Recently there has been a resurgence of two "ancient" technologies - vacuum tube (valve) amplifiers and Class-A systems. The big question is .... is there a difference? This discussion centres on the Class-A amplifier, and explains (or attempts to) how it is different from a conventional power amplifier.
Why would someone want to build or buy an amplifier which is sooo inefficient? A Class-A power amp will typically draw anything from 1/2 to about 1½ times the peak speaker current in its quiescent state (i.e. while it is just sitting there doing nothing).
To put this into perspective, for a measly 8 Watts into 8 Ohms, the RMS current is 1 Amp. The peak current is just over 1.4 Amps, so a typical 8 Watt Class-A amp will draw anything from 700mA to 2 Amps continuous. This equates to a quiescent (no signal) power dissipation of between 17 Watts and 48 Watts, based on a 24 Volt supply (+/- 12 Volts ). At very best, such an amplifier will have an efficiency of less than 35% at full power - at worst, this will be perhaps 15% or less.
The basic premise of a Class-A amp is that the output device(s) shall conduct all the time (through 360 degrees of the signal waveform). This means that in the simplest form, the power devices must conduct a continuous current which exceeds the maximum peak load (loudspeaker) current. If we use a power level of 20 Watts (hardly a powerhouse) for all further calculations, we can see the whole picture.
In contrast, a typical Class-AB power amplifier's output devices only conduct for about 182 degrees (at full power), which means that for much of the signal's duration, only one or the other device is conducting. The other is turned off. The "crossover distortion" so often referred to is nothing to do with the frequency divider in the speaker system, but is created as the signal "crosses over" the 0 Volt point (see Figure 3).
Figure 1 - The Sinewave Cycle
Let's have a quick look at some of the power amp "classes", so we have all the info:
- Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1
- Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB)
- Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1.
- Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts (single-ended operation is assumed, and yes this does work for RF).
When I first wrote this article, I had completely forgotten about the Quad "Current-Dumping" amp, which uses a low power "good" amplifier, with a push-pull Class-C type amp to supply the high currents needed for high power. Although these enjoyed a brief popularity, they seem to have faded away. I was reminded of their existence by an article by Douglas Self ("Class Distinction", in the March 1999 issue of Electronics World ), in which he quite rightly points out that the current-dumper is (at least in part) Class-C. - Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - although my original comments were valid when this was written, there have been some very significant advances since then. There are some very good sounding Class-D amplifiers being made now, and they are worthy of an article of their own.
There are many amplifier topologies which I have not mentioned above, mainly because most of them are either too bizarre, not worth commenting on, or are too complex to explain simply. Of these, Class-G and Class-H use power supply switching and modulation (respectively). This provides greater than normal efficiency and lower dissipation, but both are essentially Class-AB designs.
Although many audio amps may be called Class-B, generally they are not. Virtually without exception they are Class-AB, although most will be at the bottom end (conduction for 181 degrees for each device). Most power amps operate in Class-A up to about 5 to 10mW, after which they become Class-B.
In the device department - For the remainder of this paper, I shall use bipolar transistors for the power devices, since they exhibit highly desirable characteristics for this application. They are also far more linear than MOSFETs, and some of the newer bipolar devices are outstanding in this regard. Note that there are two types of MOSFET in common use - Lateral devices are designed for audio, and although less linear than bipolar transistors can make a very good amp indeed (see Project 101). Power switching MOSFETs are (IMHO) not suitable for use in audio except where very high power is needed and extreme linearity is not required.
| Power | 20W (continuous) |
| Load Voltage (at Speaker) | 12.65 Volts RMS (17.9 Volts Peak) |
| Load Current (through Speaker) | 1.58 Amps RMS (2.23 Amps Peak) |
| Supply Voltage | +/- 20 Volts (constant) |
| Supply Current | +/- 2.25 Amps (peak) |
In amplifier design, we are interested in the peak voltage and current, since if these are not met, then the required RMS values cannot be achieved. The ratio of RMS to peak (for a sine wave) is the square root of 2 (1.414), so RMS values must be multiplied by this constant to derive the peak values of voltage and current. (Refer to Figure 1 to see the relationship between peak and RMS voltages.)
This is how the values in the table were determined. The supply voltage needs to be slightly higher than the actual speaker peak voltage because the output devices (transistors) are not perfect, and some voltage will be lost even when they are turned on fully. (If MOSFETs were to be used, the losses may be much greater, unless an additional power supply is employed.)
Ok. We have determined that the peak speaker current is 2.25 Amps, so in the simplest of Class-A designs this will require a quiescent current of 2.25 Amps. Given that the voltage is +/- 20 Volts, this means that the power output stage will have to dissipate 40 x 2.25 = 90 Watts (45 Watts per output device).
Figure 2 - Basic Class-A Amplifier
Figure 2 shows what a simple Class-A amp looks like. The current source is a simple circuit, which will provide a current which remains constant regardless of the load placed at its output. The output transistor "dumps" any current which is not needed by the load (speaker), so when it is completely turned off, all the current source output flows through the speaker. Conversely, when it is turned on, the speaker current flows through the output transistor (as well as the current from the current source!), so its current will vary from almost 0 Amps, to a maximum of 4.5 Amps for our example. When there is no input signal, the output transistor's current must exactly equal the output of the current source. If it does not, then the difference will flow through the speaker. It is allowable (generally speaking) for an absolute maximum of 100 mV DC to be present across the speaker terminals - this equates to 1.67 mW of DC for an 8 Ohm system, assuming a 6 Ohm DC resistance for the voice coil. (Power = (V x V) / Impedance)
This simple model is not really appropriate for general use, since it wastes far too much power, although many Class-A amps still use this principle. The next step is to operate the current source at about 1/2 the speaker's peak current, and modulate its current output to ensure that both current source and power amplifier output device conduct during the entire signal cycle, but are able to vary their current in an appropriate manner. This improves efficiency (which remains dreadful, but slightly less so), and lowers the quiescent dissipation to more manageable levels.
