Amplifier FAQ
* How much power do I need?
Perhaps the most common question asked about amps.
The question can't be simply answered, except to say...it depends.
There are three important factors which need to be known before we can answer the question -
* Desired Sound Pressure Level (SPL); AKA how loud you want to play
* Speaker sensitivity; How efficient are your speakers at converting electrical power to accoustical power?
* Listening distance; How far away are your speakers
I would say that you can get pretty loud with little power. Then, as you try to get louder, power needs will rapdily increase. You may have heard that you need to double power for each 3 dB increase in SPL. If you have not heard this, it's a good fact to memorize. How much louder is 3 dB more? Some sources say things like "just" noticable. I think that's a fair explantion. You will notice a 3 dB increase, but it won't be dramatic.
To DOUBLE perceived loudness you will probably want a 10 dB increase in SPL. Note that this is perceived loudness. You may see 6 dB mentioned, which is talking about doubling sound pressure, and not about perceved loudness. An increase in 10 dB SPL requires about 10 times the power. In other words you need TEN TIMES the power for TWICE the perceived loudness. To reiterate, power needs will increase very rapidly.
The upside of power is that it requires very little power to play loudly. "loudly" is relative, but let's assume for argument's sake, I am not too far off. Only ONE Watt of power will produce 90 dB of SPL at one meter from a speaker with a sensitivity of 90 dB. Many speakers can't manage 90 dB, but most would be within 3 dB. Two meters away, that SPL number will be down 6 dB, or 84 dB. Is 84 dB loud? I would say yes. You get the idea, I hope. Very little power is needed for "loud."
Speaker sensitivity, as mentioned above can be quite helpful in being able to play your system loudly. Some speakers can be quite low. Smaller speakers, such as bookshelf speakers, for example will often have a sensitvity below 90 dB. If you had a speaker with an 87 dB sensitivity, you would need twice the power to play at the same SPL as one at 90 dB. Some speakers have a sensitivity of over 100 dB, and can play very loudly with very little power. They are likely expensive, but there's a tradeoff in that you can use a lower powered amplifier with them.
Another thing to know about speaker sensitivity is that it might be given as "in room" or "anechoic". In room would be in a "typical" room. Anechoic would indicate the measurement was taken in an anechoic chamber (which eliminates reflected sound.) If the specs don't tell you which of these two scenarios was used, I would assume in room, as that makes the spec look better.
For each doubling of distance away from a speaker, SPL will drop by 6 dB DPL. At 2 meters, SPL will be down 6 dB (from what it was at one meter). At 4 meters, 12 dB.
There are handy online calculators which can be help give you an idea (e.g. http://www.crownaudio.com/apps_htm/d...ct-pwr-req.htm
Your SPL will be higher with seven channels playing at the same time, than it would would say, two channels. I compared one channel playing to seven channels once, and got an increase of 3 dB SPL. I can't say if this is typical though.
You may ask why you need 100 or more Watts if you can play loudly with one Watt of power. I would say it's because sound is very dynamic, especially movies. If a movie's sound follows 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. You may not need this much peak power for every channel at the same time. It's (perhaps obviously,) peak levels in movies which will place the limit on how loud you can play a movie. If it's distorting/sounds harsh, you should turn it down, and consider keeping it down, or buying more power
A number people like to throw around is THX reference level. This means your system can play EACH (excepting the subwoofer) channel at 105 dB. The subwoofer would have to play at 115 dB, and that's just for LFE - it does not include any bass signals crossed over from other speakers via bass management. I personally don't think this is an important goal unless you actually want to listen to movies this loud. If you can live with less SPL, you can likely save money.
I think that about covers the basics. In summary -
* Your power needs will vary a lot depending on listening tastes, your speakers, and how your speakers are positioned in your room
* You don't need a lot of power to play loudly
* You need a lot of power if you really have to be able to play reference level without distortion
* How can I assure the best sound for my budget?
There are two answers to this.
The simple answer is buy as much power as you think you will need. That's it. If you can't afford the power you want, then you will not be able to play as loudly. The assumption here is this - if you have enough power to avoid distortion from amplifier clipping (overdriving the amplifer) than you are good to go.
The other answer is that amplifiers can sound different, and you will have to take this into account. Without spending too much time on this perpetual debate, quite a large number of blind listening tests have shown that amplifiers sound very similar. To take one extreme case, the most budget of Pioneer receivers was once involved in a listening test with much more expensive amplifiers. No one could do better than chance in identifiying the Pioneer (statistically speaking). If someone tells you they heard a dramatic or a night and day difference by switching amplifiers, you may want to consider the posibility they are overstating the situation.
ALL amplifiers distort. None is perfect.
Distortion could be grouped into three general categories - noise, harmonic distortion, and non linear frequency response (these are my own broad definitions rather than a strictly academic set of defintions.)
Noise is an inescapable fact of the physics of electronics. A well designed amp can only minimize noise. The good news is that noise can be very low when compared to the signal. A specification called Signal to Noise Ratio (SNR) is often given for amps. Say an amp has a SNR of 80 dB. That means the signal is 80 dB higher than the noise (100,000,000 times higher.) This is pretty good if you consider that an average room might have an ambient sound level somewhere in the range of 10 dB to 20 dB SPL. 80 dB above 10 dB is 90 dB, which I would call loud. And 80 dB is not a particularly good SNR for an amp. With highly sensitive speakers, you may want an amp with better than average SNR.
Harmonic distortion means that if you feed the amp a sine wave signal, for example at 1 khz, unwanted waveforms at other frequencies will appear on the output. Harmonic distortion is measured as THD (Total Harmonic Distortion.) This measures what percentage of the amplified signal is made up of harmonic distortion. Most amps will have a very low THD spec. THD is not absolute, it varies depending on power output. An amp with what looks like a high THD (> .1% THD) will almost certainly have a lower THD at lower power output. THD levels on good amps are well below the point of audibility, which is 1% THD according to some studies. 1% is not an absolute though. Humans are more sensitive if the harmonic distortion is odd order and/or higher order. Higher order just means higher multiples of the base frequency. We notice distortion more if the harmonics are higher multiples of the base frequency. A 7 khz harmonic distortion component from a 1 khz test signal would be odd order and would probably considered to be high order.
