Improving sound reproduction in the UMZCH-loudspeaker system. Encyclopedia of radio electronics and electrical engineering

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When listening, experts often prefer tube UMZCH, despite the fact that transistor ones formally have higher parameters. What's the matter? The hypothesis of the authors of the article about the occurrence of additional intermodulation distortions in the UMZCH due to the loudspeaker response was experimentally confirmed by them in the process of searching for a method for objectively assessing the quality of amplifiers. The article gives a critical analysis of the technical solutions of modern UMZCH and proposes measures that exclude the influence of the loudspeaker on the amplifier. The authors claim that transistorized UMZCH, resistant to the influence of the loudspeaker response, provide sound reproduction without specific coloring.

In classical two-channel stereophony, the quality of power amplifiers and loudspeakers has a significant impact on the realization of the potential for natural sound reproduction and the spatiality of the sound picture. Attentive listeners visiting concert halls immediately notice the difference between the sound of real musical instruments and their sound in a sound recording played through loudspeakers.

Difficulties in predicting the quality of sound reproduction are associated with the imperfection of the methods used for objective measurements of the characteristics of the sound path. Therefore, the main criterion for choosing audio equipment should be considered a subjective quality assessment (SQA).

The properties of the final links of the sound reproduction path - UMZCH and loudspeaker have the greatest influence on the results of the SOC. Consider their features and possibilities for solving existing problems.

First of all, let us evaluate the relationship between the results of the SOC and the objective characteristics of the UMZCH. focusing on only the parameters that, according to the authors, have the greatest impact on the quality of sound reproduction. Here, of great interest is the analysis of the results of the SOC of lamp and transistor UMZCH (as components between which there is the sharpest difference in estimates). As a rule, in these comparisons, the objective parameters of tube UMZCHs are significantly inferior to transistor ones, but the results of SOCs often turn out to be directly opposite. When considering, we confine ourselves to only a few basic criteria for QMS, using the wording that experts most often use.

The first sound characteristic is timbre coloring: lightness, softness, warmth or, respectively, heaviness, hardness, coldness (metallic shade). The second is the reproduction of an attack (growing sound): active, clear or sluggish, loose. The third characteristic is the localization of the signal source: good or bad panorama. Fourth - microdynamics: good detailing of complex-shaped signals with a low level or poorly distinguishable detailing of similar signals. The overall result of the SOC: a strong emotional impact or, accordingly, a weak one.

Expert assessments of the compared UMZCH are so different that there are slang expressions - "tube" and "transistor" sound. Explanations of the causes of this paradox have been repeatedly cited in the literature, but all of them give only partial answers. Let us try once again to establish the relationship between the SOC criteria considered here and the objective parameters of the compared UMZCH.

Features of timbre coloring in sound for tube UMZCH can be explained by the following main reasons:

• narrow spectrum of non-linear distortions (NI), usually limited to the second and third harmonics;
• low dependence of the NI value on the signal frequency;
• a sharp decrease in the width of the spectrum and the magnitude of NI with a decrease in the signal level;
• smooth increase in NI when the output stage is overloaded.

Features of the timbre coloring of the sound for transistor UMZCH have the following reasons:

• extended range of NI (up to the 10th harmonic and above);
• a sharp increase in NI with an increase in the frequency of the signal;
• slight decrease in the width of the spectrum and the magnitude of NI with a decrease in the signal level;
• a sharp increase in NI when the output stage is overloaded.

Reproduction of an undistorted attack of sound signals is the most important condition for accurate recognition of the source image. Obviously, the appearance of attack distortions (delay or accent) in the sound reproduction of real signals significantly affects its perception. One of the reasons for this kind of distortion is the conditions for matching the UMZCH system - an electrodynamic loudspeaker (EDG).

