L pad
Encyclopedia
An L pad is a network composed of two impedances that typically resemble the letter capital "L" when drawn on a schematic. It is commonly used for attenuation and for impedance matching.
s used to control volume while maintaining a constant load impedance
on the output of the audio amplifier
.
It consists of a parallel and series resistor in an "L" configuration. As one increases in resistance, the other decreases, thus maintaining a constant impedance, at least in one direction. To maintain constant impedance in both directions, a "T" pad must be used. In loudspeakers it is only necessary to maintain impedance to the crossover; this avoids shifting the crossover point.
A constant-impedance load was important in the days of vacuum tube
power amplifiers, because such amplifiers often did not work efficiently when terminated into an impedance greatly different than their specified output impedance. This was only true of full range speakers. Most modern applications for full range speakers use tapped transformers. Maintaining constant impedance is less important to modern amplifiers using solid state electronics
.
In high frequency horns, the L Pad is seen by the crossover, not the amp. L pads may not necessarily use infinitely variable rheostats, but instead a multi-position rotating selector switch wired to resistors on the back. Tapped transformers are not L pads—low-end manufacturers to the contrary; they are autoformers. L pads can also be used at line level, mostly in pro applications.
the signal by having two separate rheostats connected in an "L" configuration (hence the name). One rheostat is connected in series with the loudspeaker and, as the resistance
of this rheostat increases, less power is coupled into the loudspeaker and the loudness
of sound produced by the loudspeaker decreases. The second rheostat is connected between the input and ground (earth). As the first rheostat increases in resistance, the second rheostat decreases in resistance, keeping the load impedance (presented at the input of the L pad) constant. The second rheostat usually has a special taper (function of resistance versus rotation) to accommodate the need for constant input impedance.
The L network relies on a procedure known as series-parallel transformation. For every series combination of resistance, RS, and reactance, XS, there exists a parallel combination of RP and XP that acts identically to the voltage applied across the series combination. In other words, the series components and the parallel components provide the same impedance at their terminals. The transformation ratio is the ratio of the input and output impedances of the impedance matching network.
The series-parallel transformation allows the input impedance to be dropped down to lower impedances while sustaining a voltage across the circuit. This system works in reverse as well. The equations needed for this transformation are as follows:
Q = XS/RS= RP/XP = [(Rp/Rs)-1 ]^0.5 [1]
RP
= RS (Q2 + 1) and RS = RP /
(Q2 + 1) [2]
XS = QRS and XP = RP / Q [3]
For the resistance Rs and reactance Xs in series, Rp and Xp exist as a parallel combination. One simply needs to know the input impedance Rp and to choose the output impedance Rs. Or conversely know Rs and choose Rp. Keep in mind that Rp must be larger than Rs. Because reactance is frequency dependent the L network will only transform the impedances at one frequency.
Inclusion of two L networks back to back creates what is known as a T-network. T-networks work well for matching an even greater range of impedances.
There is an inherent insertion loss
where
= power dissipated by load and 
= power dissipated by the pad resisters. Large positive numbers means loss is large.
The loss is a monotonic function
to the impedance ratio. Higher ratios require higher loss.
impedances.
Speaker L pad
A speaker L pad is a special configuration of rheostatPotentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
s used to control volume while maintaining a constant load impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
on the output of the audio amplifier
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
.
It consists of a parallel and series resistor in an "L" configuration. As one increases in resistance, the other decreases, thus maintaining a constant impedance, at least in one direction. To maintain constant impedance in both directions, a "T" pad must be used. In loudspeakers it is only necessary to maintain impedance to the crossover; this avoids shifting the crossover point.
A constant-impedance load was important in the days of vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
power amplifiers, because such amplifiers often did not work efficiently when terminated into an impedance greatly different than their specified output impedance. This was only true of full range speakers. Most modern applications for full range speakers use tapped transformers. Maintaining constant impedance is less important to modern amplifiers using solid state electronics
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
.
In high frequency horns, the L Pad is seen by the crossover, not the amp. L pads may not necessarily use infinitely variable rheostats, but instead a multi-position rotating selector switch wired to resistors on the back. Tapped transformers are not L pads—low-end manufacturers to the contrary; they are autoformers. L pads can also be used at line level, mostly in pro applications.
Audio-frequency (AF) operation
The L pad attenuatesAttenuator (electronics)
An attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform.An attenuator is effectively the opposite of an amplifier, though the two work by different methods...
the signal by having two separate rheostats connected in an "L" configuration (hence the name). One rheostat is connected in series with the loudspeaker and, as the resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of this rheostat increases, less power is coupled into the loudspeaker and the loudness
Loudness
Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...
of sound produced by the loudspeaker decreases. The second rheostat is connected between the input and ground (earth). As the first rheostat increases in resistance, the second rheostat decreases in resistance, keeping the load impedance (presented at the input of the L pad) constant. The second rheostat usually has a special taper (function of resistance versus rotation) to accommodate the need for constant input impedance.
Radio-frequency (RF) operation
In RF (radio frequency) applications, the L network is the basis of many common impedance matching circuits, such as the pi network employed in amplifiers and the T network that is common in transmatches.The L network relies on a procedure known as series-parallel transformation. For every series combination of resistance, RS, and reactance, XS, there exists a parallel combination of RP and XP that acts identically to the voltage applied across the series combination. In other words, the series components and the parallel components provide the same impedance at their terminals. The transformation ratio is the ratio of the input and output impedances of the impedance matching network.
The series-parallel transformation allows the input impedance to be dropped down to lower impedances while sustaining a voltage across the circuit. This system works in reverse as well. The equations needed for this transformation are as follows:
Q = XS/RS= RP/XP = [(Rp/Rs)-1 ]^0.5 [1]
RP
= RS (Q2 + 1) and RS = RP /
(Q2 + 1) [2]
XS = QRS and XP = RP / Q [3]
For the resistance Rs and reactance Xs in series, Rp and Xp exist as a parallel combination. One simply needs to know the input impedance Rp and to choose the output impedance Rs. Or conversely know Rs and choose Rp. Keep in mind that Rp must be larger than Rs. Because reactance is frequency dependent the L network will only transform the impedances at one frequency.
Inclusion of two L networks back to back creates what is known as a T-network. T-networks work well for matching an even greater range of impedances.
Impedance matching
If a source and load are both resistive (i.e. Z1 and Z2 have zero or very small imaginary part) then a resistive L pad can be used to match them to each other. As shown, either side of the L pad can be the source or load, but the Z1 side must be the side with the higher impedance.There is an inherent insertion loss
where
= power dissipated by load and = power dissipated by the pad resisters. Large positive numbers means loss is large.The loss is a monotonic function
Monotonic function
In mathematics, a monotonic function is a function that preserves the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order theory....
to the impedance ratio. Higher ratios require higher loss.
Application notes
Speaker L pads are designed to match the impedance of the speaker, so they were commonly available with 4, 8, and 16 ΩOhm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
impedances.