The last type of distortion mentioned is frequency response non linearity. In other words, the amplifier does not respond identically to all frequencies. Ideally, if we want a perfect amplifier all frequencies would be treated equally. This is not the case in reality. Most amplifiers have excellent response linearity over the audio frequency range of 20 hz to 20 khz.
One form of disortion I did not mention above would be an amplifier interaction with the outside world. Amplifiers usually have a low output impedance. This minimizes potential interaction with speaker cables and loudspeakers. It's theoretically possible for a poorly designed amp to interact with cables and loudspeakers such that it's fidelity is adversly affected. There's numerous articles available online discussing the topic of damping factor - every one I have seen discounts this as a realistically worrisome scenario.
Now that I have mentioned these various types of distortion, I think it's safe to say a well designed amplifier will reduce these effects to the point of inaudibility. I won't discuss why so many people hear differences in amplifiers if this is the case. No one seems to have the definitive answer on this. If you hang around AV forums for awhile, you will see various theories on this topic.
In spite of some common beliefs, I think it's safe to say most amplifiers are designed to amplify sound, and not to introduce sonic "coloration." In other words, amplifiers are designed to sound the same as every other amplifier - they are not designed to sound like anything - they are designed in produce a signal identical to their input, but at a much higher amplitude. Due to perfection being impossible, they will not of course achieve this goal perfectly.
Some people will spend a lot of time and money finding that perfect amp. I can't think of any guidance here. Make sure you can return the amplifier if you don't like it, with small or no restocking fees, and try to minimize the cost of shipping where possible. Reviews may be helpful, but I have my doubts on the reliability of reviews. The methods usually used for reviewing are highly subjective relying heavily on the audio memory of the reviewer.
* Does too little power damage speakers?
You may see statements like this in various sources. "It's not too much power that damages speakers. Too little power damages speakers."
I feel this sort of statement deserves clarification.
In one sense, it's 100% wrong. Too much power damages speakers. This is not debatable.
There's two ways to damage a speaker - exceeding it's mechanical limits, or exceeding it's thermal limits. Both situations require "too much power." If you are pushing so much power to a speaker that it's woofer is experiencing overcursion, that's too much power ( it's not a pretty sound.) If you push sufficient power to a speaker for long enough, you can damage it's drivers. Voice coils in a speaker's drivers will heat up, and if they heat up too much, they will fail (this seems more common with tweeters than woofers.)
* Why do people say that too little power damages speakers?
If you play your system too loud, the amplifier will clip. Clipping means that the amplifier is trying to amplify the signal, but hits a limit. That limit is it's supply voltage. 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 amplifer from the abuse you are trying to inflict on it.
When an amplifier clips the result is that average power increases more quickly than it would if the amplifier is not clipping. This topic is non trivial, and Rodd Eliott explains it better than I ever could so if you want the gory details, read this (http://sound.westhost.com/clipping.htm)
This increase in average power is the most likely reason for speaker damage.
Modern music is highly compressed which makes the situation worse. Average signal level is so high in compressed music, that the speaker drivers don't have as much time to cool down.
If you have sufficent amplifier power to avoid clipping, your chance of doing damage to your speakers will be lower.
Can you buy an amp with too much power? This seems possible. While many sources of online info say you can't have too much power, that's clearly an oversimplification. If you had a 1000 watt amp, and you connected one of those cheap portable 8 ohm speakers to it, which is designed for less than a watt of power, I am 100% sure you could damage it, and with no clipping present. This does seems like an unlikely scenario with normal speakers and with normal music/movie sources though.
* What specs are important?
Not many, except for power. And power is measured in different ways, so direct comparisons should be taken lightly.
Hopefully, power will be measured from 20hz to 20khz into 8 ohms (ideally also into 4 ohms,) at some low THD figure, such as .01 % THD. And hopefully this is a continuous power measurement as dictated by the FTC rules (in the US.) A multi-channel (more than two) amplifier may include an all channels driven measurement. If not, it will almost certainly put out less power into all channels. Some people have criticized the all channels driven measurement. The main reason is that when playing movies, the most common use for mult-channel amplifiers, it's said that max power will never be demanded from all channels at the same time. Personally, I think it's nice to know the ACD spec, but realize that in some cases, it might not be continuous (it may have been measured with a short duration "sweep".)
THD is considered important, but consider that any good amp designed for high fidelity will have very low THD. THD is not an absolute for a given amplifier. It will vary with power, and manufacturers may squeeze out a bit more power by measuring at a higher THD than another manufacturer. A commonly stated theory is that THD of below 1 % THD is inaudible. This is also not an absolute as human sensitivity to harmonic distortion varies depending on the type of disortion. For example, if you consider a musical chord, and consider the 2nd and 3rd notes as disortion, they are actually pleasing to listen to. But if you try to play an off note, it will be perceived of as discordant. On paper, it looks like the harmonic distortion for most amps should be inaudible.
The SNR (noise) spec could be important in some cases. It's measured as Signal to Noise ratio. It will usually be very high, at least in amps I have seen. If you consider that noise in a quite room may be no lower than 20 dB, and breathing is 10 dB, a SNR of more than 80 dB seems pretty good. If you have very high efficiency speakers, you might want to pay some attention to the SNR.
There are various other measurements, none of which are of real use. Slew rate does not tell you much, unless it was abnormally low (in which case, they are not going to list it Damping factor has been critcized as being pointless in a number of articles by knowledgeable people. Occasionally you will see some max current output measurement. Not sure what that tells us with no other info. For all I know, they shorted the all the amp's output terminals and measured the peak output amperage before it shut down or blew up.
* Do I need to match an amplifier's impedance (such as 8 ohms or 4 ohms) to my speakers?
A solid state amplifier does not have an impedance as such (tube amps are a bit different in this respect.)
Almost any audio amplifier will work with 8 ohm speakers (with some exceptions such as fixed voltage amps.) As you lower the impedance of speakers you connect to a amp, you place more demand on it. Lower impedance means more current will flow through it's output devices (usually transistors). More current means more heat.
4 ohm loads present a more difficult load for amplifiers. The lower impedance means electricity will flow more easily. That means the current flow will be higher. Higher current means more heat will develop in the amplifier, especially in the output devices. Amps typically have themal protection circuits. If you your amp is shutting down due to thermal overload, you are driving it too hard, not giving it enough ventilation, or the amp is not working properly.
