You’ll save a couple of pounds but the attenuator will always run hot and you may end up blowing it. In fact, there’s not much point at all in buying a 15-watt L-Pad. Secondly, they have power ratings, so there’s no point buying a 15-watt L-Pad to use with a 50-watt amp. So if you are using an 8-ohm speaker, you will need an 8-ohm L-Pad. Firstly, you must buy one that matches the load you need. When choosing an L-Pad there are certain stipulations you should consider. Internal variable resistors are wired in series and parallel and, regardless of the position of the control knob, the total resistance of the L-Pad device remains constant. Make sure you buy the right L-Pad for your needs this one was rated at 8 ohms and can supposedly handle up to 100 watts The L-Pad we bought came with fixings and a nice big control knob. These devices are like oversized volume pots with dual wire-wound resistors inside. Everyone at G&B was massively impressed with the master control on the recently reviewed Elektra 185 amp – and that turned out to be an L-Pad. The easy solution is to use a rotary L-Pad controller. The common complaint is that the crude settings are generally a bit too quiet or a bit too loud and never quite ‘right’. In fact, plenty of commercially manufactured attenuators work exactly like that. It’s perfectly possible to combine a rotary switch with an array of resistors to build an attenuator that works in 3dB increments. With 4k and 5k in the same configuration, the attenuation is closer to 9dB, but the amplifier always ‘sees’ 8 ohms. Checking one of these tables, we see that, roughly speaking, a 20k parallel resistor coupled with a 2k series resistor attenuates the signal by 3dB. Working out the attenuation involves some complex maths, but there are several websites with tables and L-Pad calculators that can make things simple. In the example given above, there would be an attenuation of around 6dB. Adding the 4-ohm resistor brings the load back up to 8 ohms but the signal is attenuated by around 6dB Combining the speaker and the 8-ohm resistor in parallel creates a 4-ohm load. Since the speaker receives less power from the amp than it did without the added resistors, acoustic energy is lower and the amp sounds quieter. The resistors convert power into heat while the speaker converts the power into sound energy. The power from the amplifier is divided between the speaker and the resistors. If you then connect a 4-ohm resistor in series with the paralleled 8-ohm resistor and 8-ohm speaker, the load resistance comes back up to 8 ohms. If you solder an 8-ohm resistor across the terminals of an 8-ohm speaker, the result is the same – a 4-ohm load. Let’s look at a basic example if you wire two 8-ohm resistors in parallel, the combined resistance will total 4 ohms. The clever thing about the L-Pad design is the way it adds resistors to the circuit but retains the same ‘load’ at all times. In simple terms, amplifiers and speakers must be matched, so if your amp’s output is labelled 8 ohms, it should be connected to a speaker rated at 8 ohms. This method employs extra resistors to dissipate electrical energy as heat. There are myriad ways of attenuating an amplifier signal, but the vast majority of DIY attenuators are based on a straightforward circuit called an L-Pad. The complete circuit diagram for our attenuator The L-Pad All you really need is pliers, a soldering iron, an electric drill and a step cutter bit. All the parts are easy to find, and for a few hours’ work and relatively little outlay you can build an attenuator that would cost £100 or more to buy. This is one of the easiest DIY projects we have covered so far. The result is a design for a bypassable resistive attenuator that can work at 4 or 8 ohms and features treble compensation.
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