Hi-Fi 101 Part 2: The Amplifier

Now that we’ve introduced you to the hi-fi 101 series, and introduced some key terminology, it’s time to discuss hi-fi itself. In parts 2, 3, 4 and 5, we’re going to discuss the components of the hi-fi system, explaining their rolls within the system and exploring some of the features that hi-fi components typically offer as standard. Let’s get started.

To construct a hi-fi system you will require several components. These include an amplifier, some loud speakers, and 1 or more source components (such as a CD Player or a turntable). You may be familiar with the terms ’Solid State’, ‘Valve’, or even ‘Hybrid’ when talking about hi-fi components. ‘Solid State’ is simply a term used to refer to comports which use transistors, as opposed to the older (yet still widely used) ‘Valve’, or ‘Tube’ technology. Hybrid components use a mixture of transistors and tubes, in an attempt to achieve the best of both worlds.

For the remainder of this series, we’ll be focusing on solid State components. Valve and hybrid components are another kettle of fish, and tend to be reserved for the higher-end hi-fi sector. Thence it is unlikely that your first system will be equips with such technology.

It’s also important to note that while may briefly touch on multi-channel equipment such as that used in home cinema or AV (audiovisual) systems, such equipment will not be the primary focus of this guide. This guide will focus in 2-channel, stereo equipment only. Perhaps, along with valve equipment, multi-channel equipment will be covered in a future series – but for now, on with the components. First up, the amplifier.

The amplifier is the centrepiece of any system – it can, quite literally, be thought of as the heart of the system. Without an amplifier, you won’t be able to get sound from your source components to your speakers. The amplifier is responsible for taking the minute signals from your source components (such as a CD player), and amplifying them before sending them out to your speakers and thence into your listening area.

There are 3 main types of amplifier which you may be familiar with – a Preamplifier, Power Amplifier and an Integrated Amplifier. Let’s explore each of these individually.

The Preamplifier

the basic function of a preamplifier is to handle the low-level signals coming from your source components (such as a CD player), and allowing you to control the volume of the signal before it is sent on to a power amplifier. Preamplifiers typically offer the ability to select between multiple input sources, usually at line level, and some include a built-in headphone stage and / or a phono stage to allow connection of headphones and / or a turntable.

Some preamplifiers offer extra features such as a tone circuit, allowing you to adjust the output at a given frequency to compensate for the deficiencies in speakers, source material or loudspeakers. These controls are typically referred too as ‘Bass and Trebble’, as they typically are set to allow you to adjust the high and low frequencies. Often when a tone control circuit is provided, a tone defeat option is also available to enable to user to bypass the tone circuit resulting in a shorter electrical signal path – which, theoretically at least, results in better sound.

There are 2 types of preamplifier – active and passive. An active preamplifier is a preamplifier that can amplify a signal above unity gain (or a gain greater than that already produced by your source component). A passive preamp, whilst still providing the basic functions of a preamplifier, contains no circuitry capable of amplifying a line-level signal – relying entirely on the output level of the source component when matched with the gain of the power amplifier.

The Power Amplifier

The power amplifier’s job is simple – it takes the output from the preamplifier, and amplifies the signal which is then sent on to your speakers and via your speakers into your room as sound. Power amplifiers typically contain large power supplies enabling them to cope with dynamic swings in music, and come in various types, more commonly referred too as classes. The most common classes include:

Class A

Class A amplifiers are without doubt the best-sounding amplifiers available. This is due to their extremely low distortion, as in a class A amplifier all of the output devices are conducting through the full 360 degree cycle of the electrical waveform (both positive and negative). This means that there is no ‘crossover distortion’, which is the result of gaps in the waveform cycle caused by the ‘handoff’ between separate devices handling the positive and negative sides of the waveform, as seen in a class B amplifier design. Class A amplifiers are without doubt the best-sounding amplifiers – however, they are very inefficient, producing a lot of power and heat.

Class A/B

As its name suggests, class A/B is simply a combination of both A and B classes, and a largely successful attempt to combat the drawbacks of both. Where as in a push/pull class B design 1 output device caters for approximately 180 degrees (half) of the electrical waveform before it is handed over, in a class A/B design both output devices handle slightly more of the signal (between 181 and 200 degrees). This means that there is less opportunity for a gap in the cycle to occur, resulting in a significant drop in crossover distortion.