Amplifiers sold for pro audio applications, and for home audio would usually be able to handle 4 ohm loads with no issues. Receivers may shut down under heavy load. Or perhaps their longevity could be reduced, because they were not really designed for 4 ohm loads. Lower impedance is not suggested, such as connecting two 4 ohm speakers in parallel.
* How do I connect an amplifier?
For connecting an amplifier to a source such as a pre processor, there are four standard connectons -
* Unbalanced RCA jacks
* Unbalanced 1/4" jack (aka phono jack)
* Balanced 1/4" (aka TRS)
* Balanced XLR
An unbalanced connection has two wires. One is a ground, the other carries the audio signal.
A balanced connection needs three wires. One carries the audio signal, another an inverted "copy" of the audio signal, and the third is a ground.
The cheapest method is probably RCA. This should work fine when the cable length will be short, and the device being connected lacks balanced outputs.
If an amplifier lacks RCA inputs, and your device has only RCA output, such as an Audio/Video Receiver, use a cable with RCA jacks on one side, and 1/4" cables on the other side. This will be an unbalanced connection.
If the device being connected to the amp has balanced outputs, it makes sense to use them. For home audio, this is often an XLR connector requiring use of an XLR cable. You may also have the option for a 1/4" connection. For balanced 1/4" connections, you must use a cable with TRS jacks on both sides. These will have two insulators on the jack, giving it 3 contact points. Hence the acronym TRS for Tip, Ring, Sleeve. This is also known as a stereo 1/4" jack (as opposed to a mono 1/4" jack with only two contacts.)
* What do input sensitivity and gain mean?
They measure the same thing.
Input sensitvity is simple to understand. If the input sensitivity is 1 volt, the amp will be able to hit full power with an input of 1 volt. A higher input sensitivty means you need more voltage to drive the amp to full power. Many receivers have a pre amp rated for 1 volt RMS, which means they may not be able to drive all amps to full power.
Gain is measuring the same thing as input sensitvity in a different way. I will give a handy formula -
Gain (as a multiple) = 10 ^ (gain / 20)
Divide gain (dB) by 20, and use inverse logarithm (base 10) on your calculator
For 32 dB, we would get a gain of 40 times.
To convert this to input sensitivity, we would use this formula -
Input sensitvity = sqrt(max_amp_power * 8) / gain_multiple
Assuming a 200 watt / channel amp into 8 ohms with a 32 dB sensitivity -
sqrt (200 watts * 8 ohms ) / 40 = 1 volt
So a 32 dB gain would mean a 1 volt input sensitvity. Higher gain means an amp will match better with processors, preamps and receivers with lower pre amp output.
* How do I bridge an amplifier
Some amplifiers can be bridged. Don't try to bridge an amplifier not designed to be bridged. See the manual for detailed instructions.
Bridging is pretty simple. Two of the amp's outputs are used for one speaker. This will double voltage, and quadruple the power. This requires that one signal uis inverted.
In a typical bridgeable amplifier you need to set a switch for bridge mode. You will then carefully note the wiring diagram in the manual and/or on the back of the amp, and connect your speaker to the indicated outputs on the amp.
* How important is an all channels driven power measurement
This is discussed better than I can manage by other sources, such as Audioholics.com. I wanted to briefly cover it, and express my own thoughts.
For receivers, I would prefer every receiver disclose a power rating with all channels driven. Ideally, it would be done in a consistent way. I feel this would be more honest. You look at $400 receivers stating they can do 100x7 Watts into 8 ohms. Or even worse, they claim 700 Watts of power. While the math works out the same, you could at least claim, for 100x7 that was a max per channel. If the mfg. says 700 Watts total, that's lying, in my opinion.
Your typical receiver using class AB amps can be analyzed to some extent by two numbers. It's weight, and it's power consumption. A 17 pound receiver with a power consumption of 500 Watts cannot manage 700 Watts of power output. It would likely need to draw 1400 Watts of power from the wall to do that. While 500 Watts power consumption may not be it's max power consumption, and while it may be able to output more power on a short term basis because it can store power in it's filter capacitors, it can't do 700 Watts of output power.
Power amps often have better power supplies than receivers. They are often heavier, which is often a good indicator of power capability, because power for class AB amps is all about having a large power transformer. As such, an all channels driven measurement is maybe not as interesting as it would be with receivers. They may have large filter cap banks which cope better with high peak demand for short periods of time.
It's said that movies won't put peak demand on all channels at the same time. Audio signals are very dynamic. At any point in time, the amplifier will only need to deal with the demand at that point in time, which is dependent on the voltage level of every active signal going through the amp. The power supply filter capacitors used in your typical amp will help with high demands for short period of time. It's argued that the end result is that you can get by with less power than you would think. So if you thought you needed 200x7 watts continuous power handling to play your system at THX reference level, you would be overestimating your power needs. For this reason, a number of AV writers discount the need for an all channels driven rating. On the other hand, the current FTC rule would allow the amp maker to rate their amp with only two channels driven. They could save money by building what looks like a 200x7 watt amp in their specs, but is really a 50x7 watt amp with all channels driven. I think it would be helpful to know the ACD rating in this case.
There's also the amusing situation of amplifiers being rated for more power than could be pulled from the wall. Take a 400x7 Watt amplifier. That's 2800 Watts. Even a 20 amp circuit is not going to manage to supply it with enough power to meet it's specification. There's little point to an all channels driven measurement of this sort. The amp is likely limited by wall power. They may as well just list the specification as 'Puts out as much power as you wall outlet can supply minus losses.' Maybe that's overstating the situation, but not by much I suspect.
I personally think all information is interesting when looking at an amplifier. But I can accept that the all channels driven rating of an amplifier is not of more importance than other factors such as budget, reliablity and such.
* What's the difference between solid state and tube amps
Perhaps the number one reason people state for buying a tube amp is that they say tubes have a warmer sound.
Tubes are less efficient, and potentially more fragile than solid state amps. To get a tube amp with the same power as a solid state amp, you would usually pay more more. High power tube amps are uncommon. Very few people would choose tube amps in a set up designed mainly for surround sound use, as the expense would be high, and they would take up more room.
Tube amplifiers may not measure as well as solid state amplifiers, Randy Slone mentions this in his book on high power amplifiers. There's a class of tube amps known as SET (single ended triode) amplfiiers. In bench tests I have seen for this sort of amplifier, they have measured very badly when compared to solid state amplifiers.
