Tubes
At the beginning there were only tubes (thermionic tubes or valves) as active components. They then went to be used in the construction of sound amplifiers, initially common and then, with the development of special technologies for construction, both of the tubes as transformers, entered the category called HI-FI or High-Fidelity (High Fidelity ).
These amplifiers with a typical output configuration usually employed pentode tubes and ultralinear transformer, all heavy components. In all tubes, one filament heats a cathode so that it emits electrons which are attracted by an anode biased by a positive voltage (between 80 and 800 volts depending on power).
The flow of electrons between the cathode and the anode may be controlled by a voltage (or signal) is applied to a grid. The pentode tube has the other grids (suppressive and shield) used as auxiliary elements. The result of such operation is that the valve characteristic curve shows a large linear length throughout its operating range.
This means that it can operate in a very linear manner when polarized convenient leading to an amplification of audio signals with minimal distortion. This is because the dynamic range control electron flow between anode and cathode takes place at voltages ranging between 0 and a maximum.
THE BIPOLAR TRANSISTORS
The bipolar transistors, BJT, Darlington and the like operate in a different way.
These components appeared in 1948 but only became popular in amplifiers after 1955 when the types of greater power became available. The initial configurations were very similar to those used in circuits with valves. However, soon came the versions without transformers (Symmetry Complementary and Quasi-Complementary), given the lower impedance characteristics of this component, which allowed its direct connection to a speaker.
The transistor operates in a different way when compared to the valve. In transistor we have a current flowing between the collector and emitter controlled by a base current (the valve we have a voltage). Thus, while the transistor is a current amplifier typically the valve is a voltage amplifier.
But the most serious problem is that silicon bipolar transistors only start to conduct when a voltage of at least 0.6 V is applied to its base. This means that in the range of 0 to 0.6V as the transistor amplifier has a "discontinuity" in its linearity, which affects the fidelity of an amplified signal. In other words, the passage of the signal by zero (cross-over) the transistor does not operate smoothly, but gives "jump" which translates to distortion and the generation of harmonics;
However, well done special projects can reduce this effect to a minimum, and thus the transistor amplifiers can provide good power with excellent sound quality.
The MOSFETs
The MOSFETS (MOS – Power Field Effect Transistors) or VMOS, TMOS, DMOS, NMOS, PMOS, IGFET as they are called are power field-effect transistor. These components were devised before the common bipolar transistors but can be made much later and are now quite popular.
In the current power FET between the drain and source is controlled by the voltage applied to the gate. As the pentode tube this component has no discontinuity which characterizes the bipolar transistor and is therefore best considered as a component for sound amplification. In fact, many amplifiers that use power FETs have much lower distortion rates than equivalent that use bipolar transistors.
THE FINAL DIFFERENCES
But, after which the above components is best for a high quality audio amplifier? By comparison we can take the positives and negatives of each and the final decision is for the reader's account, because it also depends on the needs of each and they can be different.
Tubes
a) Advantages:
* Lower distortion
b) Disadvantages:
* Need to operate with high voltages
* Need to use heavy transformers
* The tubes themselves for this equipment are very expensive
* The tube works hot and a good part of the energy consumed by the equipment is to heat it. A smaller part is only converted into sound.
* The tubess are sensitive to vibrations
Bipolar Transistors
a) Advantages
* They are cheap
* They can provide high power at low cost
* Do not need to be heated - the circuit performance is good converting much of the energy consumed in sound.
* Do not need transformers in their circuits
b) Disadvantages:
* Distortion by the effect of cross-over (crossover)
* Fragility: when operating with high voltage and high current transistors, due to what is called the second break barier (second breakdown) can burn easily.
* Thermal drift - any imbalance in the circuit cause overheating and burns.
Power MOSFETs
a) Advantages: * They have similar characteristics to the tubes with low distortion
* They have high-performance converting most of the energy consumed in sound.
* Do not need to be heated
* Does not require heavy transformers in their circuits.
* Relatively low cost.
* They can operate with very high power.
b) Disadvantages:
* High input capacitance which limits their frequency response.
CONCLUSION
Even the transistor amplifiers can be used in various forms or "classes".
These classes may result in different sounds and yields the most sensitive ears can perceive.
The difference between the various types of amplifiers is still something that causes many discussions among the experts there who, when they find a bad amplifier, even say that it produces "transistor sound". However, using new settings amplifiers such as Class D or PWM can result in sound as good as the best equipment with tubes which means that the competition is high.
Especially if we consider that the valve supporters have not abandoned this component which is no longer found in modern appliances such as radios, computers, televisions (except for TRC) and others. These same fans are developing even today new technologies valves that appear in high quality amplifiers (and high cost!) that can be seen advertised in magazines.
In one of these magazines for some time, we found the existence of an amplifier of a few 40-watt (20 per channel) using valves with electrodes (grids, anode and cathode) coated with gold at a price exceeding $ 10,000! Anyway, if the reader want to have something different, think like a sound expert, should be aware of the differences and do not buy a product only by "Watts" that it can give output. High-fidelity sound quality is much more than that ...