The simple Class-A amplifier described by John L Linsley-Hood and the very similar looking Death of Zen (DoZ) amp on these pages use this latter approach, and it is a sensible variant of the various Class-A designs. As an example, the amplifier will only (?) need to dissipate about 50 Watts when idle, since the quiescent current is reduced to around 1.2 Amps.
Another version of the Class-A amp looks exactly the same as a standard Class-AB (Class-B) power amp, except the quiescent current is increased to just over 1/2 of the peak speaker current. This is thought by some (including me up until I was shown the error of my ways) that this is not a "real" Class-A amplifier. It is real Class-A, and is best described as push-pull (as opposed to single ended) operation. If the bias current is not high enough for the actual reactive speaker load (not some quoted nominal resistive load), it is still possible that one transistor or the other will switch off at some part of the signal cycle. This will happen at a much higher power level than is normally the case, but if this happens, then the amplifier ceases to be true Class-A.
As an extension of the above, it is possible to design an amp that looks remarkably like a conventional Class-AB amp, but with additional circuitry is biased in such a way that the output transistors do not turn off - ever. This technique can also be used with Class-AB, and supposedly reduces crossover distortion. I have not used this method, since in my experience the crossover distortion in a well designed output stage should be sufficiently low that the additional complexity is not warranted.
The last three "variants" cause the current to be modulated in each supply rail, so there is not the steady state current one expects from a Class-A amp, but a waveform that varies with the signal. When properly designed and biased, the output devices conduct at all times, but the power supply has to contend with a varying load. I have not investigated this fully, but it makes the design of the supply a little more difficult (or simpler in some ways) because of the varying load current. Tests I have performed with the DoZ amp do not show any audible effect on the sound quality - provided the supply is designed to handle the variations without any problems.
Actually, the idea that a Class-A amp draws a continuous steady current from the supply is true in one case only. A single ended amp using a current source as the collector load will draw a continuous steady current - but only if it uses a single supply. In the case of a dual supply, the same amp will draw a continuous current from one supply, and a varying current from the other. (My thanks to Geoff Moss for pointing this out - a detail that few published designs have ever mentioned !)
An amp that uses a fixed current source of (say) 2.5A from the positive supply will draw 2.5A regardless of load or signal level, but only from the positive supply. The negative supply current will vary from 2.5A at no signal, but will be almost zero at maximum positive swing, when the lower transistor is turned off, and the current flows from the current source to the load. At maximum negative signal swing, the negative supply current will be close to double the quiescent current, since the lower transistor now carries the current from both the load and the current source.
This "small" detail seems to have received scant reference in any of the articles I have read, but it will make a very big difference to the power supply. In this respect, I do not feel that the single ended version should be operated from a dual supply. If it is so important to you to eliminate the coupling capacitor, then I suggest that either a push-pull Class-A design be used, or build separate power supplies for each polarity.
There is some evidence (I refer again to Doug Self) to indicate that the distortion of a "true" Class-AB amp will often be worse than that of a Class-B design, since the switching transients are larger due to the output devices' higher gain at moderate (0.5A to 1.5A) currents. I have not been able to verify this, and the tests I have done indicate that there are definite benefits in the higher quiescent currents, provided the current is chosen reasonably carefully.
One of the biggest problems with Class-A amps is that the simple power supply used with conventional Class-AB amps is usually no good to us. The reason is that the AC ripple on the DC power rails is injected into the amp, and emerges as hum (at 120 or 100Hz, depending on location - US or elsewhere, respectively). The magnitude of this ripple is far greater than with a Class-AB amp, because a considerable amount of current is being drawn at all times, rather than during signal peaks (etc). A power supply which provides a no-load ripple of perhaps 50 mV for a Class-AB amp may have 1 Volt (or more) of ripple at a current of 1.2 Amps. This will be audible at low signal levels.
Adding capacitance helps, but by the time the ripple is reduced to a reasonable level, you have sold the car to pay for the capacitors, and no longer have a vehicle to carry them home in. You will need a ridiculous amount of capacitance to obtain reasonable hum levels (> 70dB signal to noise ratio) unless a regulated supply is used. The fact is that many Class-A power amps do not have particularly good power supply rejection (Ok, it is not generally too bad, but cannot compete with the likes of an operational amplifier), and a regulated power supply is recommended for all such amps. In case you were wondering, that does indeed mean that you need more transistors, more heatsinks, and it will cost more money. Such is the price we pay for perfection.
There is an alternative (which I have not tried for this application, but have carried out numerous spice simulations) called a capacitance multiplier, which is simpler and cheaper than a regulated supply, but should be capable of reducing the ripple to very low levels. I have had a few e-mails from readers who have built the capacitance multiplier project (see the Projects page), and the results have been very positive, so this makes the Class-A idea far more attractive from a cost and heat perspective. (Capacitance multipliers are not required to regulate, so operate with a much lower input to output differential voltage - therefore, less heat!) Indeed, the design by John Linsley-Hood referenced on these pages uses a capacitance multiplier, although its performance can be improved dramatically.
S:diyaudioproject.com
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