Please 'Boom' Responsibly
As most of you have noticed, the noise ordinances have become much tougher lately. Most of this is due to idiots, yes IDIOTS, who drive through residential areas with their windows down while their system is playing at full power. To make things worse, the music they listen to has all sorts of foul language that's not suitable for small children, (who may be playing outside). There are even a few people, who are even beyond idiot status, that play their systems at full power through residential areas after 10:00 PM (when many people go to bed). I don't believe that this type of behavior is good for the industry. If the fines get too stiff, people will stop buying large systems. If this happens, more people will get out of car audio (who wants a mediocre system). People get interested in things because they're exciting. A deck and four 6.5" speakers are not going to interest many of the younger car audio enthusiasts. If car audio enthusiasts keep annoying more and more people, the fines will keep getting tougher. All of this will only reduce interest in the equipment that fuels the industry. If you want to listen to your system at full volume, get out on the highway where there's little chance of bothering anyone. When you get to a red light, turn it down. If the only thing attractive about you is your 'system', you have some work to do. Bottom line... Think about what you're doing. Think about other people. It's not the end of the world if you have to turn the volume down for a little while.

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. Pitiful diagram of an 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. L-pad connected to a really cool speaker

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. Really cool multi-slider L-pad diagram thingy

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:

  1. The voltage across the series resistor plus the voltage across the shunt/speaker combination must equal the total voltage from the amplifier's output.
  2. The current through the series resistor is equal to the current from the amplifier's output.
  3. The current through the shunt plus the current through the speaker must equal the total current from the amplifier's output.
  4. 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.
  5. 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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