The function of the amplifier is to boost an audio signal so that it is strong enough to drive the speakers and create sound. There are different types of amplifiers, and they may be classified in a number of ways, depending upon how they are used, the range of the frequencies they cover, or the number of active devices connected to the amplifier. One can purchase an amplifier as part of unit with a pre-amplifier included. This is called an integrated amplifier. Or the buyer can purchase the amplifier and the pre-amp separately.
When in the market for a used stereo amplifier, look for used amplifiers in guitar shops or in second hand shops that sell used stereo equipment. However, audiophiles find that eBay has the large selection of used stereo amplifiers. When searching for used amplifiers, the buyer should consider the sensitivity of the amplifier, the factors that influence the amplifier's dynamic range, and the type of amplifier.
What Is an Amplifier?
An amplifier increases the power of an audio signal so that the signal can drive the speakers. With stereo amplifiers, the input signal is a current. Stereo amplifiers boost the signal from the pre-amplifier to a level that can be used by loudspeakers in a sound system. The purpose of the pre-amplifier is to act as something of a middleman between the audio source and the amplifier. The pre-amplifier boots the incoming audio signal just slightly so it can make it to the amplifier. Most stereo amplifiers come with a pre-amplifier in one unit. This is called an integrated amplifier. However, many dedicated audiophiles like to buy their preamplifiers and amplifiers as separates. Monoblock amplifiers are single channel amplifiers and are generally considered high-end amplifiers. There are several types of stereo amplifiers.
Class AB Amplifiers
This is the traditional type of amplifier and can either be tube or solid state, balanced (XLR) or unbalanced. Solid state amplifiers typically have heat sinks, large toroidal transformers and are generally quite heavy. AB amplifiers are generally the preferred type of amplifier because they are very powerful, and have a clean sound. Digital amplifiers have become much better in recent years, and there are now some hybrid digital AB amplifiers.
Class A Amplifiers
At one time Class A amplifiers were the most popular amplifier on the market, but due to their low power, extra high heat output, their large size, and the extremely high requirements for incoming power, their popularity started to wane. These amplifiers can only run at full power with no ability to run at anything less.
Tube amplifiers are the most sought-after of all amplifiers due to their warm analogue sound and the innate musical efficiency of the amplifier. However, there are some drawbacks to tube amplifiers. They create a lot of heat, do not have as much power as solid state amplifiers, and need constant adjustments to perform correctly. They must warm up before a good sound is achieved, and the tubes must be replaced after every couple hundred hours of use.
Class D Digital Amplifiers
Digital amplifiers are very popular today. They are very quiet, have low distortion, and are small. Digital amplifiers generate very little heat, yet are capable of generating high volume. Unfortunately digital amplifiers do not have the musicality or weight of other types of amplifiers, and can be very expensive.
Monaural (Monoblock) Amplifiers
This type of amplifier is often preferred by music lovers and is frequently used in demanding home theatre systems. The 'mono' means one physical amplifier is made to power one audio channel. The biggest advantage of mono amplifiers is that each audio channel has its own power supply, so as to actively and effectively provide needed power. High-end amplifiers have their own internal AC power supply, right inside the amplifier.
What to Consider When Shopping for a Used Stereo Amplifier
The nature of music is dynamic, constantly changing in volume and frequency, and the best way to hear music and fully grasp its dynamic nature is to listen to it without speakers. However a good amplifier should be able to reproduce that natural sound faithfully, from the very lows to the sparkling highs. The range of a good amplifier should allow it to use a small amount of the maximum power it possesses most of the time, while still allowing it to produce large amounts of power for short periods of time, when needed. Looking for used amplifiers on eBay is a good option as each listing has the specs listed, making it easy to find the right used stereo amplifiers. The quality of a good speaker depends upon a few specifications, which are listed below.
This is the ratio of output to input power, or amplitude. An amplifier with a higher gain is more sensitive and needs a lower number of input signals to produce a certain amount of power.
