L-Pad Schematics

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"L-Pad" is an audio electronics term that refers to a resistive attenuator network that reduces the signal to a speaker while matching the impedance to that of an amplifier. The network consists of two resistors, either fixed or variable, that are connected so that one is in series and the other is in parallel with the speaker load.

Purpose

  • The L-Pad is used to maintain a constant load on an audio amplifier channel while varying the volume level of the speaker. It ensures that the load is not too large for the amplifier output and maintains a constant impedance, which leaves the frequency response of any crossover circuitry unchanged as it adjusts the signal level.

Implementation

  • The attenuator network is a passive device made up of either fixed or variable resistors. In the variable case, the resistors must be ganged (connected to a common shaft) so that their resistances change together. The series half of the resistor pair connects from the amplifier output to the parallel combination of the other resistor and the speaker. The total resistance of the network adds up to the nominal speaker impedance, which is usually 8 Ohms.

Fixed Resistors

  • Fixed L-pads are made with two resistors of the proper value. The resistor values satisfy simultaneous equations involving the system impedance and the relative level of the output that is applied to the speaker. Solving these equations can be tricky, but there is an online calculator for finding the resistances and power ratings for L-Pad resistors (see "Resources").

Examples of Resistor Values

  • Here are some approximate values of the series and parallel resistors for various levels of attenuation in an 8-Ohm system:

    Signal level: full (no attenuation)
    Series resistor: short (0 Ohm)
    Parallel resistor: open (infinite resistance)

    Signal level: one half (-6 dB)
    Series resistor: 4.0 Ohms
    Parallel resistor: 8.0 Ohms

    Signal level: one fourth (-12 dB)
    Series resistor: 6.0 Ohms
    Parallel resistor: 2.7 Ohms

    Signal level: 0
    Series resistor: 8.0 Ohms
    Parallel resistor: short (0 Ohms)

Using Variable Resistors

  • L-Pads can be constructed using variable resistors, also known as potentiometers or rheostats. These variable networks are difficult to make but are commercially available. The rheostats must be ganged together and reversed so that the resistance of one goes up while the other goes down; they must have different ranges, and the rates of change of each resistance must match unique and nonlinear curves in order for the load resistance to remain constant for all settings.

References

  • Photo Credit Old Fashioned Amplifier image by Yali Shi from Fotolia.com
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