From an audio fidelity perspective, I could find no evidence that tube amplifiers are more accurate than solid state amplifiers. A number of sources said that tube amps can be more linear and thus would need less negative feedback. This would be an advantage if negative feedback was a bad thing. A number of audio designers have specifically addressed criticisms of negative feedback, and have rejected the criticisms. There are audio designers who seem to dislike negative feedback however.
Is there a reason the sound of tube amps is preferred to solid state amplifiers by some people?
One often stated reason is that tube amplifiers sound better due to second order hamonic distortion. Rather than being discordant, second order harmonic distortion is said to sound pleasant. So even though it's distortion, it's pleasant sounding distortion.
Vaccum tube amps are said to sound better then they are overdriven than solid state amplifiers. This would help explain their popularity as guitar amplifiers.
For more information, I suggest doing a search online for 'tube sound.' You should be able to read about studies that have been done which better explain the technical details on how solid state amplifiers and tube amplifiers can sound different.
* What are amplifier classes?
This is covered in numerous places online in better detail. And in general, it's not that important. If an amplifier is lighter than other amplifiers of the same power output, it is helpful to know what design it uses. If the lighter amp is more efficient, such as class D, the difference in weight can be understood.
Class A - Theoretically the amp with the least distortion. The downside is that it will be more expensive, heavier, run hotter, and be less efficient than a class AB amp. Two writers of books on amplifier design, Self and Slone downplay the real world advantage of this design. Put simply, the design is inefficient, because the transistors are "fed" a bias current such that they are always on when the amp is turned on. This means that with no signal, the transistors still consume a lot of power, which is wasted power, and dissapated as heat.
Class B or AB - Used in receivers, and most other power amps. The downside of class B or AB amps is crossover distortion. There are two sets of output devices (e.g. transistors.) If the audio signal is positive, one set is used for amplification, and if negative the other set is used. Both sets of output devices are not always on, making this design more efficient than class A. When the audio signal passes through 0 volts from positive to negative, or vice-versa a "glitch" occurs in the point of crossover. There are ways of dealing with this issue, which is good news as this is the most common design in amplifiers. The only difference between pure class B and class AB is that the class AB biases the output devices such that they conduct for more of the cycle. This reduces efficiency, but reduces the affects of crossover distortion.
Class D - An more efficient when compared to other all other designs. The design is interesting. The incoming signal is converted to a square wave, where the duration of the pulses in the signal carry the audio signal. This signal is then used to switch on and off the power transistors in the output stage leading to a very high efficiency. This is called switch mode, and class D amplifiers are sometimes called switch mode. Another name for class D is PWM because the audio signal is converted to a Pulse Width Modulated signal. Along with the efficiency comes a decrease in weight. These amps are very common in powered subwoofers and the pro sound amp market. They seem to be slowly becoming more common. There's one line of Pioneer receivers now using them. Time will tell if they overcome the popularity of class AB. Note that class D does not mean digital. The term digital is often applied when the class D amplifier's input is a PCM signal rather than an analog signal (the amp can directly convert the PCM signal to the PWM signal used to drive the output devices.)
Class G/H - Amplifiers which improve efficiency by using more than one supply voltage. They otherwise work like class B amplifiers. If an amplifier's power supply voltage was exactly what was needed at any instant in time, it's more efficient. Class G and H differ in how this is done, neither achieves the goal of suppling the exact voltage needed. A class H amplifier has an infinitely variable power supply voltage. A class G amplifier has two or more supply voltages.
Switching power supply amps - This is not an amplifier class. But an efficiency can be gained by using switching rather than linear power supplies. Transistors in circuits are said to operated in linear mode or switch mode. In switch mode, they are turned on or off, there's no intermediate state. In linear mode, they are fed varying amounts of current. Switching power supplies are used in computers. Switch mode power supplies are seeing more use in pro audio amplifiers. They are uncommon in home audio at this point in time. Your typical home audio amplifier uses an unregulated power supply (see the section on "How do amplifiers work")
* How do amplifiers work?
A brief explanation of how amplifiers work might be helpful.
This section will explain the most common amplifier design - a three stage class B amplifier using an unregulated power supply.
The amplifier's "heart" is the power supply. The power supply transformer puts a limit on how much power the amplifier can produce. Bigger is better in this case, and very high power amplifiers of the type we are discussing here can weigh over 100 pounds.
A typical unregulated power supply is really simple. The incoming AC voltage (120 Volts in the US,) would typically be lowered to a smaller voltage. Higher amps would need a higher voltage. A bridge rectifier then converts this smaller voltage into a postive and negative DC voltage. An audio signal will alternate between positive to negative (it's alternating current.) Class B (or AB) amps need two voltage "rails" as they have two "sides" one of which amplifies the positive part of the signal, and the other the negative part.
The typical three stage amplifier has the following stages -
* Input stage; converts the alternating voltage input to an alternating current signal
* Voltage amplifier stage; the signal will be increased to a much higher voltage
* Output stage; This could be considered to be a current amplifying stage - or to put it another way perhaps, it buffers the voltage amplifier stage from the speakers, and allows the amplifier to provide the potentially large current needed by the speakers
The goal of the input stage is to convert the input signal where a changing voltage is converted to a changing current while introducing little additional noise and distortion. It also needs to cope with ripple from the power supply, which is not providing a perfectly constant DC signal. The input stage also takes feedback from the output to lineraize (reduce distortion.) There are quite a few sources of distortion in an amplifier, and feedback is used to reduce them.
The voltage amplifier stage (VAS) is doing the real amplifying of the input signal. The input signal might be one or two volts at maximum, and the output from the VAS will be tens of Volts. For example in my receiver, it's max voltage is going to be close to 70 Volts.
The output stage does the heavy lifting. The output devices, commonly bipolar junction transistors (MOSFETs are also used,) are connected directly to your speakers. The current flowing through the transistors can be very high. Consider a 100 Watt amp connected to a 4 ohm speaker. Voltage would peak around 24 Volts. Current would be be pushing 6 Amperes peak. If you don't think 100 Watts is a lot of power, consider how you almost certainly have no desire to touch a hot 100 Watt light bulb. Don't do it, you will get a painful burn.
An amplifier is not that complex a device in some ways, and there has been little change in it's basic design for a long time (I believe the basic 3 stage amp design has been around since an RCA engineer developed it in the 1950s)
* How much power do I need?
