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Zobel network and L-pad calculator
Two small networks almost every passive crossover needs: the Zobel network flattens a driver's impedance rise, the L-pad lowers the level of a tweeter that is too loud – without the amplifier seeing a different load.
Zobel network
| Part | Position | Value |
|---|---|---|
| R1 | in series (R + C), across the driver | 5.6 Ω |
| C1 | in series (R + C), across the driver | 12 µF |
Formulas
R = Re C = Le / Re²
Opens a new project with exactly this circuit. Works as a guest without an account; then load your drivers' FRD/ZMA.
L-pad
| Part | Position | Value |
|---|---|---|
| R1 | in series (R1) | 2.4 Ω |
| R2 | across the driver (R2) | 20 Ω |
50 % of the power for this driver turns into heat in the resistors – choose R1's power rating accordingly.
Formulas
k = 10^(dB / 20) R1 = Z · (k − 1) / k (in series) R2 = Z / (k − 1) (across the driver)
Opens a new project with exactly this circuit. Works as a guest without an account; then load your drivers' FRD/ZMA.
The formulas assume a resistive load. Real drivers have an impedance that rises with frequency and peaks at resonance, and a response of their own – so the crossover behaves differently from the calculation. Only a simulation with measured FRD and ZMA data shows how it will sound.
Zobel network
Above resonance the voice coil acts like a resistance Re with an inductance Le in series – the impedance rises with frequency. An R-C branch with R = Re and C = Le/Re² across the driver turns this into a constant resistance Re on paper (Zobel's constant-resistance network). A crossover calculated for Re then works as it should.
Re and Le are in the datasheet – but Le is often given at 1 kHz only. Estimated from an impedance measurement (ZMA), the value fits the crossover region better; Crover's Zobel tool in the editor does that for you.
L-pad
An L-pad is one resistor in series and one across the driver. Sized right, the amplifier still sees the impedance Z while the driver gets less voltage. The formula assumes a constant impedance – on a real tweeter with a resonance peak, the shunt resistor also changes the Q of the high-pass.