L-PADS

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L-PADS

Function:
An l-pad is a passive device which lets you control the output level of speakers without changing the impedance seen by the amplifier. A constant impedance is not really necessary for the amplifier but if you are using passive crossovers, a constant impedance is necessary to prevent the crossover frequency from changing.

Construction:
An L-pad consists of 2 resistors connected by a sliding terminal. One resistor is in series with the load and the other (shunt) resistor is connected in parallel with the load. The combination of the series resistance and the parallel shunt/speaker loads will always present a constant load to the amplifier. The diagram below shows the schematic symbol for an L-pad. You can also see that the 2 resistors are not the same value. The sliding terminal varies the resistance in series with the speaker and also varies the resistance in parallel with the speaker load. The position of the sliding terminal is determined by the position of the volume knob on the l-pad.

Connection:
An L-pad commonly has 4 terminals. One (actually two) is a common which connects to the speaker negative and the amplifier negative terminals. One terminal connects to the speaker and the last terminal connects to the amplifier's positive output terminal.

Level control:
This diagram shows how the output level corresponds to the slider position. You can see that when the slider is closer to the amplifier positive out terminal, the output is greater than when the slider is close to the amplifier's negative output terminal.

How does it do it?
The table below shows how the shunt and series resistances (in conjunction with the speaker) maintain a constant load even though the power getting to the speaker is not constant. The table shows the values of the resistors starting at the maximum level (at the top) and ending with zero volume.

Speaker load Series
resistance Speaker and
shunt
in parallel Shunt value Load presented
to amplifier

8 ohms 0 ohms 8 ohms open 8 ohms

8 ohms .5 ohms 7.5 ohms 120 ohms 8 ohms

8 ohms 2 ohms 6 ohms 24 ohms 8 ohms

8 ohms 3.5 ohms 4.5 ohms 10.29 ohms 8 ohms

8 ohms 5 ohms 3 ohms 4.8 ohms 8 ohms

8 ohms 6.5 ohms 1.5 ohms 1.85 ohms 8 ohms

8 ohms 8 ohms 0 ohms 0 ohms 8 ohms

The calculator below will allow you to calculate the series and shunt resistors as well as the power rating for the respective resistors. Don't try to find resistors with the same exact power rating as the calculator recommends. Those values are the minimum safe value with the given amplifier power. Use the next higher available power rating. The terms 'shunt resistor' and 'parallel resistor' are interchangeable. Don't let it confuse you.

Calculates Series and Shunt Resistors

Data Input:

Speaker Impedance? Ohms

Desired Attenuation? Decibels

Amplifier Power/Channel? Watts

Data Output:

Series Resistor = Ohms

Parallel Resistor = Ohms

Series Resistor Power = Watts

Parallel Resistor Power = Watts

Speaker Power = Watts

Extra info:
Shunt || Speaker = Ohms

Amplifier Output Current (@ Full Power) = Amps

Current through Series Resistor (@ Full Power) = Amps

Current through Shunt Resistor (@ Full Power) = Amps

Current through Speaker (@ Full Power) = Amps

Amplifier Output Voltage (@ Full Power) = Volts

Voltage across Shunt and Speaker (@ Full Power) = Volts

Voltage across Series Resistor (@ Full Power) = Volts

Notes:

The voltage across the series resistor plus the voltage across the shunt/speaker combination must equal the total voltage from the amplifier's output.
The current through the series resistor is equal to the current from the amplifier's output.
The current through the shunt plus the current through the speaker must equal the total current from the amplifier's output.
The power dissipated in the speaker plus the power dissipated in the shunt resistor plus the power dissipated in the series resistor must equal the amplifier power.
The combination shunt/speaker impedance plus the series resistance must equal the speaker's impedance. Remember that we're trying to reduce the output of the speaker without changing the impedance seen by the amplifier or passive crossover.

L-pad Source:
Parts Express
Parts Express L-pad Page
4 ohm L-pad

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