Signal Levels

Electronic signals are time dependent currents in a conductor. Their voltage levels are measured in dBV. For example, comsumer line level is -10dBV. It is 750mV compared to 1 volt(reference for 0dBV). Mic level is only about 2mV, preamp quality is essential for this low of a signal. Any amplification may be expressed as a 10 times the logarithmic ratio of the output to the input, be it power or voltage.

Amplification basics

Class A

Amplifying devices operating in class A conduct over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input with no clipping. Class A designs are simpler than other classes; for example class AB and B designs require two devices (push-pull output) to handle both halves of the waveform; class A can use a single device single-ended.

Class B

Class B or AB push–pull circuits are the most common design type found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. The crossover distortion can be reduced further by using negative feedback. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Class B amplifiers are also favored in battery-operated devices, such as transistor radios.

In class AB operation, each device operates the same way as in class B over half the waveform, but also conducts a small amount on the other half. As a result, the region where both devices simultaneously are nearly off (the "dead zone") is reduced. The result is that when the waveforms from the two devices are combined, the crossover is greatly minimised or eliminated altogether. The exact choice of quiescent current, the standing current through both devices when there is no signal, makes a large difference to the level of distortion (and to the risk of thermal runaway, that may damage the devices); often the bias voltage applied to set this quiescent current has to be adjusted with the temperature of the output transistors (for example in the circuit at the beginning of the article the diodes would be mounted physically close to the output transistors, and chosen to have a matched temperature coefficient). Another approach (often used as well as thermally-tracking bias voltages) is to include small value resistors in series with the emitters. Class AB sacrifices some efficiency over class B in favor of linearity, thus is less efficient (below 78.5% for full-amplitude sinewaves in transistor amplifiers, typically; much less is common in class AB vacuum tube amplifiers). It is typically much more efficient than class A.

Class B amplifiers only amplify half of the input wave cycle, thus creating a large amount of distortion, but their efficiency is greatly improved and is much better than class A. Class B has a maximum theoretical efficiency of 78.5% (i.e., π/4). This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A single class B element is rarely found in practice, though it has been used for driving the loudspeaker in the early IBM Personal Computers with beeps, and it can be used in RF power amplifier where the distortion levels are less important. However, class C is more commonly used for this.

A practical circuit using class B elements is the push-pull stage, such as the very simplified complementary pair arrangement shown below. Here, complementary or quasi-complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch in the cross-over region - at the "joins" between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when they're not in use. This approach is called class AB operation.

Audio applications

Below is a small 25 watt amp made from LM1875T, a class AB audio amplifier IC. On the right the schematic.

Here we have the power and preamp valves for the Fender Hot Rod deluxe guitar amplifier as seen as circled dotted lines in the schematic below the picture.

Here is my initial schematic for a nice acoustic guitar amplifier composed of components most of which I currently own.

(download higher detailed version)

Filter basics

Capacitor Values for passive filters

2-way Crossover Calculator

3-way Crossover Calculator

I really want to build this one though

I built it!

Here it is with some Graphic design overlay ideas.

Amp Maker

Here is a classic project from AX84.com Schematic Theory

14 27 67 77 89


Tim Wendler timoth500@yahoo.com

Manuel Berrondo   Jean-Francois Van Huele   J. Ward Moody   Scott Bergesen  Gus Hart