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This page was last updated on 20 July 2001

Class Distinction in Audio = Amplifiers

A discussion of design problems and how to = overcome=20 them

by J. L. Linsley = Hood=20 (1)

(Wireless World, June = 1970)

Since the publication of "Simple Class A Amplifier" the author has = received=20 numerous letters asking whether it would be feasible to increase the = power=20 output to 15W, or even 20W, to provide a greater reserve for use with inefficient loudspeaker systems.

Whilst it would be possible, the problems associated with increased = heat dissipation and the provision of suitable power supplies makes this unattractive. In view of the low average power required for normal = listening, the question inevitably arose whether it would be practicable to design = an output stage which would operate in class A with an inherently low = level of high order distortion up to a watt or two, but progress further into = class B operation if and when higher powers were momentarily demanded.

There are, unfortunately, a number of snags with the class B = operation of transistor output stages, to which the answers are not fully known.

It was pointed out some years ago, by Bailey (2)and others, that the use = of quasi=20 complementary symmetry in such output stages led to an increase in = high-order=20 harmonic distortion, associated with the non-linearities in the = crossover=20 characteristics at low volume levels, and although the level of total = harmonic=20 distortion at maximum power output could be quite low, the distortion = content at=20 typical listening levels could be many times greater than this, and = would also=20 be of an audibly objectionable type.

A number of schemes have been proposed to overcome this problem, = including the use of full complementary symmetry (2 3 = 4), and=20 various methods of ensuring that there are an equivalent number of = forward=20 biased junctions in each limb have been described (5 6),=20 including the ingenious semi-complementary triples arrangement used in = the=20 "Quad" amplifier (7).

However, in the author's experience, some class B transistor = amplifiers - including those employing full symmetry, which is presumed to eliminate = the major fundamental snags of this type of operation - having an = impeccable performance on paper, did not have the tonal quality which had been = expected. Since harmonic distortion at both high and low power levels had been = found to be well below the level at which audible effects might reasonably be = expected in some of the designs tested, it seemed more probable that the audible ill-effects were due either to transient instabilities associated with loudspeaker loads -perhaps related to changes in the reactance of the base-emitter junction at the current cut-off point - or to = high-frequency crossover type distortion arising from hole-storage effects. = Hole-storage depends on the presence of holes produced when current flows in a = semiconductor - even though the current is due to majority carriers (electron flow). = The greater the current the greater the number of holes and the worse the = problems of hole storage.

Hole-storage phenomena

The expected result of hole storage in the base region of a = transistor, following the attempted termination of a high emitter collector = current, is that the transistor remains in a conducting state after the forward = base bias has been removed. This has the effect, amongst other things, that the = normal crossover discontinuity shown in Fig. 1(a) becomes displaced from the = mid-point of the transfer waveform as the frequency is increased, as shown in = Fig.=20 1(b).

These waveforms were generated in a simple complementary pair emitter-follower circuit, without additional negative feedback, driving = a resistive load. (In order to assist its display the crossover effect = was deliberately exaggerated by the use of an inadequate quiescent = current.) Provided that the peak currents flowing through the transistors are = small, this effect is innocuous. However, if the peak currents are increased, by = reducing the load resistance, the crossover waveform rapidly deteriorates as = shown in Fig. 1(c), and increasing the forward bias to give a more suitable = quiescent current has little effect in removing this prominent notch, until the = forward bias is almost equivalent to that of class A operation.

It is known from experience that these effects can be minimized by = the use of=20 transistors with good high-frequency characteristics and low-impedance base-emitter return paths. A low-impedance driver stage will also be = effective provided that it does not become cut off (as in the case of the = Darlington pair) when the input signal reverses polarity.

The effect of reducing the driver circuit impedance from 2000ohm to = 100ohm is=20 shown in Fig. 1(d).

The lack of effective symmetry between the upper n-p-n device and the = lower p-n-p is also shown in Fig. 1(c). This effective asymmetry is reduced = if the source impedance is reduced.

