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Sound on Stage

The best Rock musicians are not good because they use exotic instruments, but because they have developed a sensitivity for what is good. A cornerstone of this sensitivity is 'knowing your instrument'. This involves much more than being on intimate terms with your Stratocaster; really, 'your instrument' is you, the guitar, your amplifier and the PA — and in some exalted cases, even the audience can become your instrument. Here, we're concerned primarily with the parts of the instrument that are electronic — the instrument amplifiers and the PA. The series is a spiral — after the basics have been explained, we'll return to focus on both the more advanced and the more refined aspects of each topic.

Guitar Amplifiers

If a newcomer to Rock, your first instrument amplifier has to meet the difficult criteria of being cheap, and simple enough not to overwhelm or confuse, and at the same time, being reliable and versatile enough so as not to limit your expression. If you're already firmly committed to a well-defined style of music (eg: HM) or performance (eg: Cabaret), then choosing an amplifier can be simply a case of copying the professionals. The amp. should give you access to all the sounds your brand of music requires; end of story! However, many musicians are fired by creative aspirations, being more concerned with the en-rapturous magic of sound than rigid categories, and require an amplifier which will allow them to indulge freely in the mutual exploration of their instrument and their own mind. Paradoxically, the first stages of this exploration are best achieved with a simple amplifier; otherwise there's a danger of missing the wood for the trees. Whatever amplifier you buy, you will soon outgrow it, and until you have found your way, a simple amplifier will do no harm to creativity and won't turn out to be an unnecessarily expensive mistake if you decide to reverse direction.

Your amplifier is really as much a part of your guitar as the machine heads; what does it do? Most obviously, it makes the sound of the strings audible. Indeed, this is sometimes its sole purpose. However, the effect is little different from an acoustic guitar, yet only two other effects can vividly extend the potential of the instrument.

Firstly, the amplifier's tone controls can powerfully influence the relationships between the fundamentals and the harmonics — and therefore alter the tonality, the 'bite' and the 'roundness' of the sound. Other aspects of the amplifier — such as the overload characteristics and the input impedance can influence the upper harmonics, with similar effects.

Secondly, the amplifier can be overloaded, to produce 'dirty', 'raunchy' sound. Although the result is known and feared as distortion in musical reproduction equipment, in the context of the instrument amplifier — a generator of music — the effect is best thought of as 'harmonic generation'. The ratios of the fundamental to the odd and even and high and low harmonics generated by overdriving exerts a crucial, powerful and emphatic effect on the character of the sound. The overdriven sound can be produced in the input or output stages of an amplifier, or by the speaker, or any combination of the three; they are all different, and their 'sound' varies with level and tone control setting. The critical and complex nature of the overdriven sound largely accounts for the legendary name of certain amplifiers. Overdriving also generates spurious and therefore dissonant notes which are largely devoid of meaning in conventional western music. This effect, known as intermodulation tends to make the sound loud, 'full' and 'muddy'; it is part of the classic 'fuzz' sound.

With these modifications to the original sound, the nature of the amplifier's ability to control the sound intensity is radically altered. At very high sound levels, the amplifier can make the modified sound of the guitar seem larger and more real than life itself; the sound suddenly has the bizarre potential of multidimensionality. High levels also bring sharp, 'attackish', transient and other subtle sounds into prominence, and alter tonality, as perceived by the ear; we hear the fundamentals of the electric bass and the high harmonics of the lead guitar better at Rock concert levels. And unlike acoustic instruments, we can easily reduce the level an electric instrument, often without grossly upsetting the nature of the sound. So the volume control(s) can also give us quiet access to sounds normally only available by playing the instrument aggressively at extremely high levels.

Knobs and Sockets

Perhaps the first rule about guitar amplifiers is that they should not be shared, poverty regardless! Apart from creating a muddy sound, the result is to limit individualism and ironically, to destroy a band's coherence. Nonetheless, many guitar amplifiers feature two channels, and whilst the spare may be a useful gig-saver should another amplifier give problems, the key purpose is to extend the range of sounds available to the guitarist. The reasons begin inside the guitar. The output voltage and impedance of guitar pickups are simply very nebulous quantities — the voltage varying between 10mV and 2 volts (a 200 fold variation) depending on how you play. Your amplifier has to be able to handle these variations in input level, and, at the same time, provide a clean or dirty sound on demand. Hence the two input sockets — or a single switched input socket — marked 'high' and 'low'; the high sensitivity input can be readily overloaded for a dirty sound, whilst going into the 'low' input makes it possible to play loud and clean without overloading the amplifier.

Essentially, these inputs differ only in degree, so it may well be possible to achieve a dirty sound on the low sensitivity input if a 'loud' pickup is used, and vice versa. This underlines the desirability of checking that any amplifier you aim to buy is matched to the sensitivity of the pickup it will be working with. Ideally, the volume control shouldn't require extreme settings to achieve clean and dirty sound on the low and high sensitivity inputs respectively. If either sound cannot be readily achieved, then the pickup and amplifier are obviously incompatible in so far as sensitivity is concerned. Amplifiers are frequently chastised for being 'not 100 watts' when the pickup and/or the preamplifier are simply too insensitive to drive the amp to its full potential. This is analogous to rejecting an MGB for lack of speed without discovering the purpose of the accelerator pedal!

