Secrets of Home Theater and High Fidelity
Volume 1, Number 1, 1994
5. Amplifiers (Revised June, 2000)
Index for Amplifiers:
Introduction Tube Amplifiers Types of Tubes Single Ended Amp
Push Pull Amp Transistor Amps Types of Transistors Other Parts in an Amplifier
Fundamentals of Electricity Harmonics Harmonic Distortion Negative Feedback
Class of Operation Power Supply High Voltage and High Current
Slew Rate Unbalanced and Balanced Connectors
A preamplifier takes the source voltage (about 1 volt, from a CD player, LD player, VCR, Tuner) and amplifies it to about 3 volts, adding any treble and bass, DSP, and volume controlling that you wish. The function of modern preamplifiers is not so much to increase the voltage but to allow switching between inputs, controlling the volume, and maintaining a constant output impedance. The output of the preamplifier is fed to the power amplifier where the signal is increased to about 20 volts - 60 volts, which is sufficient to drive the speakers at high levels. Voltage amplification itself is not a complicated process, but keeping noise and distortion from being added along the way is difficult. Tube amplifiers are a product of the early twentieth century, whereas the transistor was invented in the 1940s. Tubes are very simple devices, and the designs of decades past are still being used in amplifiers of today. Many audiophiles treasure the tube sound for its warmth, and they are willing to pay high prices for the small number of tube amplifier designs available, compared to solid state (transistor) amplifiers.
A tube is much like a light bulb, as it is constructed of glass, and contains a heated filament in a vacuum. The heated filament gives off light, which we can see, just like a light bulb. However, it is not the light that is useful in tubes, but the electrons which hover around the heated filament. Tubes contain other elements which are the basis for their function. Besides the filament (called the cathode), a metal strip (called the anode or plate) is situated near the periphery of the tube. When a positive voltage (in the hundreds of volts) is connected to the plate, the electrons at the cathode, by their negative charge, are attracted to the positive charge of the plate (remember opposites attract). Flow of electrons from one point to another is called "current" (measured in amperes, or "amps"), and movement of electrons from the cathode to the anode falls into this category.
Types of Tubes
A tube with just a cathode and anode is called a diode, and since the current can only flow from the cathode to the anode, it is a tool (called a rectifier) for changing alternating current, like that which comes from the socket in your wall, and which changes direction of flow 60 times per second (60Hz), to direct current, which flows only in one direction. Diodes are the fundamental tube in power supplies, which convert alternating current (AC) to direct current (DC) for use by the amplifier. This gets rid of the 60Hz "hum" that would otherwise be passed on to the speakers, and then allows the amplifier to convert the DC current, produced by the amplifier's power supply, back to AC current that alternates at the frequencies of the music. Since rectifiers still have pulses from the AC current, even though it is only in one direction, power supply capacitors are used to store the DC and feed it to the amplifier stages as smooth (filtered) current, with no fluctuations. However, the diode can only rectify. It cannot amplify. We need to add a third unit to the tube for that, called a "grid". It is a thin metal mesh, placed between the cathode and the anode. This type of tube is called a triode. If we apply a negative voltage (and it does not take very much negative voltage to do this) to the grid, the electrons are repelled and not allowed to flow from the cathode to the anode plate. So, if the audio signal (small voltage) is applied to the grid, and its small voltage fluctuates with the sound, there will be a fluctuating amount of current flowing between the cathode and anode, in time with the music, and since the anode plate voltage is very high in comparison to the grid voltage, voila, we have amplification. Besides triodes, there are tetrodes which have a screen grid as well as the original (control) grid, and pentodes which have the control grid, screen grid, and suppressor grid.
Single Ended Amp
The function of all the grids is to modify the current flowing between the cathode and anode plate, in one way or another, and the multigrid tubes can produce higher powered amplification, but, unfortunately, more distortion. Triode based tube amplifiers were the first, the simplest, and, in the opinions of many, still the best amplifiers. Their sound is incredibly pure, in part, because they are simple. The finest in this category is called a single ended Pure Class A triode amplifier (see below for description of classes of operation). Usually one output tube is used, and the plate is connected to one wire on the primary winding of an output transformer (the secondary winding of this transformer supplies the speakers), while the other wire of the output transformer primary winding is connected to the power supply. Only a few watts of audio power (7-30 watts) can be obtained from such amplifiers, but the sound is marvelous. However, they don't have much application in home theater, because the sounds in movies often require high power to reproduce them realistically. On the other hand, this type of amplifier would be wonderful to have as part of a separate audio system for listening to music at modest volume levels. You should audition a single ended triode tube amplifier in any case, just so you know what they sound like. Models begin at about $1,000.
