A Guide to Amplifiers - Page 3 - Blu-ray Forum

Big Daddy · 11-22-2009, 01:55 PM

For a complete list of amplifier manufacturers, click HERE.

WHAT IS AN AMPLIFIER?

An amplifier’s main purpose is to take a weak signal and make it strong enough to drive a speaker. Amplifiers get the necessary energy for amplification of input signals from the AC wall outlet. If you had a perfect amplifier, all of the energy the amplifier took from the AC outlet would be converted to useful output to the speakers. However, no amplifier/receiver is 100% efficient, so some of the energy from the wall outlet is wasted in the form of heat.

COMPONENTS OF AN AMPLIFIER

Power Supply:
Amplifier/receivers need power supplies to convert the AC power from the wall to DC voltage. This conversion from AC to DC is necessary because the chips used inside the electronic equipment require DC voltage. Different types of power supplies are used in amplifiers. Some high quality amplifiers have independent power supplies for each channel.

A 4 ohm speaker is harder for an amplifier to drive than an 8 ohm speaker. The capabilities of an amplifier when driving low impedance loads is closely related to the capabilities of its power supply.

Input stage:
The purpose of the input stage (sometimes called the front end) is to receive the input signals for amplification by the output stage.

Output stage:
The output stage of an amplifier converts the weak input signal into a much more powerful signal to be sent to a speaker.

AMPLIFIER SPECIFICATIONS

Power Ratings:
Amplifiers are generally rated in watts per channel, at different impedances over a frequency range of usually 20 Hz - 20,000 Hz, at some amount of total harmonic distortion. Harmonic distortion increases with power output. Considerably more power can be delivered if distortion is allowed to increase.

Lower quality receivers/amplifiers sometimes have impressive power ratings like 1,000 watts, but if you examine the fine print, you will notice a total harmonic distortion level of 10% or so, and usually over a limited frequency range like 40-18,000Hz.

It is important to understand that two amplifiers that have the same power rating do not necessarily output the same power or sound alike. Also, power can not be amplified. Voltage and current can be amplified.

If you had a well-designed (perfect) amplifier and a perfect current source (wall outlet that had unlimited current availability), then each time you reduced the impedance by half, the power would be doubled. For example, an amplifier with a rating of 200 watts per channel at 8 ohms should be able to output 400 watts at 4 ohm and 800 watts at 2 ohm.

In the real world, amplifiers have real power supplies and their 4 ohm power rating is usually less than 100% more than the 8 ohm rating. Amplifiers with extremely good power supply will be able to do better, but eventually a limit will be reached as the AC outlet on the wall can only deliver so much current. As I said before, the capabilities of an amplifier when driving low impedance loads is closely related to the capabilities of its power supply.

Measures of Power

Power is not really something that can be amplified. Voltage and current can be amplified.

Calculation of RMS (Root Mean Squared) and Peak Power:
First, there is no such thing called RMS power. RMS refers to voltage, but erroneously people have gotten used to using it to rate power.

Power is defined as voltage squared divided by resistance.

P = V^2 / R

Let's assume we are dealing with a battery (DC voltage) and a resistor. If we apply 40 volts to an 8 ohm resistor, the voltage will always be the same (40 volts) so the power generated will always be the same. In this example, the power is equal to:

P = 40^2 / 8 = 200 watts

As a result of this power, the resistor will heat up to a certain temperature.

Now, let's change the voltage source to a 40 volt sine wave (AC current) and apply it to the same load resistor. In this case, the voltage is not constant and oscillates between +40 volts and -40 volts and as a result, the power generated will be less than 200 watts and the resistor will not get as hot.

How much sine wave voltage (AC) peak do we need to generate the same level of power and heat in the resistor as a DC voltage source?

The AC voltage required must have a higher peak than 40 volts DC to accomplish this. As it happens, the peak voltage is equal to the DC voltage, 40 volts, times the square root of 2. The detailed calculation of RMS Voltage requires some knowledge of calculus and is done HERE.

40 volts x sqrt(2) = 40 x 1.414 = 56.5 volts

Therefore, for a sine wave (AC) voltage source, we must apply 40 volts RMS to generate the same level of power and heat in the resistor.

In the above example, the RMS voltage of a pure sine wave of 40 volts is equal to 40 volts multiplied by the inverse of the square root of 2.

40 x 1/sqrt(2) = 40 x 1/1.414 = 40 x 0.707 = 28.3 RMS volts

or

The RMS voltage of a pure sine wave is approximately equal to peak voltage x 0.707.

The Following diagram should clarify the calculations above:

Diagram Created by Big Daddy

Continuous Power:
An additional factor that we must consider is the ability of the amplifier/receiver to output its full power continuously. In other words, just because an amplifier/receiver is listed as being able to output 100WPC, it does not mean that it can do so for any significant length of time.

You have to distinguish between peak power, RMS power, and continuous power. Many people wrongly assume that the RMS power rating is the same as a continuous power rating.

Remember that power is a snapshot of the amount of work being done at a given point in time. It does not have any specified time component attached to it.

For example, if we are applying a 40 volt sine wave voltage to a load resistance of 8 ohms, the peak power is calculated as:

Peak Power = 40^2 / 8 = 200 watts Peak

and

RMS power = 28.3^2 / 8 = 100 watts RMS

As you can see, the peak power rating is TWICE the RMS power output and is misleading.

RMS watts is better than peak power, but it can also be misleading as it contains no specified element of time. RMS wattage is a scientifically accurate way to measure power, but the amplifier may only be able to produce the RMS voltage into the given load for a fraction of a second.

