Audio Switch Board Grounding

Question

I'm fairly new to electronics. I'm designing a simple board with a left and right channel audio input, and two sets of left and right channel outputs. A 16Mhz AVR microcontroller controls two relays (one for each output), which allow each output to be toggled independently. The signals will be line level audio feeding into audio amps, so very little current should be flowing through the signal lines.

In order to minimize noise on the audio signal, what is the best way to handle grounding? Should I have two isolated ground planes, one for signal and one for digital circuitry, or is having one ground plane with the components laid out so that the microcontroller switching current doesn't flow near signal lines sufficient?

Secondly, should I include a relay to switch the signal grounds, or leave them connected? Are there any potential issues with leaving the two output grounds connected, even if only one output is on?

Answer 1

In order to minimize noise on the audio signal, what is the best way to handle grounding? Should I have two isolated ground planes, one for signal and one for digital circuitry, or is having one ground plane with the components laid out so that the microcontroller switching current doesn't flow near signal lines sufficient?

Ideally connect the digital and audio grounds at only one point.

Secondly, should I include a relay to switch the signal grounds, or leave them connected? Are there any potential issues with leaving the two output grounds connected, even if only one output is on?

Don't switch the signal grounds with the relay but, if possible connect the output lines to ground when switched off. If you leave them floating they will pick up stray hum.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The ground connection when the relay is off ensures that there is no noise on the output in the off condition.

If your relays are not changeover type you might reduce the noise by addition a a resistor between LEFT OUT and ground. Make it low enough to present a reasonable load to the noise but high enough that it doesn't load the previous stage excessively.

Since the previous stages may have a DC blocking capacitor on their output the voltage on the LEFT IN line may start to rise (due to leakage across the capacitor). Addition of R1 will discharge this line to ground preventing a DC thump when the relay is energised.

Answer 2

1) I would keep the analog ground separated from the digital ground.

2) The relay(s) provide galvanic isolation between the digital control signals and the audio path.

3) Leave the input and output grounds connected to each other at all times.

4) It may be necessary to ensure that the DC level remains at the identical value regardless of whether the relay is closed or open. This reduces or eliminates the chance of a pop or click when the relay changes state.