Some of the classes contain sub-classes, usually indicated by a suffix. Valve-amp-loving guitar players really only need concern ourselves with just three of these classes: A, AB, and B. The output valves in this class are conducting per cent of the time. As a sine wave can be thought of as a side-on view of a coiled spring — look at a vibrating guitar string on your guitar to see what I mean — we say that the conduction angle is degrees.
The upside to this design is that circuits can be relatively simple to build and maintain, while producing better mid-to-high-frequency performance. In this class, each output valve only conducts exactly one half of the input signal, so the conduction angle is degrees. One valve will conduct the positive half, and the other conducts the negative. As we now have a degrees rest period for each valve in-between conduction events, we can use that to cool the valve down and therefore push it harder when it is conducting.
A Class B amplifier can be much more efficient that a Class A, with a maximum efficiency of over 75 per cent. You can make big power when operating in Class B, but like every other class it has drawbacks. Brian May is one of the most prominent users of the Vox AC So what is Class AB? In Class AB, the conduction angle is greater than degress but less than degrees.
It it up to the circuit designer to choose where that point is. Usually in a guitar amplifier, the signal will stay in the Class A region where it is amplified with good fidelity and will move closer to Class AB when pushed harder. Nearly every guitar amplifier with multiple output valves will operate in Class AB. Players and amp-makers often talk of sweeter distortion in class A amps, but true class A operation actually has less distortion content at any given output level; although it displays a smoother onset of distortion when it comes, and one that is usually heard as being more harmonically rich.
Relatively few amps fit the definition for class A absolutely and beyond debate, which is not something to worry about at this juncture. The characteristic sound of different classic tube guitar amps is determined by far more than their just class definition.
Voltage vs. Bias Achieving true class-A performance in PP amps requires the careful manipulation of two factors: the DC voltage delivered to the output tubes, and the bias setting thereof. Often it is not particularly tight, punchy or bold, or at least is less so than a more efficient fixed-bias output stage, in relative terms. Yep: cathode biasing, and no negative feedback loop. Z Stangray, and others that follow the AC30 template in addition to all the smaller amps that emulate the AC You know what other amps are cathode-biased, with no negative feedback?
Single-Ended Amps In the fourth paragraph of this article I listed a number of smaller practice amps that do qualify as genuine class-A amps, by definition. In this way, Class AB amplifiers sacrifice some of the efficiency for lower distortion. Accordingly class AB is a much better option where a compromise between efficiency and linearity is needed.
To save cost, weight and power consumption, amplifiers were run in class AB, and two amplifier sub-classes were often mentioned: Class AB1 and AB2. These sub-classes are applicable to only thermionic or vacuum tube technology as they refer to the way in which the grid was biassed:.
A Class C amplifier is biassed so that it conducts over much less than half a cycle. This gives rise to very high levels of distortion, but also it enables very high efficiency levels to be achieved.
This type of amplifier can be used for RF amplifiers that carry a signal with no amplitude modulation - it can be used for frequency modulation with no issues. The harmonics created by the amplifier effectively running in saturation can be removed by filters on the output. These amplifiers are not used for audio applications in view of the level of distortion. Class C amplifiers typically use a single active device that is biased well into its off region.
As the signal is applied, the top peaks of the signal cause the device to run into conduction, but obviously for only a small portion of each input-waveform cycle.
At the output the circuit uses a high-Q, L-C resonant circuit. This circuit effective rings after it is hit by each pulse so that the output contains an approximation to a sine wave. Filtering is required on the output to ensure that the level of harmonic is sufficiently low. There is a variety of different amplifier classes which tend to be based upon switching techniques rather than using analogue approaches. In reality the actual levels attained are less, but nevertheless the efficiency levels achieved are very much higher than the other analogue classes.
One of the first class D amplifiers for audio use was introduced by Sinclair in the UK around Although the concept was good in theory, the amplifier did not work particularly well, and when it did, the amplifier tended to cause large amounts of interference to local radio and television sets as EMC precautions were not normally applied to equipment at this time.
Class G Amplifier: Class G is a form of amplifier that uses multiple power supplies rather than just a single supply. For low level signals a low voltage supply is used, but as the signal level increases, so a high voltage supply is utilised. This is gradually brought in to action up to full rated power output as required.
This gives a very efficient design as additional power is only used when it is actually required.
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