Manufacturers that only report the RMS power rating may be trying to hide the truth. Although RMS voltage calculation is technically correct, it does not tell whether the amplifier is able to generate the stated RMS voltage for a continuous period of time. When they state continuous output power, they are saying that the amplifier can easily and continuously produce the rated output power.

The RMS continuous watts is a more honest way of reporting the power of an amplifier. However, it can also be deceptive as it is carried out at just one sine wave frequency at a time. This does not put a realistic demand on the amplifier compared to real life use. A better rating is the AES (Audio Engineering Society) rating, where the equipment being tested is subjected to a broadband pink noise signal. Pink noise signal contains equal energy in each octave frequency range. Testing using such a signal is more reliable when compared with a single frequency test (RMS) as it is testing over the whole frequency range at once.

The rated power output of an amplifier is understood to be its maximum output, it in no way means that the amplifier can only be used at this output. For example, if an amplifier is rated at 100 watts, the output can be anything between zero and this maximum rated value. Chances are that the amp can probably put out more if the input signal is over driven, however the quality of the output will degrade rapidly.

Amplifier Efficiency:
Efficiency is defined as the amount of output power divided by the amount of required input power. The maximum ideal efficiency that any circuit can have is 100%. In general, audio amplifiers are not very efficient. See efficiency of different classes of amplifiers in post #2 of this thread.

Dynamic Headroom:
Dynamic headroom is the ability of an amplifier/receiver to output power at a significantly higher level for short periods to accommodate musical peaks or extreme sound effects in movies. This specification is expressed in decibels. For example, a dynamic headroom of 3dB would indicate that an amplifier can double its output for a very short time to meet the above conditions.

It requires twice the power to increase the volume of a speaker by 3dB. The table below, for a speaker with sensitivity rating of 87db, shows how much you need to increase power to get an additional 3db increase in volume.

Table Created by Big Daddy

Read this article on Dynamic Headroom: http://www.axiomaudio.com/dynamicheadroom.html

Clipping:
Clipping occurs when one tries to get a larger output signal out of an amplifier than it was designed to provide.

This is a normal sine wave:

This is a clipped sign wave:

Amplifier clipping occurs when the the signal level is exceeding the maximum capability of the amplifier. Graphically, this means that the tops of the audio signal are “clipped off” or become flat. Clipping is the enemy of speaker drivers, particularly the high frequency drivers.

During the times when a signal from the amplifier is clipped, the cones of the speaker drivers are not instructed to move in and out. It is as if they are receiving a DC signal. This means that all the amplifier's power goes into heating the voice coils instead of producing sound.

Unfortunately, the more efficient the drivers are in converting power to sound, the worst the problem becomes. High frequency drivers (tweeters and mid-range drivers) are normally more efficient than low frequency drivers in converting power to sound. During clipping all that power will be completely converted to heat. High frequency drivers do not have the mass to tolerate the excess heat and are more likely to fail.

Signal-To-Noise Ratio (S/N):
All amplifiers and all electrical circuits generate a certain amount of electrical noise. Amplifiers that are designed better minimize the amount of noise. However, no matter how good the design, there will always be some noise. Generally, the more powerful the amplifier, the more noise it will produce. An amplifier with a poorly regulated power supply can create additional noise.

The Signal-To-Noise Ratio (S/N) measures the ratio of sound to background noise. It is expressed in decibels. S/N is calculated by measuring a unit's output noise, with no signal present, and all controls set to a predetermined level. A higher number is desirable.

Distortion:
Distortion in an amplifier is the alteration of the original shape (or other characteristic) of the signal. Distortion is usually unwanted. All amplifiers alter input signals, generally in two ways: they make them stronger (amplify) them, and they add characteristics which did not exist in the original signal. These undesirable characteristics are called distortion.

Harmonic Distortion:
Harmonic distortion is one of the most common type of amplifier distortion. Harmonics of a signal are signals which are related to the original (or fundamental) by an integer (non decimal) number. Music is made of fundamental frequencies and their harmonics. For example, the note "A" (2nd string from the right on the violin, or "A" above middle "C" on the piano) consists of a fundamental frequency of 440Hz. It is also reproducing harmonics of that frequency such as 880Hz (440 x 2), 1220Hz (440 x 3) , 1760Hz (440 x 4), and so on. The volume of each harmonic frequency is lower than the fundamental frequency. Note A of other musical instruments sounds different because of their fundamental frequencies and their harmonics.

A pure tone signal has no harmonics; it consists of only one single frequency. If a pure tone signal was applied to the input of an amplifier, we would find that the output of the amplifier is not pure and consists of the an amplified version of the input signal plus distortion created by the amplifier. All amplifiers and signal processors add distortion to the signal.

Total Harmonic Distortion, or THD is specification that compares the output signal of the amplifier with the input signal and measures the level differences in harmonic frequencies between the two. The difference is called total harmonic distortion. The levels of harmonic distortion in most high quality amplifiers are very small and below audibility. It is generally accepted that a THD below 1% is not audible.

Do two amplifiers with identical THD ratings sound the same, everything else being equal? Not necessarily, but differences will be subtle and difficult to hear.

Calculation of Total Harmonic Distortion:
THD is a measurement of the total number of harmonics that are in addition to the fundamental harmonic being measured and are expressed as a percentage of the fundamental frequency. To measure THD, a reference frequency must be specified (the fundamental) and any remaining frequencies measured. For example, a test tone of 1kHz may be applied to the amplifier. The THD analyzer is designed to ignore the fundamental (in this case 1kHz) and measure anything else remaining. This is done by summing together all remaining frequencies at the output of the amplifier under test ignoring the 100% 1kHz test tone.

There are two equations for calculating THD. The first one requires calculating the power of each harmonic frequency.

