Supply pumping


Q: What is ‘supply pumping’ and do I need to take precautions?
A: During positive excursions, half-bridge class D amplifiers will draw an excess of energy from the positive rail and return the remainder to the negative rail. This charges the negative power supply capacitor. During negative excursions the pattern reverses and the charge previously stored in the negative capacitor is used up while some extra charge is delivered to the positive capacitor. Pumping occurs when a DC input is applied. When that happens, one side keeps charging and the rail voltage keeps increasing. This process can continue until the overvoltage protection trips. If the amplifier cycles on/off at low signal levels that is a sure sign of a DC input.
Under normal conditions, pumping is not an issue. Audio frequencies are high enough and the power supply capacitors large enough as to render the charge/discharge effect undetectable. Pumping can become an issue when the supply is an SMPS with small output capacitors or when the input coupling capacitors are defeated.

Q: What is the maximum allowable amplifier DC-offset?
A: Above 30mV, turn on/off pops may become audible. Above 200mV, pumping may occur.