L-Pads And Passive Level Matching
Canonical Definition
An L-pad is a series-and-shunt resistive network that reduces driver voltage while presenting a chosen nominal load to the preceding network over a stated range. Its actual attenuation and impedance vary with the driver's complex load, component tolerance, temperature, and location in the crossover.
Why Two Resistors
- The series resistor reduces driver voltage and adds source resistance.
- The shunt resistor helps control the input impedance seen by the filter.
- Together they can approximate the intended attenuation and load at the design impedance.
That approximation does not make a moving loudspeaker a constant resistor.
Starting Equations
For desired attenuation A in decibels and design load Z:
K = 10^(A / 20)
Rseries = Z × (K - 1) / K
Rshunt = Z / (K - 1)Use the equations to create a starting pad, then solve the complete filter and measured driver impedance together. Confirm the sign convention used by any calculator before ordering parts.
Network Location Matters
A pad placed before a filter changes the impedance that the filter sees. A pad placed after the filter changes the load on that filter and the driver damping. The correct location depends on the topology, desired transfer, component power, and measured result.
Resistor Power Is Frequency-Dependent
Calculate or measure the voltage and current in both resistors through the passband. Include program spectrum, crest factor, limiter behavior, duration, ambient temperature, enclosure, mounting, and pulse capability. A printed watt rating is conditional, not universal.
Thermal Drift And Compression
As resistors and the voice coil warm, resistance can change and the acoustic level match can move. Verify:
- input impedance cold and hot
- driver-terminal attenuation versus frequency
- series and shunt resistor temperature
- acoustic response and compression after thermal soak
- clearance from plastic, foam, adhesive, insulation, and trim