THD = 100 * SQRT[(P2 + P3 + P4 + ... + Pn) * R] / Vrms

where

THD = total harmonic distortion in percentage
Pi = the power of each harmonic
R = the load (output) impedance
Vrms = total RMS output voltage (containing both the fundamental and the harmonic terms)

Another way of getting the same answer would be to take the square root of the sum of the squares of harmonic voltages.

THD = 100 * SQRT[(V2)^2 + (V3)^2 + (V4)^2 + ... + (Vn)^2] / Vrms

where

THD = total harmonic distortion in percentage
Vi = the RMS voltage of each harmonic
Vrms = total RMS output voltage (containing both the fundamental and the harmonic terms)

TDH(%) is total harmonic distortion, V represents the RMS voltage of each harmonic, and Vt is the total RMS output voltage.
Note that Zout is not present in this variant of the equation;
since P = V2/Z, the impedance terms cancel.

Example: Let us assume that we use a sine-wave generator to input a certain frequency wave at the input of an amplifier, and adjust it so that it outputs 20 volts RMS into an 8-ohm load. Assume the second harmonic is measured at 0.5 volts RMS, the third harmonic at 0.4 volts RMS, and the 4th harmonic at 0.3 volts RMS. For the sake of simplicity, we assume that the rest of the harmonics are too small and can be ignored.

We can use the above formulas to calculate the THD. The results are summarized in the following tables:

Table Created by Big Daddy

Intermodulation distortion:
Inter-modulation Distortion measures non-harmonic frequencies added to the signal. This type of distortion is the result of two or more signals mixing together that are not harmonic frequencies and are undesirable. For example, if an amplifier creates a non-harmonic frequency of 300Hz along with the fundamental frequency of 440Hz (C note for violin), a third frequency of 740Hz (440 + 300) and a fourth frequency of 140Hz (440-200) will also be produced. These new frequencies are not harmonics of 440Hz. Thus, it is termed intermodulation distortion because it is between harmonic frequencies.

Intermodulation distortion is much more objectionable to the human ear than harmonic distortion.

Input Sensitivity of an Amplifier:
A pre-amplifier has two basic functions:

1. Allow input switching.
2. To achieve synergy between the source(s) and the amplifier(s).

Input switching is well understood by most users and is rather elementary. The second function is not quite as easy for most users to understand. To select an appropriate preamp, you must know two things:

1. The output voltage of the source. Most CD players, for example, have an output voltage of 2 volts.
2. The input sensitivity of the amplifier. Most amplifiers have an input sensitivity of around 1 volt.

Output Voltage: The output voltage of the source is normally a constant level unless the source has a variable output knob. This output voltage (e.g., 2 volts of CD music) drives the input stage of the preamp which in turn drives the input stage of the amplifier.

Input Sensitivity: The input sensitivity of an amplifier is defined as how many volts are required to bring the amplifier to full power. Any amount of voltage beyond that number will force the amplifier to try to output more power than it is capable of and therefore results in clipping.

So we can conclude that an important job of the preamp is to control the voltage from the source to the amplifier. This is done by adjusting the volume control. When the preamp volume is all the way down, the output signal is attenuated completely to zero volts and no sound. As the volume knob is turned up, the voltage increases as does the sound generated from the amplifier. Assuming the output voltage of the source is more than the input sensitivity of the amplifier, the ideal working range on a volume control should be where the preamp would not add any voltage gain to its input signal beyond the amplifier’s input sensitivity.

When do we need gain in a preamp?

When the input sensitivity of the amplifier is above the output voltage of the source. It is possible for some amplifiers to need up to 5 volts to bring them to full power. On the other hand, it is possible for some to only need 1/2 volt to come to full power. It is also possible for some modified CD players or Digital/Analog Converters to have less then 2 volts.

As an example, assume an amplifier with an input sensitivity of 2 volts is connected to a source with an output voltage of 1 volts. We will definitely need a preamp with some gain. Otherwise, it will not be possible to play the amplifier as loud as it it is capable of. Even if the speakers are very efficient, the music will lack dynamics and weight at low listening levels. Turntables have very low output sensitivity. That is the main reason why you need a built-in or external phono stage with turntables,

Example: Let’s say we have an 8-ohm loudspeaker that has a sensitivity of 87dB for one watt at one meter. Assume we are trying to achieve a desired maximum peak sound pressure level of 105dB. If the signal source has an output voltage of 1 volt, how much amplifier gain is required?

Remember that in order to increase the level of sound by 3dB, we need to double the amplifier’s power.

SPL, Watts
87dB, 1
90dB, 2
93dB, 4
96dB, 8
99dB, 16
102dB, 32
105dB, 64

Therefore, we need 64 watts to achieve a peak SPL of 105dB. From Ohm’s law,

Power = V^2 / R

64 = V^2 / 8, and V = 23 volts

So the amplifier needs to provide a voltage gain of 23 times which is approximately 27dBv. Please note that 0 dBv is defined as Vo = 1 volt, and

dBv = 20 . LOG(V/Vo) = 20 x LOG(23/1) = 27 dBv.

Some amplifiers have knobs that control the amplifier’s input sensitivity. When the knob is turned clockwise, the sensitivity will increase. Turning the control counter-clockwise will decrease sensitivity. This control is not a volume control for the amplifier. The amplifier can be driven to full power with a wide range of signal levels. A low level signal will require increased sensitivity for full power. A high level signal will require decreased sensitivity.

The other issue that we need to consider is output impedance of preamps and input impedance of amplifiers.

Output Impedance: All sources and preamps have output impedance. Output impedance is defined as the ability of a unit to drive difficult loads.

