Impedance Compensation And Equalization Networks
Canonical Definition
An impedance compensation network is a passive branch designed to modify the impedance presented by a loudspeaker or crossover over a stated frequency range. It is selected from the measured load and required transfer function, not from one universal formula.
Three Different Jobs
| Network | Primary job | What it does not prove |
|---|---|---|
| Series RC across a driver | Reduce part of an inductive impedance rise | flat acoustic response |
| Parallel or series RLC branch | Shape a measured resonance or narrow impedance feature | removal of mechanical distortion |
| Conjugate or multi-section network | Approximate a desired load over a wider range | constant impedance at every level and temperature |
The Familiar Zobel Approximation
Rz ≈ Re
Cz ≈ Le / Re²This can provide a useful starting value for a simple inductive model. Real voice-coil inductance can vary with frequency, position, current, and the model used. Measure the impedance before and after the branch and adjust from the required network behavior.
Compensation Changes Current And Heat
A branch that flattens impedance does so by drawing current. Record:
- network input impedance magnitude and phase
- branch voltage and current versus frequency
- resistor RMS and transient dissipation
- capacitor voltage, current, ESR, and temperature
- amplifier current and minimum supported load
Cold And Hot Are Different Conditions
Voice-coil resistance rises with temperature, component values can drift, and enclosure or mounting conditions can move resonance features. A compensation network designed from one cold low-level trace should be verified again after representative operation.
Verification Sequence
- Measure uncompensated impedance magnitude and phase.
- Define the impedance or filter-transfer problem being solved.
- Model component parasitics and branch power.
- Build and measure the branch at low level.
- Recalculate the main crossover around the compensated load.
- Measure complete network impedance, driver-terminal voltage, and acoustic response.
- Repeat at representative temperature and level.