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The information presented here is basic electronics presented in simple terms for the electronics novice. This is
intended to give a basic understanding of electronic circuits used in audio equipment. VOLTAGE AND CURRENT Voltage and current work together. Voltage is the energy source potential and current is the force behind the potential. There may always be voltage present but unless there is a circuit, or path, for the voltage to flow through then there is no current. The two types of voltage and current we are interested in is DC, direct current, and AC, alternating current. DC is an energy source that has constant polarity, positive and negative. DC can be thought of as a voltage and current that only flows in one direction. Most consider DC flowing from positive to negative although some research suggests DC flows from negative to positive. In electronic circuits it really does not matter which way DC flows, only that it flows in one direction. AC is an energy source that is constantly changing polarity form positive to negative to positive to negative....etc. Each complete swing from positive to negative is called a cycle. The number of cycles in a second is called the frequency. The voltage at an AC outlet in an American home has 60 cycles per second. In frequency terms this would be 60 hertz, or 60HZ. 1 VOLT = 1000 MILLIVOLTS or MV 1 MV = .001 VOLTS 10 MV = .010 VOLTS 100 MV = .100 VOLTS CURRENT 1 AMP = 1000 MILLIAMPS or MA 1 MA = .001 AMPS 10 MA = .010 AMPS 100 MA = .100 AMPS OHMS LAW Both DC and AC have current only when there is a circuit for the voltage to flow through. Circuits have resistance to voltage and current. In electronic circuits we use voltage, current and resistance to calculate circuit values. There are simple formulas for calculating these values known as Ohms Law. CURRENT IN AMPS RESISTANCE IN OHMS VOLTAGE = CURRENT TIMES RESISTANCE CURRENT = VOLTAGE DIVIDED BY RESISTANCE RESISTANCE = VOLTAGE DIVIDED BY CURRENT IN FORMULA TERMS E = VOLTAGE in VOLTS I = CURRENT in AMPS R = RESISTANCE in OHMS THEN 
Lets consider a simple circuit using a 9 volt battery as a DC source and a 1000 ohm resistor as the circuit the 9 volts will flow through. 
OR 9 MILLIAMPERES Since most electronic circuits have more than one current path, it may be necessary to find the total current being drawn by all circuits. In this next circuit we have 4 current circuit paths. The current from the 9 volt battery will split and flow through all 4 circuits. To find the total current first find the current flowing through each circuit (resistor). Then add all four currents to find the total. 
9 VOLTS DIVIDED BY 1000 OHMS = .009 AMPS 9 VOLTS DIVIDED BY 2000 OHMS = .0045 AMPS 9 VOLTS DIVIDED BY 800 OHMS = .01125 AMPS 9 VOLTS DIVIDED BY 1200 OHMS = .0075 AMPS .009 + .0045 + .01125 + .0075 = .03225 AMPS ROUNDED OFF TO 3 DECIMAL PLACES = .032 AMPS OR 32 MILLIAMPERES In the case of finding current flowing in circuits, if you have the test equipment you can measure these values. A standard Volt Ohm Meter, VOM, should have scales to measure voltage, current and resistance. The method for measuring voltage and current is the same for DC and AC. However, when measuring DC you must observe polarity and make sure you connect the positive meter lead towards positive in the circuit you are measuring. Measuring AC it does not matter. To measure current you must connect the meter in series with the circuit you are measuring. This requires opening up the circuit and inserting the current meter in the circuit. To measure voltage you connect the volt meter across the circuit without disconnecting anything. USE CAUTION WHEN WORKING WITH VOLTAGE AND CURRENT 
VOLTAGE DROP There are times when you need to drop voltage to a lower value. For instance, if you have a light bulb that operates on 3V and you need to use it in a circuit that has 9 volts, you need to drop the voltage from 9 volts to 3 volts. CURRENT FLOW IS THE SAME AT ANY POINT IN A SERIES CIRCUIT. We know the bulb draws .050 AMPS and the bulb requires 3 volts to operate. The current is the same value at any point in a series circuit so then the current flowing through the resistor will also be .050 AMPS. The supply voltage is 9 volts so we need to drop 6 volts. This means that there should be 6 volts across the resistor. Using ohms law to find the value of resistance needed for R to drop 6 volts. RESISTANCE = VOLTAGE DIVIDED BY CURRENT 6 VOLTS divided by .050 AMPS = 120 OHMS R = 120 OHMS 