Input Impedance: Amplifiers, preamps and receivers have input impedance. Think of it as the difficulty the amplifier imposes on the preamp and the preamp (receiver) imposes on the source.

Generally speaking, we can make the following conclusions:

The lower the output impedance of the preamp, the better it will drive difficult loads.
The higher the input impedance of the amplifier, the less difficult it will for the preamp to drive it.

For example, a preamp with an output impedance of 1,000 ohms can easily drive an amplifier with an input impedance of 100,000 ohms.

Most stock (unmodified) CD players have a relatively low output impedance. Unfortunately, amplifiers' input impedances vary quite a bit. Although the most common number is 50,000 ohms, they can vary between 10,000 ohms and 500,000 ohms.

Although 50,000 ohm is a load that most sources and preamps can drive easily, you may run into difficulty if you use extra long interconnects to connect a preamp with very high output impedance to a 50,000 ohm amplifier. The result may be either reduction in bass response, or lack of dynamics, or both. This is sometimes described as thin sound. The solution is to reduce the length of the interconnects and/or use a preamp with low output impedance.

The Slew Rate:

The slew rate in electronics is the maximum rate at which an electronic amplifier can respond to a sudden change in input level. The term is used to define the maximum rate of change of an amplifier's output voltage with respect to its input voltage. The unit of measure is volts per microsecond. A slew rate of 1V/µs, for example, means that within 1 microsecond, the amplifier can go from 0 to 1 volt. The slew rate in amplifiers is similar to acceleration in cars. This is important because it defines the maximum speed at which an amplifier can handle a transient waveform. The higher the slew rate, the faster is the amplifier.

Measurement
The slew rate can be measured using a function generator (usually square wave) and oscilloscope. Slew rate is measured by feeding an input signal that is too fast for the amplifier to cope with. It is the time an amplifier needs to go from 10% to 90% of the total output voltage in response to a step in voltage at the input. Slew rate is an overload condition, and it should not happen at all for an audio amplifier. Therefore, being concerned about the slew rate is meaningless.

This specification limits the capability of an amplifier to generate high voltage pulses with sharp rising and falling edges. This is exhibited in Figures 2, 3, and 4.

Source:Falco Systems

The higher the frequency, the faster the voltage has to rise to prevent distortion of the sine wave. If the amplifier cannot follow due to its limited slew rate, the sine wave will be distorted and its amplitude is lower than at low frequencies. Limitations in slew rate can result to non linear effects. For a sinusoidal waveform not to be subject to slew rate limitation, the slew rate capability at all points in an amplifier must satisfy the following condition:

SR >= 2Pi x f x Vpk

where f is the frequency and Vpk is the peak amplitude of the waveform.

Continued in the next post.

CONCLUSIONS

When buying an amplifier/receiver, it is important to look at the following factors:

Distortion: Total Harmonic Distortion (THD) and Intermodulation Distortion (IMD), lower numbers are better.
Signal-To-Noise (S/N) Ratio: The higher number is better.
Continuous Power: Just because the receiver/amplifier may be listed as being able to output 100WPC, doesn't mean it can do so for any significant length of time. Always make sure the watt per channel is in continuous RMS terms. Even RMS power is technically incorrect. RMS voltage is normally measured.
Dynamic Headroom: The ability of the receiver/amplifier to output power at a significantly higher level for short period of time to accommodate musical peaks or extreme sound effects in movies. Dynamic Headroom is measured in Decibels. If a receiver/amplifier has the ability to double its power output for a brief period, it would have a Dynamic Headroom of 3db.
Impedance Rating: Almost all receivers can handle 8 ohm speakers. Some can handle 6 ohm speakers. Almost none of them can handle 4 ohm speakers. Using a receiver on a low impedance speaker will result in overheating and possible damage to the receiver and/or the speaker.
Other Options: You should look at the other options that the receiver offers, such as the number of HDMI input/outputs, the kind of video processing, DSP modes, etc.

REFERENCES AND ADDITIONAL INFORMATION

Home Theater: To Separate or Not to Separate?
audioport.com
Power Amplifier Fundamental
Power Amplifier Power Specs Demystified
http://www.harmanaudio.com/all_about...werratings.asp
http://www.usedphones.com/Amplifiers.html
http://www.hometheatersound.com/feat...c_20010901.htm
http://sound.westhost.com/bi-amp.htm
http://sound.westhost.com/tweeters.htm
http://www.audioholics.com/education...r-voltage-gain
Amplifier Power Ratings
Distortion - Wikipedia, the free encyclopedia
Total harmonic distortion - Wikipedia, the free encyclopedia
THD Measurement and Conversion
What is Total Harmonic Distortion? - THD - Definition and Explanation of THD
What is Intermodulation Distortion? - IMD - Definition and Explanation of IMD
How Much Amplifier Power Do You Really Need? - Amplifier Power - What You Need To Know About Amplifiers and Amplification
Total Harmonic Distortion
PS Audio - High-Performance Home Audio Equipment
http://www.analog.com/en/content/0,2..._91250,00.html
Audio Specifications
3 ways of expressing voltage of a common AC wave form
Measurements of AC magnitude - Basic AC Theory
What is Alternating Current?
RMS Calculator
http://www.eznec.com/Amateur/RMS_Power.pdf
Sound & Communications - Audio
http://www.1388.com/articles/tech_underAmp
Deutsches Institut für Normung : Startseite DE
Amp Input Sensitivity and Gain
Setting Sound System Level Controls
Pro Audio Reference D
DECWARE - Audio Paper - How to choose the right preamp
http://en.wikipedia.org/wiki/Slew_rate
http://www.falco-systems.com/high_vo...mplifiers.html
http://www.sweetwater.com/shop/live-...ying-guide.php
http://www.psaudio.com/ps/wiki/Slew-Rate/