Push Pull Amp
To get more power out of a tube amplifier, tetrode and pentode tubes are used, and in a different configuration than single ended, a configuration called "push pull". In this type of amplifier, the output tubes (at least two are required) are utilized by connecting the anode plates to the two wires of the output transformer primary winding. One tube "pushes" the signal, and the other "pulls". Often, several tubes (in multiples of two) are connected in parallel to get more power. The push pull tube amplifier is capable of producing 1,000 watts RMS per channel, plenty for home theater, but this much power will cost in the range of $20,000 for a stereo pair because they are hand made in small quantities.
Push pull tube amplifiers with outputs of 125 watts per channel are available at more reasonable prices, and, if you decide that the tube sound is for you, such amplifiers could be worked into a home theater system. For example, a tube amplifier could be used to drive the front left and right stereo speakers, while a less expensive three channel solid state amplifier (see below) could be utilized for the center channel speaker and the two rear surround speakers. In this system, when playing music, the "bypass" mode could be selected on the surround sound receiver, which directs all the sound to the front left and right stereo speakers, just as in a standard two channel stereo system, and the center channel as well as the rear surround channel would be turned off (muted). Thus, you would have the tube sound when playing CDs in normal stereo, and a combination of tube and solid state for home theater. If this idea appeals to you, work carefully with your dealer to match the best combination of components, and listen to the complete setup before making the final purchase.
Solid state (transistor - semiconductor) amplifiers are manufactured in the millions, being assembled by electronic robots in many cases. The result is that you can purchase amplifiers with outputs in the range of hundreds of watts per channel for hundreds of dollars instead of thousands of dollars in the case of tube amplifiers. Of course, you can also spend that $10,000 tube amplifier price mentioned above for a solid state amplifier, but you don't have to unless you really want the absolute top of the line equipment. Superb sound can be purchased with budgets that most of us can handle.
Types of Transistors
Transistors are like tubes in a way. They act as valves for the flow of electrical current, and can be used to amplify by using one of the connections on the transistor to control the flow, like the grid is used on the tube as described above. There are other connections on transistors which are the equivalent to the cathode and anode of tubes. Three popular types of transistors used in amplifiers are called "Bipolar", "J-FET" (Junctional Field-Effect Transistor), and "MOSFET" (Metal Oxide Semiconductor Field-Effect Transistor). FET type transistors perform more like tubes (musical quality) than bipolar transistors, but bipolar transistors are more capable of high current output than FETs, so they both have their specific uses. B,C, and E are Base, Collector, and Emitter, respectively. G, D, and S are Gate, Drain, and Source, respectively. Transistors have several layers sandwiched together. Impurities are added to the silicon which makes it conduct under specific circumstances. This is why they are called "semiconductors". If the impurity, e.g., gallium, causes the layer to be positively charged, the layer is called "P", while impurities that result in the layer being negatively charged, e.g., antimony, make the layer "N". The sandwich defines whether the transistor is PNP or NPN. In the above illustration, P types are shown. In the case of N types, the arrow would be in the opposite direction for schematic diagrams. Darlington circuits are often used, which consists of a transistor pair, with the collectors connected together, and the emitter of one transistor connected to the base of the other. This produces an amplifier stage with current gain that is the product of the individual gains of the two transistors.
Other Parts in an Amplifier
There are many other parts in the amplifier besides the rectifying and amplifying transistors (or tubes). These include resistors (which resist the flow of current), capacitors (which store electrons), inductors (for example, transformers), conductors (the wiring which conducts electrons from one part of the amplifier to another), and insulators (plastic coating on wires and other parts which prevents electrons from flowing where they are not wanted). These parts are all placed in a chassis (metal box), some of the parts having been mounted on a circuit board, and connected to jacks (sockets), controls (on/off, volume, tone controls, input/output selectors), and indicator lights. An inexpensive amplifier will use inexpensive parts, and an expensive amplifier contains nothing but the best. Somewhere in between is what most of us are looking for.
