Lesson 4: Amplifiers

Guitar Parts

A basic sound system can contain hundreds of Amplifiers all performing particular functions. They can be used to boost the microscopic voltage of a microphone signal, or supply the comparatively gigantic power to power a speaker stack. They can have gains in 10s of thousands down to unity, and can be anything from a single stage to a complex multistage negative feedback controlled, frequency compensated piece of technology. A single integrated circuit could contain thousands of amplifiers performing different functions. Honestly though, there are only 3 main amplifiers that you need to concentrate on, and 2 of them are usually together anyway.

The first is the Preamplifier. This is usually packaged with many of the functions found in the mixer. Almost all of the controlls on the front of your common stereo hifi amplifier (like the one above) are associated with the preamplifier section. You could say the preamplifier takes any type of input and processes it so a constant signal can be applied to the output stages. Depending on the use it was designed for (professional, semi-professional, or home) the output of the preamplifier stage is usually in the ballpark of "line level".

The next stage is often overlooked because it is packaged with the last. To understand the Driver Amplifier you need to have a reasonable understanding of volts, amps, and watts from the definitions page. The driver amplfier is primarily concerned with voltage gain. It's not designed to handle a huge amount of current, so it is matched up with a final stage which is called the Power Amplifier. The power stage often has a gain of unity, but is much more robust and attached to cooling fins and heatsinks to protect it from all the heat generated by the current flowing through it.

The next thing to be discussed in this lesson is about amplifier classes. A quick look at Single Stage Transistor Amplifiers in the electronics section will help you understand this section. Classes A, B, AB, G, and H are most commonly used for audio, while C, D, E and F are more about switching and RF transmission.

Class A Class B

Class A, as discussed in the electronics section, is defined as biassing the transistor in the middle of it's working range. The advantage of this class is that the accuracy is at its maximum, but the disadvantage is that the transistor is 50% on all the time so it has a high power usage.

Class B uses no bias. The obvious disadvantage is that less than half of the wave is amplified. To increase the efficiency this type of configuration is usually set up in "Push Pull" matched pairs, where one transistor amplies the positive half of the wave, and the and the other transistor amplifies the negative half.

Class AB

Class AB gets around this problem by using the push pull configuration, but biassing each transistor with just enough voltage to turn it on and avoid the crossover distortion area of the graph.

Classes G and H are almost the same configuration. Essentially, in G class there are multiple power supply voltages that are switched in and out as the signal gets up to that level, whereas in H class, instead of set rail voltages the rail tracks the signal just a couple of volts above it. Its a way of reducing power requirements, which in turn reduces the heat generated.

Bridging

There is another one that should go in this area which is not so much a class as a mode of connection where two amplifiers amplify the same signal but in opposite phases. This is called a Bridging configuration. Any two amplifiers can do this, but the most important element is the phase splitter. This can be achieved quite simply by using the two signal wires of a balanced cable, or if the signal is unbalanced a Direct Injection (or DI) box could be used. The resultant voltage is the difference between the two outputs, and the impedance is the output impedances of the two amplifiers in parallel (see the formula for parallel resistors in the electronics section).



Also in the amplifier section we will be covering the subjects of Bi-amplification and Tri-amplification. The process of each of these two concepts is to split the signal into its different frequency components and amplify each separately. Usually the process of splitting into frequency bands is done inside the speaker box using a "passive crossover", but for more efficient use of the amplifiers and more control over the whole process it is more beneficial to use an "active crossover" which splits the signal into its frequency components, and sends the different components to different amplifiers. Multiamping is most commonly done in big sound systems, but it can also be found in many home stereo and theatre systems which have sepatate subwoofers.

Multi-amping


We're now ready to move on to Speakers. Click here to go to the lesson about speaker types and enclosures, or Click here to return to the Stage & Studio Equipment index page.