Amplifier specifications are sometimes presented in ways designed to impress rather than inform. A headline wattage figure tells you relatively little about whether an amplifier will work well with your speakers or your preamplifier. The specifications below are the ones that determine real-world compatibility and performance.
Power Output — Read It Carefully
Continuous RMS Power (Watts) Into a Stated Impedance
Power is always qualified by load impedance. "150 watts" without a stated impedance is meaningless. Always read it as "150 watts RMS per channel into 8 Ω, both channels driven simultaneously." That last phrase — both channels driven — is important. Some manufacturers rate power with only one channel driven, which can give figures 20–40% higher than real-world stereo operation.
The relationship between power and perceived loudness follows a logarithmic curve. Doubling the power adds only 3 dB of additional SPL — a modest and barely perceptible increase in most rooms. Going from 50 watts to 100 watts is not twice as loud. To produce twice the perceived loudness, you need ten times the power (a 10 dB increase). This is why speaker sensitivity matters so much more than raw amplifier wattage in most real-world situations.
Also note that many amplifiers deliver significantly more power into 4 Ω loads than into 8 Ω. A well-designed amplifier into 4 Ω should deliver approximately double its 8 Ω power, because the lower impedance allows more current to flow. An amplifier that delivers the same power into both 8 Ω and 4 Ω is likely current-limited and will struggle with demanding speakers.
Input Sensitivity — The Gain Matching Spec
Input Sensitivity (mV or V for Full Rated Output)
This is the voltage the amplifier needs at its input terminals to produce full rated power output. A sensitivity of 1 V means you need 1 V RMS from your preamplifier to drive the amplifier to its maximum power. If your preamp can only produce 0.5 V maximum, you can never reach full power. If your preamp produces 4 V and the amp needs only 0.5 V, your volume control will be useful only at very low settings.
Input sensitivity is the primary specification that determines whether a preamp-amplifier pairing will be practical. Mismatched sensitivity is one of the most common causes of systems that are either too loud at the lowest usable volume setting, or that never reach satisfying output levels.
Input Impedance — The Preamp Compatibility Spec
Input Impedance (kΩ)
The input impedance of the power amplifier determines the minimum output impedance the preamplifier can drive without frequency response degradation. Apply the 10:1 rule: preamplifier output impedance should be no more than 1/10th of the amplifier's input impedance. A power amplifier with a 10 kΩ input impedance requires a preamplifier with an output impedance of 1 kΩ or less. Higher input impedance is generally better — it places fewer demands on the preamplifier's output stage.
Damping Factor — Speaker Control
Damping Factor (dimensionless ratio)
Damping factor is the ratio of the speaker's nominal impedance to the amplifier's output impedance. A damping factor of 150 means the amp's output impedance is 1/150th of the speaker's impedance, giving the amplifier strong electrical control over the speaker cone's motion. Low damping factor results in bass that is loose and slow; very high damping factor (above 100) offers diminishing returns in most practical systems.
Peak Current Delivery
Peak Current (Amps per Channel)
The maximum instantaneous current the amplifier can deliver to the speaker load. This determines how well the amplifier controls difficult, low-impedance speakers and reproduces transient musical peaks. A 100 W amplifier with 30 A peak current delivery will sound more dynamic and composed on complex program material than a 100 W amplifier limited to 15 A, particularly with demanding 4 Ω or lower-impedance loads.
Amplifier Signal Path
Quick Reference: Amplifier Specs at a Glance
| Specification | What It Means | What to Look For |
|---|---|---|
| Power Output (RMS) | Continuous power into a stated load | Both channels driven, at your speaker's impedance |
| Input Sensitivity | Voltage needed for full output | Must match your preamp's output voltage |
| Input Impedance | Load presented to the preamp | ≥10× preamp output impedance; higher is better |
| Damping Factor | Amplifier's speaker control | >50 is adequate; >100 is good |
| Peak Current | Instantaneous current capability | Higher = better control of demanding loads |
| THD (full output) | Distortion at maximum power | <0.1% is excellent; specification must be at full output |
| SNR | Noise floor relative to rated output | >95 dB A-weighted is very good |
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