Amplifiers, Impedance and Power
Matching a speaker and amplifier is not about making two wattage numbers identical.
You need to consider output level, impedance, current demand, clipping, thermal limits, and excursion.
Impedance is not constant
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A loudspeaker’s impedance changes with frequency.
A driver normally shows:
- A low-frequency value near \(R_E\)
- A peak around mechanical resonance \(f_S\)
- A relatively low midband region
- A rise at high frequencies due partly to voice-coil inductance \(L_E\)
A bass-reflex system normally produces two low-frequency impedance peaks with a minimum near the port tuning frequency.
Check the minimum impedance
An amplifier must tolerate the speaker’s minimum impedance.
A speaker advertised as 8Ω may fall well below 8Ω at some frequencies. Use the measured impedance curve where possible.
How power changes level
In an ideal linear system:
- Doubling power adds 3 dB.
- Ten times the power adds approximately 10 dB.
- Doubling distance reduces direct sound by 6 dB in free space.
Rooms, compression, limiting, and speaker directivity alter these simple estimates.
Thermal and mechanical limits
At higher frequencies or over long durations, voice-coil heating may dominate.
At low frequencies, diaphragm excursion often becomes the first limit. A driver may exceed \(x_{max}\) with far less than its rated thermal power.
Clipping
When an amplifier is driven beyond its voltage capability, it clips the waveform. Severe clipping increases distortion and can add substantial high-frequency energy.
An “underpowered amplifier” is not automatically more dangerous than a powerful one. The risk comes from how the system is driven, the resulting waveform, and whether the speaker’s thermal is exceeded.
Protection
Useful protection methods include:
- High-pass filtering below system tuning
- Limiters
- Thermal monitoring
- Fuses in passive networks
- Amplifier clipping indicator
- DSP excursion models