Amplifiers Explained - FloridaSPL


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Old 02-12-2007, 05:58 PM   #1
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Default Amplifiers Explained

A power amplifier takes an input signal, usually a preamp level signal, which has both low current and low voltage characteristics, and produces an output which will have higher current and voltage levels. The power supply available to the audio output IC in a head unit is limited to the battery voltage of the vehicle. This means that the head unit can produce an audio signal with a limited (by the battery voltage) voltage swing, and therefore a limited power output to the speaker. Most amplifiers have a special circuit (switching power supply) to boost the available battery/charging system voltage to a higher voltage. The higher voltage developed in the amplifier's internal switching power supply will allow the audio output voltage swing to be greater. This allows the amplifier to produce more power into the speakers connected to the amplifier's output terminals. Most amplifiers will have some sort of level or "gain" control. This control is used to match the output of the head unit to an amplifier. The maximum audio output voltage from different head units will vary. If there were no gain controls, some head units would not be able to drive the amplifier to its maximum power level. Other head units may drive the amplifier to full power at a fraction of its volume control's range.
Virtually all amplifiers have battery, ground and remote connections which must be connected for the amp to operate. The battery connection is the high current +B source that's connected to the battery via a properly fused wire. The size of the power wire is determined by the current the amplifier draws and the length of the wire (from the battery to the amplifier). The ground is another high current connection and is connected to the chassis (body/floor pan) of the vehicle. The ground wire is typically as large as the power wire. The remote connection is a low current control input that tells the power supply of the amplifier to power up. The remote input current for amplifiers varies with the amplifier and the model. Some draw minimal current. Others draw a little more. The upper limit of a properly functioning amplifier is approximately 50ma (0.05 amps). If you're using/controlling more than 2 amplifiers, it is (in my opinion) much better to use a relay to control the amplifiers. Actually I really prefer having a relay in the remote circuit (no matter how many amplifiers I'm using) because it protects the head unit's remote output circuit in case of a short circuit.

The input circuit (sometimes called the 'front end') generally employs a noise cancelling circuit which compares the signal on the center conductor (the audio signal) to the signal on the RCA shield (which generally has little or no signal and is only used as a reference) and amplifies the difference between the two.

The input impedance is the impedance (that the signal source 'sees') from the center conductor to the shield on an unbalanced input circuit. A typical input impedance would be ~10,000 ohms but some amplifiers may have an input impedance of more than 50,000. If the input circuit uses a mini DIN type connector, the input impedance could be measured from one signal terminal to the other or from the signal terminals to the shield ground. Ideally, the impedance should remain constant throughout the audio band. More than a few amplifiers employ some sort of high frequency noise filter which will cause the input impedance to fall slightly at the upper end of the audio spectrum. These filters are designed to reject high frequency noise from the amplifier's switching power supply. It should also remain constant regardless of the position of the gain control. Some amplifiers (especially budget amplifiers) will have varying input impedance when the position of the gain control is changed. Head units (or equalizers, crossovers...) with low output impedance will handle these variations better than standard head units. Generally, a head unit with high output impedance will have reduced high frequency response if the amplifier's input impedance isn't consistant across the audio spectrum.

Unbalanced Input Circuit:
This type of circuit has a shield ground that's not directly connected to the chassis ground but may have only a few hundred ohms of impedance from the shield to ground. This type of circuit would be designed to accept a single ended signal (signal only on the center conductor).

Balanced Input Circuit:
Some Amplifiers have balanced inputs. This means that both the center conductor and the shield (if they're using RCA type connectors) can accept an audio signal. If the amplifier uses RCA type connectors and has balanced inputs, it likely uses the chassis ground as a reference (which is a testimony to the noise rejection abilities of a balanced input circuit). If the amp uses a mini DIN or some sort of professional audio connector, the connector will have provisions for two audio signals per channel and a dedicated ground (reference) connection.
Highly regulated amplifiers employ PWM switching power supplies. Unregulated amplifiers don't use Pulse Width Modulation to maintain a constant rail voltage. This does not necessarily make one design inherently better than the other. Both designs have their advantages and disadvantages.

A capacitor is an electronic device which consists of two plates (electrically conductive material) separated by an insulator. The capacitor's value (its 'capacitance') is largely determined by the total surface area of the plates and the distance between the plates (determined by the insulator's thickness). A capacitor's value is commonly referred to in microfarads, one millionth of a farad. It is expressed in micro farads because the farad is such a large amount of capacitance that it would be impractical to use in most situations.

Capacitor and DC voltage:
When a DC voltage source is applied to a capacitor there is an initial surge of current, when the voltage across the terminals of the capacitor is equal to the applied voltage, the current flow stops. When the current stops flowing from the power supply to the capacitor, the capacitor is 'charged'. If the DC source is removed from the capacitor, the capacitor will retain a voltage across its terminals (it will remain charged). The capacitor can be discharged by touching the capacitor's external leads together. When using very large capacitors (1/2 farad or more) in your car, the capacitor partially discharges into the amplifier's power supply when the voltage from the alternator or battery starts to fall. Keep in mind that the discharge is only for a fraction of a second. The capacitor can not act like a battery. It only serves to fill in what would otherwise be very small dips in the supply voltage.

