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Home Theater Handbook

Preamplifiers and Power Amplifiers

Part 1

See also: all questions, part 2

Is the preamplifier a separate component of an entertainment system?

The preamplifier of any Hi-Fi or Home Theater system can be a separate component or it can be integrated to the power amplifier, forming an integrated amplifier. When the integrated amplifier also includes a radio tuner then the component is called a receiver. In any case, the role of the preamplifier is very important and its quality and features determine to a great degree the quality of our system, its ease of use and its capabilities.

Why is the amplifier is the control center of a system?

The preamplifier is the control center and the brains of any system. It receives the input audio/ video signals from the various components; it selects the input, performs any desired decoding, or signal processing, and amplifies the signal to a suitable level for output to the power amplifier.

What functions does the preamplifier perform?

The preamplifier performs some very key functions in any entertainment system. It permits volume adjustment, input signal selection and signal processing. Signal processing may be something as simple as tone adjustment or something as complicated as multichannel audio decoding.

What input signal levels do power amplifiers require?

The preamplifier elevates the input signals from the various AV components, to an appropriate level for the power amplifier to amplify. Power amplifiers require input levels between 0.5 - 2.0 volts with very little current. Preamplifiers therefore only need give accurate voltage-oriented output signals with little demands on current.

What electronic components are found in a passive level control preamp?

The simplest preamplifier is the passive level control, whose only function is to attenuate the input signal of any component that provides line level output, before it is fed to the power amplifier. Passive level controls are preferred by audiophiles because the only electronic component between the source signal and the power amplifier is a variable potentiometer or a stepped resistor.

This results in sonically superior performance, but on the down side it is the output circuits of the components that must carry the burden of driving the power amplifier. All other preamplifiers buffer the components from the power amp and assume the burden of driving the input stages of the amplifier.

What is a phono amplifier?

Through the history of electronics, various types of preamplifiers where developed, which accommodated different user needs. Phono-preamplifiers where always included in any hi-fi system either integrated in the preamplifier or as a separate component. Phono-preamplifiers received the miniscule output signal of the turntable (a few millivolts at most), which is much smaller than the signal from all other components (0.5 - 2 volts).

The phono-preamplifier performs specific equalization (Rl AA equalization) on the output signal of the turntable. It boosts the bass and cuts the treble which is applied to all recorded signals, to produce a flat frequency response of 20 Hz - 20 kHz ±0.5 dB. As the popularity of turntables and records dwindles phono-preamplifiers are becoming rare and are no longer included in many modern preamplifiers or receivers.

What is the typical voltage of a line stage preamplifier input signal?

Every preamplifier has a line stage section. Line stage preamplifiers receive low level signals from the analog outputs of every audio component except for turntables. These input signals are approximately 1 volt, which is also approximately the output signal that is fed by the preamplifier to the power amplifier. The input signal is usually unbalanced but high end preamplifiers also handle balanced signals. In essence there is very limited amplification and the pre amp acts as a buffer that permits signal selection, signal processing and volume adjustment. This is precisely why you may connect any audio component directly to a power amplifier, provided the component allows volume adjustment, by varying the voltage of its output signal.

What is the function of the record output selector?

A typical preamplifier has input selectors, record output selectors, external processing loops, tone controls, loudness control and volume control. The input selectors choose the input signal to be sent to the power amplifier. It is the signal that is heard from our loudspeakers. The record output selector selects the input signal that will be recorded by our tape deck (or DAT recorder, MD recorder etc.). It is independent of what we hear from our speakers, we can therefore listen to our radio while making a copy of a CD. The external processing loops sends a fixed signal identical to the preamplifier output signal to an external processor (surround sound decoder, graphic equalizer etc.). The tone controls adjust the bass and treble of the output signal. The loudness button applies equalization to the output signal which is suitable for low volume listening. Finally the volume control varies the voltage of the signal sent to the power amplifier and as a result the level of our speakers' output.

What additional functions do A/V preamplifiers perform?

The explosive popularity of home theater resulted in the production of preamplifiers that not only support two channels stereo but also handle multichannel surround sound formats. Audio-Video preamplifiers, in addition to audio signal selection, perform multichannel surround sound decoding and video signal switching. Every Home theater system needs an A/V preamplifier either as a separate component or integrated with the power amplifier.

Why are Digital preamplifiers bound to become more popular?

Digital Preamplifiers are bound to become ever increasingly popular, as they are capable of accepting digital input signals, which are then processed in the digital domain, retaining therefore all the quality characteristics of the original signal. Digital preamplifiers also include Analog to Digital and Digital to Analog signal converters for compatibility purposes with conventional devices.

