Class A

The simplest of all voltage and current amplifiers is the single ended Class A. It uses a single device and is always turned on, irrespective of the input signal. These amplifiers have outstanding linearity and therefore very low distortion but very poor power efficiency as was seen earlier. This accounts for the class being used almost exclusively for pre-amplifiers or the voltage amplification input stage in power amplifiers in either single ended or push-pull designs.

Class A Amplifier

A MONO 100 watt Class A power amplifier would create at least 200 watts of heat even when not being driven. This is why Class A power amplifiers are generally lower power, up to 50 watts, and are very expensive.

Class B

These amplifiers behave in the exactly opposite manner to Class A. They are designed as a push-pull pair and each device only turns on for its half of the incoming signal, or 180 degrees. No input, no output and therefore no current is drawn, So they are quite efficient and produce a lot less heat.

Class B (Push-Pull) Amplifier

In the very simplified design shown the top NPN transistor only conducts during the positive part of the signal, while the lower PNP transistor only conducts during  the negative half of the incoming signal. The problem with this simple design is that as the incoming signal gets close to switching from one device to the other these very low voltage levels are driving the devices over very non-linear parts of their characteristics. This produces very high levels of distortion known as Crossover Distortion, and sounds terrible.

Pure Class B amplifiers are therefore never used for commercial audio applications. However, with a very small change to the design, crossover distortion can be virtually eliminated. This is Class AB.

Class AB

Class AB amplifiers are class B amplifiers in which a small continuos current flows through both devices at all times. This effectively biases them into Class A. So for low signal levels the amplifier appears as Class A, but as the signal increases in level, only each half handles the larger level signal. By adujusting the standing bias current through the two devices an optimal level can be selected that minimises crossover distortion and and does not creat too much heat. However the corossover distortion can never be completely removed, just reduced to an inaudable level.

Class AB (Push-Pull) Amplifier

Typical bias currents are 50-100mA, so in the above example a Class AB amplifier would not produce more than about 4-8 watts of heat. Despite this low quiescent heat generation these amplifiers still require extensive cooling when driven to their maximum designed power levels due to the high currents and attendant voltage drops across each output device (or set of output devices). Many commercial audiophile power amplifiers use this class.

The bias point for a class AB amplifier is quite critical in order to minimize the crossover distortion. Unfortunately the devices and components used to achieve this change their characteristics and they become warm. This causes the biasing to become non-optimal and results in increased crossover distortion. There are many clever ways to minimize this effect using various negative coefficient and positive coefficient devices, and types of negative and positive feedback to continually monitor and adjust the bias level.

Designed in 1975, the infamous Quad 405 power amplifier used a technique called Current Dumping. This design uses a low power pure Class A voltage amplifier to create the voltage waveform, while off-loading the high current demands to a pair of complimentary Class B power transistors.

This technique eliminates the problems of crossover distortion, bias current adjustment, thermal tracking and transistor matching. The design has  no internal adjustments or alignments and the choice of power transistor types is less restrictive. A truly innovative power amplifier design!

Simplified Current Dumper Power Amplifier Schematic

For those electronics and audiophile enthusiasts here is the schematic of the well respected Leak TL50 Plus valve power amplifier. It runs in Class AB ultra linear mode.

V1 – Low noise voltage pre-amplifier – pentode

V2 – Dual triode phase splitter to drive the upper and lower power pentodes

V3/V4 – Class AB power amplifier pentodes in ultra linear mode. (Ultra linear mode involves connecting the screen grid of the pentode to taps on the output transformer.  Its purpose is to improve the linearity of the valve and therefore reduces its output distortion.)

Class D

These amplifiers are a completely different type of design to Classes A and B. They are sometimes referred to, incorrectly, as digital amplifiers. A more correct term is a switching amplifier, but they are still analog amplifiers having very high efficiencies of almost 100%. Why? Because the output devices are either turned fully on or fully off and do not have a high current and associated high voltage drop across them, so dissipate very little power and therefore create very little heat.

Class D amplifiers usually use MOSFETS for the main power amplification stage and use a technique called Pulse Width Modulation (PWM). The incoming signal is used to rapidly turn the output devices on and off at speeds much higher than the highest audio frequency, for durations that are a function of the volume of the signal. This produces a square wave pulse train that is then low pass filtered to re-create the analog power signal.

Classes G & H

The class G&H techniques can be applied to both Class A and Class AB amplifiers.  Both classes adjust the power supply rail voltage according to the level of the incoming signal. The effect is to reduce heat and increase the amplifiers efficiency. However, there is a notable increase in the complexity of the power supply design and its control, thereby increasing cost. This maybe offset by the reduction in size of the amplifier and its associated heat sinks etc.

