Amplifier
Amplifier
The function of an amplifier (Amp) is to amplify and control suitable inputs such as CD, tape, turntable, DVD player to a level that is enough to drive a speaker system and to satisfy the listener with adequate volume. The power levels that suit speakers are generally low voltage with high current. Speakers are nearly always very low impedance, and so need high current to enable them to produce a suitable volume of sound. It also selects inputs and controls the amplitude (volume), also controls the relative amplitude (balance) between the channels (left and right). The amount of amplification is called gain. Amplifiers can be categorized according to the frequency that it operates at. Audio amplifiers operate within the frequencies of 20 – 20,000 Hz, this is known as bandwidth. IF, RF, VHF amplifiers operate at higher frequencies.
The output of an amplifier is governed by the efficiency of the speakers. For example a pair of speakers rated at 86dB/watt driven by a 60 watt amplifier will give the same sound output as 89bB/watt speakers driven by a 30 watt amplifier.
There are many specifications quoted on amplifiers and a brief explanation of some are given below.
Amplifier specifications and parameters.
dB (Deci-Bel)
Deci-Bels (dB’s) are frequently used in audio specifications and an understanding is essential when evaluating audio specifications.
The deci-Bel is a specification of magnitude that is used to measure comparable loudness levels of audio. It is a logarithmic system of measurement, a ratio and not absolute. For example a 120 watt amplifier is 1 Bell, or 10 dB higher output than a 12 watt amplifier. 1dB is the threshold of perception of the human ear, or the smallest difference that we are capable of noticing. It is derived from deci (one-tenth) of a Bel after Alexandra Graham Bell).
Noise
Because amplifiers are not perfect devices, one of the limitations of an amplifier is noise. Noise is anything that appears on the output introduced by the amplification process that is not present on the signal. It is usually quoted as a ratio, noise factor or signal to noise ratio.
Slew rate
Slew rate is the rate of change of the signal that the amplifier is capable of. It’s how quickly the amplifier can react to change of signal. Often amplifiers are slew rate limited by the impedance of the drive current overcoming the capacitive effect of part of the circuit. This will limit the full power bandwidth to frequencies well below the small signal response.
Rise time
The rise time of an amplifier is closely related to slew rate but is quoted in volts per second and is the maximum rate of change of the output to change from 10% to 90% of its final level.
Dynamic range
The dynamic range of an amplifier is the range between the highest and lowest useful output levels. The lowest level is limited by noise and highest is limited by distortion usually clipping.
Bandwidth
Bandwidth, sometimes called frequency response of an amplifier refers to the range of frequencies that the amplifier is capable of reproducing to give satisfactory performance. It measures how uniformly it reproduces sounds ranging from the lowest to the highest frequency. Amplifiers should reproduce the relative loudness of various instruments and not over emphasize any particular frequency or instrument. It’s specified as a range such as 20 – 20 kHz. This particular example also happens to be the range that the human ear is capable of hearing. It is also given limits such as +/-3dB with reference to the mid point eg. 1kHz.
Whilst it is desirable to have a flat response there is no point in having a response much above 20kHz, in fact it is generally desirable to limit the extreme high frequency response above 20kHz to deal with any problems with RF or AM radio interference.
There is also no point in reproducing frequencies below 10 Hz, these can cause problems such as ‘breathing’ action of woofer cones moving in and out when using a vinyl source due to wow or rumble. Not only can this cause considerable power usage, but can also result in damage to speaker suspensions.
Efficiency:
This amplifier statistic is a ratio measured by how much input power is usefully applied to the amplifier compared to power wasted as heat. If you consider the efficiency of a class AB amplifier as 50%, that means that half of the power consumed is turned into output then the other half is wasted as heat. This wasted heat has to go into the heatsink and for a powerful amplifier means a large heatsink. Class A amplifiers can achieve 10 to 20% at best so proportionately need larger heatsinks and do run hot. Class B amplifiers are more efficient, between 35 – 50%. A more recent addition to amplifiers is known as Class D and are very efficient, between 90 – 95%.
