Amplifier Efficiency, Heat, And Thermal Limiting

Canonical Definition

Efficiency is the ratio of real audio output power to DC input power for a stated signal, load, supply, channel condition, temperature, and time. The difference becomes internal loss, mostly heat.

η = Paudio,out / Pdc,in
Ploss = Pdc,in - Paudio,out

Operating Point Matters

Efficiency changes with output level, load, supply voltage, modulation, frequency, switching method, idle current, output filter, and temperature. A class-D amplifier can exceed 90 percent in one test and be much less efficient at low level, difficult load, or another frequency.

“Class D” is a topology family, not a universal efficiency or sound-quality result.

Example

If an amplifier receives 120 A at 13.5 V while delivering a measured 1200 W of real audio power:

Pdc,in = 13.5 × 120 = 1620 W
η = 1200 / 1620 = 74.1%
Ploss = 1620 - 1200 = 420 W

The heat budget is therefore hundreds of watts, not the small difference suggested by quoting an unrelated peak-efficiency number.

Where Heat Appears

Output devices and switching losses
Power-supply transformers, inductors, rectifiers, and capacitors
Output filters and current-sense elements
Input, driver, and control circuitry
Power and ground terminals, fuses, holders, lugs, and cable resistance

Thermal Time Behavior

Short bursts can be clean while sustained use produces temperature rise, output reduction, earlier clipping, fan activation, protection cycling, or shutdown. Record the complete warm-up and recovery timeline rather than only the first seconds.

Observation	Possible mechanism
Output falls gradually while waveform stays clean	Thermal gain or supply limiting
Clipping begins earlier as temperature rises	Rail, supply, or current capability has changed
Protect cycles on and off	Thermal or monitored fault threshold with hysteresis
Terminal or holder heats before case	Series resistance in the electrical path

Installation Review

Leave the manufacturer-specified clearance around heatsinks and fans.
Do not bury the amplifier against carpet, insulation, or enclosure walls.
Verify intake and exhaust paths after trim panels are installed.
Measure terminal and fuse-holder temperature as well as case temperature.
Retest the real daily or demonstration condition after heat soak.

Fans Are Not The First Answer

A fan can improve convection, but it does not correct an unsupported load, low supply voltage, hot connection, clogged internal path, failed internal fan, or amplifier operated beyond its continuous design.