Rule 6 - components
Part 3: amplifiers
Select and integrate audiophile-quality components
Music extracted either from an analog or digital source will produce electrical signals which flow along the various audio interfaces including a preamplifier where it is finally filtered to the power amplifier. The power amplifier's job is to convert the low-level signal into a high-level signal and ideally pump enough juice to drive the loudspeakers efficiently
The amplifier is the engine that drives an audio system - there is no wastage of wattage in an amplifier
Types of amplifier design and applications – for audio signal management and amplification
Headphone amplifier – a fix for the headphone junkies
This device as the name implies amplifies a weak signal coming from its source (audio output) to drive headphones. Some headphone designs are not as efficient (e.g., planar and electrostatic) that would therefore need greater amplification from such a device. Some headphone amps may also have outputs and volume control which may also be doubled as a preamplifier to drive a full audio component system.
Receiver – retro product
An integrated amplifier that incorporates a tuner for AM or FM bandwidth tunning
Tuner – old school
A dedicated component for AM or FM bandwidth tunning. Need to be connected to a preamplifier.
AV amplifier – for the Home Théâtre enthusiast
An integrated amplifier that incorporates a digital surround processor with relevant industry surround sound standards to facilitate surround sound integrated with multiple amplifiers (minimum 5 channels) built-in. They may also incorporate a tuner and more recently they may also include a DSP sound correction equalization circuit to enable room correction.
Phono amplifier – a necessary evil
Also known as a phono stage, or Head amplifier, is an electrical component that converts a weak low-level signal from the cartridge to a line-level signal for the preamplifier. It may incorporate both active and or passive RIAA (Recording Industry Association of America) equalization circuitry. This is necessary because vinyl is cut with the low frequencies compressed, and the high frequencies boosted for a practical cutting of the lathe and subsequent vinyl stamping production, otherwise, the record grooves would be too deep. Therefore, the RIAA equalization circuitry during playback is necessary to normalize (flatten the response curve) the low and high frequencies during playback. The phono stage usually incorporates either a MM (moving magnet - 47K ohm) or MC (moving coil) selection or both. For MC, it would have various adjustments (capacitance and gain) to optimize the cartridge outputs as the manufacturer intended.
Pre-amplifier – silence is golden
A preamplifier, also known as a preamp, is an electronic amplifier that may use solid-state devices, vacuum tubes, or both (hybrid). It will incorporate many inputs to receive multiple signal sources manage by a selector switch. The primary function of the preamp is to increase a weak electrical signal from various media sources into an output signal (line-level) strong enough with low noise to drive a power amplifier. Inputs and outputs may be single-ended or balanced. Perhaps an important feature of the preamp is its volume control knob (attenuator pot) which also determines its sound quality. Without a preamplification as an intermediary, the final signal would be noisy or distorted affecting the signal-to-noise (S/N) ratio, therefore, the S/N ratio of a preamplifier in this respect is critical to enable good sound.
An ideal preamp should be linear i.e., have a constant voltage gain e.g., from 10 mV to 1 V through its operating range, with no significant current gain. It should have high input impedance i.e., requiring only a minimal amount of current to sense the input signal, and a low output impedance i.e., when current is drawn from the output there is minimal change in the output voltage. Some lower-end preamps will have tone controls for bass and treble adjustments. Most would be fitted with a balance knob for the left and right channels. And some may even feature an old-style loudness switch and a phono stage albeit for the MM cartridge. Most modern-day preamps have a remote to control source selection and even volume.
Integrated amplifier – a cost-effective and space-saving proposition
As the name suggests, it’s a combination of pre and power amplification. The Integrated Amplifier may be a hybrid design i.e., vacuum tubes in the preamplification section and solid-state at the power amplification section. The main advantage of the integrated amplifier is that you don’t have to buy costly separates or match both units’ impedances or purchase an extra connecting cable. However, you still have to match the amplifier power section to the efficiency of the loudspeakers used. You can also choose one that has certain features that you would require such as a phono stage, subwoofer out, etc. However, the reason preamps are kept separate in high-end audio is to keep the noise generated by the large transformers in the power amplifier from interfering with the preamp circuits. Moreover, there would be enough space for a beefier transformer and enough room to design a smart circuit layout that is mirror-image. A preamplifier moreover uses little electricity, unlike a power amplifier that draws current and generates heat as well.
