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Using L-Pads |
L-Pads are dual resistive devices that are specially made to maintain a consistent load on a power amplifier's output, while allowing you to adjust the actual signal level going to a speaker. In most cases, you would set your levels at the mixer, however, in many crossover networked systems you will encounter drivers that have different SPL levels - typically Midrange and Tweeters - that have noticeably higher output levels than the Woofer.
There are a number of ways to correct this, however it often takes experimentation to find out just how much attenuation is required to even things out. The L-Pad is a specialty device that handles this task by allowing you to set your levels with the turn of a knob. Many of the L-Pad on the market are intended for home audio applications and cannot take the abuse commonly seen in Pro-Audio Gear. Always buy L-Pads that are rated at a higher wattage than you expect them to see. The ones That I use are rated at 100 watts.
Internally there are 2 high current ganged potentiometers that provide the constant load. The speaker driver is attached to the wiper arm of these 2 ganged potentiometers and signal ground. The Power amplifier (or the output of crossover network) connects across the 2 resistors. Typically, you cannot pump more than 100 watts into an L-Pad without burning it up, so these rarely are used to control full range speaker cabinets by themselves, however, you do see these on Midrange Drivers and High Frequency Horns. Once you know how much attenuation you need, you can measure the resistances for each side (in these drawings connection 1 and 3) and replace the L-Pad with high current wire-wound resistors. This could save you from having to buy a number of L-Pads and and the possibility of them getting out of adjustment during transit.
This 100 Watt L-Pad is fairly large and is made of ceramic, with a vented metal cover. It may get quite warm if you adjust the levels to "low output volume" when you have a high level signal input to the L-Pad inputs. Some L-Pads are made of plastic - these often cannot survive the power requirements of a Pro-Audio system and are intended for less demanding home audio applications.
You can see the outer Wire-Wound resistor clearly in this picture. The dual wiper is connected to pin 2 of the L-Pad. If you look carefully, you'll see the inner Wire-Wound resistor (its between the openings in the dual wiper). The inner resistor is typically made of thinner wire and cannot handle as much current. If the L-Pad fails, it will usually burn out the inner resistor and the level will no longer be adjustable. If it fails it will also alter the impedance of the speaker and this will alter the cross-over point of any passive crossover network that is driving this speaker. If this occurs, look at replacing the L-Pad with high current Wire-Wound resistors.
Note: L-Pads, unless otherwise indicated on the packaging, are set up for 8 ohm drivers.
L-Pads are also referred to as an Lpad or Lpads.
Photos taken using a Kodak DC25 Digital Camera using a Tiffen clip on close up lens.
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