Fundamentals of Electricity
In order to understand what the various parts in an amplifier do, it is necessary to know some fundamentals of electricity. Electricity involves the storage and flow of electrons. The electrons flow from one region where there is a relative excess, to a region where there is a relative deficiency (one region relative to the other). The difference in the relative number of electrons between the two regions is called the potential difference, or more commonly, the voltage. It is sort of like the water behind a dam. The more water, the more pressure that will be exerted at the opening near the bottom of the dam. It is the same with electrons. The more the difference in the number of electrons between the region of excess and the region of deficiency, the greater the voltage, or pressure for electrons to flow. When the two regions are connected by a conductor, electrons flow from the region of excess, called the negative pole or cathode, to the region of deficiency, called the positive pole or anode. This flow of electrons is called current, and is measured in amperes (abbreviated "amps"). In some textbooks the flow of current is designated as positive to negative, but, in fact it is from negative to positive, which is the way you will see it in up to date texts. The reason for the mistake dates back to the discovery of electricity, and it was not known which way current flowed. Taking a 50/50 chance, the wrong guess was made, and it is surprising how long this mistake has stayed in the literature (it was in my high school textbooks). For a current of 1 amp to flow, 6.28 x 1018 electrons (this number is called a "coulomb") have to pass through the conductor in one second. The electrons move from atom to atom in the conductor (for example, copper atoms) like a brigade of people in a line handing buckets of water from person to person in order to extinguish a fire at the end. The people remain stationary, but the buckets of water move. With electrical current, the atoms of the conductor (copper) remain in one place, and pass the electrons along. There is resistance to the flow of electrons in any conductor (except for super conductors, which are being researched at present), and this resistance is measured in ohms. To put the three factors together (volts, amps, and ohms), a mathematical definition states that 1 volt applied to a conductor with a resistance of 1 ohm, will result in 1 ampere flowing in the conductor.
A good conductor (copper wire) has low resistance, and a poor conductor (the insulation surrounding the wire) has high resistance. Energy is expended in overcoming the resistance in conductors, and this energy is converted to heat. Even a good conductor like copper will heat up substantially if too much current is passed through it. This is why we have circuit breakers in our homes, usually set at 20 or 30 amps. Otherwise, the resistance of the copper would cause the wire to get extremely hot if too much amperage was flowing, and we would have to call the fire department. Resistance is what makes the cathode filament in a vacuum tube heat up, so that we can use the electrons surrounding this hot filament to produce amplification. Resistors are used in electronic circuits for specific purposes. Getting rid of the heat that is generated in all electronic equipment by the resistance to flow of current is a problem that has to be seriously addressed at the design stage. This requires the use of "heat sinks" which provide large surface areas for the heat to be transferred to the air surrounding them. Adequate ventilation of your equipment must be taken into account when stacking components on top of one another, and when placing them in cabinets with doors.
While a conductor allows the flow of electrons, and an insulator blocks the flow, there is also a material called a semiconductor, where electrons flow only under certain conditions. Semiconductors (transistors) form the basis of an entire industry in electronics. Silicon and germanium are semiconductors used for this purpose.
Capacitors are one way to store electricity (electrons), and inductors are another. Inductors store electrical energy in a magnetic field, in the process of inductance. This process (inductance) occurs even in a single wire conducting an electrical current. When the wire is formed into a coil, more inductance results. Such coils are used in electrical circuits, and are called inductors, or "chokes". However, by definition, any element in an electric circuit having a magnetic field is an inductor. The unit of inductance is the henry, and 1 henry permits a current increase of 1 ampere per second when 1 volt is applied across an inductor's connectors (terminals).
