How Amps Work
Pre and Power Amplifiers
Continuing the series on how things work, this time we will cover pre and power amplifiers. Various configurations are available. A pre-amplifier is basically a signal source selector and volume control and a power amplifier boosts the small signals to large enough levels to driver loudspeakers. Various devices are available which combine functions of both. An integrated amplifier combines a pre-amp and a power amp in the ones box. A receiver is an integrated amp with the addition of a radio tuner and commonly these days a surround sound processor. There are many different configurations such as multi-channel amplifiers for surround sound and video processing as well coming more popular.
Pre-amplifiers come as two main types, passive or powered. The latter are the most popular and use active (or powered) circuitry to keep the signals at a stable level; also know as buffering. Several inputs are usually provided and can be selected one at a time to be fed to the output. Some have recording or monitoring loops as well. Inputs can be the common RCA type which have a single signal and a shield or balanced (XLR) which have two signals; each in the opposite direction of each other (out of phase) and a shield. The balanced ones reduce the noise that can affect the signal but are a lot less common in home audio, generally reserved for high-end equipment. Once the input source is selected, the signal goes through to the controls, such as tone or balance and of course the volume control. The first two are not always fitted as they add extra circuitry to the audio signal path that can reduce the quality of the audio. After this is usually the output buffer circuit that makes sure it is boosted to the right level for power amplifiers to handle. This exact layout does vary a lot, otherwise all will be the same and some may have extra circuitry to improve the sound, large power suppliers and good filtering to lower noise and more. The signal is then sent to the output connectors.
Passive pre-amps are not very common but are usually either transformer based or just use a volume control. Same as power transformer can cut the mains voltage down, a transformer volume control can do the same but of course on a much smaller scale. Miniature transformers are wound very carefully with high-grade wire and have the signal coming in one end and then multiple points on the output. Each output point is at a different level hence by using a switch to select between each output you can control the volume. Due to the fine work involved, these tend to cost more than active pre-amps but many people believe they can sound better. The other way mentioned is simply by putting a volume control directly between the source unit and the power amp. While very simple it often does not sound very good due to mismatches between the two units and you have to be lucky to get it to sound the best.
Power amplifiers boost the small signals to higher levels suitable for speakers to playback. Speakers need the large power levels as they need to move a lot of air. Amps start at under a few watts and go to many thousands but most domestic ones range from about 10 to 400W RMS. Power is not everything and a low power amp can sound a lot better than a high powered one; or visa versa! It all comes down to the quality of the parts and the design. The power amplifier takes the signal from the input connections and usually feeds it through a buffer circuit to ensure it is at the right level for the main circuitry. Some use several stages of boosting the signal. The main amplification stage uses most commonly transistors or valves; each has their advantages and disadvantages which we wont go into here. Needless to say there are other types of amps such as the newer digital style ones too but they are a story for another time. So this final stage boosts the signal to enable it to go to the speakers and make beautiful music. How much power is produced is often also based on the power supply and the design. A good power supply needs to be able to deliver as much power as is necessary to reach maximum listening levels without running out of steam. It also has to filter out noises from the 50HZ mains supply and any other noises that are now very common in mains powerlines. Finally it needs to be able to react quickly to the demands of the music signal, a crescendo can happen very fast and if the power supply cannot deliver the power fast enough to the amplifier output stages, then the music will fall flat and distort.
Some amplifiers use push-pull designs, this is also known as Class A-B, where two parts work together, one amplifies the top half of the signal, the other amplifies the bottom half. These tend to produce more power but may not be as good in audio quality (all else being equal) as a Class A style amp. Class A uses a single part to cover the signal from top to bottom and does not suffer from switching distortion in the middle of the signal. The down side is they run hotter, use more power and tend cost more.