What Are Stereo Amplifiers and How Do They Work?

Before we start talking about stereo amplifiers, let’s say a few words about amplifiers in general because we are sure many people don’t completely understand the working principle of an amplifier.

What Are Amplifiers?

The first amplifier was invented by an American engineer Lee de Forest in the early 1900’s. The invention was, in fact, the electron vacuum tube that was named ‘’Audion’’ and its purpose was to amplify weak current. That, however, wasn’t the final design of the amplifier as the followers of Mr. de Forest (Irving Langmuir and Edward Armstrong) managed to really understand how the amplifier worked and perfect it based on the new knowledge. They made the triode – vacuum tube with no air that contains three components (positive terminal – platinum plate, negative terminal – cathode and the platinum grid in the middle). The grid was something we call input and the cathode was the output. When the cathode is heated (when the input is supplied with power), electrons receive negative voltage and they start moving towards the positive plate (the output). In the process, the current flow is established. Negative voltage hits the platinum grid, breaks the current flow and the voltage of the cathode is changed. That’s how the amplification is made.

The first amplifier was too large and unstable and the scientists didn’t manage to make a smaller and safer device (transistor) until 1947. After that, the shape of amplifiers changed a lot and they became smaller and more portable. One thing modern amplifiers have in common with the oldest one is the working principle that has remained the same for all these years.

All amplifiers in the world receive smaller electrical signal (input) from the power source (phone, player, etc.) and amplify it (make the same signal larger).

What Kinds of Amplifiers Exist?

Amplifiers can be classified in numerous ways (by construction, by frequency, by voltage, power, by class from A to D, etc.). That’s not so strange considering the fact that amplifiers are widely used – they are built into all kinds of devices from hearing aid devices to the home theater speaker systems. Because of that, we have many different categories of amplifiers and different types of amplifiers. One can easily get confused when reading amplifier classification but that’s not our topic, so we won’t go into details. We are just going to say that one of the ways of classifying amplifiers is by their function.

This type of classification gives us servo amplifiers, linear and non-linear, wideband, buffer, RF and audio amplifiers. The group called audio amplifiers is the most important for us right now. These amplifiers are used to amplify audio frequencies and we can divide them into 3 subgroups: preamplifiers, power amplifiers, and stereo amplifiers.

Preamplifiers (also known as preamps) are there to receive very weak signals (mic-level signals) and amplify them to the higher level (line-level signal) where the regular amplifiers take over.

Power amplifiers are used to amplify the low-power signal (such as radio signals) so that some other devices can use it (loudspeakers for example).

Stereo amplifiers have two output channels that they use to receive stereo output signal coming from some device and send it to the speakers that are connected to the amplifiers.

How Do Stereo Amplifiers Work?

Stereo amplifiers are one of the most popular ways of improving the sound quality in different situations and conditions. They can be used at home or office and they can amplify the signal coming from CD and DVD players, TVs, etc. Stereo amplifiers have the same working principle as those old amps and we will explain it here thoroughly.

Stereo amplifiers have 3 ports/connectors (input port for connecting the signal source, the output port that’s used to send the signal to the speakers and power source connector or power input). These amplifiers receive power from the wall socket and they convert alternating current to the direct current thanks to the device called transistor. That device is some kind of valve that enables the current to flow in specific amounts. The amount of current that the transistor allows to flow depends on the signal received from the signal source (TV, PC, player, etc.). Smaller signals will enable less current to flow and the amplification will be smaller than the amplification of larger signals that enable more current to flow.

The transistor controls not only the amount of current. It also controls its frequency. Depending on the frequency of the input signal, the transistor will work slower or faster (if the frequency is 50 Hz, the transistor will open and close 50 times per second).

How to Choose the Right Stereo Amplifier?

As we have previously said, stereo amplifiers are the most popular way of improving the sound quality when using speaker systems. However, making a decision which model to buy is not an easy task, so we have prepared a few pieces of advice in order to help you. That way you will know which characteristics to pay attention to when purchasing a stereo amp.

Pre/power or integrated amplifier

Stereo amps can be bought in two forms – in one or two boxes.

Integrated amps come in one box and they combine two devices (pre-amp and power amp) into one. They are better than the second type because they take up less space and you get everything already combined and matched together, so you don’t have to worry about these things – the perfect combination is already achieved. On the other hand, they offer lower sound quality in comparison to the pre/power amplifiers.

Pre/power amps consist of two boxes. In one box, you will find the pre-amp that has input port and volume controls while the power-amp is in the separate box. The pre-amp can be very sensitive because the signal that goes through the pre-amp is sensitive. Power-amp, however, is noisy and a great amount of current goes through it. This is why some think it is wise to keep them separate. It won’t save you any space, but the sound quality will be better. The important thing is to try to buy both devices from one manufacturer in order to achieve better results.

Compatibility (impedance/power output/sensitivity)

The most important thing about buying an amplifier is probably matching it perfectly with your speakers. In order to do that, you have to consider these three characteristics.

Power output is measured in watts (W) and it is important because it generally defines how loud your music will be. The power output you need depends on the size of the room you want to fill with music. In case you have a small apartment, even 10W will be enough but if you have really big house, you should consider some serious wattage (100 W and more). In case you use 10W amp, each of the channels will get 5W.

Impedance is measured in ohms (Ohm) and it can span from 8 to 600 ohms. Impedance is the other name for the electrical resistance and it shows what voltage you need for the best performance of the certain device. The higher the impedance of a speaker, the bigger voltage will be required by the amp to make the speaker work.

Sensitivity is measured in decibels (dB) and it shows how loud your speakers can be.

Connection (inputs/outputs)

The last but not the least is the content of the rear panel. All the input and output ports are there and based on the number and type of the connections you will know which devices you can use with that specific model. Apart from that, the rear panel tells you if your amp supports upgrading.

Let’s start with the input ports. Most stereo amplifiers have RCA inputs that can be used to connect them to different types of players (even turntables) or some other low-signal devices. This kind of connection requires high equalization level. Three-pin XLR connectors are in charge of balanced audio and they are usually used to connect the amp to the high-level devices that are often found in studios.

Although stereo amps usually use analog connection, some modern amplifiers use digital connection and have some digital inputs. Some of them can also have USB ports that come in handy when connecting your amp to your PC, notebook, phone via USB cable.

You should also pay attention to output ports. They can be used to connect some additional devices to the amplifier. For example, you can connect separately your subwoofer to add some extra bass, connect headphones using 3.5mm or 6.3mm output ports or you can connect two pairs of speakers to one amp simultaneously. However, the last one can lead to poor performance.

Many modern amps have built-in Bluetooth so you can even try connecting them to your devices using this technology and forget about all those annoying wires.

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Hello, my name is James Longman.

I’m a writer and editor at AudioReputation. I disassembled my first portable AM/FM radio when I was only 8. At the age of 11, I burned the circuit board on my old boombox cassette player. I’m not going to explain how but it was reckless and stupid.

Since then, I have become much more careful around radios, boomboxes, and other audio devices (at least, I like to think so) but I have never lost the passion for audio equipment. Throughout 20 years of my professional career, I’ve been working for various audio equipment manufacturers and even started building speakers on my own in my little workshop.

I love the work we do here at AudioReputation. Testing, comparing, and evaluating all kinds of audio devices (speakers, soundbars, headphones, home theater systems, etc.) is something I truly enjoy. I try to be unbiased and give you my honest opinion on every piece of equipment I test. Still, you should take my reviews with a pinch of salt and always be just a little bit skeptical. The fact that I liked some speaker or soundbar doesn’t mean that you are going to love it. If you have the opportunity, you should test it/hear it before buying it.

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