Bandwidth represents the range of frequencies the amplifier gives satisfactory performance over. Bandwidth is defined as the difference between the lower and upper half power points, because satisfactory performance is defined as the point at which the power drops down halfway from its peak value on an output versus frequency curve.
Efficiency represents the amount of power that is usefully applied to an amplifiers output. Class A amplifiers are quite inefficient, lying within a range of 10 to 20 percent and having a maximum efficiency of 25 percent. Class B amplifiers have a high efficiency, but are not useful for audio work because of the high levels of distortion. The most efficient amplifiers run cool, needing no fans to keep them cool, and generally are found in the high-end amplifier categories, making them quite expensive.
In a perfect world, an amplifier is totally linear. However, real amplifiers have limited linear abilities. When a signal to the amplifier increases, the output also increases until at some point the amplifier cannot produce any more sound. This is known as 'clipping' and results in distorted sound. When reduction in gain takes place before clipping, compression occurs, which is a non-linear effect that does not sound good at all. Linearisation attempts to counteract the undesirable, non-linear feedback.
Output Dynamic Range
This is the decibel range between the smallest and largest useful output levels. The lowest useful amplified level is limited by noise, while the highest is limited by distortion. The ratio of the two is the amplifiers dynamic range.
Settling Time and Ringing
Settling time refers to the time it takes for the output sound level to settle to within a predetermined percentage of the final value. Ringing is an output variation that cycles below and above the final value and causes a delay in reaching the stable settling point.
This is an issue encountered by all amplifiers with feedback, whether or not that feedback was intentional. There are several levels of stability, such as stem stability and Linvil stability.
Speaker sensitivity is also known as speaker efficiency and is a measure of the sound produced by a speaker. Speaker sensitivity is measured in decibels (dB). A speaker with 85 dB efficiency uses two times the amplifier power to reach the same volume that an 88 dB speaker can put out, and an 88 dB speakers require ten times more power than a speaker with a 98 dB efficiency rating. When searching for an amplifier that performs to the right specifications, amplifier efficiency, or sensitivity, is an extremely important factor to consider. Because the most efficient amplifiers tend to be on the high-end scale of amplifiers, buying one can be quite expensive. However looking for an efficient, used amplifier on eBay can be the perfect solution.
How to Find a Used Stereo Amplifier on eBay
When you are ready to search for a used stereo amplifier, the easiest way to start is by typing in specific keywords into the search bar on any page on eBay. For instance, if you type in "used stereo amplifier", the local search engine is prompted to go through all the current listings and pull out all the listings that meet that description. If you want to narrow your search, you can type in a more specific search phrase like "Class A used stereo amplifier". Consider using the search tool on the results page to narrow your search to local areas because some amplifiers can be quite heavy. By searching locally, you can save money and time by arranging to pick up the used stereo amp yourself.
Be sure to read the listings carefully to determine whether the seller has a return or refund policy. If you need to ask any questions, you should navigate to the seller's member profile and click on the contact link. While on the seller's member profile, you might want to read the feedback left by other buyers. This is important to do as it allows you to see how well the seller has fulfilled all their obligations in relation to eBay's seller policies. The best sellers are rewarded with a "Top-Seller" title. You are able to sort listings to only show those posted by eBay Top-Sellers.
There are different types of stereo amplifiers. There are Class A, AB, Class D digital stereo amplifiers, as well as monaural amplifiers. Each type of amplifier has specific advantages and disadvantages that directly affect the sound they can produce and how well they can be meshed with existing sound equipment. Amplifier size can be an important issue as well, and researching types of amplifier enables one to find an amplifier that is the perfect size. When an amplifier is required, choose a type that provides the exact sound quality that one is looking for by picking the correct sensitivity and dynamic range.
While searching, think about buying a set of used speakers to go with the stereo amplifier you have chosen. No matter what type of amplifier one needs, or what sound quality one is looking for, it is easy enough to find a used amplifier on eBay that matches an audiophile's specific requirements, but at a price that is substantially lower than what one would pay for a new stereo amplifier.