Perhaps the most common question asked about amps.
The question can't be simply answered, except to say...it depends.
There are three important factors which need to be known before we can answer the question -
* Desired Sound Pressure Level (SPL); AKA how loud you want to play
* Speaker sensitivity; How efficient are your speakers at converting electrical power to accoustical power?
* Listening distance; How far away are your speakers
I would say that you can get pretty loud with little power. Then, as you try to get louder, power needs will rapdily increase. You may have heard that you need to double power for each 3 dB increase in SPL. If you have not heard this, it's a good fact to memorize. How much louder is 3 dB more? Some sources say things like "just" noticable. I think that's a fair explantion. You will notice a 3 dB increase, but it won't be dramatic.
To DOUBLE perceived loudness you will probably want a 10 dB increase in SPL. Note that this is perceived loudness. You may see 6 dB mentioned, which is talking about doubling sound pressure, and not about perceved loudness. An increase in 10 dB SPL requires about 10 times the power. In other words you need TEN TIMES the power for TWICE the perceived loudness. To reiterate, power needs will increase very rapidly.
The upside of power is that it requires very little power to play loudly. "loudly" is relative, but let's assume for argument's sake, I am not too far off. Only ONE Watt of power will produce 90 dB of SPL at one meter from a speaker with a sensitivity of 90 dB. Many speakers can't manage 90 dB, but most would be within 3 dB. Two meters away, that SPL number will be down 6 dB, or 84 dB. Is 84 dB loud? I would say yes. You get the idea, I hope. Very little power is needed for "loud."
Speaker sensitivity, as mentioned above can be quite helpful in being able to play your system loudly. Some speakers can be quite low. Smaller speakers, such as bookshelf speakers, for example will often have a sensitvity below 90 dB. If you had a speaker with an 87 dB sensitivity, you would need twice the power to play at the same SPL as one at 90 dB. Some speakers have a sensitivity of over 100 dB, and can play very loudly with very little power. They are likely expensive, but there's a tradeoff in that you can use a lower powered amplifier with them.
Another thing to know about speaker sensitivity is that it might be given as "in room" or "anechoic". In room would be in a "typical" room. Anechoic would indicate the measurement was taken in an anechoic chamber (which eliminates reflected sound.) If the specs don't tell you which of these two scenarios was used, I would assume in room, as that makes the spec look better.
For each doubling of distance away from a speaker, SPL will drop by 6 dB DPL. At 2 meters, SPL will be down 6 dB (from what it was at one meter). At 4 meters, 12 dB.
There are handy online calculators which can be help give you an idea (e.g. http://www.crownaudio.com/apps_htm/d...ct-pwr-req.htm
Your SPL will be higher with seven channels playing at the same time, than it would would say, two channels. I compared one channel playing to seven channels once, and got an increase of 3 dB SPL. I can't say if this is typical though.
You may ask why you need 100 or more Watts if you can play loudly with one Watt of power. I would say it's because sound is very dynamic, especially movies. If a movie's sound follows 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. You may not need this much peak power for every channel at the same time. It's (perhaps obviously,) peak levels in movies which will place the limit on how loud you can play a movie. If it's distorting/sounds harsh, you should turn it down, and consider keeping it down, or buying more power
A number people like to throw around is THX reference level. This means your system can play EACH (excepting the subwoofer) channel at 105 dB. The subwoofer would have to play at 115 dB, and that's just for LFE - it does not include any bass signals crossed over from other speakers via bass management. I personally don't think this is an important goal unless you actually want to listen to movies this loud. If you can live with less SPL, you can likely save money.
I think that about covers the basics. In summary -
* Your power needs will vary a lot depending on listening tastes, your speakers, and how your speakers are positioned in your room
* You don't need a lot of power to play loudly
* You need a lot of power if you really have to be able to play reference level without distortion
* How can I assure the best sound for my budget?
There are two answers to this.
The simple answer is buy as much power as you think you will need. That's it. If you can't afford the power you want, then you will not be able to play as loudly. The assumption here is this - if you have enough power to avoid distortion from amplifier clipping (overdriving the amplifer) than you are good to go.
The other answer is that amplifiers can sound different, and you will have to take this into account. Without spending too much time on this perpetual debate, quite a large number of blind listening tests have shown that amplifiers sound very similar. To take one extreme case, the most budget of Pioneer receivers was once involved in a listening test with much more expensive amplifiers. No one could do better than chance in identifiying the Pioneer (statistically speaking). If someone tells you they heard a dramatic or a night and day difference by switching amplifiers, you may want to consider the posibility they are overstating the situation.
ALL amplifiers distort. None is perfect.
Distortion could be grouped into three general categories - noise, harmonic distortion, and non linear frequency response (these are my own broad definitions rather than a strictly academic set of defintions.)
Noise is an inescapable fact of the physics of electronics. A well designed amp can only minimize noise. The good news is that noise can be very low when compared to the signal. A specification called Signal to Noise Ratio (SNR) is often given for amps. Say an amp has a SNR of 80 dB. That means the signal is 80 dB higher than the noise (100,000,000 times higher.) This is pretty good if you consider that an average room might have an ambient sound level somewhere in the range of 10 dB to 20 dB SPL. 80 dB above 10 dB is 90 dB, which I would call loud. And 80 dB is not a particularly good SNR for an amp. With highly sensitive speakers, you may want an amp with better than average SNR.
Harmonic distortion means that if you feed the amp a sine wave signal, for example at 1 khz, unwanted waveforms at other frequencies will appear on the output. Harmonic distortion is measured as THD (Total Harmonic Distortion.) This measures what percentage of the amplified signal is made up of harmonic distortion. Most amps will have a very low THD spec. THD is not absolute, it varies depending on power output. An amp with what looks like a high THD (> .1% THD) will almost certainly have a lower THD at lower power output. THD levels on good amps are well below the point of audibility, which is 1% THD according to some studies. 1% is not an absolute though. Humans are more sensitive if the harmonic distortion is odd order and/or higher order. Higher order just means higher multiples of the base frequency. We notice distortion more if the harmonics are higher multiples of the base frequency. A 7 khz harmonic distortion component from a 1 khz test signal would be odd order and would probably considered to be high order.
