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Power-supply rejection ratio (PSRR) describes how much a circuit suppresses a disturbance on its supply before that disturbance appears at its output. It matters to phone audio, but a single PSRR figure cannot tell you whether a phone will sound clean: the result also depends on frequency, regulator noise, load transients, available voltage headroom, circuit layout, and the audio output measured under realistic conditions.
How supply disturbances can reach a phone’s audio
A phone’s battery and power-distribution network are not ideal voltage sources. Radio transmission, display use, processor activity, and audio playback all change current demand. Because the battery and connections have impedance, changing current can create voltage fluctuations. Those fluctuations may pass through a regulator or couple through shared power and ground paths into an audio codec, headphone driver, speaker amplifier, or microphone front end.
This is a circuit-level possibility, not proof that every phone has audible interference. Whether a disturbance reaches the output depends on the device’s power architecture, filtering, layout, operating state, and audio path.
A historical cellular-radio example
An EDN article published July 17, 2008, described a GSM/TDMA example in which transmitter activity switched at 217 Hz and drew high current pulses. In that example, current could reach 1.7 A, with a possible battery voltage drop of up to 500 mV through the battery’s equivalent series resistance. These are figures from that historical example, not measurements or universal specifications for current phones. The mechanism illustrates how radio activity can disturb a shared supply; it does not establish that a contemporary handset will produce an audible buzz.
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What PSRR tells you—and what it leaves out
PSRR is commonly expressed in decibels as a ratio between a disturbance on a power rail and the corresponding disturbance at a circuit’s output. A higher rejection value means less of the specified supply disturbance appears at the output, under the stated test conditions.
But PSRR varies with frequency and operating conditions. A value measured at one frequency is not a promise of the same rejection across the audio band or across every radio-related disturbance. Analog Devices describes AC PSRR measurement as applying a signal to the supply and measuring the resulting output error component. The outcome depends on factors such as frequency, load, output voltage, signal path, and regulator headroom.
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When comparing two designs, look for PSRR across relevant frequencies and for the test conditions behind each number: supply-ripple amplitude and frequency, load current, output voltage, and available headroom. A headline figure without those details is incomplete.
Other power-supply factors that affect audio
Regulator output noise
PSRR describes rejection of disturbances arriving through the supply; it does not describe all noise produced by the regulator itself. An LDO can reject incoming ripple well and still contribute internally generated output noise. Analog Devices’ application note on LDO noise treats these as separate considerations. For audio, both the incoming disturbance spectrum and the regulator’s own noise matter.
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Transient response and load changes
A regulator’s small-signal PSRR does not show how it behaves when current demand changes quickly. Radio transmission, processor activity, or an audio amplifier’s changing load can provoke a transient voltage dip or recovery. A design therefore needs to be considered under changing load as well as with steady input ripple.
Dropout and headroom
A regulator needs sufficient input-to-output voltage margin to maintain regulation. If the supply approaches the regulator’s dropout region, performance can change; a good PSRR figure measured with comfortable headroom may not describe behavior near dropout. Battery voltage, regulator choice, load, and operating state all affect that margin.
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Layout and coupling paths
Not all interference travels through a regulator’s input-to-output path. Shared power or ground impedance can couple current changes into sensitive audio circuitry directly. Consequently, regulator specifications alone cannot characterize the complete phone design: the distribution network, grounding, filtering, and placement of audio and power components also shape the result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to assess whether power noise affects audio
The relevant question is not simply which regulator has the largest PSRR number, but how much supply-related error reaches the audio output in the operating conditions that matter. Audio Precision describes a PSRR test as measuring an amplifier’s ability to prevent power-system noise from affecting audio output.
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- Identify the supply disturbances of interest and their frequencies, including ripple and changes associated with load activity.
- Check PSRR over the relevant frequency range and record the test frequency, ripple amplitude, load, output voltage, and headroom.
- Measure regulator output noise separately; rejection of input ripple does not account for noise generated inside the regulator.
- Evaluate transient behavior and regulation near the expected load and battery conditions.
- Measure the audio output under equivalent operating conditions, rather than inferring audible quality from a regulator specification alone.
Audio analyzers are laboratory tools for injecting or measuring supply-related audio artifacts. Their role is to characterize a circuit or device, not to serve as a consumer add-on or evidence that an accessory will improve playback.
What PSRR cannot tell you about a phone
The sources cited here explain circuit mechanisms and measurement approaches; they do not compare current handset models, establish a universal audible threshold for supply artifacts, or show that a particular phone sounds better than another. Nor do they demonstrate that an external charger or accessory improves playback quality. A phone’s audio performance has to be assessed from measurements of that device and its complete signal and power paths, not from PSRR in isolation.
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