GND when building multi-board audio equipment

[pipthepilot]

Hello Everyone,

I'm designing a 4 channel audio mixer and to make it easier to build and test, I decided to build different sections on separate PCBs. However, by separating the boards, I realise I'm not sure how to manage the grounding as my design currently has ground loops.

The design uses the following PCBs that i'm designing...

• Power board: Distributes ±15V and 48V phantom power
• Balanced input/output board: XLR sockets, phantom power switch and coupling caps
• 4x Channel boards: Stereo Preamp, Level and EQ
• Bus & Mixer board: L&R master BUS and mixer

The diagram below is an approximate layout of my design.


Power is supplied to the input & output board which sends power and balanced input signal to each of the channel PCBs. The bus/mixer board is also supplied power from the power board. Each channel send Left & Right signal to the bus/mixer board and the master left/right signal is sent back to the output.

I guess my question is should I include a GND signal for the audio out from each channel board to the mixer, and mixer to output or are those unnecessary?

Thanks in advance for the help,
Philip

 

[ccaudle]

should I include a GND signal for the audio out from each channel board to the mixer

A good first step is to remove the term "GND" from your vocabulary, and instead think in terms of function and current flow.
You have two needs: a current path for power supply return current, and a stable reference for the audio signal.
Mixing those two functions together thoughtlessly is a recipe for superimposing power supply noise onto your audio signal.

This paper is a good explanation for a more useful way to think of the signal and power flow.
Putzeys: The G Word, or How to Get Your Audio off the Ground

Read that paper a couple of times to let it sink in, then rephrase your question as "should I have a reference signal for my audio signal which has low loop area and is less likely to pick up noise, or should I let the reference for the audio signal take a poorly defined circuitous path around the entire chassis, forming a huge loop antenna to pick up magnetic fields" and the optimal approach will probably become more obvious.

But note that you are providing the audio reference signal, not "including a GND," which requires a change in how you think about the circuitry on the bus and mixer PCB.

 

[pipthepilot]

A good first step is to remove the term "GND" from your vocabulary, and instead think in terms of function and current flow.
You have two needs: a current path for power supply return current, and a stable reference for the audio signal.
Mixing those two functions together thoughtlessly is a recipe for superimposing power supply noise onto your audio signal.

This paper is a good explanation for a more useful way to think of the signal and power flow.
Putzeys: The G Word, or How to Get Your Audio off the Ground

Read that paper a couple of times to let it sink in, then rephrase your question as "should I have a reference signal for my audio signal which has low loop area and is less likely to pick up noise, or should I let the reference for the audio signal take a poorly defined circuitous path around the entire chassis, forming a huge loop antenna to pick up magnetic fields" and the optimal approach will probably become more obvious.

But note that you are providing the audio reference signal, not "including a GND," which requires a change in how you think about the circuitry on the bus and mixer PCB.

Thank you, haven't had chance to read yet but definitely will. You're absolutely spot on that I don't have the correct vocabulary to think of the circuitry correctly.