As is known, when a pulsed signal acts on a voice coil (VC), a force arises in the EDH that tends to change its position in the magnetic field, i.e., to move. However, the back EMF of induction that occurs in this case, closing on the output resistance of the UMZCH, creates a current that prevents the change in the position of the ZK and is directed towards the current that causes this change, i.e., the output current of the UMZCH. The flow of "countercurrent", on the one hand, reduces the quality factor of mechanical resonance and enhances damping [1], the effectiveness of which depends on the output resistance of the UMZCH, and on the other hand, this leads to a delay in the reproducible attack of the musical signal. Thus, this process is directly dependent on the value of the "countercurrent", which, with a constant value of the back-EMF, is the greater, the lower the output resistance of the UMZCH. Any change in the value of the output impedance (for example, due to the frequency dependence of the OOS depth) leads to a change in the "backflow" and distortion of the attack. Similar distortions arise due to a change in the inductance of the ZK [1] in its various positions inside the magnetic system and excitation of the EDH from a voltage source. Comparison of the values ​​of the output impedance of a tube (0,5 ... 1,5 Ohm) and transistor (usually 0,1 Ohm or less) amplifiers allows us to conclude that a larger resistance value is preferred.

One should not exclude the influence on the accuracy of attack reproduction and little-studied distortions from thermophysical processes in active and passive elements of UMZCH, EDG and "acoustic" cables.

Localization of signal sources and microdynamics are considered to be the next important characteristics of RNS. These characteristics, according to the authors, are determined mainly by the magnitude and spectrum of intermodulation distortion (II) in the UMZCH-EDG system.

Thus, at the first stage, the following conclusions can be drawn:

1. The results of the SOC of the UMZCH - EDG system are determined by the totality of its technical characteristics and formally do not depend on the type of active elements used in the amplifier.

2. The greatest influence on the timbre coloration is exerted by the magnitude and width of the NI spectrum, as well as their dependence on the frequency and level of the sound signal.

3. The accuracy of reproduction of the sound signal attack depends, in particular, on the current caused by the back-EMF of the EDH induction and distortions from thermophysical processes in active and passive elements of high-current circuits.

4. Localization of signal sources and microdynamics are determined mainly by the magnitude and spectrum of the IR.

Now let's analyze the possibilities of improving the UMZCH parameters that have the greatest impact on the SOC.

Let's start with methods for reducing the magnitude and spectrum of NI. Studies of these types of distortions have established two main causes of their occurrence - the nonlinearity of the characteristics of active elements and the operating mode of the output stage. Some of the linearity advantages of vacuum tubes compared to transistors are well known and well documented in the literature. Improving the transistor UMZCH in this parameter is most effective when using the operating modes of the output stage transistors without cutting off the collector current, for example: Super A, New class A, Non switching [2, 3], etc. In these operating modes, there is not only a significant reduction in the NI spectrum (up to the fourth-fifth harmonic) and their values, but also their sharp decrease with a decrease in the signal level. The frequency independence of NI is usually achieved by choosing the appropriate circuitry and elements. A compensatory method known as "feed forward error correction" - correction of distortions using a direct connection has a high efficiency in reducing NI [4, 5]. Quite promising methods for reducing NI include compensatory with feedback on the subtraction of distortions - OSVI [6].

When designing transistor UMZCH, it is necessary to take into account the peculiarities of the operation of transistors of the UMZCH output stage when operating on a real load. The reasons for the appearance of various distortions and methods for their reduction are detailed in [7–9], but the methods for controlling distortions proposed there are extremely complex and require expensive measuring equipment. The probability of distortions can be significantly reduced using recommendations, for example, in [10]. The best results in reducing NI in transistorized UMZCH are achieved by using the operating mode of the output stage in class A with a minimum depth of the overall OOS. At the same time, NI can be much lower than in tube amplifiers, due to the absence of an output transformer in them - a source of distortion at low frequencies.

A smoother increase in NI when the output stage is overloaded in transistor UMZCH is achieved by reducing the depth of the overall OOS - the effect is higher, the smaller its depth.