If we needed to know the resistance of the light bulb when on, we can use ohms law to find that value. The same is true if the light is a bulb, LED or any other device, if you know the voltage and the current values you can find the resistance value. 3 VOLTS divided by .050 AMPS = 60 OHMS RESISTANCE OF BULB WHEN ON = 60 OHMS WATTS When there is resistance to current there is power produced, usually in the form of heat. You may have noticed that a 100 watt light bulb gets hotter than a 25 watt light bulb, more wattage generates more heat. Current flowing through a resistance produces heat. A resistor in a circuit must be able to handle the heat generated as current flows through it. In the circuit above, a 120 ohm resistor is used to drop 9 volts down to 3 volts. There will be .050 AMPS flowing through the resistor. A resistor must be used that can handle the heat produced as the current flows through the resistor. CURRENT SQUARED TIMES RESISTANCE or  THEN .050 X .050 = .0025 (CURRENT SQUARED) .0025 X 120 = .3 WATTS The 120 ohm resistor will produce .3 of a watt heat. You would want to pick a resistor that can handle .3 of a watt. A 1/2 watt resistor would work. A 1 watt resistor would be better with a bigger safety factor. It is always best to over-size the wattage rating of a resistor. 1,000 = 1K 4,700 = 4.7K 10,000 = 10K 100,000 = 100K 1,000,000 = 1MEG TRANSFORMERS The type of transformer we are concerned about are transformers with a core, usually iron core. The core is used in power and audio transformers. As current flows through one winding of a transformer, an electro-magnetic field is produced when the current is first applied. This field cuts through the second winding of the transformer and produces a voltage. Since the field is only produced when the current is first applied, the only way to sustain voltage in the second winding is to keep the current in the first winding constantly changing. It then can be seen that DC will not work with a transformer because DC is a constant current. Transformers are for use with AC. The constant cycling of AC from positive to negative to positive to negative, etc, will produce a voltage in the second winding. The winding the voltage is applied to is the input winding or primary winding. The second winding is the output winding or secondary.
output has less winding's than input output voltage will be less than input voltage  SECONDARY WITH CENTER TAP a tap in the center of the output winding  TRANSFORMER WITH MULTIPLE SECONDARY'S 
Power transformers used in power supplies have a rating for their secondary winding's. The secondary is usually selected for a required voltage. It is important to also select a transformer that is rated to deliver the current needed. Using a transformer rated at a current less than the current demand will cause the transformer to get hot and fail. It is also a fire hazard to use an under-rated transformer. It is best to use a transformer that is rated more than the current demand. CONVERTING AC TO DC CONSTANT When we measure AC voltage, the value we measure is the average of the cycles and not the value of maximum peak. However, it is the peak value that the resulting DC voltage will have after rectifying the AC to DC. Therefore, the resulting DC voltage from rectified AC will be higher than the AC voltage. There is a constant that we can use to determine what value AC voltage we need to get a required DC voltage. We can also find what DC voltage a transformer AC voltage will produce by using 1.414 constant. Multiply the transformer voltage by 1.414 to find the DC voltage. For example, a transformer with a 24 volt AC secondary will produce 34 volts DC (33.936 rounded off). RECTIFIERS Rectifiers, usually a diode, are used to rectify AC to DC. A diode conducts in one direction only. Depending on which way the diode is facing determines if the resulting DC voltage is positive or negative.  NEGATIVE  The resulting DC voltage above is not a constant DC voltage. The diode has produced positive (or negative) humps. This is known as ripple and in this case very severe ripple. This would not be a usable DC voltage with such high ripple. To reduce the ripple we add a capacitor after the diode. The capacitor will charge on each hump and hold the voltage until the next hump re-charges the capacitor again. 
The capacitor will discharge some from hump to hump depending on the load drawing current. The higher the value of capacitance the less ripple there will be. The examples shown above are called a half wave rectifier because it only rectify's half of the AC voltage. FULL WAVE RECTIFIER A full wave rectifier circuit uses a transformer with a center tapped secondary. The center tap is connected to common ground and a diode is used on each end of the winding. 