11-22-2009, 01:55 PM	#1
Big Daddy Blu-ray Champion Top ranked HT gallery Member since:Jan 2008 Location:Southern California Home Theater Gallery:79 Blu-ray collection:122 Trading Score:1	A Guide to Amplifiers For a complete list of amplifier manufacturers, click HERE. WHAT IS AN AMPLIFIER? An amplifier’s main purpose is to take a weak signal and make it strong enough to drive a speaker. Amplifiers get the necessary energy for amplification of input signals from the AC wall outlet. If you had a perfect amplifier, all of the energy the amplifier took from the AC outlet would be converted to useful output to the speakers. However, no amplifier/receiver is 100% efficient, so some of the energy from the wall outlet is wasted in the form of heat. COMPONENTS OF AN AMPLIFIER Power Supply: Amplifier/receivers need power supplies to convert the AC power from the wall to DC voltage. This conversion from AC to DC is necessary because the chips used inside the electronic equipment require DC voltage. Different types of power supplies are used in amplifiers. Some high quality amplifiers have independent power supplies for each channel. A 4 ohm speaker is harder for an amplifier to drive than an 8 ohm speaker. The capabilities of an amplifier when driving low impedance loads is closely related to the capabilities of its power supply. Input stage: The purpose of the input stage (sometimes called the front end) is to receive the input signals for amplification by the output stage. Output stage: The output stage of an amplifier converts the weak input signal into a much more powerful signal to be sent to a speaker. AMPLIFIER SPECIFICATIONS Power Ratings: Amplifiers are generally rated in watts per channel, at different impedances over a frequency range of usually 20 Hz - 20,000 Hz, at some amount of total harmonic distortion. Harmonic distortion increases with power output. Considerably more power can be delivered if distortion is allowed to increase. Lower quality receivers/amplifiers sometimes have impressive power ratings like 1,000 watts, but if you examine the fine print, you will notice a total harmonic distortion level of 10% or so, and usually over a limited frequency range like 40-18,000Hz. It is important to understand that two amplifiers that have the same power rating do not necessarily output the same power or sound alike. Also, power can not be amplified. Voltage and current can be amplified. If you had a well-designed (perfect) amplifier and a perfect current source (wall outlet that had unlimited current availability), then each time you reduced the impedance by half, the power would be doubled. For example, an amplifier with a rating of 200 watts per channel at 8 ohms should be able to output 400 watts at 4 ohm and 800 watts at 2 ohm. In the real world, amplifiers have real power supplies and their 4 ohm power rating is usually less than 100% more than the 8 ohm rating. Amplifiers with extremely good power supply will be able to do better, but eventually a limit will be reached as the AC outlet on the wall can only deliver so much current. As I said before, the capabilities of an amplifier when driving low impedance loads is closely related to the capabilities of its power supply. Measures of Power Power is not really something that can be amplified. Voltage and current can be amplified. Calculation of RMS (Root Mean Squared) and Peak Power: First, there is no such thing called RMS power. RMS refers to voltage, but erroneously people have gotten used to using it to rate power. Power is defined as voltage squared divided by resistance. P = V^2 / R Let's assume we are dealing with a battery (DC voltage) and a resistor. If we apply 40 volts to an 8 ohm resistor, the voltage will always be the same (40 volts) so the power generated will always be the same. In this example, the power is equal to: P = 40^2 / 8 = 200 watts As a result of this power, the resistor will heat up to a certain temperature. Now, let's change the voltage source to a 40 volt sine wave (AC current) and apply it to the same load resistor. In this case, the voltage is not constant and oscillates between +40 volts and -40 volts and as a result, the power generated will be less than 200 watts and the resistor will not get as hot. How much sine wave voltage (AC) peak do we need to generate the same level of power and heat in the resistor as a DC voltage source? The AC voltage required must have a higher peak than 40 volts DC to accomplish this. As it happens, the peak voltage is equal to the DC voltage, 40 volts, times the square root of 2. The detailed calculation of RMS Voltage requires some knowledge of calculus and is done HERE. 40 volts x sqrt(2) = 40 x 1.414 = 56.5 volts Therefore, for a sine wave (AC) voltage source, we must apply 40 volts RMS to generate the same level of power and heat in the resistor. In the above example, the RMS voltage of a pure sine wave of 40 volts is equal to 40 volts multiplied by the inverse of the square root of 2. 40 x 1/sqrt(2) = 40 x 1/1.414 = 40 x 0.707 = 28.3 RMS volts or The RMS voltage of a pure sine wave is approximately equal to peak voltage x 0.707. The Following diagram should clarify the calculations above: Diagram Created by Big Daddy Continuous Power: An additional factor that we must consider is the ability of the amplifier/receiver to output its full power continuously. In other words, just because an amplifier/receiver is listed as being able to output 100WPC, it does not mean that it can do so for any significant length of time. You have to distinguish between peak power, RMS power, and continuous power. Many people wrongly assume that the RMS power rating is the same as a continuous power rating. Remember that power is a snapshot of the amount of work being done at a given point in time. It does not have any specified time component attached to it. For example, if we are applying a 40 volt sine wave voltage to a load resistance of 8 ohms, the peak power is calculated as: Peak Power = 40^2 / 8 = 200 watts Peak and RMS power = 28.3^2 / 8 = 100 watts RMS As you can see, the peak power rating is TWICE the RMS power output and is misleading. RMS watts is better than peak power, but it can also be misleading as it contains no specified element of time. RMS wattage is a scientifically accurate way to measure power, but the amplifier may only be able to produce the RMS voltage into the given load for a fraction of a second. Manufacturers that only report the RMS power rating may be trying to hide the truth. Although RMS voltage calculation