Let us further consider possible methods for increasing the accuracy of reproducing an attack of an audio signal, taking into account the reasons that have a great influence on it.

Like transient intermodulation distortion, attack distortion is reduced quite effectively as the depth of the overall feedback is reduced. The expansion of the frequency response of the UMZCH without a common OOS to 300 ... 500 kHz also contributes to the reduction of the signal establishment time in the UMZCH.

However, a particularly effective reduction in the distortion of the attack from the current in the load circuit, caused by the induction back-emf, is achieved in the UMZCH with a high output impedance (RplL>> Rh). The results of improving the characteristics of the audio path are described in detail in [11 - 13]. On fig. Figures 1 and 2 show the spectrograms of harmonic distortions (12) when the EDH is excited from an UMZCH with a low output impedance and from an UMZCH with a high output impedance. The total harmonic distortion for a 3 kHz signal is about 3% and 0,2%, respectively.

An analysis of the modeling of distortions caused by thermophysical processes occurring in active and passive elements of the sound path made it possible to practically implement a passive device that improves the accuracy of attack reproduction [14].

The methods listed above for improving the quality of attack reproduction show their influence on the final result and explain the reasons for unsuccessful attempts to achieve this only by increasing the slew rate of the UMZCH output voltage.

Considerable difficulties are caused by the decrease in IS due to the multiplicity of causes of their occurrence and the complexity of detection [15-20]. To a large extent, the solution of the problem is hindered by the measurement methods used, which do not allow predicting the expert assessment with sufficient accuracy. In [21], a more informative method for measuring the noise intermodulation coefficient (NIR) was proposed. However, the analysis of the results of the SOC and with this measurement method also does not explain the reasons for the sharp difference in the estimates: for example, for a lamp UMZCH - 9 points, and for a transistor - 5. And this is with minor differences in KSI - 0,8% and 0,9%, respectively . Therefore, this method also needs to be improved.

An attempt to explain subjective assessments for this case of measurements led the authors to experimentally test the hypothesis about the possible effect on IS in UMZCH of the response (impulse response) of the EDG (1). For this, the same method of measuring the CSI was used, but instead of the resistive load of the UMZCH, a real EDH was used. Special attention should be paid to the need to use in these measurements the real EDD and not its equivalent, which does not take into account the nonlinearities of the signal transformation. At the same time, a sharp increase in CSI was found only for a transistor UMZCH with a low output resistance: instead of 0,9%, it became 9,7%, i.e., there was an increase of more than 10 times. For the lamp UMZCH, these values ​​were 0,8% and 1,2%, respectively.

The main difference when replacing a resistive load equivalent with a real EDG is that it is in the OOS circuit. in addition to the output voltage of the UMZCH signal and its distortion, the response from the EDG additionally penetrates. In the OOS loop, they are combined and a signal for compensating for UMZCH distortions and a response from the EDD with the corresponding magnitude and phase is formed. The frequency spectrum of the compensation signal in this case can be 10-30 times higher than the upper limit of the audio signal.

Obviously, the main requirement for eliminating distortions is their exact compensation, which is practically impossible to implement. The limitations are related to the real frequency response and phase response of the UMZCH, with the level of distortion and noise. In addition, the compensation regime is also significantly affected by the non-linearity of the EDH characteristics. Thus, the compensation is incomplete. The best compensation in this case is achieved only for the relatively low-frequency components of the spectrum of the products of UMZCH distortion and the response from the EDD, and the high-frequency components of the spectrum of these oscillations again fall into the OOS circuit, causing new pre-emphasis in the amplifier. There is a vicious circle that generates a sharp increase in the high-frequency components of the distortion. Increasing the depth of the overall OOS of the amplifier leads only to a further expansion of the spectrum of distortion and, accordingly, to an even greater deterioration in the quality of sound reproduction.