Most conductors are surrounded by insulators to prevent current from flowing where it is not wanted. Capacitors are made from thin sheets of conductors wrapped between layers of plastic insulators. Capacitors are used to store electrons as an electrostatic field, and the unit of capacitance is called a farad. Any two surfaces having different electric potentials (voltage) that are close together so that an electrostatic field forms, is a capacitor. One farad of capacitance occurs when 1 volt is applied across the terminals of the capacitor and 1 coulomb (see above) is stored. It may seem rather simple when all the formulas use the number 1 in their relationships, but the design of an amplifier or other electronic component is extremely complicated, and the choice of resistors (in ohms), inductors (in henries - also spelled henrys), and capacitors (in farads) is just as important as the choice of tubes or transistors. It is interesting that some of the best sound comes from equipment with the least number of electrical parts in the signal path. This is probably another of the several reasons that tube amplifiers have such an excellent reputation. There are not very many parts in them. In some very expensive solid state equipment, the engineers have eliminated certain parts from the signal path (parts that are necessary in less sophisticated designs), in order to improve the sound quality. This is very high technology, and it shows up in the listening room.
In the world of acoustics, there are discussions of "harmonics", and this has application to amplifiers and other components in sound reproduction. Sounds of nature, as well as musical instruments, are distinguished from one another not simply on the basis of whether they are high or low in pitch, but their harmonic content. A sound has what is called a "fundamental" frequency. Humans can hear, by normal air conduction through the ear, frequencies between 20 Hz and 20 kHz. This is only an average, and as we age, we tend to lose the ability to hear high frequencies above about 12 kHz. Secondly, the sensitivity throughout the range is not smooth. We are most sensitive to the range of about 500 Hz to 5 kHz. The fundamental frequency of a sound is the lowest frequency component in that sound. The sound also has harmonics, which are multiples of the fundamental frequency. For example, a sound might have a fundamental frequency of 500 Hz. The second harmonic (there is no first harmonic unless you want to call the fundamental frequency as such) would be twice the frequency of the fundamental, or 1 kHz, the third harmonic three times the fundamental, or 1.5 kHz, and so on.
Natural sounds (not recorded) contain many harmonics, and primarily even ordered (second, fourth, sixth, etc.). It is the combination of harmonics that allows us to distinguish a trombone from a french horn when they are both playing the same note. Amplifiers produce an artifact called "harmonic distortion", which means that they create harmonics where they don't exist, or exaggerate harmonics that are already there. Even ordered harmonics (second, fourth, sixth, etc.) are pleasing (consonant) to the ear, while odd ordered harmonics (third, fifth, seventh, etc.) are irritating (dissonant). Think of it in terms of the sound of a barber shop quartet. If the three harmony parts are all singing in tune with each other and the soloist, all is well as far as the listener is concerned, no matter how loud the harmony parts are singing. This is consonance. However, if just one of the harmony parts sings out of tune, even quietly, the listener finds this irritating. This is dissonance.
Tube amplifiers, particularly the single ended type, produce even ordered harmonic distortion, primarily second order. Push pull tube amplifiers, particularly solid state amplifiers, tend to produce odd ordered harmonic distortion. If you are purchasing a single ended triode tube amplifier, 1% total harmonic distortion (THD), which means 1% of the sound is harmonically distorted, is actually pleasing to the ear, because it is even ordered. However, with a solid state amplifier, THD as low as 0.5% can be irritating, because it is odd ordered distortion. Therefore, when purchasing a solid state amplifier, make sure that the THD is no more than 0.5%. All high quality solid state amplifiers meet this specification, but you should note the fine print as to the THD in the rear channel of surround sound integrated amplifiers for the distortion factor. Often, it is larger than 0.5%. On the other hand, the bottom line in all cases, is whether the sound pleases you or not. "Specsmanship" as it is called (emphasizing the specifications on the technical data sheet supplied with a component) should not be the deciding factor. Decide with your ears, but ask to see the technical data sheet before you write the check.
Negative feedback is the process of taking a portion of the output, electrically inverting it, and then feeding it back to the input. The purpose of negative feedback is to flatten the frequency response, reduce harmonic distortion, lower the output impedance, and also to reduce the effects of "parasitic oscillation" that can occur when parts of the circuitry cause an induced current to flow in other circuits where it is not wanted. However, too much negative feedback can sometimes be used to produce an artificially low harmonic distortion specification. Second order harmonics will be reduced, but fifth order harmonics will increase, and this odd ordered distortion is much more noticeable than the original second order distortion. A tell tale sign of too much negative feedback is an excess of sibilance in the human voice (the "s" is exaggerated).
