Index LoudspeakersL-Pad Attenuator

L-Pad Attenuator

Pad a tweeter down while the crossover still sees the same load.

R1, series2.95Ωnearest E12: 2.7 Ω

R2, parallel13.7Ωnearest E12: 15 Ω

Heat in R118.5W

Heat in R218.5W

Circuit

Where the power goes

Driver31.5 W · 63%

R1, series18.5 W · 37%

R2, parallel18.5 W · 37%

The crossover still sees 8 Ω, so its corner frequency does not move.

Input

Driver impedance (Ω)

Attenuation

A

Power arriving from the crossoverW

Used to size the resistors. Choose parts rated for at least twice the dissipation shown.

How it works

Two conditions, two resistors

Tweeters are usually more sensitive than the woofers they are paired with. A single series resistor would lower the level but also change the load the crossover sees, shifting its corner. The L-pad uses two resistors to satisfy both requirements at once: the wanted attenuation and an unchanged total impedance.

With $k = 10^{A/20}$ as the voltage ratio:

R_1 = Z\,\frac{k - 1}{k}

R_2 = \frac{Z}{k - 1}

Checking the result

The driver in parallel with $R_2$ has impedance $Z/k$ . Adding $R_1$ in series brings the total back to $Z$ , and the divider leaves $1/k$ of the voltage across the driver:

Z_{in} = R_1 + \frac{Z\,R_2}{Z + R_2} = Z

\frac{V_{driver}}{V_{in}} = \frac{1}{k}

Practical notes

Place the pad between crossover and driver, never before the crossover.
Use non-inductive resistors. Wire-wound types add inductance that tilts the treble.
A lower source impedance for the tweeter slightly changes its damping near resonance; with the crossover well above resonance this is negligible.