The last type of distortion mentioned is frequency response non linearity. In other words, the amplifier does not respond identically to all frequencies. Ideally, if we want a perfect amplifier all frequencies would be treated equally. This is not the case in reality. Most amplifiers have excellent response linearity over the audio frequency range of 20 hz to 20 khz.
One form of disortion I did not mention above would be an amplifier interaction with the outside world. Amplifiers usually have a low output impedance. This minimizes potential interaction with speaker cables and loudspeakers. It's theoretically possible for a poorly designed amp to interact with cables and loudspeakers such that it's fidelity is adversly affected. There's numerous articles available online discussing the topic of damping factor - every one I have seen discounts this as a realistically worrisome scenario.
Now that I have mentioned these various types of distortion, I think it's safe to say a well designed amplifier will reduce these effects to the point of inaudibility. I won't discuss why so many people hear differences in amplifiers if this is the case. No one seems to have the definitive answer on this. If you hang around AV forums for awhile, you will see various theories on this topic.
In spite of some common beliefs, I think it's safe to say most amplifiers are designed to amplify sound, and not to introduce sonic "coloration." In other words, amplifiers are designed to sound the same as every other amplifier - they are not designed to sound like anything - they are designed in produce a signal identical to their input, but at a much higher amplitude. Due to perfection being impossible, they will not of course achieve this goal perfectly.
Some people will spend a lot of time and money finding that perfect amp. I can't think of any guidance here. Make sure you can return the amplifier if you don't like it, with small or no restocking fees, and try to minimize the cost of shipping where possible. Reviews may be helpful, but I have my doubts on the reliability of reviews. The methods usually used for reviewing are highly subjective relying heavily on the audio memory of the reviewer.
* Does too little power damage speakers?
You may see statements like this in various sources. "It's not too much power that damages speakers. Too little power damages speakers."
I feel this sort of statement deserves clarification.
In one sense, it's 100% wrong. Too much power damages speakers. This is not debatable.
There's two ways to damage a speaker - exceeding it's mechanical limits, or exceeding it's thermal limits. Both situations require "too much power." If you are pushing so much power to a speaker that it's woofer is experiencing overcursion, that's too much power ( it's not a pretty sound.) If you push sufficient power to a speaker for long enough, you can damage it's drivers. Voice coils in a speaker's drivers will heat up, and if they heat up too much, they will fail (this seems more common with tweeters than woofers.)
* Why do people say that too little power damages speakers?
If you play your system too loud, the amplifier will clip. Clipping means that the amplifier is trying to amplify the signal, but hits a limit. That limit is it's supply voltage. 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 amplifer from the abuse you are trying to inflict on it.
When an amplifier clips the result is that average power increases more quickly than it would if the amplifier is not clipping. This topic is non trivial, and Rodd Eliott explains it better than I ever could so if you want the gory details, read this (http://sound.westhost.com/clipping.htm)
This increase in average power is the most likely reason for speaker damage.
Modern music is highly compressed which makes the situation worse. Average signal level is so high in compressed music, that the speaker drivers don't have as much time to cool down.
If you have sufficent amplifier power to avoid clipping, your chance of doing damage to your speakers will be lower.
Can you buy an amp with too much power? This seems possible. While many sources of online info say you can't have too much power, that's clearly an oversimplification. If you had a 1000 watt amp, and you connected one of those cheap portable 8 ohm speakers to it, which is designed for less than a watt of power, I am 100% sure you could damage it, and with no clipping present. This does seems like an unlikely scenario with normal speakers and with normal music/movie sources though.
* What specs are important?
Not many, except for power. And power is measured in different ways, so direct comparisons should be taken lightly.
Hopefully, power will be measured from 20hz to 20khz into 8 ohms (ideally also into 4 ohms,) at some low THD figure, such as .01 % THD. And hopefully this is a continuous power measurement as dictated by the FTC rules (in the US.) A multi-channel (more than two) amplifier may include an all channels driven measurement. If not, it will almost certainly put out less power into all channels. Some people have criticized the all channels driven measurement. The main reason is that when playing movies, the most common use for mult-channel amplifiers, it's said that max power will never be demanded from all channels at the same time. Personally, I think it's nice to know the ACD spec, but realize that in some cases, it might not be continuous (it may have been measured with a short duration "sweep".)
THD is considered important, but consider that any good amp designed for high fidelity will have very low THD. THD is not an absolute for a given amplifier. It will vary with power, and manufacturers may squeeze out a bit more power by measuring at a higher THD than another manufacturer. A commonly stated theory is that THD of below 1 % THD is inaudible. This is also not an absolute as human sensitivity to harmonic distortion varies depending on the type of disortion. For example, if you consider a musical chord, and consider the 2nd and 3rd notes as disortion, they are actually pleasing to listen to. But if you try to play an off note, it will be perceived of as discordant. On paper, it looks like the harmonic distortion for most amps should be inaudible.
The SNR (noise) spec could be important in some cases. It's measured as Signal to Noise ratio. It will usually be very high, at least in amps I have seen. If you consider that noise in a quite room may be no lower than 20 dB, and breathing is 10 dB, a SNR of more than 80 dB seems pretty good. If you have very high efficiency speakers, you might want to pay some attention to the SNR.
There are various other measurements, none of which are of real use. Slew rate does not tell you much, unless it was abnormally low (in which case, they are not going to list it
Damping factor has been critcized as being pointless in a number of articles by knowledgeable people. Occasionally you will see some max current output measurement. Not sure what that tells us with no other info. For all I know, they shorted the all the amp's output terminals and measured the peak output amperage before it shut down or blew up.* Do I need to match an amplifier's impedance (such as 8 ohms or 4 ohms) to my speakers?
A solid state amplifier does not have an impedance as such (tube amps are a bit different in this respect.)
Almost any audio amplifier will work with 8 ohm speakers (with some exceptions such as fixed voltage amps.) As you lower the impedance of speakers you connect to a amp, you place more demand on it. Lower impedance means more current will flow through it's output devices (usually transistors). More current means more heat.
4 ohm loads present a more difficult load for amplifiers. The lower impedance means electricity will flow more easily. That means the current flow will be higher. Higher current means more heat will develop in the amplifier, especially in the output devices. Amps typically have themal protection circuits. If you your amp is shutting down due to thermal overload, you are driving it too hard, not giving it enough ventilation, or the amp is not working properly.