Class of Operation
High Fidelity Amplifiers operate in what is known as Class A, Class AB, Class D, or Class H. In Class A, the bias (the amount of voltage applied to the grid in tubes - see discussion of tubes above - and to the corresponding controlling connector in transistors) is such that there is current flowing through the system whether there is an input signal (music) or not. This makes the tubes or transistors get hot, as the power not being used for music is dissipated as heat. In fact, a Class A amplifier at idle (no music signal) dissipates about 4 times the rated audio power as heat. So, if the amplifier is rated at 10 watts per channel of audio power, it dissipates 40 watts of heat per channel at idle. When a signal is applied, the power that was being dissipated as heat becomes audio power to the speakers. So, it runs hot when no music is playing, and cooler when you crank it up! The value of Class A amplification is that, because current is always flowing in the system, the musical signal is "instantly" diverted to the output, rather than current flow having to be turned on and off, so the response to the input signal is extremely fast, and a very clean sound results. Class AB amplifiers operate in Class A for a small portion of their potential power output, then transfer to Class B (where additional current has to be drawn) as higher power demands are made.
Class A amplification is not very efficient, and the quality of components has to be high, especially if it produces high power. If the amplifier operates in Class A all the way to the limits of its power output, and, if all stages of the amplifier are operating in Class A, then the amplifier is said to be Pure Class A. This is something to watch for in shopping for an amplifier, since some will advertise Class A operation, but do not operate in Class A throughout their output range, nor operate in Class A in all stages of their circuitry. Class AB is much more efficient, allowing Class A operation when soft passages in the music are being played, and then switching to the more efficient Class B operation when loud passages come along. The single ended triode tube amplifier mentioned above is usually a Pure Class A amplifier.
There are numerous solid state as well as tube Pure Class A amplifiers, and they are all quite expensive. The majority of separate component high fidelity amplifiers (power amplifier separate from the preamplifier) are Class AB. Many of the amplifiers (specifically, the integrated amplifiers, i.e., preamplifier and power amplifiers in one chassis) sold in large home electronics chain stores are likely to operate almost entirely in Class B. That does not mean that they sound bad, only that you don't get any Class A, even at low volume. However, a few of the integrated units will be Class AB, and they will advertise that fact, because the sound quality is improved. They will say something to the effect of Class A operation, but actually meaning that only some of the power is in Class A. If you become interested in one of these amplifiers, including surround sound integrated packages, check to see how much of the power is devoted to Class A. Also, feel the top of the amplifier after it has been sitting with the power on for a while. It may be quite warm to the touch, and if you buy such a unit, you will need to be careful as to where you place it in your home. Better than average ventilation is important in this case.
Class D amplifiers are switching amplifiers. The + and - output transistors are switched on and off at varying frequencies, with the higher frequencies producing a larger voltage on the waveform (more volume). The + and - transistors are never on at the same time, but when they are switched on and off, they are completely on and off, rather than somewhere in between. Class D is extremely efficient, so almost no heat is produced. However, distortion at the crossover point (+ to -) can be introduced because of "dead time" which is caused by the fact that the + and - transistors cannot ever be on at the same time. Class D amplifiers are often used at concerts for musical groups because they put out lots of power but don't weigh a lot. However, these models don't make good amplifiers for home use because they have noisy fans, and they usually have 1/4" phone jacks for inputs rather than RCA jacks, so adapters are needed. The switching frequency makes a lot of of difference to the high fidelity, and the best ones have a high switching frequency. These are the ones to consider in home audio systems. Class D amplifiers are also great for subwoofers, and there are many available with 500 watts of power and more.
Class H amplifiers have several rail voltages and depending on the intensity of the signal, the low rail voltage wil be used (low volume) or the high rail voltage (high volume). Because the output transistors always "see" the rail voltage, and have to dissipate the difference between the rail voltage and the voltage across the speakers multiplied by the current (which equals watts of heat), Class H is efficient and not much heat from the amplifier is generated. Its drawback is that there is finite time to switch between one rail voltage and the others, so in the mid volume level, there can be distortion when the rails are moving back and forth.
