An Amp is a device for making a larger, more powerful signal out of a small, weak signal.
It was on February 18, 1908 that Lee DeForest was granted U.S. Patent No. 879,532 titled “space telegraphy,” but in actuality, the patent was for a vacuum tube triode amplifier that DeForest called the audion amplifier. For the first time it was possible to amplify signals.
The first Amps were developed to support radio. Later, other applications were developed. All electronics depend in some way on amplification. Our industry uses Amps for audio.
An Amp consists of two basic elements:
• A power supply that supplies a large energy source and
• An amplifier section that modulates the large energy source in accordance with the small signal
The result is a large, powerful signal.
Classes of Amps
A: A small signal modulates a larger current. This larger current is present when the small signal is not present. Efficiency up to about 26%. Excellent quality sound.
B: Uses a push-pull arrangement where one amplification device operates on the positive side of the waveform and another operates on the negative side. Efficiency up to 75%. Sounds bad because of distortion caused by switching from one device to another.
C: A small signal turns a larger signal on or off. There is no in-between state. Efficiency up to 90%. Not usable for audio. Audio requires accurate reproduction of all levels—not just no power and full power levels.
D: A variation of class C. Class D is a way of modulating a class C amplifier to allow it to carry audio information. Sounds very good with latest technology. Efficiency up to 90%. Produces Electromagnetic Interference (EMI)
AB: A variation of Class B. Always has a small current flowing (class A operating region) and this eliminates the switching distortion inherent in Class B. Efficiency up to 65%. Sounds excellent—if well-designed
H: A variation of Class AB. Changes the power supply voltage to the amplifier depending on the signal level. Improved dynamic efficiency. Requires complex power supply. Single tone efficiency up to 65% Sounds excellent—if well-designed
Power Supplies are of two classes:
1: Standard (Analog) Power Supply: Efficiency to 80%. Heavy, for large amounts of power. Components are large
2: Switching Power Supplies: Efficiency to 90%. Lightweight—even for large amounts of power. Components are small. Can support universal line input voltages. Can support regulated output w/no power loss. Produces EMI
Efficiency is the power output of a device divided by the power input to the device. Power input that is not outputted by a system is dissipated as heat from the system.
The greater the efficiency of an Amp system, the less AC power is required to deliver the same output power to the load.
A typical Class AB amplifier with a 65% efficiency, used with a standard analog power supply running at about 80% efficiency yields an overall Amp system efficiency of approximately 50%. Efficiency is typically rated at full power, continuous tone levels. At lower power levels, efficiency is much worse. It is at low signal levels where Class D and Class H designs offer significant improvements. Class D maintains almost the same efficiency for all power levels while Class H switches to a lower power supply at lower signal levels in order to maintain good efficiency.
How Switching Supplies Work
• A switching supply consists of four basic building blocks:
• A DC supply that operates directly off the AC line
• A power oscillator which converts the DC supply to a very high frequency, typically 50 kHz—200 kHz
• A transformer that changes the high frequency power signal to the various outputs needed
• Rectification and filtering stages that produce the DC outputs needed
Smaller & Lighter
Since the transformer in a switching supply is operating off a very high frequency power signal—instead of 50/60 Hz—it can be much smaller and lighter.
This is the major advantage of a switching supply. Since switching supplies are more complex than a standard supply, their circuitry usually costs more. In addition, more parts have to be added to control the EMI produced by switchers. But these costs are usually more than compensated for by the reduction in size, weight (and cost) of the transformer. In addition, costs of the chassis can be reduced because the weight it has to support is much less. Since the power supply is the heaviest part of an Amp, the entire product is much lighter and easier to handle.
Trends in Amplifier Design
For many years, the standard Amp was a Class AB design with a standard analog power supply. Manufacturers have been experimenting with other types of both amplifier design and power supply design in order to support customer needs better.
The primary customer needs today for Amps are:
– High Power
– Good Sound
– Low Cost
– Lightweight Construction
As we will see, what is good sound is very controversial. There is so much confusion about technical terms for rating good sound—and how these terms actually relate to the listening experience—that the focus today for most consumers is power rating and cost.
Satisfying Customer Needs
Twenty years ago the rated output power of an Amp was the continuous tone output level of the Amp; a 300 W Amp could produce a 300 W tone all day long. Then it was recognized that most audio Amps are not used on continuous tones, but are used on audio signals. Audio signals consist of many tones of different power levels. So customers didn’t need an Amp that produced continuous tones all day; they needed an Amp that could produce audio signals all day. This is easier and cheaper to do. Manufacturers today have adopted various methods for rating the power output of their products. This is causing much customer confusion.
Manufacturers, industry and standards groups have contributed to defining how to rate power output for audio Amps. There have been many methods proposed for rating amplifier power. Many of these proposals—such as tone bursts tests—attempt to rate the instantaneous or short duration power levels. Since audio signals vary in duration and level, the validity of these rating methods depends on how the Amp is used and the characteristics of the signals it is amplifying. Some methods use non-audio signals, such as square waves, to determine the amplifier power rating; this causes the number to be higher.
