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Amplifier Classes (Topology)

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by: Kyle Duffy

A car audio amplifier increases the power of the signal being fed to it by taking energy from its power supply and matching the signal but increasing its amplitude. Whoa, yeah that may be hard to swallow for some people. To put it in every day terms, an amplifier takes an electric signal and makes it bigger which translates into louder and better sounding music for us. Thank you engineers! Amplifiers use many different methods for converting these electrical signals and as you research amplifiers you will often see “Class A/B” or “Class D” and sometimes other classes all together. What does the class letter mean and which one is right for you? Let’s find out!

Class A
The input signal is used 100% of the time in class A amplifiers. What this means is that the output signal (to your speakers) is an exact amplified replica of the input signal (from your headunit) with almost no clipping or distortion. However, these amplifiers run extremely hot because the transistors in the amp are on all the time. These amplifiers have a very simple circuit design compared to the other classes and have the least distortion. In a nutshell, for the price of running hot, you get great sound quality. These types of amplifiers are used in low-power applications such as to drive headphones. You will most likely not find car audio Class A amplifiers because 1000 watts of heat could be generated just to get a few hundred watts of audio. Not a great compromise.

Class B
In most Class B amplifiers, there are two output devices, each of which conducts alternately (meaning in a push-pull fashion).  50% of the input signal is used so they only amplify half of the wave cycle. What this means is that class B amplifiers tend to create a lot of distortion but they are far more efficient than class A amplifiers. The positive and negative halves of the input signal are dealt with by different parts of the circuit and then the output continuously switches. Still, you may never see a class B amplifier in the audio world. These amplifiers are favored in battery-operated devices, such as transistor radios.

Class A/B
These amplifiers are the most commonly used amps of today in car audio. They combine the best of both worlds with class B efficiency and the great sound quality of class A with low distortion.  Class A/B amplifiers run very similar to class B in that half of the signal is amplified and the other half is “off”, however the deadzone has been reduced. This means that a bit more of the signal is being amplified, greater than 50%, so the instance where both would be “off” has been reduced. Class A/B is a good compromise for audio amplifiers, since the average listening volume is relatively low, keeping the amplifier in A range in terms of sound quality. When you crank the volume up the distortion levels are less than class B.

Class D
The main benefit of Class D amplifiers is the efficiency, often times at or over 90% is attainable. This efficiency is extremely high compared to class A/B amps which run less than 78% efficient. This translates into smaller heatsinks, less power draw from your vehicles charging system, and smaller internal components.  These amplifiers switch their output devices “on” and “off” instead of an always “on” which creates a square wave as opposed to an analog sine wave. This square wave contains inaccurate pulse frequency and harmonics which must be removed from the signal which is done by a technique called PWM (Pulse Width Modulation).  Cleaning up the signal of a class D amplifier is relatively difficult so they are best used in applications where quality is not a factor, such as driving subwoofers. Class D amplifiers have inherently low output impedance, usually down to a 1 ohm load, which makes them ideal for running subwoofers. High quality class D amplifiers are starting to appear and some even rival class A/B amplifiers in terms of quality. Do not mistake a class D amplifier as being “Digital”; this is not usually the case. Class D is simply next in the list after C. Digital class D amplifiers are controlled by digital circuits which convert the signal into zeros and ones and then digitally process the signal to remove errors. Class D amplifiers appear to be the way of the future.
 
Other Amplifier Classes
Some manufactures have started producing amplifiers with class names such as bD, GH, etc. These class names do not reflect any breakthrough in technology, but rather reflect tweaks to improve on the current class A/B and D designs. For example, class GH amplifiers deliver the efficiency of a class D amp with the sound quality of an AB design. It does this because it is not a switching amplifier (on and off) and employs different “rail” voltages with a special control circuit that switches back and forth between the rails as the signal fluctuates. This reduces the amount of wasted output power; however these amplifiers are sophisticated and costly to produce. As for Class bD amplifiers they have an efficiency rating closer to that of class D amplifier (75-80%) vs. the tradition ratings of class A/B amplifiers (20-60%). These amps are more complex than the tradition class A/B and filtering problems are simplified. As time goes by you will see more amplifier classes appear that offer improvements over the general class A/B and class D designs.

Conclusion
So if the above mentioned information is giving you a headache let’s just get down to the basics. Which one is right for you? The most common amplifiers in the car audio world are class A/B and class D. You will find most speakers are run using class A/B amplifiers and subwoofers are driven by class D amplifiers. It is difficult not to stick to that pattern of play as you will find few deviations of it. Soon we will start seeing all class D systems, until then let A/B hold its rein.

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Written By: Kyle Duffy