is technically correct, it does not tell whether the amplifier is able to generate the stated RMS voltage for a continuous period of time. When they state continuous output power, they are saying that the amplifier can easily and continuously produce the rated output power. The RMS continuous watts is a more honest way of reporting the power of an amplifier. However, it can also be deceptive as it is carried out at just one sine wave frequency at a time. This does not put a realistic demand on the amplifier compared to real life use. A better rating is the AES (Audio Engineering Society) rating, where the equipment being tested is subjected to a broadband pink noise signal. Pink noise signal contains equal energy in each octave frequency range. Testing using such a signal is more reliable when compared with a single frequency test (RMS) as it is testing over the whole frequency range at once. The rated power output of an amplifier is understood to be its maximum output, it in no way means that the amplifier can only be used at this output. For example, if an amplifier is rated at 100 watts, the output can be anything between zero and this maximum rated value. Chances are that the amp can probably put out more if the input signal is over driven, however the quality of the output will degrade rapidly. Amplifier Efficiency: Efficiency is defined as the amount of output power divided by the amount of required input power. The maximum ideal efficiency that any circuit can have is 100%. In general, audio amplifiers are not very efficient. See efficiency of different classes of amplifiers in post #2 of this thread. Dynamic Headroom: Dynamic headroom is the ability of an amplifier/receiver to output power at a significantly higher level for short periods to accommodate musical peaks or extreme sound effects in movies. This specification is expressed in decibels. For example, a dynamic headroom of 3dB would indicate that an amplifier can double its output for a very short time to meet the above conditions. It requires twice the power to increase the volume of a speaker by 3dB. The table below, for a speaker with sensitivity rating of 87db, shows how much you need to increase power to get an additional 3db increase in volume. Table Created by Big Daddy Read this article on Dynamic Headroom: http://www.axiomaudio.com/dynamicheadroom.html Clipping: Clipping occurs when one tries to get a larger output signal out of an amplifier than it was designed to provide. This is a normal sine wave: This is a clipped sign wave: Amplifier clipping occurs when the the signal level is exceeding the maximum capability of the amplifier. Graphically, this means that the tops of the audio signal are “clipped off” or become flat. Clipping is the enemy of speaker drivers, particularly the high frequency drivers. During the times when a signal from the amplifier is clipped, the cones of the speaker drivers are not instructed to move in and out. It is as if they are receiving a DC signal. This means that all the amplifier's power goes into heating the voice coils instead of producing sound. Unfortunately, the more efficient the drivers are in converting power to sound, the worst the problem becomes. High frequency drivers (tweeters and mid-range drivers) are normally more efficient than low frequency drivers in converting power to sound. During clipping all that power will be completely converted to heat. High frequency drivers do not have the mass to tolerate the excess heat and are more likely to fail. Signal-To-Noise Ratio (S/N): All amplifiers and all electrical circuits generate a certain amount of electrical noise. Amplifiers that are designed better minimize the amount of noise. However, no matter how good the design, there will always be some noise. Generally, the more powerful the amplifier, the more noise it will produce. An amplifier with a poorly regulated power supply can create additional noise. The Signal-To-Noise Ratio (S/N) measures the ratio of sound to background noise. It is expressed in decibels. S/N is calculated by measuring a unit's output noise, with no signal present, and all controls set to a predetermined level. A higher number is desirable. Distortion: Distortion in an amplifier is the alteration of the original shape (or other characteristic) of the signal. Distortion is usually unwanted. All amplifiers alter input signals, generally in two ways: they make them stronger (amplify) them, and they add characteristics which did not exist in the original signal. These undesirable characteristics are called distortion. Harmonic Distortion: Harmonic distortion is one of the most common type of amplifier distortion. Harmonics of a signal are signals which are related to the original (or fundamental) by an integer (non decimal) number. Music is made of fundamental frequencies and their harmonics. For example, the note "A" (2nd string from the right on the violin, or "A" above middle "C" on the piano) consists of a fundamental frequency of 440Hz. It is also reproducing harmonics of that frequency such as 880Hz (440 x 2), 1220Hz (440 x 3) , 1760Hz (440 x 4), and so on. The volume of each harmonic frequency is lower than the fundamental frequency. Note A of other musical instruments sounds different because of their fundamental frequencies and their harmonics. A pure tone signal has no harmonics; it consists of only one single frequency. If a pure tone signal was applied to the input of an amplifier, we would find that the output of the amplifier is not pure and consists of the an amplified version of the input signal plus distortion created by the amplifier. All amplifiers and signal processors add distortion to the signal. Total Harmonic Distortion, or THD is specification that compares the output signal of the amplifier with the input signal and measures the level differences in harmonic frequencies between the two. The difference is called total harmonic distortion. The levels of harmonic distortion in most high quality amplifiers are very small and below audibility. It is generally accepted that a THD below 1% is not audible. Do two amplifiers with identical THD ratings sound the same, everything else being equal? Not necessarily, but differences will be subtle and difficult to hear. Calculation of Total Harmonic Distortion: THD is a measurement of the total number of harmonics that are in addition to the fundamental harmonic being measured and are expressed as a percentage of the fundamental frequency. To measure THD, a reference frequency must be specified (the fundamental) and any remaining frequencies measured. For example, a test tone of 1kHz may be applied to the amplifier. The THD analyzer is designed to ignore the fundamental (in this case 1kHz) and measure anything else remaining. This is done by summing together all remaining frequencies at the output of the amplifier under test ignoring the 100% 1kHz test tone. There are two equations for calculating THD. The first one requires calculating the power of each harmonic frequency. THD = 100 * SQRT[(P2 + P3 + P4 + ... + Pn) * R] / Vrms where THD = total harmonic distortion in percentage Pi = the power of each harmonic R = the load (output) impedance Vrms = total RMS output voltage (containing both the fundamental and the harmonic terms) Another way of getting the same answer would be to take the square root of the sum of the squares of harmonic voltages. THD = 100 * SQRT[(V2)^2 + (V3)^2 + (V4)^2 + ... + (Vn)^2] / Vrms where THD = total harmonic distortion in percentage Vi = the RMS voltage of each harmonic Vrms = total RMS output voltage (containing both the fundamental and the harmonic terms) TDH(%) is total harmonic distortion, V represents the RMS voltage of each harmonic, and Vt is the total RMS output voltage. Note that Zout is not present in this variant of the equation; since P = V2/Z, the impedance terms cancel. Example: Let us assume that we use a sine-wave generator to input a certain frequency wave at the input of an amplifier, and adjust it so that it outputs 20 volts RMS into an 8-ohm load. Assume the second harmonic is measured at 0.5 volts RMS, the third harmonic at 0.4 volts RMS, and the 4th harmonic at 0.3 volts RMS. For the sake of simplicity, we assume that the rest of the harmonics are too small and can be ignored. We can use the above formulas to calculate the THD. The results are summarized in the following tables: Table Created by Big Daddy Table Created by Big Daddy Intermodulation distortion: Inter-modulation Distortion measures non-harmonic frequencies added to the signal. This type of distortion is the result of two or more signals mixing together that are not harmonic frequencies and are undesirable. For example, if an amplifier creates a non-harmonic frequency of 300Hz along with the fundamental frequency of 440Hz (C note for violin), a third frequency of 740Hz (440 + 300) and a fourth frequency of 140Hz (440-200) will also be produced. These new frequencies are not harmonics of 440Hz. Thus, it is termed intermodulation distortion because it is between harmonic frequencies. Intermodulation distortion is much more objectionable to the human ear than harmonic distortion. Input Sensitivity of an Amplifier: A pre-amplifier has two basic functions: 1. Allow input switching. 2. To achieve synergy between the source(s) and the amplifier(s). Input switching is well understood by most users and is rather elementary. The second function is not quite as easy for most users to understand. To select an appropriate preamp, you must know two things: 1. The output voltage of the source. Most CD players, for example, have an output voltage of 2 volts. 2. The input sensitivity of the amplifier. Most amplifiers have an input sensitivity of around 1 volt. Output Voltage: The output voltage of the source is normally a constant level unless the source has a variable output knob. This output voltage (e.g., 2 volts of CD music) drives the input stage of the preamp which in turn drives the input stage of the amplifier. Input Sensitivity: The input sensitivity of an amplifier is defined as how many volts are required to bring the amplifier to full power. Any amount of voltage beyond that number will force the amplifier to try to output more power than it is capable of and therefore results in clipping. So we can conclude that an important job of the preamp is to control the voltage from the source to the amplifier. This is done by adjusting the volume control. When the preamp volume is all the way down, the output signal is attenuated completely to zero volts and no sound. As the volume knob is turned up, the voltage increases as does the sound generated from the amplifier. Assuming the output voltage of the source is more than the input sensitivity of the amplifier, the ideal working range on a volume control should be where the preamp would not add any voltage gain to its input signal beyond the amplifier’s input sensitivity. When do we need gain in a preamp? When the input sensitivity of the amplifier is above the output voltage of the source. It is possible for some amplifiers to need up to 5 volts to bring them to full power. On the other hand, it is possible for some to only need 1/2 volt to come to full power. It is also possible for some modified CD players or Digital/Analog Converters to have less then 2 volts. As an example, assume an amplifier with an input sensitivity of 2 volts is connected to a source with an output voltage of 1 volts. We will definitely need a preamp with some gain. Otherwise, it will not be possible to play the amplifier as loud as it it is capable of. Even if the speakers are very efficient, the music will lack dynamics and weight at low listening levels. Turntables have very low output sensitivity. That is the main reason why you need a built-in or external phono stage with turntables, Example: Let’s say we have an 8-ohm loudspeaker that has a sensitivity of 87dB for one watt at one meter. Assume we are trying to achieve a desired maximum peak sound pressure level of 105dB. If the signal source has an output voltage of 1 volt, how much amplifier gain is required? Remember that in order to increase the level of sound by 3dB, we need to double the amplifier’s power. SPL, Watts 87dB, 1 90dB, 2 93dB, 4 96dB, 8 99dB, 16 102dB, 32 105dB, 64 Therefore, we need 64 watts to achieve a peak SPL of 105dB. From Ohm’s law, Power = V^2 / R 64 = V^2 / 8, and V = 23 volts So the amplifier needs to provide a voltage gain of 23 times which is approximately 27dBv. Please note that 0 dBv is defined as Vo = 1 volt, and dBv = 20 . LOG(V/Vo) = 20 x LOG(23/1) = 27 dBv. Some amplifiers have knobs that control the amplifier’s input sensitivity. When the knob is turned clockwise, the sensitivity will increase. Turning the control counter-clockwise will decrease sensitivity. This control is not a volume control for the amplifier. The amplifier can be driven to full power with a wide range of signal levels. A low level signal will require increased sensitivity for full power. A high level signal will require decreased sensitivity. The