Class G amplifiers make two or more pairs of supplies available to the output devices which can be discretely switched between and/or modulated up and down according to the level of the incoming signal.

Very Simplified Class G Power Amplifier

Class H amplifiers use just one dual power supply whose voltage is changed either in discrete steps or modulated up and down according to the level of the incoming signal.

Very Simplified Class H Power Amplifier

Audio power amplifiers are designed to drive loudspeakers whose impedance (resistance) is low, over a wide range of frequecies, typically 10Hz to 50KHz, at extremeley low distortions, typically 0.005%. This requires the amplifier to simultaneously deliver large voltage swings and lots of current. If we assume a typical power amplifier of 100 watts driving a purely resistive 8 ohm speaker (very rare) with a pure sign wave (also very rare), its peak output voltage must  exceed +/- 40 volts at +/-5 amps peak. Real speakers do not have fixed impedances of 8 ohms and can vary between 2 ohms and 20 ohms. MUCH worse than that, a speaker is NOT purely resistive, and are said to have an impedance. Impedance is the AC equivalent to DC resistance. Unfortunately, this means that the speaker causes the power amplifier to have to provide voltage and power peaks that are not in phase, as the speaker ‘behaves’ as a capacitive and/or inductive load. This type of non-linear power demand can reek havoc on a power amplifiers technical performance. It is one of the issues that can make what appears on paper to be two technically identical power amplifiers, sound different. In order to support these varying load conditions modern 100 watt power amplifiers need to provide voltages as high as +/-50 volts with peak currents as high as +/- 12 amps.

Depending upon the class of the amplifier the heat generated by these amplifiers can be very high and require huge heat sinks, fans and large arrays of active devices. A pure Class A 100watt power amplifier would need to consume 2.5 amps at 80 volts, even when not driving an 8 ohm speaker. This would involve dissipating over 200 watts of heat. It is clear from this that those 1, 2 and 3KW sub amps are therefore not Class A, being designed around Class D.

Basic Class AB Power Amplifier Schematic

Clearly the efficiency of the power amplifier is very important, and is one of the primary features the designer will take into account when choosing a class for a specific application and market.

My 2 Cents

So in conclusion what do we buy? That is the million dollar question, and we haven’t even touched upon wether BJT, FET or valve amplifiers sound better (different) when compared to each other. I have owned all classes of power amplifier and hand built many of them. Even at the audiophile level there are real audible differences between the various designs and types of output devices. This is true for both voltage (pre-amplifiers/receivers) and power amplifiers.

To make things much more complex most modern multi-channel pre-amps use enormous amounts of digital signal processing as do some power amps. The conversion of analog signals to and from digital is not transparent and this process can easily hide subtleties between analog designs, giving the amplifier its own characteristic ‘digital’ sound.

In my experience, and with state of the art power amplifier designs, these audible differences often come down to the way the amplifiers interact with the complex impedances of the speaker load and just how well the amplifier can maintain its peak and full power without increased distortions and loosing its damping factor.  Methods, types and amounts of feedback deployed within the amplifier and overall design topology has real impacts on the amplifiers behavior when driving these complex loads. Cost effective all analog power amplifiers that sounded great to me included; the Quad 405, Leak TL50 Plus and the Denon AVR5803. Recently I have moved to using all active Genelec studio speakers. Being mounted within the speaker, these, just like my SVS subs, use Class D power amplifiers due to their efficiency and low heat creation. They do however tend to have a more clinical and ‘harder’ sound than my original all classically linear analog system using the Quad 405 and Leak TL50 bi-amped arrangement driving passive speakers.

For decades, and until my migration to multi-channel sound to support movies, I used the discontinued stereo Radford ZD22. I have auditioned various other audiophile pre-amplifiers but always found that the Radford met my needs. Right up their with the Radford, is the now discontinued Denon AVP A1 HDCI with its all balanced, all analog audio chain. It’s an outstanding integration of analog and digital techniques for both stereo and multi-channel listening and currently forms the center piece for my A/V room.

So what do we all do? I said it before. If you can get the amplifier on approval, do so, and listen to it in your system.

Want to know more about the semiconductors, transistors and tubes that go into these amplifiers designs? Please read these posts:

• Semiconductors and Transistors 101 – Part 1 of 3
• Valves Or Tubes To You – An Introduction

For those looking for a more detailed technical introduction to many aspects of amplifier design, click here for Electronics Tutorials.