Power and Impedance

In the world of high-end audio, the relationship between amplifier power and speaker impedance is often used as a benchmark for “quality.” However, the phenomenon where an amplifier produces the same power at 4 Ohms as it does at 8 Ohms is a fascinating intersection of physics and intentional engineering.

Here is a technical breakdown of how and why this occurs.

1. The Theoretical Ideal: Ohm’s Law

In a “perfect” voltage-source amplifier, the output voltage remains constant regardless of the load. According to Ohm’s Law, the relationship between Power ( $P$ ), Voltage ( $V$ ), and Resistance ( $R$ ) is defined as:

P = frac{V^2}{R}

If the Voltage ( $V$ ) stays the same and the Resistance ( $R$ ) is halved (from 8 Ohms to 4 Ohms), the Power ( $P$ ) must mathematically double. This is why many audiophiles look for “doubling” as a sign of a robust power supply.

2. The Bottleneck: Power Supply Rails

In the real world, an amplifier is limited by its Power Supply Unit (PSU). The PSU provides the “rails” (the maximum voltage and current) that the amplifier circuits can draw upon. There are two primary reasons why power might not double:

Current Limiting: Doubling power requires doubling the current ( $I$ ). If the PSU or the output transistors cannot safely swing that much current, the amplifier will “clip” or engage protection circuits.
Voltage Sag: Under heavy loads (4 Ohms), a less-than-perfect power supply’s voltage will “sag” or drop. Since power is proportional to the square of the voltage, even a small drop in voltage significantly reduces the power output.

3. The “Optimized” Design: Constant Power Output

When you see a high-end amplifier like the NuPrime Evolution STA-SE rated at 400W for both 8 Ohms and 4 Ohms, it indicates a specific engineering philosophy: The “Maximized Envelope” Design.

In this scenario, the amplifier’s output stage is capable of handling much more than the power supply provides. The designers have tuned the system so that the power supply delivers its absolute maximum stable, clean wattage at 8 Ohms.

When the load drops to 4 Ohms, the power supply is already at its “ceiling.” It cannot provide more voltage or current without compromising the ultra-low distortion levels (THD+N) or thermal stability. Therefore, the output is governed by the maximum capacity of the PSU rather than the load of the speaker.

4. Why is this beneficial?

While “doubling” sounds impressive on paper, a constant power rating across loads offers several technical advantages:

Thermal Management: In compact chassis designs, doubling power at 4 Ohms would generate four times the heat. By capping the output at a stable 400W, the amplifier operates within its most efficient thermal window.
Consistency: The amplifier’s sonic signature remains more consistent. Because the power supply isn’t being “stressed” harder at 4 Ohms than it is at 8 Ohms, the distortion profile stays uniform.
Efficiency via GaN: In the case of the Evolution STA-SE, using Gallium Nitride (GaN) allows the power supply to be much smaller and faster. The design choice to provide 400W across the board ensures that the “Active Noise Cancellation” and “Only Distortion Cancellation” technologies work perfectly regardless of which speakers you plug in.

Summary: If an amplifier “doubles” its power into 4 Ohms, it often means the 8-Ohm rating was “under-built” relative to the power supply’s potential. If an amplifier provides the same power into both, it means the power supply is being utilized to its maximum optimized potential at the higher impedance.