Capacitors and AC voltage:
Generally, if an AC voltage source is connected to a capacitor, the current will flow through the capacitor until the source is removed. There are exceptions to this situation and the A.C. current flow through any capacitor is dependent on the frequency of the applied A.C. signal and the value of the capacitor.

Amplifier mounting:
DO NOT mount an amplifier on your subwoofer box. I know that there has been a great deal of discussion over mounting an amplifier to an enclosure and many people do it all of the time with no problems but those people probably build good enclosures from 3/4" (or thicker) MDF with extensive bracing. Most people (especially young impatient people) are too lazy to do that and build unbraced enclosures from 5/8 MDF. These enclosures will flex considerably more than a proper enclosure and will likely cause amplifier failure if the amp is mounted to the enclosure.

When the woofer(s) moves in or out, the box flexes and therefore causes the sides of the box to vibrate. This vibration is transferred to the amplifier mounted to the box. All of the electrical components in the amplifier have mass. Inertia (an object in motion tends to stay in motion, an object at rest tends to stay at rest) tells them to stay at rest, the box vibration is trying to make them move. The energy from the box's vibration is transferred to the components through the electrical leads which are soldered into the circuit board. All of this will cause the components to break loose and therefore cause the amplifier to fail prematurely. Basically, the amplifier will commit suicide! I'm not telling you this because someone told me it was bad. I've been repairing amplifiers since ~1985. Virtually every amplifier that's come into my shop with parts rattling around inside them have been mounted on the speaker box. It causes the legs of the semiconductors to break (which causes amplifier failure). It causes the capacitors to break off of the board (which can cause catastrophic amplifier failure). It causes solder joints to break on the semiconductors mounted to the heat sink. It causes transformer windings to grind into one another (which causes lots of smoke to pour out of your amplifier). People who repeatedly tell others to mount their amps on the speaker box because they've never had a problem remind me of people who drink and drive and say there's nothing wrong with it because they've never crashed their vehicle. Eventually, in both cases, problems will arise.


When installing an amplifier:
----Disconnect the ground wire from the battery. It doesn't really matter which one is removed because removing either connection from the battery (positive or ground) will break the circuit but if you let the wrench touch to ground (any metal surface) when removing the positive wire, you may do significant damage or seriously injure yourself. If you let the wrench ground out when removing the ground wire, you won't have any problems (except maybe scratching the paint).
---- If you don't remove the wire from the battery, at the VERY least remove the fuse (or open the breaker) from the power wire which delivers power to the amplifiers.
-----When making the power and ground connections on the amplifier, connect the ground wire first. I know it is tempting to connect the RCA cables first because it is instant gratification (having made a connection) but you may damage the head unit or the input section of the amplifier if the amplifier tries to ground through the RCA shield connection.
-----If the amplifier has screw down terminal blocks which are designed to accept either bare wire or spade terminals, use the spade terminals. If you insert bare wire into the blocks, you may have a strand or two of wire touch to the neighboring terminal which is easily enough to convert the amplifier into a paperweight.
-----Mount the amplifier down before moving the vehicle. If the amplifier falls or slides against anything, there is a chance that it will be damaged seriously enough to warrant a trip to a repair shop. I know how cool you are (because I know how cool I was at 15 or 16 years old) and nothing could possibly happen but... mount it down anyway.
-----When making the ground connection for the amplifier, the floor pan of the vehicle is a better choice than some of the braces and other metal structures that you may want to use for ground. Braces and other such structures are sometimes connected to the vehicle's chassis (body) by a few spot welds which will provide a less than optimum ground return path.
------If the amplifier's ground is properly connected to the body of the vehicle, it will provide a better return path to the charging system's ground than will a ground wire run back to the battery. This is especially true if the ground strap from the engine block to the chassis is upgraded.

For a good ground:
Get a 3/8 inch bolt, nut and lock washer, find a place on the body that can be accessed from the inside of the vehicle and out. You must be able to get to both sides so that you can hold the nut from turning when tightening it up. Drill a 3/8" hole for the bolt, making sure NOT to drill through any fuel lines, brake lines, the gas tank or anything else. Scrape the area under the bolt (inside the vehicle) to remove ALL paint and primer then bolt the ground wire's ring terminal down with the 3/8 inch bolt.

As a side note:
For grounding devices that draw only a few amps (like crossovers, head units and equalizers), you can use virtually any type of screw. Many people warn against using the black oxide coated screws but it won't make a big difference because the electrical connection is between the ring terminal and the metal surface that's been sanded clean and not through the screw. The screw simply holds the ring terminal to the metal.

Amplifier Classes:
Most mobile amplifiers use complementary transistor pairs to drive the speakers. In this configuration there is a transistor (or group of transistors) which conducts current from the positive power supply voltage for the positive half of the audio waveform and a different transistor (or group of transistors) which conducts current from the negative power supply voltage for the negative half of the waveform. There are some amplifiers which use the same transistor(s) to drive both the positive and the negative halves of the waveform.

