Audio PCB Design: Noise, Grounding and Power Supply

Audio circuits are judged by what cannot be heard. A preamplifier with a signal to noise ratio twenty decibels worse than expected may still measure correctly on a bench meter, because the problem is a ground current or a coupling path rather than a component value. An audio PCB is therefore designed around the quiet parts of the circuit, and the layout does most of the work.

What Makes Audio Different

The signals are small, the bandwidth is narrow and the listener is sensitive to effects that no specification captures well. A switching harmonic that would be invisible in a digital system can be audible as a whistle, and a ground current in a shared conductor becomes hum at the mains frequency or at twice it. The design has to keep those currents away from the conductors that carry the signal.

The supply is the second distinguishing feature. A single ended audio stage has poor rejection of supply noise, so what appears on the rail appears in the output, attenuated only by the power supply rejection ratio of the amplifier. That makes the supply part of the signal path rather than a supporting subsystem.

The Noise Floor

Noise reaches the output through several paths. It can be conducted from the supply, coupled capacitively from an adjacent trace, induced magnetically by a current loop, or generated by the components themselves. Each has a different fix, and the first step is to identify which one dominates in a given design rather than to apply all the remedies at once.

Thermal noise sets the floor that cannot be removed, and it depends on resistance and bandwidth. A high impedance input with a large feedback resistor will be noisier than a low impedance design with the same gain, which is why low noise audio stages are built around low values and low impedance sources rather than around quiet op amps alone.

Grounding Strategy

The central question in an audio layout is where the return currents flow. A star arrangement, in which each stage has its own return conductor meeting at one point, prevents the current from one stage from developing a voltage across the return of another. Where a continuous ground plane is used instead, the plane performs the same function provided no large current is allowed to flow across it.

In practice the two approaches are combined. A ground plane provides a low impedance reference for the small signals, and the high current returns from the output stage and the supply are routed so that they do not cross the region where the sensitive circuits sit. The reasoning is the same as in any mixed signal design and is described in mixed signal PCB design guidelines.

Power Supply and Decoupling

The supply for an analogue stage should be as quiet as the design allows, which in practice means either a linear regulator with adequate rejection or a switching regulator followed by filtering. Where a switching converter is used, its switching frequency and its harmonics must be kept out of the audio band, and the output filter must attenuate them below the noise floor of the amplifier.

Decoupling follows the same logic as everywhere else but with different priorities. The capacitor beside the amplifier pin supplies the transient current the stage draws, and its loop must be small. Bulk capacitance supplies the slower demand, and the regulator supplies the average. The layout discipline for the converter itself is described in DC to DC converter layout and routing.

Analogue Layout

Keep the input of a high gain stage as short as possible and surround it with a ground that belongs to the same reference. The feedback network should be placed next to the amplifier rather than at the far end of the trace, because the input capacitance of the trace and the impedance of the feedback path interact to produce a pole that can compromise stability.

Where a signal leaves the board, the connection should be made carefully. A single ended output referenced to a ground that also carries digital current will inject that current into whatever is connected. Where the connection is long or the environment noisy, a differential or a balanced arrangement is worth the extra complexity, because it removes the dependence on the ground reference at the far end.

Digital and Analogue Together

Most audio products contain both. The conversion devices are the boundary: they have an analogue side and a digital side, and the layout should treat them as two separate circuits joined only at the device. The analogue ground pin connects to the analogue reference, the digital ground pin to the digital reference, and the two meet at the converter as the datasheet requires.

Digital signals should not be routed over the analogue region, and no digital return current should flow through the analogue ground. Where the two must cross, they should cross at right angles on different layers so that the coupling is minimised. Clock and converter signals are the most important to keep away, because their frequency content reaches into the audio band through intermodulation. Keeping the conversion devices as far as possible from the sensitive analogue stages, and giving each its own local decoupling, reduces the interaction. Where the board must be small, the converter can be placed at the edge of the analogue region so that its return current has a direction to leave rather than crossing the sensitive area.

Component Selection and Layout Choices

The components set the limit that layout cannot exceed. A resistor with a high temperature coefficient drifts, a capacitor with the wrong dielectric becomes nonlinear, and an operational amplifier chosen for its low offset may have a noise density that is poor for the impedance level in use. Selecting for the parameter that matters in the stage, rather than for the headline number, saves a great deal of iteration later.

Layout then protects what the components provide. Keep the high impedance nodes small, keep the low impedance returns short, and keep the input and the output of a gain stage physically separated so that feedback does not occur through the board itself. Where a design is on the edge of the noise target, a guard ring around the input node, tied to the same reference, is a cheap measure that often brings it back.

Measurement and Verification

Verification of an audio board is measurement to a defined bandwidth and weighting, with the input terminated and the gain set as it will be used. A spectrum of the output with no signal applied shows which components are present, and it distinguishes a supply harmonic from a ground related hum by its frequency and by how it changes when the supply is altered.

Where a noise source cannot be identified by ear or by a simple measurement, a current probe or a near field probe is more informative than a voltmeter. The goal is to establish whether the noise is conducted, capacitively coupled or magnetically induced, because the three require different changes to the layout and applying the wrong one wastes a board revision. The wider set of characteristics that distinguish a quiet board from a merely conforming one is discussed in PCB design quality characteristics.

FAQ

Is a ground plane good for audio? It is good for the small signal reference, provided the high current returns are kept out of the region where the sensitive circuits sit.

Why does a star ground help? Because it stops the return current of one stage from developing a voltage across the return path of another, which would appear as crosstalk or hum.

Can a switching supply be used for audio? It can, with adequate filtering and a switching frequency chosen so that its harmonics do not fall in the audio band. A linear regulator afterwards usually makes the design much easier.