Fig. 1. Crossover = distortion in=20 a class B stage employing transistors with an fT=20 of about 2MHz. (a) Low frequency sine wave at 10mA. (b) High frequency = sine wave=20 showing the effect of hole storage on the crossover discontinuity under = light=20 load conditions. (c) Influence of hole storage and n-p-n/p-n-p asymmetry = under=20 high current conditions at 200kHz. (d) Improvement of conditions in (c) = by=20 reducing source impedance.

It was noted that this effect did not become apparent, even under = high emitter current conditions, until the operating frequency approached = 0.05=20 fT. At 0.1 fT, the problem was severe and this = argues that=20 the occurrence of high transient currents - which may = arise =20 with certain loudspeaker systems - and high driver stage = output=20 impedances, is most undesirable unless the highest frequency components = of the=20 waveform are low in relation to the transition frequency of the output=20 transistors. With the availability of power transistors having = transition=20 frequencies of the order of 4MHz (such as the MJ480/490 series) it is = unlikely=20 that hole-storage phenomena will be troublesome at the rates-of-change = of signal=20 voltage likely to be encountered in audio amplifier practice so long as = the=20 driver stage does not leave the output transistor base open-circuited on = cut-off. However, the use of a driver output, or base circuit, impedance = not in=20 excess of a few hundred ohms appears prudent. With earlier designs using = germanium diffused junction power output transistors, which usually have = very=20 poor h.f. performance, this problem could be important, and Dinsdale has = referred to a "subjective audible improvement" resulting from the = replacement of=20 low transition frequency output transistors with types having better = h.f.=20 characteristics.

Transient instabilities on loudspeaker loads

Phase-angle measurements made with a variable frequency sine wave = input, from=20 a high impedance source, reveal that even a simple single-unit = loudspeaker can=20 present quite complex characteristics. The reactance - which is normally inductive - changes rapidly, and sometimes even becomes capacitive, at frequencies in proximity to cone and structure resonances.

In general, the characteristics of most of the common designs of = transistor power amplifiers are such that instability problems do not arise with = inductive loads, and the inclusion of a small choke, of a few microhenries = inductance, in the speaker output lead is a well known technique for avoiding = instabilities under adverse load conditions. However, capacitive loads can frequently = impair the stability margins of the feedback loop, and it is in this respect = that

the reactive characteristics of the loudspeaker load are most = significant Since it was suspected that the region of the output waveform where = this might arise most readily was that at which the output transistors were being = driven from the conducting to the cut-off state, an input waveform which = provided a transient of controllable steepness (by varying the input amplitude), = but arrested at the mid-point, was provided by the circuit of Fig. 2.

Fig. 2. Circuit for = generating=20 the test waveform shown in Fig.3.

The waveform generated by this device is shown in Fig. 3 and the = result of introducing such a waveform into an amplifier of poor stability = margins, coupled to a resistive load shunted by an appropriate value of = capacitance is shown in Fig. 4(a). (The broadening of the oscilloscope trace in the = horizontal regions at the mid-point of the waveform was due to inadequately = recorded h.s. oscillation.)

Fig. 3. Test = waveform for providing arrested transient input.

The output waveform obtainable from a design with better stability = margins and improved bandwidth is shown in Fig. 4(b). In both cases the = magnitude of the input signal was adjusted so that clipping occurred on both = negative- and positive-going peaks.

Since the h.f. instability shown in Fig. 4(a) - which did not occur = in the absence of a large input signal, and which required a particular range = of shunt capacitance to provoke it at all - also occurred on parts of the = waveform preceding the arrested transient, it was concluded that the change in = reactance of the base-emitter junction at cut-off or switch-on, was not a major = cause of the transient induced instability observed in this particular = design.

Fig. 4. Amplifier = performance using 10kHz test waveform. (a) Response of amplifier showing inadequate = stability with reactive load. (b) Response of improved amplifier with = reactive=20 load.