Power amplifiers – watts more important
An amplifier is an electronic device that is used to increase (or amplify) the current, voltage, or power of an analog electrical signal. It is fed an electrical line-level signal at its input, and at its output, amplifies the signal to a sufficient level (higher current necessary) so that it could drive the loudspeaker (load) to an intended output level (SPL). An amplifier’s power output is measured in Watts. In all amplifier designs, banks of output transistors, and in the case of a vacuum tube design, the tubes, each a little amplifier by itself, add their collective power together to provide the amplifier's final output. Measured distortion specification in amplifiers is not as important because they are so nuanced that you can’t differentiate (hear) between them. After selecting your loudspeakers, there are two factors to consider when choosing your amplifier; power and impedance which are both interrelated. This is because the amplifier must be able to drive the loudspeaker in its most linear form.
Power output
We need to describe how an amplifier interacts with loudspeakers because the loudspeakers behave as a resistive, and sometimes a reactive load. All loudspeakers are rated in impedance commonly at 8 ohms as an average. However, the impedance fluctuates depending on the frequency being fed as the character of the music is dynamic. The lower the impedance when it drops at certain frequencies, the more current is required from an amplifier, failing which the amplifier goes into clipping and distortion occurs when it is unable to supply the power required, and worse still overheating may cause the loudspeaker voice coil to be damage. The woofer may also be overdriven to the extent of bottoming out when they reach their full excursion point and introduce mechanical noise. Therefore, all loudspeakers have a maximum number of watts they can cope with, and a minimum number required to comfortably drive them. These numbers are detailed in the manufacturer’s specification sheet. Suffice it to say, an amplifier will be capable of providing much more power to a lower-impedance loudspeaker than a higher-impedance loudspeaker. In the loudspeaker’s specifications, you might find a few different power ratings such as Peak power and Continuous power among them that are significant. Peak power refers to the maximum short-term power (instantaneous burst) a loudspeaker can handle without damage and is limited by the maximum excursion of a loudspeaker cone (ability to exert outward and inward movement). However, sustained sounds are perceived as being louder than short transients at the same level.
Therefore, we should be more concerned about how much power a loudspeaker can sustain over an extended period but is limited by the heat tolerance of a loudspeaker’s voice coil. This is the continuous power rating found in the specification sheet. It is characterized as the RMS (Root Mean Square) response, which is a mathematical means of determining average signal levels known as continuous power. You need to choose a higher power rating amplifier than that of the loudspeakers to allow headroom so that you can get the most out of your loudspeakers without clipping. Choosing additional amplifier power within limits will not cause loudspeaker damage as many have mistakenly believed, but choosing an amplifier that can’t supply enough power is more likely to cause damage when the amplifier clips, the waveform gets distorted (you will hear) and the loudspeaker’s midrange and tweeter voice coils may start to fry.
Impedance
Impedance is measured in ohms and it refers to the opposition the circuit has to the flow of electrical current. You need to find the nominal impedance for your loudspeakers which is also characterized by their impedance usually rated between 4 to 16 ohms. The impedance of the loudspeaker will determine the load that will be powered by the amplifier. This means a loudspeaker with a “nominal impedance” of 8 ohms and a power rating of 350 watts (RMS) will require an amplifier that can produce 700 watts per channel into an 8-ohm load to allow for headroom. However, if the amplifier is powering a 4-ohm load loudspeaker instead might lead to more current than the amplifier can safely provide. The amplifier may overheat and may shut down. That’s why it’s important to make sure that the amplifier you choose is rated to provide adequate power to whichever loudspeakers you are using. Therefore, it is important to understand the amount of current an amplifier is capable of delivering to meet the requirements of the loudspeaker. Ohm’s law states that as impedance decreases, electrical current will increase at a given voltage.