Amplifiers sold for pro audio applications, and for home audio would usually be able to handle 4 ohm loads with no issues. Receivers may shut down under heavy load. Or perhaps their longevity could be reduced, because they were not really designed for 4 ohm loads. Lower impedance is not suggested, such as connecting two 4 ohm speakers in parallel.
* How do I connect an amplifier?
For connecting an amplifier to a source such as a pre processor, there are four standard connectons -
* Unbalanced RCA jacks
* Unbalanced 1/4" jack (aka phono jack)
* Balanced 1/4" (aka TRS)
* Balanced XLR
An unbalanced connection has two wires. One is a ground, the other carries the audio signal.
A balanced connection needs three wires. One carries the audio signal, another an inverted "copy" of the audio signal, and the third is a ground.
The cheapest method is probably RCA. This should work fine when the cable length will be short, and the device being connected lacks balanced outputs.
If an amplifier lacks RCA inputs, and your device has only RCA output, such as an Audio/Video Receiver, use a cable with RCA jacks on one side, and 1/4" cables on the other side. This will be an unbalanced connection.
If the device being connected to the amp has balanced outputs, it makes sense to use them. For home audio, this is often an XLR connector requiring use of an XLR cable. You may also have the option for a 1/4" connection. For balanced 1/4" connections, you must use a cable with TRS jacks on both sides. These will have two insulators on the jack, giving it 3 contact points. Hence the acronym TRS for Tip, Ring, Sleeve. This is also known as a stereo 1/4" jack (as opposed to a mono 1/4" jack with only two contacts.)
* What do input sensitivity and gain mean?
They measure the same thing.
Input sensitvity is simple to understand. If the input sensitivity is 1 volt, the amp will be able to hit full power with an input of 1 volt. A higher input sensitivty means you need more voltage to drive the amp to full power. Many receivers have a pre amp rated for 1 volt RMS, which means they may not be able to drive all amps to full power.
Gain is measuring the same thing as input sensitvity in a different way. I will give a handy formula -
Gain (as a multiple) = 10 ^ (gain / 20)
Divide gain (dB) by 20, and use inverse logarithm (base 10) on your calculator
For 32 dB, we would get a gain of 40 times.
To convert this to input sensitivity, we would use this formula -
Input sensitvity = sqrt(max_amp_power * 8) / gain_multiple
Assuming a 200 watt / channel amp into 8 ohms with a 32 dB sensitivity -
sqrt (200 watts * 8 ohms ) / 40 = 1 volt
So a 32 dB gain would mean a 1 volt input sensitvity. Higher gain means an amp will match better with processors, preamps and receivers with lower pre amp output.
* How do I bridge an amplifier
Some amplifiers can be bridged. Don't try to bridge an amplifier not designed to be bridged. See the manual for detailed instructions.
Bridging is pretty simple. Two of the amp's outputs are used for one speaker. This will double voltage, and quadruple the power. This requires that one signal uis inverted.
In a typical bridgeable amplifier you need to set a switch for bridge mode. You will then carefully note the wiring diagram in the manual and/or on the back of the amp, and connect your speaker to the indicated outputs on the amp.
* How important is an all channels driven power measurement
This is discussed better than I can manage by other sources, such as Audioholics.com. I wanted to briefly cover it, and express my own thoughts.
For receivers, I would prefer every receiver disclose a power rating with all channels driven. Ideally, it would be done in a consistent way. I feel this would be more honest. You look at $400 receivers stating they can do 100x7 Watts into 8 ohms. Or even worse, they claim 700 Watts of power. While the math works out the same, you could at least claim, for 100x7 that was a max per channel. If the mfg. says 700 Watts total, that's lying, in my opinion.
Your typical receiver using class AB amps can be analyzed to some extent by two numbers. It's weight, and it's power consumption. A 17 pound receiver with a power consumption of 500 Watts cannot manage 700 Watts of power output. It would likely need to draw 1400 Watts of power from the wall to do that. While 500 Watts power consumption may not be it's max power consumption, and while it may be able to output more power on a short term basis because it can store power in it's filter capacitors, it can't do 700 Watts of output power.
Power amps often have better power supplies than receivers. They are often heavier, which is often a good indicator of power capability, because power for class AB amps is all about having a large power transformer. As such, an all channels driven measurement is maybe not as interesting as it would be with receivers. They may have large filter cap banks which cope better with high peak demand for short periods of time.
It's said that movies won't put peak demand on all channels at the same time. Audio signals are very dynamic. At any point in time, the amplifier will only need to deal with the demand at that point in time, which is dependent on the voltage level of every active signal going through the amp. The power supply filter capacitors used in your typical amp will help with high demands for short period of time. It's argued that the end result is that you can get by with less power than you would think. So if you thought you needed 200x7 watts continuous power handling to play your system at THX reference level, you would be overestimating your power needs. For this reason, a number of AV writers discount the need for an all channels driven rating. On the other hand, the current FTC rule would allow the amp maker to rate their amp with only two channels driven. They could save money by building what looks like a 200x7 watt amp in their specs, but is really a 50x7 watt amp with all channels driven. I think it would be helpful to know the ACD rating in this case.
There's also the amusing situation of amplifiers being rated for more power than could be pulled from the wall. Take a 400x7 Watt amplifier. That's 2800 Watts. Even a 20 amp circuit is not going to manage to supply it with enough power to meet it's specification. There's little point to an all channels driven measurement of this sort. The amp is likely limited by wall power. They may as well just list the specification as 'Puts out as much power as you wall outlet can supply minus losses.' Maybe that's overstating the situation, but not by much I suspect.
I personally think all information is interesting when looking at an amplifier. But I can accept that the all channels driven rating of an amplifier is not of more importance than other factors such as budget, reliablity and such.
* What's the difference between solid state and tube amps
Perhaps the number one reason people state for buying a tube amp is that they say tubes have a warmer sound.
Tubes are less efficient, and potentially more fragile than solid state amps. To get a tube amp with the same power as a solid state amp, you would usually pay more more. High power tube amps are uncommon. Very few people would choose tube amps in a set up designed mainly for surround sound use, as the expense would be high, and they would take up more room.
Tube amplifiers may not measure as well as solid state amplifiers, Randy Slone mentions this in his book on high power amplifiers. There's a class of tube amps known as SET (single ended triode) amplfiiers. In bench tests I have seen for this sort of amplifier, they have measured very badly when compared to solid state amplifiers.