Class AB amplifiers are far more efficient than those of a pure class A configuration, as only half of the output devices are conducting at any given moment. So while their distortion figures may not be as impressively low as their Class A counterparts due to the constant switching of transistors, they’re more efficient in terms of their power usage figures and they produce far less heat. A large majority of the amplifiers on the market today, particularly budget designs, are of a class A/B design.

Class D

Class D is the most efficient amplifier design, achieving as high as 90% efficiency. Whereas the previously mentioned class A, B and A/B amplifiers have 1 or more output device constantly active, even when the amplifier is idle (when no signal is applied), in a class D circuit the output devices are rapidly switched on and off usually by a pulse width modulation (PWM) circuit. This results in an amplifier that is, as mentioned, incredibly efficient, and that is able to run cooler than a class A, B or A/B design.

There are other classes of amplifier design, including class G and H (variations on class A/B) and class T, a variation of class D designed by the tripath company. However, classes A, A/B and D are the 3 types you’re most likely to encounter on your hi-fi journey.

Power amplifiers come in a few guises – including stereo power amplifiers (containing 2 channels of amplification in the same enclosure), Mono amplifiers (usually referred too as ‘Monoblocks’, containing a single channel of amplification in an enclosure), and multi-channel power amplifiers (containing, you guessed it, multiple channels of amplification in a single enclosure, usually intended for home theatre use). Some stereo amplifiers can be configured to run in mono or ‘bridged’ mode, whereby both channels of amplification are combined to serve 1 musical channel, requiring 2 amplifiers to form a stereo pair.

The Integrated Amplifier

The integrated amplifier is the hi-fi amplifier in its simplest, and most common form – it is simply a preamplifier and a power amplifier contained within a single enclosure. When assembling your first system, you’ll more than likely be utilising an integrated amplifier.

Internally, integrated amplifiers typically share common circuitry such as the power supply and power supply transformer. As with a preamplifier, integrated amplifiers typically offer a selection of inputs to connect your source components, and all integrated amplifiers include a volume control. Many also include a headphone stage, and some include an internal phono stage to allow you to connect your turntable. Some more modern designs include a built-in DAC (Digital to Analogue Converter) to directly process the digital signal from a CD player, streamer or other digital component (covered in part 3)

Integrated amplifiers feature at least 1 pair of speaker outputs, some go as far as to feature a second set to allow you to run a second pair of speakers in another location, or to allow you to bi-wire your speakers (see the loudspeakers section).

The Headphone Amplifier

The headphone amplifier is, as its name suggests, designed to take the signal from a source component, and amplify the sound to a level required by the speaker drivers in a pair of headphones (usually measured in milliwatts or MW). Headphone amplifiers come in 2 forms – either integrated into your preamplifier or integrated amplifier, or as an external, accessory component.

Headphone amplifiers typically feature a line-level input designed to be connected to the line-level output of your amplifier, usually that used to send audio to a recording device. The majority of headphone amplifiers feature their own volume control to allow you to adjust the sound level, and a few also feature gain settings to adjust their output to best suit your desired headphones.

The Phono, or Turntable, Preamplifier

A turntable preamplifier, more commonly referred too as a ‘Phono Preamplifier’, a ‘Phono Stage’ or a ‘RIAA Preamplifier’ is a device designed to take the low-level signals from your turntable cartridge, and to amplify them to line level (the same level as output by, for example, your CD Player). As with the headphone amplifier, these devices can either reside inside your preamplifier or integrated amplifier depending on the components you’ve chosen, or as an external component.

External phono stages are typically of a higher quality than those integrated into an amplifier or preamplifier. When you reach a certain level of component quality in your system, it’s generally advised to opt for an external phono preamplifier. Common features include amplification for the various types of turntable cartridge (discussed in part 3 of this series), and loading options to match the phono preamplifier to your phono cartridge by adjusting the impedance and in some cases the capacitance.

