Q Hi.I guess I'll start off like everyone else and tell you how great the magazine is. I have been a subscriber since 1996, but this is my first time writing in. I like all of the features that are put in the magazine, especially the do-it-yourself articles (please put more). I am writing because I have a small problem. I want to make a remote sub-level controller, but I don't know how. I know there is a way to do it with an L-pad, but I need some advice. That's where you guys come into the picture. I have asked a few installers around the area, but I get the same shady response: "I really can't tell you, but if you bring it here I can hook you up for a few dollars."
I have been performing my own installations and a few for others since I became interested in car audio, but this is something that I have not tried yet. I usually use an in-dash equalizer, but this is for my work car and I want to cut costs (at least for now). I have a Rockford Fosgate 200a4 bridged to two channels (rated by Rockford @ 100 watts per channel) going to a pair of Pioneer 12s. I appreciate any advice that you may have for me, and please keep up the good work.Jimmie Harris, Gary, INVia the Internet
A An L-pad is easily recognized by its layout, which is made of two resistors in an inverted "L" configuration.
There are two main ways to "pad" or attenuate for the purpose of reducing a signal to a device such as a tweeter. You could design a certain amount of resistance in series with the tweeter, which would lower its output, but the resistance of the tweeter is now the sum of the tweeter plus the resistor (see Figure 1). If you have already built a crossover, you now need to rebuild it since the load impedance has changed (which changes the characteristics of the crossover).
The other way is to use an L-pad, which has the unique characteristic that it doesn't change the impedance no matter what size of L-pad you use (see Figure 2). All you need to do is punch two formulas into a scientific calculator to determine the right values for the two resistors.
There are several more calculations you need to make to determine the wattage required for each resistor. Even in the case of a tweeter, you can easily burn your car to the ground by not designing the right power factors into your L-pad.
The next concern for you is "where in the signal path will your L-pad be installed"? As a remote bass control, you could either install an L-pad in the subwoofer wires from the amplifier (a very bad idea) or a totally different L-pad in the preamp line to your subwoofer amplifier (the right way). A preamp-based level control is a variable resistor that tracks two resistive elements simultaneously (see Figure 3) to adjust volume while maintaining the correct impedance.
I can understand the reluctance of your local installers to give away free information. The self-preservation instinct causes installers to want to get paid to install, and the more an installer stops work to tell the public how to install, the less reason there is for their boss to keep him on the payroll. On the other hand, I've been given accurate advice by my local home improvement store, only to go home and screw up the job because I did not have the required background knowledge or training to correctly understand what I was being told. The risk is that the installer will become liable for your mistakes because he advised you. If you have any difficulty understanding this information, it would be worth the "few dollars" to prevent a costly problem.