other issue that we need to consider is output impedance of preamps and input impedance of amplifiers. Output Impedance: All sources and preamps have output impedance. Output impedance is defined as the ability of a unit to drive difficult loads. Input Impedance: Amplifiers, preamps and receivers have input impedance. Think of it as the difficulty the amplifier imposes on the preamp and the preamp (receiver) imposes on the source. Generally speaking, we can make the following conclusions: The lower the output impedance of the preamp, the better it will drive difficult loads. The higher the input impedance of the amplifier, the less difficult it will for the preamp to drive it. For example, a preamp with an output impedance of 1,000 ohms can easily drive an amplifier with an input impedance of 100,000 ohms. Most stock (unmodified) CD players have a relatively low output impedance. Unfortunately, amplifiers' input impedances vary quite a bit. Although the most common number is 50,000 ohms, they can vary between 10,000 ohms and 500,000 ohms. Although 50,000 ohm is a load that most sources and preamps can drive easily, you may run into difficulty if you use extra long interconnects to connect a preamp with very high output impedance to a 50,000 ohm amplifier. The result may be either reduction in bass response, or lack of dynamics, or both. This is sometimes described as thin sound. The solution is to reduce the length of the interconnects and/or use a preamp with low output impedance. The Slew Rate: The slew rate in electronics is the maximum rate at which an electronic amplifier can respond to a sudden change in input level. The term is used to define the maximum rate of change of an amplifier's output voltage with respect to its input voltage. The unit of measure is volts per microsecond. A slew rate of 1V/µs, for example, means that within 1 microsecond, the amplifier can go from 0 to 1 volt. The slew rate in amplifiers is similar to acceleration in cars. This is important because it defines the maximum speed at which an amplifier can handle a transient waveform. The higher the slew rate, the faster is the amplifier. Measurement The slew rate can be measured using a function generator (usually square wave) and oscilloscope. Slew rate is measured by feeding an input signal that is too fast for the amplifier to cope with. It is the time an amplifier needs to go from 10% to 90% of the total output voltage in response to a step in voltage at the input. Slew rate is an overload condition, and it should not happen at all for an audio amplifier. Therefore, being concerned about the slew rate is meaningless. This specification limits the capability of an amplifier to generate high voltage pulses with sharp rising and falling edges. This is exhibited in Figures 2, 3, and 4. Source:Falco Systems The higher the frequency, the faster the voltage has to rise to prevent distortion of the sine wave. If the amplifier cannot follow due to its limited slew rate, the sine wave will be distorted and its amplitude is lower than at low frequencies. Limitations in slew rate can result to non linear effects. For a sinusoidal waveform not to be subject to slew rate limitation, the slew rate capability at all points in an amplifier must satisfy the following condition: SR >= 2Pi x f x Vpk where f is the frequency and Vpk is the peak amplitude of the waveform. Continued in the next post. CONCLUSIONS When buying an amplifier/receiver, it is important to look at the following factors: Distortion: Total Harmonic Distortion (THD) and Intermodulation Distortion (IMD), lower numbers are better. Signal-To-Noise (S/N) Ratio: The higher number is better. Continuous Power: Just because the receiver/amplifier may be listed as being able to output 100WPC, doesn't mean it can do so for any significant length of time. Always make sure the watt per channel is in continuous RMS terms. Even RMS power is technically incorrect. RMS voltage is normally measured. Dynamic Headroom: The ability of the receiver/amplifier to output power at a significantly higher level for short period of time to accommodate musical peaks or extreme sound effects in movies. Dynamic Headroom is measured in Decibels. If a receiver/amplifier has the ability to double its power output for a brief period, it would have a Dynamic Headroom of 3db. Impedance Rating: Almost all receivers can handle 8 ohm speakers. Some can handle 6 ohm speakers. Almost none of them can handle 4 ohm speakers. Using a receiver on a low impedance speaker will result in overheating and possible damage to the receiver and/or the speaker. Other Options: You should look at the other options that the receiver offers, such as the number of HDMI input/outputs, the kind of video processing, DSP modes, etc. REFERENCES AND ADDITIONAL INFORMATION Home Theater: To Separate or Not to Separate? audioport.com Power Amplifier Fundamental Power Amplifier Power Specs Demystified http://www.harmanaudio.com/all_about...werratings.asp http://www.usedphones.com/Amplifiers.html http://www.hometheatersound.com/feat...c_20010901.htm http://sound.westhost.com/bi-amp.htm http://sound.westhost.com/tweeters.htm http://www.audioholics.com/education...r-voltage-gain Amplifier Power Ratings Distortion - Wikipedia, the free encyclopedia Total harmonic distortion - Wikipedia, the free encyclopedia THD Measurement and Conversion What is Total Harmonic Distortion? - THD - Definition and Explanation of THD What is Intermodulation Distortion? - IMD - Definition and Explanation of IMD How Much Amplifier Power Do You Really Need? - Amplifier Power - What You Need To Know About Amplifiers and Amplification Total Harmonic Distortion PS Audio - High-Performance Home Audio Equipment http://www.analog.com/en/content/0,2..._91250,00.html Audio Specifications 3 ways of expressing voltage of a common AC wave form Measurements of AC magnitude - Basic AC Theory What is Alternating Current? RMS Calculator http://www.eznec.com/Amateur/RMS_Power.pdf Sound & Communications - Audio http://www.1388.com/articles/tech_underAmp Deutsches Institut für Normung : Startseite DE Amp Input Sensitivity and Gain Setting Sound System Level Controls Pro Audio Reference D DECWARE - Audio Paper - How to choose the right preamp http://en.wikipedia.org/wiki/Slew_rate http://www.falco-systems.com/high_vo...mplifiers.html http://www.sweetwater.com/shop/live-...ying-guide.php http://www.psaudio.com/ps/wiki/Slew-Rate/ Last edited by Big Daddy; 05-07-2013 at 06:26 AM.

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