What is the role of the amplifier?

The amplifier's role is to amplify the low level audio signal it receives from the preamplifier, to a level that can drive our speakers. The power amplifier must deliver sufficient current to our loudspeakers at an output voltage required for an appropriate volume from the loudspeaker (approximately 10 volts). The technology of making power amplifiers is mature.

What is the ideal behavior of an amplifier?

The ideal amplifier amplifies the signal without imparting any distortion or external sound characteristic to it. In practice of course this is unattainable; however the continuous improvement of amplifiers has yielded some very impressive models. The basic requirement on any amplifier is to supply the necessary power and current needed by the speakers in order to reproduce the audio signal with high fidelity. An amplifier approaches the ideal behavior if its basic components, power supply and amplifying circuitry are carefully designed.

What components of the amplifier influence the quality of its sound?

Amplifiers are a critical link in the chain of sound reproduction and have received a great deal of attention and technological development. Modern speakers are capable of reproducing excellent sound and in the process they can reveal any shortcomings of the amplifier. Many hi-fi speakers place great demands on the amplifier's current and power supply capability. Therefore the amplifier's transformers and power supply stages, the circuit design and the quality of the electronics, determine the sound quality of the amplifier and its ability to meet the demands of difficult loudspeakers.

What is the most expensive stage of an amplifier?

The power supply is the most expensive stage of an amplifier. If the power supply cannot meet the loudspeakers' demand for current or if the electronics are of low quality, the sound will be inferior. The transformer must be capable to supply the needed current to the transistors; otherwise the amplifier can be driven to clipping and the speakers to destruction.

Why are capacitors used in amplifiers? Can you describe their function?

The size of the capacitors utilized in the power supply stage is a determining factor of the amplifier behavior. The electrolytic capacitors are the power storage devices of the amplifier and they must cover the speaker current needs continuously. If the storage capacity is not adequate, the capacitors will run out of power reserves and we will experience clipping.

Why is «clipping» dangerous to speakers?

The behavior we call "clipping" gets its name from the shape of the graph of the power output of the amplifier, which we observe when it occurs. Instead of being a smooth curve the curve is flat, as if someone "clipped" its top. Clipping is indicative of the inability of the power supply and the amplification circuits to meet the demands for power and current placed by the speakers.

When driven to the extremes of clipping, DC current may be driven to the speakers, causing their immediate destruction.

What determines the capability of an amplifier to handle low speaker impedance?

As we have already described, the size of the capacitors and the power supply determines the capability of the amplifier to meet the demands of low impedance speakers.

When is a speaker a difficult load, for the amplifier?

It is a well known fact that the impedance of a loudspeaker changes depending on the frequency of the audio signal it is called to reproduce. This variance of impedance and the sensitivity of the loudspeaker, i.e. how loud it plays for a given input, are the two attributes that determine if a speaker is an easy or difficult load for the amplifier. The frequency at which the lowest impedance is observed is also important. If the frequency of lowest impedance is high this is easier on the amplifier because in this area the amount of audio information is small. If the lowest impedance frequency is in the mid or low range, then the speaker is considered a difficult load, since there is a great amount of audio information in this range.

Ideally how should an amplifier behave, when the speaker impedance is halved?

The ideal amplifier would double its power output when driving speakers with half the impedance. For example, if an amplifier delivers 50 W to a load of 8 Ohm, then it should deliver 100W to a load of 4 Ohm and 200 W to a load of 2 Ohm.

Amplifiers with an ideal behavior that double the power delivered when impedance is cut in half do not exist. The usual behavior for good amplifiers with careful design and adequate power reserves is a 60-70% increase in power output.

How do we define class A amplifiers?

Power amplifiers amplify the weak audio signals that are supplied by preamplifiers, to a level that can drive speakers. The audio signal received by an amplifier at its inputs has a sinusoidal form, which means that it has both positive and negative polarity components. The operation of an amplifier is characterized by the way it handles these positive and negative polarity components. A favorite class of amplifiers for audiophiles is class A. When an amplifier is operating in class A each output transistor or vacuum tube amplifies both the positive and negative polarity components and therefore it is always in operation. Class A operation is favored by audiophiles due to the sweet and musical sound it produces. The main problem of class A operation is low output power ratings and huge heat generation that necessitates special cooling considerations. In addition class A type amplifiers have increased power consumption (they consume the most when at rest).

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