NOTE:Amplifiers in classes A, B, and AB operate their output transistors in a 'linear' mode. Class 'D' amplifiers operate their outputs in 'switch' mode.

Mode examples:

Linear mode:
Imagine that you are the amplifier's output device(s) and you must support a 10 pound iron weight (the speaker load). The most difficult method (linear mode) would be to hold the weight straight out in front of you. This would very roughly simulate the linear mode architecture. Your muscles would start to ache in a short amount of time. Think of this pain as the power dissipation in output transistors.

Switch mode:
In this example, you can support the weight in one of two positions. In the first position, you can hold the iron weight directly over your head with your elbows locked so that your're not really using very much effort to support the weight. In the second position, you would let the weight hang down by your side. This would also use very little effort from your muscles. If you held it directly over your head half of the time and by your side for the other half of the time, it's position would 'average' out to be the same as if you held it out straight in front of you like in the previous (linear mode) example. This would roughly simulate the switch mode which we will discuss later in this page. You can see that with this method (switch mode), there would also be little pain (power dissipation) involved in supporting the weight.

Many class A amplifiers use the same transistor(s) for both halves of the audio waveform. In this configuration, the output transistor(s) always has current flowing through it, even if it has no audio signal (the output transistors never 'turn off'). The current flowing through it is D.C. A pure class 'A' amplifier is very inefficient and generally runs very hot even when there is no audio output. The current flowing through the output transistor(s) (with no audio signal) may be as much as the current which will be driven through the speaker load at FULL audio output power. Many people believe class 'A' amps to sound better than other configurations (and this may have been true at some point in time) but a well designed amplifier won't have any 'sound' and even the most critical 'ear' would be hard-pressed to tell one design from another.
NOTE: Some class A amplifiers use complimentary (separate transistors for positive and negative halves of the waveform) transistors for their output stage.

A class 'B' amplifier uses complimentary transistors for each half of the waveform. A true class 'B' amplifier is NOT generally used for audio. In a class 'B' amplifier, there is a small part of the waveform which will be distorted. In a pure class 'B' amplifier, the output transistors are not "biased" to an 'on' state of operation. This means that the the part of the waveform which falls within this .6 volt window will not be reproduced accurately. The output transistors for each half of the waveform (positive and negative) will each have a .6 volt area in which they will not be conducting. The distorted part of the waveform is called 'crossover' or 'notch' distortion. Remember that distortion is any unwanted variation in a signal (compared to the original signal).

As we said earlier, a class 'A' amplifier is very inefficient. This is not good for a Car audio amplifier. We also said that a class 'B' amplifier will cause a signal to be distorted, which is not good in any audio amplifier. A class 'AB' amplifier is the best compromise. A class 'AB' amplifier is a class 'B' amplifier which has a small amount of "bias" current flowing through the output transistors at all times. This eliminates virtually all of the crossover distortion. The bias current is flowing because the output transistors are always conducting current (even without an audio signal). This differs from a pure class 'A' amplifier in the amount of current flow. A pure class 'A' amplifier has an enormous amount of current flowing through its output transistors with NO audio signal. A pure class 'B' amplifier has NO current flowing through its outputs with no input signal. A class 'AB' amplifier is much more efficient than the class 'A' but without the distortion of the class 'B'. MANY of the car audio amplifiers which claim to be a class 'A' amplifier are just a high bias class 'AB' design. These amplifiers are only class 'A' at very low power output levels. At higher power levels, one of the output transistors will switch off while the other output transistor is conducting. I don't want you to think that I am telling you that there are no class 'A' amplifiers. There are a few high quality mobile amplifiers which are a true class 'A' design.

We said that class 'A' amplifiers were VERY inefficient. Class 'AB' amplifiers are also inefficient but are more more efficient than class 'A' amplifiers. Class 'AB' mobile amplifiers are generally 60% efficient when driving a 4 ohm load at maximum power (just before clipping). The reason that these amplifier configurations are inefficient is because there is a difference of potential (voltage) across the output transistors and current flowing through the output transistors. When you have voltage across the device and current flow through the device, there will be power dissipation in the form of heat. The power needed to produce this heat is wasted power. When there is (virtually) no voltage drop across a device (such as a large piece of wire or a transistor), there can be a significant amount of CURRENT flow through the device with (virtually) no power dissipation. This means that there is virtually no heat given off (highly efficient). The inverse is also true. If you have a significant amount of VOLTAGE across the device (transistor, wire...) but no current flow through the device, again, there will be no wasted power.
OK, now to the point. A class 'D' amplifier, which may also be known as a switching amplifier or a digital amplifier, utilizes output transistors which are either completely turned on or completely turned off (they're operating in switch mode). This means that when the transistors are conducting (switched on) there is virtually no voltage across the transistor and when there is a significant voltage across the transistor (switched off), there is no current flowing through the transistor. This is very similar to the operation of a switching power supply which is very efficient.
Anyone can hold the helm while the sea is calm.

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