15. Performance Standards
It has always been accepted in the past, and is still held as axiomatic among a very large section of the engineering community, that performance characteristics can be measured and that improved levels of measured performance will correlate precisely, within the ability of the ear to detect such small differences, with improvements that the listener will hear in reproduced sound quality.
Within a strictly engineering context, it is difficult to do anything other than accept the concept that measured improvements in performance are the only things that should concern the designer.
However, the frequently repeated claim by journalists and reviewers working for periodicals in the hi-fi field-who, admittedly, are unlikely to be unbiased witnesses -- that measured improvements in performance do not always go hand in hand with the impressions that the listener may form, tends to undermine the confidence of the circuit designer that the instrumentally determined performance parameters are all that matter.
It is clear that it is essential for engineering progress that circuit design improvements must be sought that lead to measurable performance improvements. However, there is now also the more difficult criterion that those things that appear to be better, in respect to measured parameters, must also be seen, or heard, to be better.
FIG. 41: Method of fabrication of components in a silicon-integrated circuit.
15.1 Use of ICs
This point is particularly relevant to the question of whether, in very high-quality audio equipment, it is acceptable to use IC operational amplifiers, such as the TL071, or some of the even more exotic later developments such as the NE5534 or the OP27, as the basic gain blocks, around which the passive circuitry can be arranged, or whether, as some designers believe, it is preferable to construct such gain blocks entirely from discrete components.
Some years ago, there was a valid technical justification for this reluctance to use op-amp ICs in high-quality audio circuitry, as the method of construction of such ICs was as shown, schematically, in FIG. 41 , in which all the structural components were formed on the surface of a heavily ' P ' doped silicon substrate, and relied for their isolation from one another or from the common substrate on the reverse-biased diodes formed between these elements.
This led to a relatively high residual background noise level, in comparison with discrete component circuitry, due to the effects of the multiplicity of reverse diode leakage currents associated with every component on the chip. Additionally, there were quality constraints in respect to the components formed on the chip surface-more severe for some component types than for others-that also impaired the circuit performance.
A particular instance of this problem arose in the case of PNP transistors used in normal ICs, where the circuit layout did not allow these to be formed with the substrate acting as the collector junction. In this case, it was necessary to employ the type of construction known as a " lateral PNP, " in which all the junctions are diffused in, from the exposed chip surface, side by side.
In this type of device the width of the ' N ' type base region, which must be very small for optimum results, depends mainly on the precision with which the various diffusion masking layers can be applied. The results are seldom very satisfactory. Such a lateral PNP device has a very poor current gain and HF performance.
In recent IC designs, considerable ingenuity has been shown in the choice of circuit layout to avoid the need to employ such unsatisfactory components in areas where their shortcomings would affect the end result. Substantial improvements, both in the purity of the base materials and in diffusion technology, have allowed the inherent noise background to be reduced to a level where it is no longer of practical concern.
15.2 Modern Standards
The standard of performance that is now obtainable in audio applications, from some of the recent IC op-amps, especially at relatively low closed-loop gain levels, is frequently of the same order as that of the best discrete component designs, but with considerable advantages in other respects, such as cost, reliability, and small size.
This has led to their increasing acceptance as practical gain blocks, even in very high quality audio equipment.
When blanket criticism is made of the use of ICs in audio circuitry, it should be remembered that the 741, which was one of the earliest of these ICs to offer a satisfactory performance-although it is outclassed by more recent types-has been adopted with enthusiasm, as a universal gain block, for the signal handling chains in many recording and broadcasting studios.
This implies that the bulk of the program signals employed by the critics to judge whether or not a discrete component circuit is better than that using an IC will already have passed through a sizeable handful of 741-based circuit blocks, and if such ICs introduce audible defects, then their reference source is already suspect.
It is difficult to stipulate the level of performance that will be adequate in a high-quality audio installation. This arises partly because there is little agreement between engineers and circuit designers, on the one hand, and the hi-fi fraternity, on the other hand, about the characteristics that should be sought and partly because of the wide differences that exist between listeners in their expectations for sound quality or their sensitivity to distortions. These differences combine to make it a difficult and speculative task to attempt either to quantify or to specify the technical components of audio quality or to establish an acceptable minimum-quality level.
Because of this uncertainty, the designer of equipment in which price is not a major consideration will normally seek to attain standards substantially in excess of those that he supposes to be necessary, simply in order not to fall short. This means that the reason for the small residual differences in the sound quality, as between high-quality units, is the existence of malfunctions of types that are not currently known or measured.
16 Audibility of Distortion
16.1 Harmonic and Intermodulation Distortion
Because of the small dissipations that are normally involved, almost all discrete component voltage amplifier circuitry will operate in class ' A ' (that condition in which the bias applied to the amplifying device is such as to make it operate in the middle of the linear region of its input/output transfer characteristic), and the residual harmonic components are likely to be mainly either second or third order, which are audibly much more tolerable than higher order distortion components.
Experiments in the late 1940s suggested that the level of audibility for second and third harmonics was of the order of 0.6 and 0.25%, respectively, which led to the setting of a target value, within the audio spectrum, of 0.1% THD, as desirable for high-quality audio equipment.
However, recent work aimed at discovering the ability of an average listener to detect the presence of low-order (i.e., second or third) harmonic distortions has drawn the uncomfortable conclusion that listeners, taken from a cross section of the public, may rate a signal to which 0.5% second harmonic distortion has been added as " more musical " than, and therefore preferable to, the original undistorted input. This discovery tends to cast doubt on the value of some subjective testing of equipment.