The second task of the phono preamplifier is to perform RIAA equalisation on the signal. RIAA (and acronym for the Recording Industry Association of America) is a standard specification for the record and playback of phonograph records. When a record is recorded or ‘cut’, the low frequencies are reduced and the high frequencies boosted. This results in a narrower groove width and therefore greater recording time on a vinyl record, though in order to achieve the correct sound the reverse operation (known as RIAA Equalisation) must be performed when the vinyl is played back. Reducing the low frequencies also limits the excursions made but he cutting head when cutting the groove, which in turn minimises stress caused to the playback stylus which might otherwise cause distortion or excessive groove damage.

Some phono preamplifiers also feature a selection of filters, including a subsonic and high-pass alter. A subsonic or ‘high-pass’ filter is designed to filter very low-frequency information which can cause damage to sub woofers. Such information is below the musical frequency level. A low-pass filter aids to filter the high-frequency sounds, useful to help reduce the crackle from old records.

Why the need for separate components?

At this point, you may be asking yourself this very question. Why, for example, would one want a separate pre and power amplifier? Why have 2 mono amplifiers when a stereo amplifier will do the job? And why buy separate pre, power, phono and headphone amplifiers when an integrated amplifier has it all?

The answer is simple and surprisingly logical, and is the reason that separate hi-fi components offer better sound quality (and features) than their one-box counterparts. An integrated amplifier, priced at the same price point as a preamplifier (say £1000 for example) will contain a greater number of components. Whereas a preamplifier need only contain the components for low-level amplification, an integrated amplifier must also contain the components of the power amplifier, a larger power supply, and usually the components required to implement the extra features (such as a phono or headphone amplifier).

Obviously, this incurs a cost, meaning the components in the integrated amplifier will usually be of a lower quality, or meaning that compromises must be made in the design of the integrated amplifier so as to enable the manufacturer to produce it at the given price point.

Of course, there are scientific reasons also. Power amplifiers generate a lot of electrical interference, due to the large amounts of electrical current flowing through their electronics, in particular their output devices and power supplies. This interference can have a negative affect on the low-level signals within a preamplifier. Similarly, a phono amplifier, which deals with lower than line-level signals (sometimes the microvolt range) is further susceptible to interference.

When separated, each component will be encased in their own enclosure, and circuits which are commonly shared from one another in integrated devices (such as the power supply) are separated and electronically isolated from one another. Putting this in the context of a system – if the user purchases a preamplifier, stereo power amplifier and a phono preamplifier, each device will be a separate unit, each with their own power supply and dedicated audio circuitry. The power amplifier will have no affect on the low-level signals flowing through the preamplifier, and neither will the preamplifier affect the phono preamplifier in the same way.

Similarly, the power supply circuits can be optimised for each component, resulting in a decrease in electrical noise, and an increase in power and headroom. Naturally, there are exceptions to the rule. As discussed in part 3, some high-end digital components (such as a CD player) are constructed in such a way that their internal digital and analogue sections are separated, yet still contained within the same physical enclosure.

Such is the case with some amplifiers. Though they’re still referred too as integrated amplifiers, some higher end integrated amplifier feature separate preamplifier and power amplifier sections, some going as far as to separate the power amplifiers themselves creating what is known as a ‘dual mono’ setup where a separate power amplifier circuit powers each channel. Some integrated amplifiers take things a step further, and separate the power supply circuitry (some even utilising multiple transformers), and some may even offer discrete, or ‘separate’ headphone or phono stages inside.

To summarise part 2 – in this part, we’ve explored the function and various types of Amplifier – the centrepiece of your hi-fi system. In part 3, we’ll begin exploring source components – the components designed to play back your music. Until then…
Continue to Part 3

By Ashley

I founded Audio Appraisal a few years ago and continue to regularly update it with fresh content. An avid vinyl collector and coffee addict, I can often be found at a workbench tinkering with a faulty electronic device, tweaking a turntable to extract the last bit of detail from those tiny grooves in the plastic stuff, or relaxing in front of the hi-fi with a good album. A musician, occasional producer and sound engineer, other hobbies include software programming, web development, long walks and occasional DIY. Follow @ashleycox2

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