Damping factor
The damping factor (DF) describes how much control (damping) the amplifier has over the load (loudspeaker) it is driving. It is said to be high when an amplifier’s impedance is low. A typical Solid-State Amplifier’s output impedance is usually less than 0.1 ohm, so, with an 8-ohm loudspeaker, and an output impedance of 0.1 ohms you get a DF of 80. The benefits of having a low output impedance amplifier (or high damping factor) indicate the degree to which the amplifier has control over the cone stopping (decaying longer than the instrument’s natural decay) and its current capacity. Damping is most evident at frequencies below 150 Hz where the size and weight of the cone become significant. A low damping factor mostly affects low-frequency performance, hence, low-frequency transients such as kick drums will sound “muddy” instead of “punchy”.
Power supply
In high-end audio amplifiers, much of the cost of an amplifier is from the power supply, that provides the current to the circuitry. The power supply does its job by modulating this current, and every anomaly present in the current would be delivered to the outputs. A well-designed amplifier would produce a clean, steady flow of current even during musical peaks and at the same time filter out noise coming from the power line.
Power efficiency
Amplifiers always put out less power than they consume. An amplifier's efficiency is the ratio of what it puts out divided by what it draws from the electrical system. No amplifier is 100% efficient, i.e., putting out exactly what it draws, or more power than it draws. The power that doesn't make it to the output terminals is wasted energy that is converted into heat. Too much heat if not sufficiently controlled will corrupt the amplifier's output signal, and damage the internal components. This is why heat sinks and in the case of pro-audio, cooling fans are part of its integral design. Amplifiers should ideally be working at the most around 50% of their capacity, 60% would inevitably introduce distortion. Loudspeakers usually break down because the amplifier lacks the power when push to its limits. Therefore have an amplifier that is overkill for your selected loudspeakers so you would have enough headroom for the amplifier to function at its most linear.
Matching Pre-amp to Amplifiers
You don’t have to worry if using an integrated amplifier or a pre and power amplifier built by the same manufacturer to be paired with each other. Same-brand components have synergy in impedance matching and are also less prone to ground loops. Otherwise check the specification sheet, which is why the amplifier should be the last component purchased.
Power Amplifier Classes
The way an amplifier combines power, and signal (which also determines the output stage distortion) defines its Class. The art of power amplifier design and implementation can vary dramatically, and the design is always evolving.
Class-A amplifiers are considered to be the least efficient because both output stages (transistors) constantly run at full power (constant bias) regardless of an input signal. With no signal, the transistors' power turns into heat with no turn-off or cooling cycles. Further, each output transistor, amplifies both the negative and positive voltage of the signal's AC waveform, generating even more heat. Class-A amps are said to have about a 20% (50% at best, theoretically) average operating efficiency with the remaining power dissipated as heat. However, the transistors perform in their most linear mode with the least amount of distortion under these thermal conditions. Moreover, because there's no crossover or switching, hence no induced high-frequency distortion. Pure Class-A amplifiers run hot to the touch, cost more for the same power rating, and often are large and heavy to meet operating conditions where form follows function. They are not common in the marketplace and find applications that require low power and low distortion, such as horn-design loudspeakers or loudspeakers with an efficiency rating of over 100db. Despite their quirks, they are the best-sounding amplifier designs bar none. It is pertinent to note that tube-design amplifiers such as a single-ended triode design have a similar philosophy to their solid-state counterparts.
Class-AB amplifiers offer both power efficiency and sound quality. The output transistors are push-pull pairs, allowing for both output stages to be on at the same time. However, one output will receive the current flow between one-half and one full cycle, while the other output receives just enough current flow to remain on. This way, it can respond instantly to the input signal, and would not switch suddenly. This hybrid design leverages the strengths of both worlds, with greater efficiency than Class-A, without the non-linear crossover distortion of Class B. There's an optimum bias current, that manages the amount of time when both transistors are passing current, which is said to minimize crossover distortion. Class AB amps are said to have a 60% operating efficiency. For this reason, most audio amplifiers are Class AB designs having a positive cost-benefit and are widely used in high-end audio. It is pertinent to note that tube-design amplifiers have push-pull designs that have a similar philosophy to their solid-state counterparts.
