CHAPTER 13
Amplifiers
In the world of audio, amplifiers have many applications. They can be designed to amplify, equalize, combine, distribute or isolate a signal. They can even be used to match signal impedances between devices. At the heart of any amplifier (amp) system is either a vacuum tube or a semiconductor-type transistor series of devices. Everyone has heard of these regulating devices, but few have a grasp of how they operate, so let’s have a basic look into these electronic wonders.
AMPLIFICATION
To best understand how the theoretical process of amplification works, let’s draw on an analogy. The original term for the tube used in early amplifiers is valve (a term that’s still used in England and other Commonwealth countries). If we hook up a physical water valve to a high-pressure hose, large amounts of water pressure can be controlled with very little effort, simply by turning the valve (Figure 13.1). By using a small amount of expended energy, a trickle of water can be turned into a high-powered gusher and back down again. In practice, both the vacuum tube and the transistor work much like this valve. For example, a vacuum tube operates by placing a DC current across its plate and a heated cathode element (Figure 13.2). A wire mesh grid separating these two elements acts like a control valve, allowing electrons to pass from the plate to the cathode. By introducing a small and varying signal at the input onto the tube’s grid, a much larger electrical signal can be used to correspondingly regulate the flow of electrons between the plate and the cathode (Figure 13.3a).
FIGURE 13.1
The current through a vacuum tube or transistor is controlled in a manner that’s similar to the way that a valve tap can control water pressure through a water pipe: (a) open valve; (b) closed valve.
FIGURE 13.2
An example of a triode vacuum tube.
FIGURE 13.3
A simple amplifier schematic: (a) showing how small changes in voltage at the tube’s grid can produce much larger, corresponding amplitude changes between its cathode and plate; (b) showing how small changes in current at the transistor’s base can produce much larger, corresponding amplitude changes through the emitter and collector to the output.
The transistor (a term originally derived from “trans-resistor” meaning a device that can easily change resistance) operates under a different electrical principle than a tube-based amp, although the valve analogy still applies. Figure 13.3b shows a basic amplifier schematic with a DC power source that’s placed across the transistor’s collector and emitter points. As with the valve analogy, by presenting a small control signal at the transistor’s base, the resistance between the collector and emitter will correspondingly change. This allows a much larger analogous signal to be passed to the device’s output.
As a device, the transistor isn’t inherently linear; that is, applying an input signal to the base won’t always produce a corresponding output change. The linear operating region of a transistor lies between the device’s lower-end cutoff region and an upper saturation point (Figure 13.4a). Within this operating region, however, changes at the input will produce a corresponding (linear) change in the collector’s output signal. When operating near these cutoff or saturation points, the base current lines won’t be linear and the output will become distorted. In order to keep the signal within this linear operating range, a DC bias voltage signal is applied to the base of the transistor (for much the same reason a high-frequency bias signal is applied to an analog recording head). After a corrective voltage has been applied and sufficient amplifier design characteristics have been met, the amp’s dynamic range will be limited by only two factors: noise (which results from thermal electron movement within the transistor and other circuitry) and saturation.
FIGURE 13.4
Output curves of a transistor: (a) proper operating region; (b) a clipped waveform.
Amplifier saturation results when the input signal is so large that its DC output supply isn’t large enough to produce the required, corresponding output signal. Overdriving an amp in such a way will cause a mild to severe waveform distortion effect known as clipping (Figure 13.4b). For example, if an amp having a supply voltage of +24 volts (V) is operating at a gain ratio of 30:1, an input signal of 0.5 V will produce an output of 15 V. Should the input be raised to 1 V, the required output level would have to be increased to 30 V. However, since the maximum output voltage is limited to 24 V, levels above this point will be chopped off or “clipped” at the upper and lower edges of the waveform. Whenever a transistor and integrated circuit design clips, severe odd-order harmonics are often introduced that are immediately audible as distortion. Tube amp designs, on the other hand, tend to lend a more musical sounding, even-order harmonic aspect to a clipped signal. I’m sure you’re aware that clipping distortion can be a sought-after part of a tube instrument’s sound (electric guitars thrive on it); however, it’s rarely a desirable effect in quality studio and monitoring gear. The best way to avoid undesirable distortion from either amp type is to be aware of the various device gain stages throughout the studio’s signal chains.
The Operational Amplifier
An operational amplifier (op-amp) is a stable, high-gain, high-bandwidth amp that has a high-input impedance and a low-output impedance. These qualities allow op-amps (Figure 13.5) to be used as a basic building block for a wide variety of audio and video applications, simply by adding components onto the basic circuit in a building-block fashion to fit the design’s needs. To reduce an op-amp’s output gain to more stable, workable levels, a negative feedback loop is often required. Negative feedback is a technique that applies a portion of the output signal through a limiting resistor back into the negative or phase-inverted input terminal. By feeding a portion of the amp’s output back into the input out of phase, the device’s output signal level is reduced. This has the effect of controlling the gain (by varying the negative resistor value) in a way that also serves to stabilize the amp and further reduce distortion.
