This page introduces some basic technical concepts related to audio amplifier design, introducing the basic building blocks that are needed when building a system.
Overview
This diagram shows the block diagram of a typical integrated amplifier. Note that there are many possible ways to build an amplifier, and this represents just one possible layout.
This diagram is not a schematic - it shows signal flow, rather than specific details of the individual components. Only one channel is shown - for a stereo amplifier, everything is duplicated, with the possible exception of the power supply.
The input signals are applied to an input selector which routes the required source through to the next stage. This selector might be a simple switch - many forms are possible, including push-buttons, rotary switches, or perhaps a toggle switch. Or, it could be more complicated - perhaps relays or an analogue switch IC, under logic or microprocessor control. The exact implementation doesn't matter here, as long as you understand the function.
Note that simple amplifier might only have one input, in which case this selector is unnecessary.
Next, the chosen signal is presented to the volume control. As before, this can be very simple, or more involved. The most frequently chosen option is a potentiometer, but others are available.
The power amplifier has the job of raising the signal level, and providing an interface to the loudspeaker. Before chip-amps, this was alway the most difficult part of the job!
Finally, an important part of any power amplifier is the power supply. All the energy that is applied to the loudspeaker comes from the power supply. Amplifiers are typically mains powered, but there are exceptions. For example, Final Labs make a range of battery powered amplifiers.
Signal levels
Note the voltages written in red - these represent typical signals voltages found in the amplifier.
A CD player can typically provide 2V RMS, but note this is the highest possible level available, and in practice the observed voltages depend entirely on the recording. Contrast that to the tuner, which is only 400mV in this example. Such variations in signal level are normal, and we must cater for that in the circuit design.
The level coming from the input selector depends entirely on the source selected, as shown. However, the signal level after the volume control depends on the source, and the setting of the control. When switching from CD to Tuner, you might well increase the volume control slightly to achieve a match in sound levels.
Note the signal level coming from the volume control will always be smaller the incoming signal, unless the volume control is set to the highest position - in that case it will be the same. In other words, the volume control has no gain. Indeed, normally this stage has negative gain, or attenuation.
(Note that this rule might not apply to more advanced electronic control circuits, but these are outside the scope of this introduction. To see an example of such a product, check out my hi-fi preamp.
From the volume control, the power amplifier provides voltage gain. To produce 50 watts with an 8 ohm loudspeaker, you need a voltage swing of 20 volts RMS. So at the very least, the power amplifier must make the input signal 10 times larger in the case of the CD input. However, it's normal to incorporate more gain than is necessary to accomodate "quieter" sources such as our tuner - figures of gain from around 25 to 40 are typically used.
Also, the power amplifier provides current gain. Ohms law tells you that 20V across an 8 ohm load results in a current of 2.5 amps. This current must be supplied from the power supply.
Alternative gain structures
The layout consided above is relatively common these days, especially as the nominal output level of other common sources has crept up in recent years to match CD players. But sometimes, a designer might prefer to add another gain stage, thus meaning that he can ask for less gain from the output stage. A typical value of gain might be 3 or 4, with the power amplifier gain reduced to say 20.
This extra gain stage can be placed between the input selector and the volume control. This means that any noise generated by this stage is not present on the output at low volume settings. However, there is a danger of this input stage overloading with an unusual source component that has a particularly high output level.
Moving this gain stage to after the volume control removes the overload problem, but might cause an increase in the noise observed at the output of the amplifier. In practice, with modern devices and techniques, this is rarely a problem.
Unity gain "amplifiers", or Buffers
Another technique is to use a circuit with a gain of 1 (or unity). But why would we do this? The output voltage is the same as the input, so what have we achieved? Such a circuit is called a buffer, and while it might not provide voltage gain, it will give current gain.
This is a good thing for all sorts of reasons. For example, some sources are unable to provide much current because they have a high source-impedance. When asked to deliver current beyond their abilities, distortion can result. This distortion might not be gross enough to sound like distortion, but the sonic character of the source component could be altered.
Some power amplifier designs present a low input impedance to the previous stage - the IGC (inverting Gainclone) is a good example of this. Implimenting a high quality buffer prior to the amplifier can improve the measured and subjective performance in this situation.
Pre and Power Amplifiers
The explanation so far has assumed that all the circuitry is in built inside the same box. Such an item is called an integrated amplifier, and the majority of amplifiers sold are in this form. But there are arguments for splitting the two functions - power amplifiers contain large mains transformers that generate magnetic radiation, whereas a preamplifier might have a moving-coil phono preamp that amplifies micro-volt signals - physically seperating these two items seems like a good idea.
Of course, this approach increases the cost of the system. Cases are often the most expensive part of a project - even if you're making them yourself, they represent a lot of extra effort. But against that, you can develop power and preamps seperately, refining each unit in turn until you have the ultimate system!