Class-D amplifiers, also known as switching power amplifiers use active transistors to function as electronic switches that can be either on or off. It has the highest efficiency but is rather shy on sound quality. The onboard circuitry creates high-frequency (over 100 kHz) pulses of DC. The width of each pulse is then modified by the input signal into a stream of pulses, the wider the pulse, the louder the signal - called "pulse width modulation", “delta-sigma modulation”, or “pulse-density modulation”. These DC pulses are fed to the output transistors (MOSFETs) creating the high-power output. The transistors are either on at full power or off with no power which is why they are said to have a 90% operating efficiency at the expense of linearity and distortion. Amplifier designers have developed ways of addressing distortion such as building full-bridge (as opposed to half-bridge) circuit topologies that cancel even order harmonic distortion and the circuit’s DC offset. After amplification, a passive low-pass filter smoothens the output signal for continuous power instead of pulses and back to an analog signal removing noise generated by those high-frequency DC pulses.
Class D amplifiers excel at band-limited high-power applications, they have become popular because they are lighter, and run cooler than the other classes of amplifiers with the same power output. Its small form factor makes it ideal for use in-car audio applications, active subwoofers, soundbars, and active loudspeakers. They are also applied in the pro-audio industry for amplifiers and PA systems where sound quality is not an essential factor. It is also said that class D amplifiers may not be able to handle difficult loudspeaker loads because they do not necessarily provide high currents. Notably, most audiophiles have not fully embraced those designs because of their questionable sound quality. This is partly due to the amplifiers using integrated circuits instead of discrete ones with inherent pulse distortion. Refer to my article on Discrete vs Integrated circuits in the 10 guiding audiophile principles for a better understanding of this subject.
Vacuum tubes vs Transistors
This is another contentious debate where there are 2 camps in this regard. There are two main types of transistors; the bipolar junction transistor (BJT) and the junction field-effect transistor (JFET). The metal–oxide–semiconductor field-effect transistor (MOSFET) largely superseded both the bipolar transistor and the JFET. These are solid-state devices that come in many shapes and sizes having the same conceptual function as vacuum tubes. They can act as a switch to control circuits, and can also be made to amplify signals.
Life expectancy - tubes have a shorter life expectancy that may last between 3,000 to - 10,000 hours of play depending on the circuit designs, handling, tube type, and manufacturer. Moreover, they tend to lose their electrical properties as they age. Transistors are permanently soldered onto the circuit board, thereby having a longer life expectancy, but intolerant to high temperatures, therefore, requiring suitable heat sinks i.e., a slight temperature change can cause a significant change in the character of the transistor circuit.
Low voltage supply - transistor amplifiers require a lower D.C supply to run their applications while vacuum tubes require a higher D.C voltage ranging from 400 V to 500 V on the rails. Because power consumption is comparatively high, there is more heat wastage in vacuum tubes. Transistors are more power-efficient than tubes.
No heating element is required - transistor amplifiers do not require heaters, whereas vacuum tubes only work with a heater. The heater requirement in vacuum tubes makes the power supply bulky, heat dissipation difficult, and has a limited shelf-life for the tubes because of heat generated from within.
Size - transistor amplifiers are solid-state devices that are relatively smaller than vacuum tubes which have larger output transformers that are much heavier and bulky. The little transistor is an integrated component design with resistors, and other diodes or electronics components that allow the transistor to be small. Therefore, transistor circuits are generally lightweight and portable.
Durability – the outer covering of the vacuum tube is made of glass whereas transistors are made out of silicon. Unlike mechanically strong transistor amplifiers, vacuum tubes are susceptible to breaking. However, the vacuum tubes if damaged or spent may be replaced easily with the same spec tube but must be biased accordingly.
Flexibility - An advantage of vacuum tubes is that different manufacturers or compatible model tubes may change the sound that you may prefer, therefore, trials (tube rolling) are possible to find your ideal sound. NOS (New Old Stock) tubes are also available. These vintage tubes are much sought after for their better ‘sound’ quality but may be costly.
Variation of parameters – Transistors, when manufactured have poor absolute precision values between transistors due to process variation because it's difficult to get components of absolute values into a transistor. Tubes have electrodes that control the flow of electrons that may be had in matched pairs, triodes, or quads to attain perfect linearity between channels or push-pull requirements. Moreover, transistors are temperature dependent and act differently every time there is a slight change in the temperature unlike tubes having consistent results because of their built-in heating element.