What is not in dispute is that the intermodulation distortion (IMD), which is associated with any nonlinearity in the transfer characteristics, leads to a muddling of the sound picture so that if the listener is asked not which sound he prefers, but which sound seems to him to be the clearer, he will generally choose that with the lower harmonic content.
The way in which IMD arises is shown in FIG. 42 , where a composite signal containing both high-frequency and low-frequency components, fed through a nonlinear system, causes each signal to be modulated by the other. This is conspicuous in the drawing in respect to the HF component, but is also true for the LF one.
This can be shown mathematically to be due to the generation of sum and difference products, in addition to the original signal components, and provides a simple method, shown schematically in FIG. 43 , for the detection of this type of defect. A more formal IMD measurement system is shown in FIG. 44 .
With present circuit technology and device types, it is customary to design for total harmonic and IM distortions to be below 0.01% over the range 30 Hz-20 kHz, and at all signal levels below the onset of clipping. Linear IC op-amps, such as the TL071 and the LF351, will also meet this specification over the frequency range 30 Hz-10 kHz.
FIG. 42: Intermodulation distortions produced by the effect of a nonlinear
input/output transfer characteristic on a complex tone.
FIG. 43: Simple HF two-tone intermodulation distortion test.
FIG. 45: Effect of amplifier slew-rate saturation or transient intermodulation
distortion.
FIG. 46: Typical amplifier layout causing slew-rate saturation.
FIG. 47: Transient "ringing."
FIG. 48: Circuit design aspects that may cause slew-rate limiting.
FIG. 49: Input HF limiting circuit to lessen slew-rate limiting.
16.2 Transient Defects
A more insidious group of signal distortions may occur when brief signals of a transient nature, or sudden step type changes in base level, are superimposed on the more continuous components of the program signal. These defects can take the form of slew-rate distortions, usually associated with a loss of signal during the period of the slew-rate saturation of the amplifier-often referred to as transient intermodulation distortion or TID.
This defect is illustrated in FIG. 45 and arises particularly in amplifier systems employing substantial amounts of negative feedback when there is some slew-rate limiting component within the amplifier, as shown in FIG. 46 .
A further problem is that due to "overshoot, " or "ringing," on a transient input, as illustrated in FIG. 47. This arises particularly in feedback amplifiers if there is an inadequate stability margin in the feedback loop, particularly under reactive load conditions, but will also occur in low-pass filter systems if too high an attenuation rate is employed.
The ear is very sensitive to slew-rate induced distortion, which is perceived as a "tizziness" in the reproduced sound. Transient overshoot is normally noted as a somewhat overbright quality. The avoidance of both these problems demands care in the circuit design, particularly when a constant current source is used, as shown in FIG. 48 .
In this circuit, the constant current source, CC 1 , will impose an absolute limit on the possible rate of change of potential across the capacitance, C1 (which could well be simply the circuit stray capacitance), when the output voltage is caused to move in a positive-going direction. This problem is compounded if an additional current limit mechanism, CC 2 , is included in the circuitry to protect the amplifier transistor (Q1) from output current overload.
Since output load and other inadvertent capacitances are unavoidable, it is essential to ensure that all such current limited stages operate at a current level that allows potential slewing to occur at rates that are at least 10 greater than the fastest signal components.
Alternatively, means may be taken, by way of a simple input integrating circuit, ( R1C1 ), as shown in FIG. 49 , to ensure that the maximum rate of change of the input signal voltage is within the ability of the amplifier to handle it.
16.3 Spurious Signals
In addition to harmonic, IM, and transient defects in the signal channel, which will show up on normal instrumental testing, there is a whole range of spurious signals that may not arise in such tests. The most common of these is that of the intrusion of noise and alien signals, either from the supply line or by direct radio pick up.
This latter case is a random and capricious problem that can only be solved by steps appropriate to the circuit design in question. However, supply line intrusions, whether because of unwanted signals from the power supply or from the other channel in a stereo system, may be reduced greatly by the use of circuit designs offering a high immunity to voltage fluctuations on the DC supply.
Other steps, such as the use of electronically stabilized DC supplies or the use of separate power supplies in a stereo amplifier, are helpful, but the required high level of supply line signal rejection should be sought as a design feature before other palliatives are applied.
Modern IC op-amps offer a typical supply voltage rejection ratio of 90 dB (30,000:1). Good discrete component designs should offer at least 80 dB (10,000:1).
This figure tends to degrade at higher frequencies, which has led to the growing use of supply line bypass capacitors having a low effective series resistance. This feature is either a result of the capacitor design or is achieved in the circuit by the designer's adoption of groups of parallel connected capacitors chosen so that the AC impedance remains low over a wide range of frequencies.
A particular problem in respect to spurious signals, which occurs in audio power amplifiers, is a consequence of the loudspeaker acting as a voltage generator, when stimulated by pressure waves within the cabinet, and injecting unwanted audio components directly into the negative feedback loop of the amplifier. This specific problem is unlikely to arise in small signal circuitry, but the designer must consider what effect output/line load characteristics may have, particularly in respect to reduced stability margin in a feedback amplifier.
In all amplifier systems there is a likelihood of microphonic effects due to vibration of the components. This is likely to be of increasing importance at the input of " low-level, " high-sensitivity preamplifier stages and can lead to coloration of the signal when the equipment is in use, which is overlooked in the laboratory in a quiet environment.
16.4 Mains-Borne Interference
Mains-borne interference, as evidenced by noise pulses on switching electrical loads, is most commonly due to radio pick up problems and is soluble by the techniques (attention to signal and earth line paths, avoidance of excessive HF bandwidth at the input stages) that are applicable to these.
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