Guitar AmplifiersIN THE STUDIO and on stage, instrument amplifiers are vitally important to the musician. Specialised equipment is a necessity, and the instruments themselves are becoming more and more demanding of the amplifier-speaker combination with which they are used,. The size and power ratings of such systems has grown considerably in recent years; it is not ,uncommon to find instrument amps today, which are more complex and larger than a band's entire PA system of a few years ago. As one of the most controversial pieces of amplification ever produced, we shall first take a look at the guitar amplifier, which has evolved from a general purpose "as long as it works" device, to a sophisticated piece of equipment - at least in the professional sector, where the so-called "universal" amplifier is no longer relevant.
Most guitarists will say that, with a given amplifier, a certain volume level is required to obtain the right sound. While this is to a point psychological, there are very good technical reasons to support this statement. The "right sound' usually means a fairly high level of harmonic distortion, both from amplifiers and speakers - distortion which gives much needed assistance to an electric guitar by providing the missing harmonics and increased sustain. A certain amount of sustain is obtained by acoustic feedback, particularly with semi-acoustic guitars, however the majority is the result of the amp being driven into "clipping", or distortion. This has the effect of maintaining the level at a (more or less) constant level for much longer than would normally be the case (Figure 1).
If an amplifier is overdriven, the sound produced is usually considered objectionable, so special measures must be taken to obtain the desired characteristics.
Firstly, there is the bandwidth restriction of the speaker cabinet. For the low-end this may be achieved by using an open-back box, to allow bass cancellation, or to contain the speakers in an enclosure of limited internal capacity. High-end roll-off is usually a characteristic of the speakers themselves: speakers - without "whizzer" cones or aluminium domes will generally sound smoother because of their limited high frequency response. A broad peak in the speaker's response at 2kHz to 4kHz adds "presence" and bite, and the response should roll off beyond that point. It is preferable that frequencies above about 7kHz are not reproduced at all, as this will exclude the 7th harmonic (which is discordant) of all strings of the guitar. Damping or venting of guitar speaker enclosures is usually avoided like the plague because damping will lower colouration, a desirable feature in musical instrument amplification. Without colouration all plucked string instruments, for example, would sound much the same.
- Vented boxes are avoided because they emphasise (or allow to be reproduced) the lower frequencies - these are usually not desired, since they tend to muddy the sound. This is especially true for "heavy metal" playing styles, since severe amp overdrive is commonly used to achieve the sound, and even small amounts of low frequency material (such as string handling) will cause large amounts of bass "waffle".
- The majority of guitar speaker boxes (as noted above) are either open backed, or are relatively small sealed enclosures. There are naturally exceptions, but even 20 years after this article was first written, are still uncommon.
Additional bandwidth restrictions are usually designed into the output transformer of valve amplifiers. Transistor amplifiers usually do not have an output transformer, so this method of filtering is precluded. The effect of all this manipulation of response is to "clean up" the distorted output waveform of the overdriven amplifier - the sound will become less harsh, and the transition from overdriven to "clean" becomes less apparent.
In fact in a valve amp the clean or "no distortion" state exists only when there is no input. Distortion is present even at low levels, and increases with the input signal.
The above situation is in contrast with the majority of transistor amps, which generally have very low distortion up to the point of clipping, after which distortion rises rapidly. Maximum distortion is the same as for a valve amp - i.e. a square wave, but the transition is such as to be very noticeable; this can be objectionable when the guitarist is playing clean but for the odd note or chord which overdrives the amp, causing a marked change in tonality (Figure 2).
Output impedance and "dynamic output" are other major contributors to the sound of a guitar amp. Output impedance, in this instance, has nothing to do with speaker "ohmage", but is the actual source impedance of the amplifier, as seen by the speaker. In a valve amp, the source impedance may be as high as 200 ohms for a nominal 8 ohm output. This allows the speaker freedom to exhibit its own natural resonances, plus the colouration provided by the enclosure itself. A low output impedance on the other hand, will repress colouration by damping the speaker, in much the same way as would fibreglass in the cabinet. This tends to make the amp sound flat and lifeless, lacking the subtle colouration and tonality which makes a fine musical instrument - and after all, a guitar amplifier is as much a musical instrument as the guitar itself. Transistor amps usually have a low source impedance, typically less that 1 Ohm.
- It should be noted that the power rating for guitar amps is (very) commonly quoted at 10% distortion. This means the amp is typically about 80W at the onset of clipping (usually taken as the "real" maximum power level). This is justified on the basis that few guitarists will operate their amps with no distortion at all. This assumption is not necessarily valid, but has prevailed for a very long time, so is unlikely to change now.
- The source impedance of valve amplifiers is generally lower than that quoted above, and in fact it will generally match the speaker impedance (assuming no feedback is applied). I appear to have goofed in the original article.
- From the very first guitar amplifiers I ever built using transistor output stages, I used current feedback to increase the output impedance of the amp. This restores much of the "valve sound", and provides a simple but effective method of providing short-term short circuit protection (for example while plugging 6.5mm phone jacks into their sockets while the amp is on). This technique has now been adopted by nearly all guitar amp manufacturers. Regrettably, this technique does little to improve dynamic output. Output impedance of my amps was typically 200 Ohms.
Dynamic output in this context means the ability of an amplifier to deliver power to the load (the loudspeaker). Because of the high source impedance and low efficiency of valves, output transformer and the power supply, a valve amp is capable of developing approximately 75% of its rate power into double its rated impedance, and up to 50% of rated power into four times rated load. This means the amp can provide relatively constant power regardless of the variations of impedance with frequency which are a characteristic of speaker loads.
In contrast, a transistor amp can usually deliver only 55% or so of its rated power into double rated load, and about 30% into four times rated load. These variations may not appear to amount to very much, but they do make a big difference in terms of spectral balance and dynamic performance. Such is the difference in fact (and this also applies to valve hi-fi amps, bass amps, etc) that a 100 watt valve amp will generally sound as loud as a 150 watt transistor amp.
The technical reader with an understanding of dB and the logarithmic response of the human ear may be sceptical about the foregoing paragraph, but differences of 1dB are audible when one has a suitable point of reference; in musical groups, where the balance of instruments is critical, such differences are very noticeable.
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Post By：2008-1-12 15:50:38 [只看该作者]
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