with a box with 6 inputs and 6 outputs, all on XLR, all wired to a PCB with standard twisted pair mic cable.    What is best practice in terms of grounding each input/output for prevention of ground loops?  All inputs and outputs are THAT chips.

Click to expand...

If the inputs and outputs are balanced (which they are if they're THAT chips) then that is what minimizes group loop noise. There is no "ground" in balanced lines. That's the point. So unless you've done something really screwy, you should have no ground loop issues at all.

Pin 1 is for shielding (and for 48V phantom return current in the case of a mic) and has nothing to do with ground loop hum if the signal wires are balanced. Thus each pin 1 of every XLR should be connected to the chassis directly at the entry point. This is because it's for shunting electrostatic noise like RF (as opposed to electromagnetic noise like mains hum from nearby power cables) so it's important that pin 1 is connected to the chassis through the shortest length of wire possible. The length of that wire determines the wavelength (aka frequency) of RF that will be filtered. This is why many XLR connectors have a metal spike next to the screw hole so that when you tighten the screw it stabs the metal panel.

Note that it's common to add small ceramic capacitors to the signal wires to the chassis as well. This is why I make little circular boards that solder directly to the XLRs that have little SMD caps for shunting RF and low-capcitance TVS diodes for shunting high voltage transients on line inputs and line outputs (can't use 'em on a mic input because of phantom power).

And of course if you're shunting things into the metal panel, it should not be overlooked that said panel must have good conductivity throughout the entire chassis, sides, top, bottom and then finally to where the mains earth terminal is connected to the chassis. This point is usually a screw with a toothed washer screwed in from the inside with two wires - the mains earth wire and a wire going to the filter cap ground(s) of the power supply. So there should be one single point where the chassis and circuit ground are connected. Meaning don't also connect pin one of any XLRs to the ground of the circuit.

This topic has been discussed many times on this site and elsewhere so you can search around (use 'site:groupdiy.com' when Googling) and find more info but it is so important it's worth repeating over and over.

Good stuff squarewave!

If you look in the Power folder in the DIY tabeof my web site:

http://www.customtubeconsoles.com/diy

you will find a pdf file called Grounding 101 which covers most of what has be said so far.

Cheers

Ian

squarewave said:

...... Thus each pin 1 of every XLR should be connected to the chassis directly at the entry point. This is because it's for shunting electrostatic noise like RF (as opposed to electromagnetic noise like mains hum from nearby power cables) so it's important that pin 1 is connected to the chassis through the shortest length of wire possible. .....

Click to expand...

IMO, ESI isn't the same as EMI/RFI ( or I didn't understand you correctly).

moamps said:

IMO, ESI isn't the same as EMI/RFI ( or I didn't understand you correctly).

Click to expand...

My understanding (which I hope someone will correct me if I'm wr-wr-wrong) is that Radio Frequency Interference (RFI) is a specific class of ElectroStatic Interference (ESI) which refers to high voltage / low current AC transients (such as from a radio transmitter). ElectroMagnetic Interference refers to low voltage / high current magnetic field transients (such as from a nearby power cable).

squarewave said:

My understanding (which I hope someone will correct me if I'm wr-wr-wrong) is that Radio Frequency Interference (RFI) is a specific class of ElectroStatic Interference (ESI) which refers to high voltage / low current AC transients (such as from a radio transmitter). ElectroMagnetic Interference refers to low voltage / high current magnetic field transients (such as from a nearby power cable).

Click to expand...

rather it's:
(ESI) which refers to high frequency / low current AC transients
and
ElectroMagnetic Interference refers to low frequency / high current magnetic field transients

The EU specs call up an electrostatic discharge test as part of the susceptibility sections. Perhaps this is what is meant by ESI?

Cheers

ian