Sound - Transistor amplifiers are limiting at the onset of clipping, unlike vacuum tubes which when clipped are less abrupt. During overload conditions, transistors present a steep rising edge harmonic that is not pleasant to the ears. Transistors are made up of PN junctions whereas vacuum tubes are made up of wires. The circuit board of vacuum tubes made is usually hard-wired point-to-point. I believe these differences contribute to the perceived sound quality, moreover, vacuum tubes are more forgiving when matched with loudspeakers than their transistor counterparts. The consensus is that vacuum tubes sound better overall and especially in the midrange where they excel.
A dozen recommended tips and tweaks
Place the subwoofer amplifier as close as possible to the passive subwoofer to reduce the cable length or use an active subwoofer instead.
Use mono-block amplifiers instead to facilitate a short loudspeaker cable enabling good transient response. Mono-blocks further mitigate heat and crosstalk between amplifier channels.
Never use amplifiers with a built-in fan, UV meters, or an LED display panel as they may induce noise into the circuit boards unless there is a switch to turn them off during listening. Ideally, choose truly balanced (common mode rejection) connections if available.
Power amplifiers spec at less than 0.1%THD from 20Hz to 20kHz with 1 W over an 8 Ohm load is preferred, however when the amp starts to get overdriven, the distortion rises quickly.
Measure the outputs of each stereo channel with an SPL meter at the sweet spot by introducing pink noise, and adjust the balance if the amplifier has gained control knobs, otherwise adjust at the pre-amplifier end if there is a balance adjustment. The gain controls are usually calibrated in dB increments and volts.
The higher the current and damping factor of the power amplifier the better. Whether your power amplifier can supply current fast enough to reproduce the music faithfully depends partially on the amp's slew rate (how fast its output can change). Slew rates of 100V/micro sec and damping factors above 100 (referenced with a 4-ohm load) are good.
Amplifier power, (watts into ohms) as a rule of thumb amplifiers should not be underpowered but have at least two times the RMS (continuous) power rating value of the loudspeaker (read the loudspeaker spec sheet). Remember a doubling of power is only a 3dB change (2 times power = +3 dB). The lower your loudspeaker operating impedance e.g., lower than 8 ohms, the more watts would be required by your amplifier to drive the load. Therefore, the amplifier output impedance has to be significantly lower than the load (loudspeaker). This will allow the amplifier to provide adequate power to the loudspeaker while maintaining some extra headroom (e.g., Pop/Rock 14dB, Classical 20dB) to avoid the tendency to overdrive the input of an amplifier. You may refer to the Resource section "Amplifier power requirements calculator" for help.
Make sure that your pre-amp maximum output voltage (normally between 1 to 2 volts – read the spec sheet) is twice that of your Amplifier’s input voltage. The reason it should be twice as much is to allow for headroom to cater to peaks in the musical program.
The power amplifier has to have a higher input impedance than the preamplifier. Make sure that the input impedance of the power amplifier is at least 10 times more than the preamplifier’s output impedance.
A heavy gauge conductor (at least 14 AWG, better 12 AWG) for the power cord should be used with power amplifiers running directly off a dedicated wall outlet instead of a power line conditioner to handle instantaneous higher current deliveries during the transient bursts in music.
The beefier the power supply (transformer) in the amplifier the better.
When pairing an amplifier to your loudspeakers, you’ll want to find an amplifier that can provide adequate power to match the efficiency of your loudspeakers at a given distance and the desired sound pressure level. An amplifier power requirements calculator is a quick and sure way of getting there. Refer to RESOURCES/Amplifier power calculator.
Conclusion
A good preamplifier design will be quiet with very low signal-to-noise distortion. A good power amplifier design should typically deliver more current and therefore have more control over the behavior of the loudspeaker, and a greater ability to react to variations in the impedance presented by the loudspeaker. In summary, the preamplifier and the power amplifier have to be matched, and the power amplifier has to be matched with the loudspeakers so that sound distortion or component damage may be alleviated. Therefore, amplifier matching is crucial to good sound and may even prevent amplifier or loudspeaker damage. A vehicle’s engine, like the amplifier, must be able to carry the load of passengers including its weight and be able to maneuver through undulating terrain comfortably without any strain. This is why they say there is no replacement for displacement in a car engine, similarly, there is no wastage of wattage in a power amplifier, and the more power it has the merrier it will be.