How an Amp works
There was no “basis” of how amplifiers work. This article is for car audio enthusiasts who might be wondering how the amp is not what he was doing. We will not get into the flow of electrons, the history of the transistor or the inner workings of a transformer, but we’ll see what amp with no signal received from a unit source in general and (hopefully) easy to understand way.
car audio enthusiasts who have an interest in the field but have not studied this topic at the school know that the amp to improve or enlarge small signals coming into it. This is true to a certain extent, but in fact create a new signal amplifier which is a COPY of the original signal. Let’s look at an analogy.
Compare an audio amp and a photocopier. You’re probably wondering how we can compare these two dissimilar pieces of technology. Both make a copy of something and, as anyone who has used a copier knows, you can set the copier to enlarge. If you put a piece of paper in the copier with a 1″ by 1″ image on it, you can tell the copier to spit out an enlarged copy of that image on another piece of paper. So now you have an image that is larger, say 3″ by 3″. The new image is a larger copy of the old image, but it’s on a different piece of paper, so it’s not he SAME image. Now let’s take this crazy comparison to an amplifier. It takes an original signal and produces a larger signal at the other end. The signal coming out of the amp is a larger version of the signal entering the amp. However, like the copier, the signal leaving the amp is not the SAME signal that came into the amp. Having said that, nothing’s perfect. The signal leaving the amplifier is bigger than the original in height only and not in length. If the wavelength were longer, the frequency would have to change—let’s leave that for another article! For now, this analogy should give you a general idea of what an amplifier does to an input signal.
So in general, what a car audio amplifier does is take a very small signal from a “source unit” such as a CD player or what have you and increases its stregnth to a level that can drive a speaker. Although they aren’t the same signals, the only difference between the input signal and the output signal is the strength of the signal.
Signals
The first step to understanding how an amp works is to understand what a signal is. A signal is used to convey data from one place to another. There are two types of signals, analog and digital. Our example uses an analog signal sent down an RCA cable, which is a voltage level representing an air pressure waveform of the sound. Although this may sound confusing, it really just means that the source unit sends an electrical signal representing sound (music) down its RCA cables to the amplifier.
To see what the amp does with the signal, let’s investigate the three stages in most amps.
•The Input Stage
•The Power Supply
•The Output Stage
Input Stage
Source units differ in the voltage they send down the cables. For example Source Unit A might send a 1-volt signal down its RCA while B might send a 3-volt signal, as illustrated below.
Amps need to be able to receive input signals at different voltages. Some amps are manufacturerd to receive only limited inputs (say RCA only), but most allow at least two levels of inputs, high-level (speaker wires from a source unit) and low-level (RCA from source unit). In this article we will concentrate on the low-level input.
The sensitivity of the amp’s input must be matched to the source unit’s output. This is the function of the Gain control in an amplifier. Don’t mistake the adjustment for a volume control. The gain control is only used to match signal levels between the various components in the audio system. In other words, if turning the source unit’s volume control to maximum results in max output from the amp (without distortion), you have set the gain control properly.
The Power Supply
The power supply is responsible for converting the supply voltage (vehicle’s battery voltage) to a higher voltage. A typical car battery can supply 13.8 volts of DC power. This small voltage isn’t enough to drive speakers to volume levels typically required by users.
All car audio speakers have a certain amount of built-in resistance. This resistance is measured in ohms. The majority of car speakers have a resistance of 4 ohms.
If we were to base our amp’s output voltage on the battery’s voltage of 13.8 volts and a typical speaker’s resistance of 4 ohms, then the maximum power we could expect the amp or source unit to output would be 49 watts. This is due to what is called Watt’s Law. This law states that Power (P) equals Voltage squared (V2) divided by Resistance (R). Our car battery represents the Voltage (V) of 13.8 volts DC. By squaring this voltage (13.8), we get 190. Our speakers represent the Resistance (4 ohms) in this equation. By dividing 190 (V2) by 4 (R), the maximum Power we can expect from our amplifier 47.5 watts, assuming the amp is 100 percent efficient.
If the output stage of an amplifier could handle a lower impedance (ohm) load, say 2 ohms, then figuring out the potential of our amp would require the (R) value in our equation to change to 2. Squaring the voltage (13.8) remains the same at 190, but dividing by 2 now gives us a maximum power rating of 95 watts. Still not near enough for our big 15″ subs!
So how can we get more power? The answer is to raise the voltage. Obviously we cannot raise the vehicle’s voltage, so the amp will have to perform that task. In fact, raising and controlling the voltage is the majority of the amplifier’s work.
Raising or “stepping up” the voltage is performed by the amp’s power supply. A bigger and more robust power supply means the output stage can do its work better and produce higher power levels to the speaker. To “step up” the voltage, the amp’s power supply uses a transformer to boost the supply (vehicle) voltage.
A transformer is a device that takes electricity of one voltage and changes it to another voltage. Most transformers are step-up or step-down transformers, meaning they take a smaller voltage and make it higher or take a higher voltage and make it lower. A typical step-down transformer is found on top of utility poles. These transformers reduce the voltage from the utlity’s sub-station to a much lower voltage that can be used in your home. The type of transformer found in a car audio amp is a step-up transformer (see the figure at the bottom). This transformer takes the available voltage from the vehicle’s batery and steps it up to a higher voltage, which can power our speakers.
Because music is an AC (alternating current) signal, we’ll need both positive and negative voltage to motivate our speakers. To do this, the transformer’s output is rectified and filtered to two DC “voltage rails.” The voltage rails are inverse of each other. In other words, one rail handles the positive swing of the signal while the other handles the negative. When combined, this swing effect results in AC signal.
If we have a power supply that can output +25 volts, it must also output -25 volts. These are the positive and negative voltage rails. The difference between the two voltage rails in this example is 50 volts. Following Watt’s Law, this amplifier would deliver 625 watts (50 volts squared divided by 4 ohms). To put it another way, this amplifer would have a peak-to-peak power limit of 625 watts.
The higher the “rail voltage” the more power the amplifier can deliver to the speakers. A common term in the industry refers to an amplifier with more rail voltage as having more “headroom” than a comparable amplifier with lower rail voltage.
Output Stage
The output stage is what drives our speakers. The main components of the output stage are the power transistors. The most popular type of transistor used today is the MOSFET. Transistors act as electronic “gateways” for delivering the higher rail voltage to the amp’s outputs. To do this, they pass the voltage produced by the power supply in a controlled manner.
Remember the term “signal” from earlier in this article? Well, it’s this signal that controls the “opening and closing” of these output transistors. So, in fact, the original signal controls the doorway for the higher voltage signal to go through. It also controls the switching on and off of the transistors in a way that they reproduce the original, but more powerful signal, to the output terminals of the amplifier and onto the speaker wires and speakers.
