Why you don't get the power you expect from your amplifier

Why you don't get the power you expect from your amplifier

Donell Perry |

I’m going to explain impedance rise and its significance for amplifiers.

Impedance rise occurs when the impedance of a subwoofer goes above its nominal value. This can be caused by various factors, including free air or what most people know as “box rise.”

Let’s consider a hypothetical scenario with made-up numbers to illustrate the concept, though the fundamentals are real.

Imagine we have a 1000W amplifier that delivers 1000W at 4 ohms. This means that at any impedance above 4 ohms, the wattage output will be less than 1000W.

Each frequency within the bandwidth that your subwoofer and box combination can play will present the amplifier with different impedances, typically higher than the nominal impedance. The frequency that presents the amplifier with an impedance closest to 4 ohms is generally around the tuning frequency of the subwoofer in the box.

The rise is lower at the tuning frequency because there is significant pressure, and the subwoofer works with the airspring, loading the cone. This resistance, known as rise, increases as the subwoofer moves within its excursion. The subwoofer, essentially a coil in a magnetic field, generates a back EMF (electromotive force) that opposes the amplifier’s power, creating resistance. The less the subwoofer moves, the less force is transferred back to the amplifier, and therefore, the less the impedance rises.

Many people don’t realize that the wattage rating on their amplifier is rarely achieved. Typically, you will get around 60% of the rated power of the amplifier when wiring a subwoofer to its nominal impedance. This is due to the actual impedance being higher than the nominal value during operation, which reduces the amplifier’s power output.

Donell Perry (SINEWAVE02)