What is the use of an amplifier ?


What is the use of an amplifier?

 

If you have purchased a pair of loudspeakers then you will definitely need an amplifier to power them up. There are many variations of amplifiers. Amplifiers differ by shape, size and configuration.

 

There are high and low power amplifiers, single channel, stereo and multi-channel amplifiers.

 

When referring to channels we are speaking about the number of loudspeakers that your amplifier can power up at any given moment.

 

When speaking of mono amplifiers, we are speaking of amplifiers that can only power one speaker at a time.

 

Stereo amplifiers can power 2 speakers at a time, whilst multi-channel speakers as you may have guessed can power multiple speakers at one time.

 

Now the multiple speakers does have a limit and that limit is usually around 5 or 7 speakers.

 

I am sure you have heard TV commercials advertising 5 channel or 7.1 channel systems for home theater systems.

 

This just means that you have a speaker system powered by a receiver (which includes an amplifier with 5 or 7 speakers).

 

Choosing the appropriate amplifier can be tricky, but I will attempt to make the decision easy for you.

 

Prior to choosing your amplifier there are a few terms and issues that you must be aware of. First you must understand the term impedance in as far as it relates to your system/speakers.

 

Impedance is measured in what are called ohms.

 

Most amplifiers will be rated for 4-8 ohms. So therefore, if a loudspeaker falls between 4 and 8 ohms you will mostly be fine. Your problems begin when your amplifier’s impedance dips to below 2 ohms.

 

If you ever have a loudspeaker with an impedance of less than 4 ohms then get in touch with your amp manufacturer to check the stability of your amp.

 

Once we have established the impedance of the loudspeaker, we must make sure that our amplifier has an impedance in line with the loudspeaker.

 

Therefore, we need an amplifier that has an impedance between 4 and 8 ohms.

 

The next thing that we must look into is the loudspeaker sensitivity. This is denoted by means of a series of numbers and letters.

 

For example, let’s say 85 db at 1 watt at 1 meter. This implies that with 1 watt of power, and if you are sitting 1 m/3ft away from the loudspeaker then, the expected volume output level is 90 decibels.

 

I understand that not all of us are mathematically inclined, but there is an easy way to understand this concept.

 

There is an inverse relationship between the sensitivity and the power required to power a loudspeaker. The higher the DB number the less power you need to power the loudspeaker.

 

So for example a speaker that has a sensitivity of 100 db will require LESS power than a speaker with a sensitivity of 90Db, all else equal.

 

There is a financial trade off though between sensitive speakers the more DB they can produce.

 

Can you have too much power in an amplifier?

 

The simple answer is not really. Not really because the only circumstance I can think of is highly unlikely.

 

This could only happen if you had say a 2000 watt amp and purchased cheap portable (designed for less than a watt of power) speakers and connected them.

 

But this is an unlikely set up so I will not pay more attention to it.

 

We mistakenly believe that too much power is will cause damage to our speakers. But the opposite is true. Most of damage caused to your speaker will be caused by your speaker having too little power.

 

We can compare power in an amp to horse power in a car When you want to overtake another car, you hit the accelerator and the cars horse power helps the car accelerate.

 

Therefore, when your music reaches what is known as a dynamic passage, where it suddenly becomes very loud very quickly, you want the right amount of power to handle the extra power.

 

Let me explain with an example. Say there is a loud sound that happens suddenly whilst you are watching a movie or listening to music. Say there is a loud explosion.

 

If your amp does not have enough power then you can hit the ceiling of the power of the amplifier and your loud speaker can get damaged (clipped).

 

Clipping means that the amplifier is trying to amplify the signal , but then it hits  a limit.

 

Such a limit is the supply voltage of the amplifier. This supply voltage could be reduced by load on the power supply or the amplifier’s limiter circuits kicking in to try to save the amplifier from the abuse being inflict on it.

 

With modern music being so highly compressed it exacerbates the situation. The signal level in compressed music is quite high that the speaker drivers do not have sufficient time to cool down.

 

So make sure that your amplifier and speakers have some decent power to avoid damaging your speakers or causing distortion.

 

What does enough power mean?

 

The typical speaker produces 90 DB of sound pressure level (SPL) , with one watt of power and 1m from the speakers. 90 db is pretty loud.

 

For example a normal conversation is 60 db, a lawn mower roughly 90 db and a  concert is around 120db.

