Understanding and Calibrating Your System: Amplifiers
Welcome back for another round of system calibration. So far in the series we have covered everything from general setup, to basic calibration, to speaker designs, to bass management, and even some basic acoustics. So what then is left? Amplifiers and converters!
Also available in this series:
- Understanding and Calibrating Your System: An Understanding
- Understanding and Calibrating Your System: Monitor Speakers
- Understanding and Calibrating Your System: Amplifiers
- Understanding and Calibrating Your System: Convertors
This next part in the series is dedicated to analyzing just what exactly goes into these power amplifiers, why they have such an impact on the sound, and ultimately what would work best for us as audio engineers and musicians. And if you are worried that this tutorial will get too technical since it involves electronics, fear not. Everything will be kept clear, simple, but still detailed so that everyone can follow along.
So with that in mind, lets get amplified!
There are many ways in which amplifiers can be designed, and each design has their own pros and cons. These different designs are typically organized into a class system using letters.
If you have the luxury to do a shootout and compare these amplifiers side by side then by all means you should do just that. However shootouts can be either pricey or impossible to get together because of the rarity of some designs.
While we as audio engineers and musicians normally only care about how they sound, having a working knowledge of amplifier design can help us figure out, roughly, the sound of an amp before even hearing it. This is particularly useful if you are looking for new a new amp for your new speakers but have no way to "try before you buy".
Also keep in mind that while a particular amp design may have certain characteristics, a little change here or there in design could lead to a huge change in sound (particularly when cheap parts are used in a good design). Again nothing beats hearing the amp for yourself!
So with that in mind lets start by taking a look at a few typical amplifier class designs.
These types of amplifiers are considered by many to be the holy grail of amplifier design. Without getting into too much technical jargon, a Class A design operates over the entire input signal as it amplifies.
What does this mean exactly? It means the amplifying element (transistor or tube) is never turned off as it amplifies the signal; which results in a very clean sound stable sound since it is constantly operating. This produces a very tight high frequency response and generally less harmonic distortion.
However everything comes at a price and with Class A, efficiency is always the problem. Class A designs run very hot and waste a lot of power in order to produce the amplification. The general rule with Class A amps is that at best you will get 50% efficiency which results in needing large expensive heat sinks and power supplies.
In addition, if you turn on a Class A amp but feed it no input signal, you will still draw the same amount of power regardless! If you need a powerful amplifier for your speakers, be prepared for sticker shock a large Class A amp!
These amplifiers were created in order to compensate for the disadvantages of Class A amplifiers. The premise here is that we have what is known as a push pull system in which two elements are used so that one amplifies only the positive half of a waveform and the other only amplifies the negative and the two are recombined before output.
When an amplifying element in Class B does not have input it turns completely off to save power consumption, which results in substantially greater efficiency and in turn reduces cost since less powerful power supplies and heat sinks are needed.
However Class B designs usually lack in what matters most for audio amplifiers, how they sound. Since you have two amplifying elements that are working together one needs to turn on as soon as the other ends in order to create a smooth waveform. However, this transition does not happen perfectly which results in crossover distortion at the zero point of a waveform.
Many consider this kind of distortion very unpleasant, which is why you do not often see Class B amplifiers used in audio.
This amplifier design is nothing more than a compromise between the quality of a Class A design and a Class B design. Most amplifiers in today's market are AB since they are reasonably cheap to make but do not compromise too much of the audio quality. In fact some believe modern Class AB designs are just as high quality as a Class A; shootouts help here!
The way a Class AB amp works is at its core it still a Class B design, but it still constantly conducting a tiny bit to smooth over and correctly connect the crossover point that is missing in a Class B design.
There are of course Class C, D, and E designs for amplifiers, but you will often not find these used for audio amplifiers; although Class D has become more common for live sound. Generally speaking your choices will probably be between Class A and AB when working with audio as we need the highest possible audio quality possible and these designs are as close linear as it gets.
If you have the option of getting a quality Class A I would highly recommend it since it will almost guarantee you linearity but there are plenty of high quality AB designs that will work fine for audio engineering.
Aside from what choosing a type of power amplifier design to use, there are other more practical considerations that we have to account for such as matching a amplifier to a speaker. First and foremost we need to determine exactly how many watts per channel we are going to need to efficiently power our speaker and not risk blowing the drivers or the amp. Here are a few things you will need in order to figure this out.
- The continuous or RMS rating of the speaker (in Watts)
- The impedance of the speaker (in Ohms)
The RMS rating of the speaker is essentially the minimum power you will need to power your speakers. Others will tell you that as long as you do not try and crank the volume that you will be fine but as audio engineers we will have the need to listen loudly from time to time so to be safe this rating should be the minimum.
But this rating is only half the story as we need to know what impedance of the speakers are. This rating is the opposition to current and the lower the impedance, the better the current will flow. What this means for our speakers is that if our RMS is 200 W at 8 Ohms then our amp will need to be at minimum 200 W at 8 Ohms. However when dealing with amplifiers they usually can operate at multiple impedances which will change the amplifiers power output.