Square-wave performance and tonal quality

In view of the fact that a loudspeaker system can present a reactive = load, of=20 a type which is found in certain circumstances to cause signal induced instability, and since this instability could be provoked by a = square-wave input into an amplifier with a suitable reactive load, a series of = tests and comparative listening trials was conducted to determine whether there = was any audible relationship between the two. In the event, it was found, = beyond doubt, that an amplifier system which did not show any sign of instability = over the range of load shunt capacitances up to, say, 0.33uF had a better tonal = quality on even a simple loudspeaker system than one in which some shunt = capacitor value could cause h.f. oscillation. Moreover, in a more complex = loudspeaker system, with a crossover network and high-frequency capacitively = coupled "tweeter", it was possible to hear the difference between systems which = would,=20 in the lab., with some RCload combination, give a square-wave = response=20 such as that of Fig. 5(a) and those which had a response like that shown = in Fig.=20 5(b). No positive distinction could be drawn in listening trials between = a=20 system giving a waveform such as Fig. 5(b) and one in which a = square-wave input=20 could produce a single overshoot "spike".

Since the frequency of the "ring" waveform in Fig. 5(a) is well = beyond the=20 upper limits of the audible spectrum, it is clear that it is not this of = itself=20 which produces the undesired sound quality, but rather that this type of = behaviour is symptomatic of a different and more objectionable effect = when the=20 amplifier is used with a loudspeaker load.

Fig. 5. Amplifier = response=20 driving a reactive load (15ohm, 0.47uF) with a 10kHz = square wave.=20

(a) The ringing = gives evidence=20 of instability. (b) No transient ring indicates better = stability.

The conclusions which have been drawn from this series of experiments = are these: (1) that it is desirable to employ output power transistors in = which the transition frequency is at least ten times higher than the highest = signal frequency component which is passed to the amplifier from preceding = stages; (2) that it is preferable to drive the output transistors from a source = which has a low impedance over the whole signal voltage swing, or at least to = provide a reasonably low-resistance base-emitter current path; and (3) that the phase/frequency characteristics of the feedback loop should be such = that a square-wave output devoid of overshoots is obtained when the amplifier = is bench tested with a wide range of shunt capacitance values in an = RCdummy load.=20 This latter requirement probably implies either a fairly limited number = of=20 stages within the feedback loop or a relatively restricted h.f. = bandwidth.

When these requirements had been met, and when the harmonic = distortion levels=20 over the range 40mW up to the maximum rated power output were of a = suitably low=20 level, there was no audible difference, in the most careful listening = trials,=20 between several different designs. However, it is difficult in class B = systems=20 to obtain the desired low level of harmonic distortion at low signal = levels=20 without the use of substantial amounts of negative feedback, and this = leads to a=20 worsening of the amplifier response to signals containing = transients.

The use of a class AB system, if the problems in maintaining the = correct forward bias level can be solved satisfactorily, should facilitate the attainment of these desired standards, particularly if the h.f. negative-feedback loop can be made fairly simple.

Next month full details will be given of a 15-20W class AB amplifier = with the=20 following characteristics:-

Power output: 15W into 15ohm, or 18W into 8ohm (20W with = modified output circuit component values.)

Bandwidth: 10Hz-100kHz +/- 0.5dB at 2V output; 20Hz-50kHz +/- = 1.0dB at=20 maximum power output.

Output impedance: 0.03ohm (at 1kHz).

Total harmonic distortion: 0.02% at 15W/15ohm or 18W/8ohm; = less than 0.02% at all power levels below maximum output.

Intermodulatlon distortion: Less than 0.1% at 10W (12.3V = r.m.s. into 15ohm) and 70Hz, and at 1V r.m.s. at 10kHz.

Square-wave transfer distortion: Less than 0.2% at 10kHz.

REFERENCES

1. Linsley Hood, J. L., "Simple Class A Amplifier", Wireless = World,=20 April 1969.

2. Bailey, A. R., "30-watt High Fidelity Amplifier", Wireless = World,=20 May 1968.

3. Williamson, R., Hi-Fi News,Feb.1969, pp. 320-329.

4. Hardcastle, I., and Lane, B., "Low-cost 15-W Amplifier", = Wireless=20 World, Oct. 1969.

5. Shaw, I. M., "Quasi-complementary Output Stage = Modification", Wireless World, June 1969.

6. Baxandall, P. J., "Letters to the Editor", Wireless World, Sept.1969.

7. "Low Distortion Class B Output", Wireless World,April 1968.

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HISTORY:=20 Page created 20/07/2001