 

So you can see that you need very low power (wattage) to produce loud sound.

 

However with sound, as you increase the volume past a certain point the increase in power far outweighs the increase in loudness (db).

 

As a general rule for every 3DB increase in SPL, you need double the power. 3 DB is just noticeable. To double perceived loudness you need a 10 db increase in SPL.

 

A 10 DB increase requires 10 times the power requirement.

 

I suppose you may be wondering why are there 1000 watt speakers then if you only need 1 watt of power to play a fairly loud movie or music.

 

It is because sound is very dynamic especially movie sound.

 

If a movie’s sound adheres to the THX standard, it’s average level would be 20 dB below peak level. This means you need 100 times the power at peak level than average level.

 

A very high-power amplifier could even weigh upwards of 100 pounds.

 

An ordinary unregulated power supply is simple. The incoming AC voltage (120 volts in the US) would commonly be reduced to a lower voltage.

 

Higher AMPS need a high voltage. A bridge rectifier changes the lower voltage into negative and positive DC voltage. An audio signal will alternate between positive and negative (it’s alternating current.)

 

Different types/classes of amplifier

 

Class A amplifier

 

The simplest type of amplifier is the class a amplifier.

 

Nothing matches a class a amplifier in terms of linearity and distortion. The first problem with a class A amplifier is that it is inefficient.

 

I would only suggest a class A amplifier at between 25 and 50 watts. Efficiency can range from 10-25% with a typical class A amplifier.

 

So if you have a 25 watt amp at 25% efficiency you would consume 100 watts of power. Trying to do that with 5 speakers would be too energy intensive.

 

Class A amplifiers should thus be used for low power applications or headphone pre amps. Don’t get me wrong the sound is amazing, but they just consume way too much power. Technology has made some better alternatives.

 

Class B amplifier

 

Following on from class A are class B amplifiers. These amps try to solve the efficiency problem of class A amplifiers.

 

So with class B amplifier, one output device conducts on the positive waveform and one on the negative wave form.

 

With class B there is a push pull system at work, whereby a positive waveform goes into one transistor and the negative into the other.

 

In theory, an efficiency of 78.5% is possible, but unlikely. You are more likely to get 70% when the amp is at max capacity.

 

Class B is not commonly used by audiophiles. That is because of cross over distortion. This occurs at the time of the cross over from the positive output to the negative, and is extremely undesirable.

 

 

Class A-B amplifiers

 

Class A-B amplifiers are a hybrid between class A and class B. Class A-B overcomes the inefficiency of class A and the cross over distortion of class B amplifiers.

 

The output devices are shifted, so that they are not ceasing conduction at between 180 and 200 degrees, conduction is shifted higher, and output devices conduct consistently at a very low power level of between 1-5 watts.

 

You will therefore be running at low-class amplification, eliminating cross over distortion. You get a 2 in 1 solution, edging very close to class B efficiency and getting close to class A linearity.

 

This is what makes class A amplifiers so popular. The efficiency extends beyond just being better than class A or class B, class A-B amplifiers are also very well designed.

 

Everything fits so well into the chassis, and they sound very consistent as well.

 

Class G and Class H

 

This class of amplifier would fall in the class of subset of class A-B amplifiers. Additional circuit components are included, making the amplifiers more efficient, especially at both lower and high levels of power.

 

The disadvantage of class G and especially class H amplifiers is that the circuit is complex to make, difficult to service, heavy with a big power supply and it is not a modular design.

 

Class D Amplifiers

 

Some refer to this as digital amplification, however it is typically analogue. You have an amplifier that is being modulated with pulse width modulation.

 

This makes the output devices switch quickly and because they are switching quickly, it makes the amplifier very efficient you don’t have to use as big of a heatsink at all in this case because a typical Class D amplifier has anywhere between 90-97% efficiency.

 

This is a great solution when you are doing high channel density or when you are doing car audio applications.

 

Class D Amplification has come a long way over the last few years. This is a viable option to be looking at.

 

There are usually two types of Class D amplifiers, you have the ones that have analog output in terms of the feedback that they use, and then you have the ones that are digitally controlled with a DAC.

The only problem I find you get with some of these class D amplifiers is the output filter.

 

If this isn’t matched correctly you can get some erroneous frequency response output and it changes how the amplifier sounds depending on the load or the loudspeaker that you are driving.

 

 

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