Dual input sockets may also provide a variation in the amp's treble again, possibly in conjunction with high/low sensitivity sockets, but more commonly in the form of a 'bright'/ 'mellow' switch. When a guitar pickup is fed into a low impedance, the signal is damped, making the sound 'rounder', quieter and more mellow. The latter effect is often superfluous because the treble response (where the upper harmonics lie) is usually already awkwardly attenuated by the shunting effect of the guitar tone and volume controls and the curly connecting lead; the 'bright' input or switch is a means of correcting this deficiency — it simply boosts the high frequencies. The result can also be attained by avoiding the treble attenuation in the first place — or equally it can be achieved with well designed tone controls.

Some amplifiers have only a single channel — 'bright' versus 'normal' and 'high' versus 'low' sensitivity, plus any built in effects being selected by switches or sockets common to the channel. Strictly there is no need to provide separate channels for these functions, but it makes life more convenient if rapid changes in effect can be achieved simply by plugging the guitar into the alternative channel, with the controls preset and therefore ready for action. The multichannel amplifier is essentially a convenience for pre-ordained, high speed acts; it's not essential if your forte is laid back, improvised Reggae for instance!

The tone (or 'equalisation') controls on a guitar amplifier are fundamentally similar to those on a hi-fi system. However, the guitar amplifier's controls should provide a greater range of gain variation to enable you to boost — or cut — with emphasis. And the frequencies at which these controls have most effect are attuned to the electric bass/guitar rather than a composite music signal. Whilst a midrange control is very useful, the effect of carefully designed and sensitively applied bass and treble controls alone can be effective enough. It is easy to feel, with a few broad and magnaminous test sweeps, that these controls are limited in their ability to create original sounds — but exploration of the whole instrument will usually reveal that the creative limitations lie mainly in the mind. In other words, simple tone controls which you can feel instinctively on stage will initially give you a far greater fluency and insight into the potential of your instrument than other, more versatile yet complicated methods of tone control.

Figure 1. Block Diagram of a typical guitar amplifier.

The master volume control has a considerable mystique which it does not deserve. It is usually nothing more esoteric than a pot wired between the equalisation and output stages of the amplifier. It's used to limit the output power and hence the loudness of the amplifier, whilst the channel (or input) volume control is wound up to produce distortion in the stages prior to the power amplifier. Thus, dirty sound can be produced without the usual byproduct — deafening sound levels. Ignoring the humanitarian aspects of the master volume control, it greatly enhances the range and precision of overload effects. For decibel deprived dirty sound, only the input stages will be overloaded, but brain damage regardless, output stage overload has a character of its own, and the two extreme control settings (channel volume down, master control up, versus channel volume up and master control down) can give very different sounds, with a wide range of cacophonous melodies to explore at the intermediate control settings. It's far better to choose an amplifier with a master volume control than one with an esoteric equaliser or other gimmick.

Lurking on the rear panel of your amp is an array of sockets, the functions of which may be a source of bewilderment to you owing to the frequently idiosyncratic labelling. The sockets variously termed 'slave', 'line output', 'link out' are all similar in function — they provide an output of intermediate (or line) level which can be fed to auxiliary power amplifiers ('Slaves' or 'Slave amplifiers') to extend the power capabilities of your amplifier. Equally, they may be used to feed a tape machine, a demo-studio mixing desk or a PA system.

Many effects boxes work admirably when linked between the guitar and amplifier, but if for instance you require echo or reverb, it's beneficial to add these after the signal has been amplified to line level. The signal must pass through the effects unit and then back into the amplifier — usually via a socket marked 'echo return' or 'return'. Obviously, the signal requiring reverb or echo must be prevented from passing through the amplifier directly, thus the socket from which the signal is derived (usually marked 'echo', 'echo send' or 'send') is arranged to break the signal path when a plug is inserted. These sockets operate at essentially the same level as the slave output and are usually wired immediately before the master volume control. Slave output sockets, however, are usually connected after the master volume control, so their output level mimic's the level from the amplifier's own speaker. Unfortunately, if any part of your music's character depends on overdriving the amplifier's output stage or speaker, then this aspect will obviously be missing from the 'slaved' output signal unless you are fortunate enough to be using an identical guitar amp as a 'slave'. Some instrument amplifiers derive a 'slave' signal from the loudspeaker socket, via an attenuator which prevents the high power herabouts from vapourising all your auxiliary gear! This method has the advantage of including the sound of the overdriven output stage in the signal sent to the 'Slave' amps, so it's useful for recording (see the E&MM Direct Inject Box Project in last month's issue), or boosting the merchants of dirty sound; but the conventional 'slave' output, wired immediately after the master volume control will always provide a cleaner, quieter signal, essential for more cultured guitar sounds. And clearly, neither type of slaving can include the sound of an overdriven speaker.

In the next part, we'll look at loudness, power and loudspeakers.

Naming the knobs and sockets
Input gain = Volume
Tone = Top cut, Treble cut
Mid = Presence
Tone controls = Equalisation
Sensitivity = Boost
Master volume = Output gain, Volume
Line output = Slave output, Line, 'Link Out'
Echo Send = Send, Effects Send
Echo Return = Return, Effects Return
Power Amplifier = Output stage

Previous Article in this issue

Tim Souster - Electronic Music Composer

Next article in this issue

Guide to Electronic Music Techniques

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - May 1981



Feature by Ben Duncan

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