Articles

Current

In order to understand what the various parts in an amplifier do, it is necessary to know some fundamentals of electricity. Electricity involves the storage and flow of electrons. The electrons flow from one region where there is a relative excess, to a region where there is a relative deficiency (one region relative to the other). The difference in the relative number of electrons2000-06-current-primer-electronflow-1.gif between the two regions is called the potential difference, or more commonly, the voltage. It is sort of like the water behind a dam. The more water, the more pressure that will be exerted at the opening near the bottom of the dam. It is the same with electrons. The more the difference in the number of electrons between the region of excess and the region of deficiency, the greater the voltage, or pressure for electrons to flow. When the two regions are connected by a conductor, electrons flow from the region of excess, called the negative pole or cathode, to the region of deficiency, called the positive pole or anode. This flow of electrons is called current, and is measured in amperes (abbreviated "amps"). In some textbooks the flow of current is designated as positive to negative, but, in fact it is from negative to positive, which is the way you will see it in up to date texts. The reason for the mistake dates back to the discovery of electricity, and it was not known which way current flowed. Taking a 50/50 chance, the wrong guess was made, and it is surprising how long this mistake has stayed in the literature (it was in my high school textbooks). For a current of 1 amp to flow, 6.28 x 1018 electrons (this number is called a "coulomb") have to pass through the conductor in one second. The electrons move from atom to atom in the conductor (for example, copper atoms) like a brigade of people in a line handing buckets of water from person to person in order to extinguish a fire at the end. The people remain stationary, but the buckets of water move. With electrical current, the atoms of the conductor (copper) remain in one place, and pass the electrons along. There is resistance to the flow of electrons in any conductor (except for super conductors, which are being researched at present), and this resistance is measured in ohms. To put the three factors together (volts, amps, and ohms), a mathematical definition states that 1 volt applied to a conductor with a resistance of 1 ohm, will result in 1 ampere flowing in the conductor.

A good conductor (copper wire) has low resistance, and a poor conductor (the insulation surrounding the wire) has high 2000-06-current-primer-resistorsymbol-2.gifresistance. Energy is expended in overcoming the resistance in conductors, and this energy is converted to heat. Even a good conductor like copper will heat up substantially if too much current is passed through it. This is why we have circuit breakers in our homes, usually set at 20 or 30 amps. Otherwise, the resistance of the copper would cause the wire to get extremely hot if too much amperage was flowing, and we would have to call the fire department. Resistance is what makes the cathode filament in a vacuum tube heat up, so that we can use the electrons surrounding this hot filament to produce amplification. Resistors are used in electronic circuits for specific purposes. Getting rid of the heat that is generated in all electronic equipment by the resistance to flow of current is a problem that has to be seriously addressed at the design stage. This requires the use of "heat sinks" which provide large surface areas for the heat to be transferred to the air surrounding them. Adequate ventilation of your equipment must be taken into account when stacking components on top of one another, and when placing them in cabinets with doors.

While a conductor allows the flow of electrons, and an insulator blocks the flow, there is also a material called a semiconductor, where electrons flow only under certain conditions. Semiconductors (transistors) form the basis of an entire industry in electronics. Silicon and germanium are semiconductors used for this purpose.


Capacitors are one way to store electricity (electrons), and inductors are another. Inductors store electrical energy in a 2000-06-current-primer-inductorsymbol-3.gifmagnetic field, in the process of inductance. This process (inductance) occurs even in a single wire conducting an electrical current. When the wire is formed into a coil, more inductance results. Such coils are used in electrical circuits, and are called inductors, or "chokes". However, by definition, any element in an electric circuit having a magnetic field is an inductor. The unit of inductance is the henry, and 1 henry permits a current increase of 1 ampere per second when 1 volt is applied across an inductor's connectors (terminals).

Most conductors are surrounded by insulators to prevent current from flowing where it is not wanted. Capacitors are made from thin sheets of conductors wrapped between layers of plastic insulators.2000-06-current-primer-capacitorsymbol-4.gif Capacitors are used to store electrons as an electrostatic field, and the unit of capacitance is called a farad. Any two surfaces having different electric potentials (voltage) that are close together so that an electrostatic field forms, is a capacitor. One farad of capacitance occurs when 1 volt is applied across the terminals of the capacitor and 1 coulomb (see above) is stored. It may seem rather simple when all the formulas use the number 1 in their relationships, but the design of an amplifier or other electronic component is extremely complicated, and the choice of resistors (in ohms), inductors (in henries - also spelled henrys), and capacitors (in farads) is just as important as the choice of tubes or transistors. It is interesting that some of the best sound comes from equipment with the least number of electrical parts in the signal path. This is probably another of the several reasons that tube amplifiers have such an excellent reputation. There are not very many parts in them. In some very expensive solid state equipment, the engineers have eliminated certain parts from the signal path (parts that are necessary in less sophisticated designs), in order to improve the sound quality. This is very high technology, and it shows up in the listening room.