Power Supply Rail Health Check

Created on 2008-07-01 21:03:00

Using your AP to do a Power Supply Rail Health Check

by Kendall Castor-Perry, special to Audio.TST

Modern professional-quality audio systems are commonly put together from sub-assemblies which are frequently designed, qualified, manufactured and tested independently.  The hierarchical test process, where sub-units are tested separately to a module specification, substantially eradicates expensive whole-system rework caused by building in defective units.  The final system test generally concentrates on proving connectivity, full functionality and key audio specifications.

Some potential interactions between sub-assemblies can remain unexplored by this degree of testing, however.  These interactions may not be detected in the final, “goods-out” testing of a completed unit.  One such interaction to be wary of is that between an audio circuit or module and its power supply.  Differences in the characteristics of the supply used for upstream module testing compared to the one installed in the final product can cause distortion and coloration of the audio signal in the channel, as well as crosstalk between channels.

A source of such problems is the difference in output impedance versus frequency characteristics; this depends in detail on the power supply topology and the values of some of the key components employed.  The supply current of the audio circuits is almost invariably modulated by the signal being handled (an exception being  very carefully designed, single-supply class A amplifying stages).  This varying supply current then causes a varying signal voltage to appear on the power supply rails because the output impedance of the power supply is finite.  This residual voltage in turn can sometimes be inadequately rejected by the audio circuits themselves.

Your AP analyzer can help in several ways.  Firstly, the effect of finite power supply impedance reacting with signal-varying load current can be monitored directly.  This is done by driving the generator output signal through one of the audio channels and monitoring the power supply rail (with a DC value of up to 230 Volts for 2700 Series & APx520/525 or 200V for ATS-2) directly with an analyzer input (note that you can't, however, do this on the APx585/586 analyzers, which have DC-coupled input circuits).

When you use a sine-wave test signal, expect to see the signal on the supply rail contain both the fundamental frequency and a spread of harmonics, with the 2nd harmonic usually being particularly strong.  Plotting both the fundamental level and the THD+N as functions of the input signal level as the signal frequency is swept over the audio band delivers a signature for the response of that particular power supply responding to the demands of that particular circuit.

Changing components in either the power supply (a regulator or capacitor, for instance) or the audio circuitry (substituting an op-amp, maybe), can affect this signature.  If audio circuits weren't sensitive to what happens on their supply rails, this would be an interesting check of production consistency but wouldn't worry us on audio quality grounds.  But they are sensitive to it.  Some of this residual nastiness on the power supply rail will find its way into the output signal of the channel you're driving – and into others too.  This causes a distorted form of crosstalk – which gives us an ideal method for quantifying the situation using an AP analyzer.

To make use of this, make a crosstalk measurement between two channels in the equipment, paying particular attention to the harmonic components in the 'victim' channel.  Now, simple crosstalk caused by signal coupling between cables, circuit board traces and components is generally a linear process.  The presence of harmonics in the crosstalk signal is highly likely to be due to coupling through the supplies.  It's easy to see this distortion signal on the 'victim' channel because it has no signal of its own, but this signal will also be present on the main channel.  As it is likely to have a strong frequency dependent behavior originating in the interaction with the power supply, this can cause detectable timbral changes in the audio signal path, which may be detectable by critical users of the equipment.

Keep a careful eye on these interactions and make sure that subtle changes in module build specifications don't lead to a change in audio performance.  Your AP analyzer is a useful partner in keeping this under control!

Guest contributor Kendall Castor-Perry is an audio expert and longtime AP user with 21 years experience as an analog designer and systems engineer. He was also a runner up in AP's recent Test Bench of the Year competition.