FIGURE 13.5
Basic op-amp circuit and 741-type pin configuration.
PREAMPLIFIERS
One of the mainstay amplifier types found at the input section of most professional mixer console and outboard devices is the preamplifier (preamp). This amp type is often used in a wide range of applications, such as boosting a mic’s signal to line level, providing variable gain for various signal types and isolating input signals and equalization, just to name a few. Preamps are an important component in audio engineering because they often set the “tone” of how a device or system will sound. Just as a microphone has its own sonic character, a preamp design will often have its own “sound.” Questions such as “Are the op-amps designed from quality components?” “Do they use tubes or transistors?” and “Are they quiet or noisy?” are all-important considerations that can greatly affect the overall sound of a device.
EQUALIZERS
You might be surprised to know that basically, an equalizer is nothing more than a frequency-discrimi nating amplifier. In most analog designs, equalization (EQ) is achieved through the use of resistor/capacitor networks that are located in an op-amp’s negative feedback loop (Figure 13.6) in order to boost (amplify) or cut (attenuate) certain frequencies in the audible spectrum. By changing the circuit design, complexity and parameters, any number of EQ curves can be achieved.
FIGURE 13.6
Basic E equalizer circuit: (a) low-frequency; (b) high-frequency.
Summing Amplifiers
A summing amp (also known as an active combining amplifier) is designed to combine any number of discrete inputs into a single output signal bus, while providing a high degree of isolation between them (Figure 13.7). The summing amplifier is an important component in analog console/mixer design because the large number of internal signal paths requires a high-degree of isolation in order to prevent signals from inadvertently leaking into other audio paths.
FIGURE 13.7
A summing amp is used to provide isolation between various inputs and/or outputs in a signal chain.
Distribution Amplifiers
Often, it’s necessary for audio signals to be distributed from one device to several other devices or signal paths within a recording console or music studio. In this situation, a distribution amp isn’t used to provide gain but instead will amplify the signal’s current (power) that’s being delivered to one or more loads (Figure 13.8). Such an amp, for example, could be used to boost the overall signal power so that a single feed could be distributed to a large number of headphones during a string or ensemble session.
FIGURE 13.8
Distribution amp.
Power Amplifiers
As you might expect, power amplifiers (Figure 13.9) are used to boost the audio output to a level that can drive one or more loudspeakers at their rated volume levels. Although these are often reliable devices, power amp designs have their own special set of problems. These include the fact that transistors don’t like to work at the high temperatures that can be generated during continuous, high-level operation. Such temperatures can also result in changes in the unit’s response and distortion characteristics or outright failure. This often requires that protective measures (such as fuse and thermal protection) be taken. Fortunately, many of the newer amplifier models offer protection under a wide range of circuit conditions (such as load shorts, mismatched loads and even open “no-load” circuits) and are usually designed to work with speaker impedance loads ranging between 4 and 16 ohms (most speaker models are designed to present a nominal load of 8 ohms). When matching an amp to a speaker set, the amp should be capable of delivering sufficient power to properly drive the speakers. If the speaker’s sensitivity rating is too low or the power rating too high for what the amp can deliver, there could be a tendency to “overdrive” the amp at levels that could cause the signal to be clipped. In addition to sounding distorted, clipped signals can contain a high-level DC component that could potentially damage the speaker’s voice coil drivers.
FIGURE 13.9
Mackie FRS-2800 professional power amplifier. (Courtesy of Loud Technologies, Inc., www.mackie.com)
Voltage and Digitally Controlled Amplifiers
Up to this point, our discussion has largely focused on analog amps whose output levels are directly proportional to the signal level that’s present at its input. Several exceptions to this principle are the voltage-controlled amplifier (VCA) and the digitally-controlled amplifier (DCA). In the case of the VCA, the overall output gain is a function of an external DC voltage (generally ranging from 0 to 5 V) that’s applied to the device’s control input (Figure 13.10). As the control voltage is increased, the analog signal will be proportionately attenuated. Likewise, a digitally controlled external voltage can be used to control the amp’s overall gain. Certain older console automation systems, automated analog signal processors and even newer digital console designs make use of VCA technology to digitally store and automate levels.
FIGURE 13.10
Simplified example of a voltage-controlled amplifier.
With the wide acceptance of digital technology in the production studio, it’s now far more common to find devices that use digitally controlled amplifiers to control the gain of an audio signal. Although most digital devices change the gain of a signal directly within the digital domain, it’s also possible to change the gain of an analog signal using an external digital source. Much like the VCA, the overall gain of an analog amp can be altered by placing a series of digitally controlled step resistors into its negative feedback loop and digitally varying the amount of resistance that’s required to achieve the desired gain.