High Voltage and High Current
You will find Pure Class A and exceptional quality Class AB equipment mostly at "High End" (High Performance) audio stores, and some at the large home electronics chain stores. You should listen to as much of this equipment as you have time for - disregard the price for the moment - before buying anything. There are many excellent brands to listen to. Most are separate components (preamplifier, power amplifiers in separate chassis), but a few are integrated. In general, separate components perform better than integrated units because the circuitry from one section (power amplifier) will have its own power supply, and will not cause electrical interference with another section (preamplifier). This is particularly so with high powered systems. When evaluating amplifier power output, the numbers to look for are watts continuous (RMS or Root Mean Square) per channel from 20 Hz - 20 kHz into 8 ohms. If it says 6 Ohms, this value will be a little higher than it would be at 8 Ohms. If it says @ 1 kHz, this is not as good as stating the value at 20 Hz - 20 kHz.
A good power supply is extremely important, and can be one of the major factors which distinguishes the quality of two amplifiers that have the same specified output. The main components in a power supply are the power transformer, rectifiers, and some large capacitors. If the power supply is inadequate, then distortion will result when high power demands are made, such as during loud passages in the music. Whereas one amplifier rated at 100 watts per channel may "clip" (go into high distortion) at just over 100 watts, a different 100 watt per channel amplifier with a better power supply may give much more than 100 watts, at low distortion, for short bursts (a kettle drum thud in a musical passage, or a gunshot in movie, for example). Well built power supplies not only provide noise free electrical power to the amplifier parts, but are capable of delivering high voltage and high current. You will see why this is important in the section on speakers.
The "slew rate" is an interesting factor that is sometimes listed on the specification sheet. It is stated in Volts per microsecond or V/microsec., and refers to how fast the voltage rises from 0 to maximum power. The slew rate is measured by using a square wave and calculating the number of volts that the square wave rises vertically on the Y axis, over the amount of time it takes for this voltage rise to occur, on the X axis. If the slew rate is high, the sound will tend to be "crisp" with a bit of "edge" to it, while a low slew rate gives a sound that has less of these characteristics to it. Tube amplifiers usually have low slew rates (except for the Class A single ended triode amplifier, which is very fast). This may be one of the reasons tube amplifiers have a characteristic soft edged quality about them. Solid state amplifiers, on the other hand, often have very high slew rates, and this may contribute to the sharp edged sound that they can have. However, the precise effects of the slew rate on sound quality are difficult to define. The term just seems to be one of those things that appear on specification sheets, and makes an interesting topic for discussion.
Unbalanced and Balanced Connectors
Some of the newer design preamplifiers, power amplifiers, and CD players have both "unbalanced" and "balanced" jacks (sockets) to connect them together. An unbalanced jack is the standard two connection jack. Typically, it is an RCA type found on just about every home electronic component made. The balanced jack has three connections - one for the positive, one for the negative, and a third for the ground (in the unbalanced connector, the ground is combined with the negative conductor). Balanced connections can eliminate electrical interference that gets into the cable which links the components together, especially if the cable has to be long (10 - 15 feet).
Fully balanced products are balanced throughout the circuitry in the chassis. This means balanced from input to output. Quasi-balanced products may have balanced inputs and/or outputs, but are not balanced throughout the circuit. Fully balanced products will always have less noise than unbalanced or quasi-balanced products. Quasi-balanced products may or may not have improved noise characteristics. What is important here is to know that if the product brochure or advertisement says it has balanced inputs and outputs, that does not mean it is fully balanced.
You will find that many of the high end preamplifiers don't have any tone controls (treble/bass), and practically none of them have DSP. What you will also discover is that with such high quality equipment, tone controls are not really necessary because the signal is not altered significantly as it passes through the system, and, thus, sounds quite natural. However, tone controls, and DSP, may be something you prefer to have. Equalizers are tone controls that allow the user to adjust not only the treble and bass, but many frequencies in between. If used in a surround sound system, they have to be placed after the surround sound decoding in the signal path. They are condemned by some audio purists, but they can be useful to fine tune the sound of a system so that it is just right for you. Therefore, listen to as wide a variety of setups as possible. Then you will be able to truly evaluate what it is you are trying to achieve in your home theater/home audio environment before you make the investment.
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