From an audio fidelity perspective, I could find no evidence that tube amplifiers are more accurate than solid state amplifiers. A number of sources said that tube amps can be more linear and thus would need less negative feedback. This would be an advantage if negative feedback was a bad thing. A number of audio designers have specifically addressed criticisms of negative feedback, and have rejected the criticisms. There are audio designers who seem to dislike negative feedback however.
Is there a reason the sound of tube amps is preferred to solid state amplifiers by some people?
One often stated reason is that tube amplifiers sound better due to second order hamonic distortion. Rather than being discordant, second order harmonic distortion is said to sound pleasant. So even though it's distortion, it's pleasant sounding distortion.
Vaccum tube amps are said to sound better then they are overdriven than solid state amplifiers. This would help explain their popularity as guitar amplifiers.
For more information, I suggest doing a search online for 'tube sound.' You should be able to read about studies that have been done which better explain the technical details on how solid state amplifiers and tube amplifiers can sound different.
* What are amplifier classes?
This is covered in numerous places online in better detail. And in general, it's not that important. If an amplifier is lighter than other amplifiers of the same power output, it is helpful to know what design it uses. If the lighter amp is more efficient, such as class D, the difference in weight can be understood.
Class A - Theoretically the amp with the least distortion. The downside is that it will be more expensive, heavier, run hotter, and be less efficient than a class AB amp. Two writers of books on amplifier design, Self and Slone downplay the real world advantage of this design. Put simply, the design is inefficient, because the transistors are "fed" a bias current such that they are always on when the amp is turned on. This means that with no signal, the transistors still consume a lot of power, which is wasted power, and dissapated as heat.
Class B or AB - Used in receivers, and most other power amps. The downside of class B or AB amps is crossover distortion. There are two sets of output devices (e.g. transistors.) If the audio signal is positive, one set is used for amplification, and if negative the other set is used. Both sets of output devices are not always on, making this design more efficient than class A. When the audio signal passes through 0 volts from positive to negative, or vice-versa a "glitch" occurs in the point of crossover. There are ways of dealing with this issue, which is good news as this is the most common design in amplifiers. The only difference between pure class B and class AB is that the class AB biases the output devices such that they conduct for more of the cycle. This reduces efficiency, but reduces the affects of crossover distortion.
Class D - An more efficient when compared to other all other designs. The design is interesting. The incoming signal is converted to a square wave, where the duration of the pulses in the signal carry the audio signal. This signal is then used to switch on and off the power transistors in the output stage leading to a very high efficiency. This is called switch mode, and class D amplifiers are sometimes called switch mode. Another name for class D is PWM because the audio signal is converted to a Pulse Width Modulated signal. Along with the efficiency comes a decrease in weight. These amps are very common in powered subwoofers and the pro sound amp market. They seem to be slowly becoming more common. There's one line of Pioneer receivers now using them. Time will tell if they overcome the popularity of class AB. Note that class D does not mean digital. The term digital is often applied when the class D amplifier's input is a PCM signal rather than an analog signal (the amp can directly convert the PCM signal to the PWM signal used to drive the output devices.)
Class G/H - Amplifiers which improve efficiency by using more than one supply voltage. They otherwise work like class B amplifiers. If an amplifier's power supply voltage was exactly what was needed at any instant in time, it's more efficient. Class G and H differ in how this is done, neither achieves the goal of suppling the exact voltage needed. A class H amplifier has an infinitely variable power supply voltage. A class G amplifier has two or more supply voltages.
Switching power supply amps - This is not an amplifier class. But an efficiency can be gained by using switching rather than linear power supplies. Transistors in circuits are said to operated in linear mode or switch mode. In switch mode, they are turned on or off, there's no intermediate state. In linear mode, they are fed varying amounts of current. Switching power supplies are used in computers. Switch mode power supplies are seeing more use in pro audio amplifiers. They are uncommon in home audio at this point in time. Your typical home audio amplifier uses an unregulated power supply (see the section on "How do amplifiers work")
* How do amplifiers work?
A brief explanation of how amplifiers work might be helpful.
This section will explain the most common amplifier design - a three stage class B amplifier using an unregulated power supply.
The amplifier's "heart" is the power supply. The power supply transformer puts a limit on how much power the amplifier can produce. Bigger is better in this case, and very high power amplifiers of the type we are discussing here can weigh over 100 pounds.
A typical unregulated power supply is really simple. The incoming AC voltage (120 Volts in the US,) would typically be lowered to a smaller voltage. Higher amps would need a higher voltage. A bridge rectifier then converts this smaller voltage into a postive and negative DC voltage. An audio signal will alternate between positive to negative (it's alternating current.) Class B (or AB) amps need two voltage "rails" as they have two "sides" one of which amplifies the positive part of the signal, and the other the negative part.
The typical three stage amplifier has the following stages -
* Input stage; converts the alternating voltage input to an alternating current signal
* Voltage amplifier stage; the signal will be increased to a much higher voltage
* Output stage; This could be considered to be a current amplifying stage - or to put it another way perhaps, it buffers the voltage amplifier stage from the speakers, and allows the amplifier to provide the potentially large current needed by the speakers
The goal of the input stage is to convert the input signal where a changing voltage is converted to a changing current while introducing little additional noise and distortion. It also needs to cope with ripple from the power supply, which is not providing a perfectly constant DC signal. The input stage also takes feedback from the output to lineraize (reduce distortion.) There are quite a few sources of distortion in an amplifier, and feedback is used to reduce them.
The voltage amplifier stage (VAS) is doing the real amplifying of the input signal. The input signal might be one or two volts at maximum, and the output from the VAS will be tens of Volts. For example in my receiver, it's max voltage is going to be close to 70 Volts.
The output stage does the heavy lifting. The output devices, commonly bipolar junction transistors (MOSFETs are also used,) are connected directly to your speakers. The current flowing through the transistors can be very high. Consider a 100 Watt amp connected to a 4 ohm speaker. Voltage would peak around 24 Volts. Current would be be pushing 6 Amperes peak. If you don't think 100 Watts is a lot of power, consider how you almost certainly have no desire to touch a hot 100 Watt light bulb. Don't do it, you will get a painful burn.
An amplifier is not that complex a device in some ways, and there has been little change in it's basic design for a long time (I believe the basic 3 stage amp design has been around since an RCA engineer developed it in the 1950s)