BRIDGE RECTIFIER The bridge rectifier using 4 diodes provides full wave rectifying without a center tap. The entire secondary winding of the transformer is used for both cycles of rectifying so when calculating the value of AC voltage needed at the secondary, you do not need to double the value. Since the entire secondary winding is used for rectifying both cycle halves, there is a higher current demand on the winding. You should use a transformer rated double the load current draw. For example, if the load will draw 1 AMP, use a transformer rated at 2 AMPS. 
CAPACITORS A capacitor is a device that contains two plates separated by an insulator and not electrically connected together. Since the plates have no electrical connections, current will not flow between them. This completely blocks DC from passing through. However, AC will pass, or at least appear to pass through. Actually, the AC signal will change polarity on both sides of the plates as the AC cycles positive and negative. This does require a circuit path on both sides (plates) of the capacitor. Capacitors are wonderful simple devices. They can be used to pass AC signals and block DC voltages in amplifier and other circuits. Large value capacitors will hold a large charge of current and smooth out AC ripple in power supplies. 
In the drawing above, if R1 and R2 were equal values of resistance, then the voltage in the middle would be 50 volts. The circuit requires a complete current path in order for current to flow so the bottom part of the circuit is ground. When the 100VDC is first applied, the capacitor will charge through R3 and while the capacitor is charging there will be voltage on the R3 side of the capacitor. But once the capacitor has charge there will no longer be any DC voltage on the R3 side. When the AC signal is applied, the capacitor will charge and discharge on each cycle of the AC signal. This charging and discharging will allow the AC signal to appear on the R3 side of the capacitor. R3 is required for the capacitor to charge and discharge otherwise there is no circuit for current to flow. R3 can be any resistance up to the millions of ohms. However, if the R3 value is too low then loading on the capacitor may cause the voltage to drop on the R3 side of the capacitor. Loading on a capacitor is why you will find that capacitor values in a transistor amplifier may be 10MFD or higher but in vacuum tube amplifiers may only be .01 mfd. As the load resistance on a capacitor lowers, it approaches the value of reactance (resistance) of the capacitor. Then the capacitor and resistor become a voltage divider and as the load resistance drops, the voltage on the resistor side of the capacitor also drops. So for loads with a lower resistance you need a larger value capacitor. A transistor may have a 10K base resistor load on its input capacitor where a vacuum tube grid may have 100K to 1MEG (million). This loading effect is frequency sensitive. Depending on the amount of loading on the capacitor, higher frequencies will pass unaffected but lower frequencies will be affected by the loading and roll off as the loading reduces the voltage. This can be used to advantage if you want to roll off frequencies after a certain point. 
AMPLIFIER STAGES A one transistor, or tube, or IC amplifier is usually referred to as an amplifier stage. Tube and transistor amplifier stages require special bias arrangements where IC amplifier stages usually do not. Bias is a method of placing the tube or transistor in the center of its operating range. At the extreme low end of the operating range, the tube or transistor is off and not conducting, no output. At the extreme high end of the operating range, the tube or transistor is completely on, full output saturated. Bias is a voltage applied to the input, tube control grid or transistor base, that makes the amplifier conduct just enough to place it in the center if its operating range. 
Without bias, the amplifier will conduct only on the half cycle portion of the input signal as each cycle turns the amplifier on and off like a switch or non-linear. The result is a distorted output. 
Vacuum tubes and transistors bias a little different. A vacuum tube has continuous electron flow so the grid needs to be made a little negative to reduce the electron flow. A transistor needs to have its base biased somewhat positive to slightly turn on the electron flow. There are two ways to bias a vacuum tube amplifier. Below, tube A is self biased. The 3.3K resistor between the cathode and ground causes the cathode to have a positive voltage. The source of the positive voltage is the 300 volt supply voltage linked through the vacuum tube, the 100K plate resistor and the 3.3K cathode resistor becomes a voltage divider. With the cathode being positive, the grid not having a source for positive voltage becomes negative with respect to the cathode. Tube B is fixed bias meaning it has a negative voltage supplied from a power supply, it is a fixed voltage. In this case the cathode is connected directly to ground. 
The drawing below shows the basic way to bias a transistor. There may be variations but the most stable is to use a voltage divider on the base. The 82K and 10K resistors form a voltage divder that will hold the base at proper bias. 
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