Spectrum analysis could be used to sort the ripple from the signal under drive, but it is simpler to leave the amplifier undriven and artificially provoke ripple on the HT rails by loading them with a sizeable power resistor in my work I have standardized on drawing 1 A. If a Class-B amplifier is measured for ripple output when quiescent, there will be a very low amplitude on the supply rails and the measurement may be very good, but this gives no assurance that hum will not be added to the signal when the amplifier is operating and drawing significant current from the reservoir capacitors. with a substantial ripple amplitude on the rails. This bold statement does, however, require a couple of qualifications.įirstly, the output must be ripple-free under load, i.e. It is my experience that if the amplifier is made ripple-proof under load, then it is proof against distortion components from the rails as well. This must not be confused with distortion caused by inductive coupling of half-wave supply currents into the signal path – this effect is wholly unrelated and is completely determined by the care put into physical layout I labeled this Distortion 6 (induction distortion).Īssuming the rail bypass capacitors are connected correctly, with a separate ground return, ripple and distortion can only enter the amplifier directly through the circuitry. The most common defect seems to be misconnected rail bypass capacitors, which add copious ripple and distortion into the signal if their return lines share the signal ground this was denoted Distortion 5 (rail decoupling distortion) on my list of distortion mechanisms in Chapter 3. The second possibility, the intrusion of distortion by supply-rail injection, can be eliminated in practice, at least in the conventional amplifier architecture so far examined. In a Class-B amplifier this will be in the form of half-wave pulses, as the output current is drawn from the two supply rails alternately if this enters the signal path it will degrade the THD seriously. The rails also carry a signal-related component, due to their finite impedance.Most people find this much more disturbing than the equivalent amount of distortion. A proportion of the 100 Hz ripple on the rails will appear at the output, degrading the noise/ hum performance.My aim is not just the production of hum-free amplifiers, but also to show that there is nothing inherently mysterious in power-supply effects, no matter what subjectivists may say on the subject. I want to confirm this by examining just how supply-rail disturbances insinuate themselves into an amplifier output, and the ways in which this rail injection can be effectively eliminated. ![]() I have (I hope) shown in earlier chapters that regulated power supplies are just not necessary for an exemplary THD performance. The literature on power amplifiers frequently discusses the importance of power-supply rejection in audio amplifiers, particularly in reference to its possible effects on distortion ! Power-Supply Rail Rejection in Amplifiers
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