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Home Theater Handbook

Preamplifiers and Power Amplifiers

Part 2

See also: all questions, part 1

What is class B operation?

In class B operation, different transistors or vacuum tubes amplify the audio signal's positive and negative polarity components. As a result each transistor or vacuum tube works only half the time. Class B amplifiers however generate a lot of distortion and are not used for Hi-Fi.

Why class A/B amplifiers are popular?

Practically most class A/B amplifiers initially work in class A operation and switch to class B when there is an increase in power demand. Class A/B amplifiers often come close to class A performance and sound quality with out the cost of manufacture and operation of the later. This is the reason why most amplifiers are class A/B amplifiers.

Why vacuum tube amplifiers are favored by audiophiles?

Solid-state power amplifiers and vacuum tube amplifiers are quite similar. The major difference is the use of vacuum tubes instead of transistors in the amplification circuits. Vacuum tube amplifiers are famous for their musicality. These are the first and oldest types of amplifiers. Because of the warm and musical sound they reproduce, vacuum tube amplifiers are highly regarded by audiophiles. So even though they are generally very expensive and made in small quantities, they are still available for sale by high-end audio stores.

What are the main advantages and disadvantages of each type of tubes used in amplifiers?

Vacuum tube amplifiers are characterized as triode or pentode amplifiers, depending on the type of vacuum tubes used in their amplification stages. Triode tubes have superior audio performance but are limited to 60 Watt RMS power output. Pentode tubes produce more power with a very small compromise in audio quality. However even this small compromise is very important to some audiophiles.

What are the two types of digital power amplifiers?

The audio sources of home entertainment systems become ever increasingly digital based. This results in an increase in popularity of digital power amplifiers. There are two methods to making a digital amplifier. True digital power amplifiers and power amplifiers which incorporate digital technology. The later type accepts a digital signal at its input which is then converted to an analog signal by its integrated Digital to Analog (D/A) converters, and is amplified in the traditional analog way.

What form is the digital input signal converted to by a true digital power amplifier?

The true digital power amplifier converts incoming digital signals from the Pulse Code Modulation (PCM) form to a Pulse Width Modulation (PWM) form. In simple terms, the PCM data represents the audio signal in binary form (ones and zeros) of fixed length pulses with varying magnitude, while the PWM data with of varying length pulses of fixed magnitude. The length of the PWM pulse determines the audio signal amplitude.

How is the PWM data converted to analog audio signal?

The conversion of PWM data to audio is performed at the power amplification stage. The PWM data switches conventional transistors on and off depending on the length of the data pulse. The signal amplitude depends on how long the transistors are on or off. The on / off output of the transistors, represents the audio wave.

Why is the multichannel A/V integrated amplifier, a common choice for setting up a Home Theater system?

The use of a multichannel A/V integrated amplifier is very popular for setting up a home cinema system. The multichannel A/V amplifier has numerous analog and digital audio inputs and outputs and analog video inputs and outputs and can therefore be the control center of a home theater system. In addition the multichannel A/V amplifier has built-in the necessary amplification stages to drive all the speakers of a typical home cinema system.

Why are some typical audio adjustments that can be performed by an A/V amplifier?

The A/V amplifier permits various adjustments to the sound to improve the home cinema experience. First of all we can declare each speaker as large or small and in effect route the bass either to the speaker or to a separate subwoofer. We can also adjust the level and the delay of each speaker to match it with our listening room requirements.

Why is a typical A/V amplifier capable of being the control center of a Home Theater system?

Most A/V amplifiers have various sound format decoders and Digital Signal Processors that allows them to reproduce, in our room, an impressive three-dimensional sound stage from the soundtrack of a movie or the content of any stereo program. In essence an A/V amplifier is a fully equipped audiovisual laboratory, that can be the control center of a home entertainment system. It allows the process of all audio and video sources and the improvement of the final audio output.

What are the technical characteristics of an amplifier capable of driving low impedance speakers?

  1. The size of the capacitors and the transformer utilized in the power supply section and the implementation of the amplified stages determines the capability of the amplifier to drive low impedance speakers.
  2. The most critical factor is the amplifier capability to increase the output current that it can supply to the speakers. Amplifiers with carefully implemented power supplies, large transformers and capacitors, do not run out of the needed power that they must deliver. If the amplifier can not supply the required current, then the sound of our system will be inferior and we may even damage our speakers,
  3. Weak bass, reduced dynamics and harsh highs characterize the sound of a system whose amplifier is not up to the task. Especially dangerous for the speakers is the occurrence of "clipping".
  4. In general all semiconductor amplifiers can drive any speaker as long as it is not clipping. Of course the sound level may not be the desirable and in this case we need an amplifier with greater power reserves.
  5. Vacuum tube amplifiers are even more sensitive to speaker loads. We must never connect a vacuum tube amplifier to speakers whose impedance is lower than the specified by the amplifier manufacturer.

How many speakers can my amplifier drive?

  • Most regular amplifiers have outputs for two pairs of speakers, while multichannel amplifiers have outputs for at least five speakers and a subwoofer.
  • We must always remember some basic facts, when connecting speakers to an amplifier: First of all, if we drive low impedance speakers with high level signals the amplifier will overheat and the circuit protection systems will be activated, or we may destroy the amplifier.
  • Secondly we must remember Ohm's law which states that if we add a second pair of speakers in parallel to the first pair, the amplifier sees half the impedance load. For example, if we connect in parallel two 4 Ohm pairs of speakers, the amplifier will see 2 Ohms at its outputs. We therefore must be very careful not to exceed the capabilities of our amplifier when adding speakers to an amplifier.

How much power do I need?

  • The power requirement for an amplifier is dependent on how loud we listen to music, how big is our room and how sensitive are our speakers. It is desirable for our amplifier to have as much power as possible, in order to overcome our demands.
  • Our amplifier must deliver the desired sound levels with out clipping. When comparing the power output of two amplifiers make sure it is stated for the same frequency range from 20 - 20.000 Hz. Many times manufacturers will state power output over smaller range (40 - 20.000 Hz) as the low end puts demands on the amplifier that they can't meet.

My amplifier does not have a subwoofer output. How can I connect my subwoofer?

  • Subwoofers have speaker-level input/output jacks. Just connect the subwoofer speaker-level inputs with the amplifier speaker outputs and the subwoofer speaker level outputs with the main speakers.
  • The crossover filter of the subwoofer routes the mid and high frequencies to the main speakers and the bass to the subwoofer.

My amplifier delivers 100 W per channel. Do I need to buy speakers with the same power rating?

  • Unless you listen to music at very high levels, there is no concern if your speaker's power rating is lower.
  • The specifications of speakers are for the continuous RMS power input that will not damage the speakers. The fact that the amplifier has a greater power output is an advantage, since the amplifier will not clip as easily.

What is amplifier bridging?

  • Bridging of the amplifier outputs means that we use the positive output of one channel and the negative output of the other channel to drive a speaker. We must follow the instructions described in the owner's manual if we want to bridge the outputs of an amplifier.
  • The bridging of a two-channel amplifier converts it to a mono amplifier with theoretically four times the power. In reality we will get approximately three times the power per channel.

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