Efficiency plays a large part in the design of an amplifier. Just consider a 100 watt amplifier, in class A it would have to dissipate around 85 watts into the heatsink. The same amplifier designed as class AB would need to dissipate 50 watts into the heatsink. But in class D would only need to dissipate 10 watts, so it’s easy to imagine the different size of heatsinks for each type.
Linearity
A perfect amplifier would be linear, but in the real world all amplifiers have limitations. An amplifier will amplify a signal until a point where saturation occurs in some part of the amp, usually the output stage. This is a consequence of trying to get more volume than the amplifier was designed for, but because music is very dynamic it happens more often than we would like. This is heard audibly as clipping but just before clipping occurs, a reduction in volume is also heard. This reduction of volume causes a compression effect which is sometimes deliberately employed in the design to reduce the very noticeable effects of hard clipping.
Settling time
Settling time of an amplifier is the time taken for the output to stabilize to a certain percentage of its final level. More often this is quoted for oscilloscope amplifiers but has crept in to some Hi Fi amps.
Stability
An amplifier can suffer from stability as a consequence of using feedback and can be an issue when employing multiple amplifier stages. It’s mainly concerned with RF and microwave amplifiers.
Output power
This is usually the first and most used specification to be scrutinised. Most often stated as RMS (root mean square) and with an easy job to perform such as a resistive load. It’s always specified just before the onset of clipping. It’s also important to note the specified impedance because if this is not the impedance of your speakers then it is not as impressive. In an effort test the amplifier under strenuous conditions most are specified with both channels driven.
Loudspeaker loads are a very different prospect because the impedance of speaker systems is very dependant on frequency and consequently some amplifiers that perform admirably in the lab fall apart driving certain loudspeakers.
Total Harmonic Distortion
This is a measure of the distortion that an amplifier produces and generally it goes up along with power levels. THD is achieved by putting a high quality signal into the amp, amplifying it and then subtracting the original signal and analysing the result. Unfortunately it falls well short of telling the truth about the quality of sound of an amplifier. Many amplifiers with a relatively high level of distortion sound excellent, whilst others with lower distortion sound much worse.
Damping Factor
This is a factor often overlooked when choosing amplifiers. Damping factor is the ratio of the speaker impedance to the source resistance. It describes how much control the amplifier has on the speaker. Because speaker impedance is quoted as say 8 Ohms, this is purely a nominal figure, and on a real speaker impedance is often well below this. When amplifiers are driving a very low impedance the speaker sound is boomy and not controlled. More expensive amplifiers have a high damping factor and as such have greater control of the speaker. It’s rather like amplifiers with low damping factor allows the speaker to do what it likes, whereas ones with high damping factor are making the speakers do what the amplifier wants – like it or not.
Input Sensitivity
Sensitivity is not quite so important with modern equipment, it just determines what position the volume control needs to be to achieve adequate listening levels. Most modern amplifiers have what is called line level inputs typically quoted as 100 mV. If vinyl is being used then with a magnetic pick up the sensitivity needs to be 2 or 3 mV and RIAA equalised. Moving coils cartridges have a much lower output and sensitivity needs to be much lower.
Input Impedance
This is the impedance as seen by the amplifier at the input sockets at a frequency of 1 kHz. With modern equipment at line level this is not so critical unless you’re using vinyl when it needs to be matched to the cartridge being used.
Hum and Noise
With the amplifier switched on and no source applied this is what is heard when close to the speakers. Again with line level inputs there is very little that is heard but when using vinyl there is a noticeable increase in noise levels. It’s specified as a ratio of signal to noise, and in practice levels of 60 dB and above are quite acceptable
Gain
Gain is simply a ratio of input to output and is controlled by the volume. It is also preset for different input for instance a low output device such as a turntable needs a higher level of preset gain to achieve a suitable level of control by the volume.
Overshoot
This refers to the initial response or over reaction of an amplifier before it settles down to its steady state.