Today, the primary standards are dictated by the Federal Trade Commission (FTC). Many manufacturers also use standards developed by Industry associations—such as the Electronics Industry Association (EIA). Some manufacturers use other methods.
Safety agencies (particularly those in the European Union, and Underwriters Laboratories here in the US) have developed standards for measuring average continuous power of an amplifier. These are used in turn to measure maximum AC line power consumption and to confirm maximum temperatures. Safety groups have determined that typical worst-case power for an amplifier amplifying an audio signal occurs at one-eighth of the non-clipped output power (measured with a 1 kHz tone). The Amp is cooked with a bandwidth-limited (20-20 kHz) pink noise signal whose power is equal to 1/8 of the tone full power. Measurements are then made for temperature and AC line power consumption. Thus 1/8 of the maximum tone power before clipping represents realistic worst-case continuous power levels for an audio amplifier.
Standards bodies are always reviewing and updating industry standards. We can expect standards for audio amplifier testing and ratings to change as our customers’ needs and usage change—and as our knowledge and technology improves.
Good sound is achieved by good design tailored to the application and user preferences. But good sound is a subjective thing; different users prefer different characteristics to their sound. Different applications may require a different sound character. A presentation application typically requires good sound accuracy, also called sound clarity.
The industry has developed various solutions to satisfy different user preferences and different applications. The primary way of rating sound clarity is by rating distortion. Distortion compares the accuracy of the large output signal to the small input signal. The lower the distortion, the more accurate the sound.
THD Distortion is rated by specifying THD (Total Harmonic Distortion). A harmonic is a tone whose frequency is an integral multiple of another tone, called the fundamental (pure) tone. If a pure tone is inputted to an amp, the output should be the same pure tone. In practice, the output contains small levels of tones that are integral multiples of the input pure tone. These extra tones are distortion.
The THD rating indicates the percentage level of distortion tones in the output relative to the input signal. Today’s THD measurement equipment measures the output signal only. It measures the amount of harmonic frequencies relative to the fundamental frequency in the Amp output. It is therefore important that the measurement input signal be a pure tone—a tone with only a single, fundamental frequency and no harmonics.
Some measurement techniques include some noise—hence the parameter THD + Noise. It is important for the equipment setup to minimize noise as well as have a very low distorti input tone in order to correctly measure the THD of the Amp.
It is generally accepted that 1% THD is the maximum acceptable distortion for high-fidelity sound reproduction.
ODD or Even Distortion is not always bad. This is because some user preferences for sound character imply certain types of distortion. If the distortion tone frequencies are multiples of odd numbers (3,5,7 for example) the distortion is said to be odd-order.
If the distortion tone frequencies are multiples of even numbers (2,4,6 for example), then the distortion is said to be even-order.
Odd order distortion sounds very bad (dissonant). Even order distortion is like hitting octave keys on a piano; it sounds good—and some users may prefer to have some of this type of distortion. Tube amplifiers produce mainly even-order distortion, even when they clip. Solid-state amplifiers produce mainly odd order distortion—especially when they clip.
A very low THD rating is therefore more important for a solid-state amplifier than for a tube amplifier in order for some users to feel the Amp sounds good. Some users who operate their amps at clipping levels a lot (musicians, for example) may prefer tube amplifiers.
Transient Distortion is a way of rating how quickly an amplifier can react to changes in the input signal. If an Amp takes a little time to react to a change (as always happens), then its output is not faithful to the input signal for the time it takes to react.
Damping Factor is a way of rating how well an Amp can control the movement of a loudspeaker. Since a loudspeaker is a mechanical device, it will follow the basic laws of physics: when it is put into motion by some stimulus it will tend to stay in motion after the stimulus is removed. This extra motion produces distortion of the sound. A high damping factor enables the Amp to better control the speaker and minimize these extra movements.
Good (high) damping factor is important to achieving good transient response—especially transient bass response. A low damping factor will result in the speaker not reacting quickly to a bass signal, causing the bass to be “mushy”. But some users and applications may require a “mushy” characteristic to the sound.
For example, if the Amp is used for background music, the user may want the sound to be “mushy” or “mellow” and non-dominating. In a presentation application, the sound is dominant.
“Mellowing” of the sound can be achieved by degrading transient response and damping factor. Tube amps have much lower damping factors than solid state amps. Therefore, users who prefer a “mellow” sound may prefer a tube amp. Users with very old source material (records or 78’s) may prefer an Amp with a “mellow” sound. This is because the signal information from these old sources does not contain fast transients in the program material. An Amp that has good transient response will only reproduce more recording defects and surface noise — not additional program information.
Users that want accurate, realistic sound are likely to prefer a solid-state amplifier; good solid-state designs achieve the best accuracy.
The basic principle in designing for low cost is to not over design! Products must be designed so that they are adequate for their intended application—and no more. Products must be protected from stresses that will exceed their intended application in order to stay reliable.
Making products just adequate for their application saves size, weight and cost. Additional savings in size, weight, and cost are possible by using the new technologies of Class D and H amplifiers and switching power supplies.