For example, if our power amplifier is 200 W at 8 Ohms and we were to plug it into a 4 Ohm speaker, our amplifier would then theoretically run at 400 W since it is now twice as efficient. You will never quite get true doubling of our power, but will get marginal increases in power output.
Generally speaking most amps will work at 8 Ohms and 4 Ohms, while others can handle up to 2 Ohms and others may go down to 16 Ohms. So those with 4 Ohm speakers will generally have more options available to them if they need higher wattage power amplifiers.
Up until now however we have only discussed bare minimum ratings for power amplifiers. In truth, your power amplifier should be 1.5 to 2 times greater than the RMS rating of your speaker (at matched impedances of course).
"But that will blow your speaker!" you say. But in truth the volume will probably be so loud that you will never get close to blowing your speaker.
Ironically, under-powering your speaker with a weaker amp will be more likely cause a speaker driver to blow. Why is this? Because the more we cause the amp to work, the better chance we have of causing square wave style distortion, which is what will burn out a driver.
So by doubling the RMS rating of the speaker to get our amplifiers rating will ensure we never reach that distortion.
So far we have covered the most crucial aspects of power amplifiers that will get you up and running but that is not to say that we have taken into account all considerations. There are a various other features and designs that could effect how the power amplifier sounds.
So with that mind, lets take a look at a list of various other features that we should consider.
Mono or Stereo
Some amplifiers will be able to run in both mono and stereo and this is know as bridged mode. This will produce more power for powering a single speaker and in fact there are strictly mono amplifiers known as monoblocks.
Generally speaking monoblocks will be more efficient than stereo amps since they only have to drive one speaker. You can get them in either true mono form with one amp in a chassis, dual mono with two separate amps in one chassis (this includes separate power cords and everything), or dual mono that shares a few parts but separates the rest; the first two can potentially sound a little different from the last since they are truly separate.
Some claim that they also have lower distortions, cleaner sound, and better stereo imaging. Again do a shootout if possible.
Digital or Analog
Some amplifiers are in fact digital in nature and include a slew of other features that may or may not be useful for audio engineers. Some of the major digital features you will find are limiters, temperature and voltage metering, and sub harmonic enhancement. Most all of these features are handy for live sound engineers who most likely do not want to distort, need extra bass content, and need to monitor their system closely.
Studio engineers however usually do not have need for these features since they need to create a truly neutral sound that can be listened to anywhere, not just in the venue. In addition since a studio engineers setup is usually constant so being able to monitor an amps status is usually not necessary. However the choice is yours as to what you need.
While it would seem inconsequential, having a gain knob on am amplifier could be a necessity or it could be a danger. Generally speaking, most amplifiers designed for live use will have stereo gain knobs on the front as a reference level control and a stereo balancer.
Some hifi audio amplifiers will have these or more commonly have either a single gain knob or none at all. Those that do have this single knob have it generally for convenience sake. However many high end audio amplifiers do not have a gain knob because it could theoretically degrade the audio signal from having to pass the audio through the potentiometer.
In addition, studios who like to set up their system once and forget about it do not want someone to accidentally bump the gain knob and throw off the system and drive the engineer crazy as to why he had to turn his other gains up so much.
If your convertors for some reason do not have a gain controllable output then by all means you will need one on your amplifier or need to get a preamp (not the mic level kind). Your other option if you do have an output gain control on your convertor is to use that as your reference level and not have a gain control on your power amplifier.
Tube or Transistor
It goes without saying but the solid state vs tube debate still rages on to this day. Any of the class designs are possible using either transistors or tubes but which one is used will affect the sound.
Some argue that tubes give a more natural warm sound and that solid state is too dead and clinical sounding. Others say that solid state is the most accurate and cleanest sounding and every engineer should be using those as their reference so as not to skew the mix.
Truth be told there can be very clean tubes and very muddy tubes just as their can be very clean solid states and very cloudy solid states. The only real consideration is how well they are implemented, as they both require their own design.
I recommend to try and shoot them out if you can and walk in with no bias as to what sounds better. (If you got the biasing joke then you know too much about amplifiers).
This rating on a amplifier is its ability to react to change. A low slew rate means that the amplifier is slow to react and will skew the transients of your music so that they are not as accurate as they should be.
Some argue that slew rate is over rated and has minimal effect on the sound while others believe it is essential for clear articulations. Generally speaking amps with better slew rates will cost more but if you can afford it, having the better slew rates will not hurt your sound in the least, possibly even make it better.
Conclusion for Now
So what have we learned here? We covered just how many different options and designs go into power amplifiers and how it could or could not affect the sound of our system. Being one of the three key ingredients in a modern setup you need to make sure you get it right if you are using passive speakers. Just remember to make sure your amp is powerful enough, as a linear frequency response, and is in your budget and you will be good to go!
Next time we will take a closer look at the final piece to the puzzle, the convertors. Until next time!