In addition, conditions are created under which it becomes possible that a simple conductor, such as the UMZCH-EDG connecting cable, due to differences in its distributed parameters, is able to influence the results of the SOC, increasing or weakening certain harmonics from their rich variety. At the same time, another hypothesis appears, proposed by the authors to explain the mysterious reasons for the influence of acoustic cables on the results of the SOC: it becomes possible to consider them as a "sonic valve" - ​​LPF, which weakens the penetration of the response from the EDG to the UMZCH output.

Now we will show the reasons for the small influence on the AI ​​of the response from the EDG in tube UMZCH, which, as a rule, have a matching output transformer and a relatively shallow OOS depth. If we take into account that all the troubles from the EDD response signal are caused by the penetration of the high-frequency components of its spectrum, i.e., interference, then it is obvious that the leakage inductance of the output transformer can play a useful role as a low-pass filter, significantly attenuating the amount of high-frequency interference penetrating into the amplifier. In addition, the shallow FOS depth also contributes to a decrease in the effect of the response from the EDG. The authors believe that the processes described here in the UMZCH-EDG system largely explain the difference in the SOC of lamp and transistor UMZCHs obtained in the experiment [21].

The results of the analysis indicate the possible effect of two components of AI in the UMZCH - EDG system. One is its own AI in UMZCH, which can be objectively measured (KSI) with a resistive load equivalent. The second one is the IS induced in the UMZCH under the influence of the EDD response. The detection of the second component occurs when the UMZCH is loaded on the real EDD by repeated measurement of the CSI.

This allows us to recommend the design of the UMZCH in such a way that the circuitry provides the minimum own AI in the UMZCH. To analyze their spectrum, you can use a slightly modified technique for measuring the CSI, analyzing the noise in one-third octave bands. At this stage, the close relationship between NI and AI should be taken into account, using known methods to reduce them.

As can be seen from the above, the most effective method to reduce the influence of the response from the EDD on the increase in IS in the UMZCH is to exclude the conditions for its interaction with other signals in the FOS loop. There are various methods to accomplish this task. For example, a passive matching device, called a dissipator, has a high efficiency [14]. However, there are significant losses in signal power. Another example of a simpler implementation is UMZCH on field-effect transistors using an output transformer. In this case, the achieved effect is much inferior to the dissipator, but the output power losses are reduced. The maximum effect of reducing the effect of the EDG response on NI is achieved while maintaining high efficiency and the absence of the influence of UMZCH-EDG acoustic cables only by using UMZCH with a high output impedance [12, 13]. active and passive elements, changes in dynamic range and signal intermodulation due to thermal compression.

With this solution, the accuracy of attack reproduction is significantly improved. The distortions that occur in the EDD are also significantly reduced for the following reasons:

• the non-linearity of its impedance due to the displacement of the voice coil and the change in its active resistance due to current at high signal levels;
• non-linear nature of the interaction of an alternating magnetic field around the voice coil with a constant magnetic field in the gap;
• the presence of additional forces of attraction between the magnetic circuit and the voice coil when it is displaced from the core.

Based on the foregoing, it is possible to draw the following conclusions:

1. The results of objective measurements of the CSI in the UMZCH when it is loaded on a real EDG make it possible to predict the results of the SOC of the UMZCH - EDG system.

2. A decrease in the magnitude and spectrum of NI and IS, their frequency independence and smooth increase during overloads are necessary conditions for achieving high fidelity of sound reproduction in the UMZCH - EDG system. The sensitivity of UMZCH to the EDH reaction should be minimal.

3. The greatest effect in improving the quality of sound reproduction can be achieved by using an EDG with an UMZCH having a high output impedance.

Literature

1. Aldoshina I. A. Electrodynamic loudspeakers. - M .. Radio and communications. 1989, p. 15, 119.
2. Kondo Hikaru. Nuevo concepto en arnplificatores cle polencia para audio sistema "super A" cle JVC. - Mundo electromco, 1980, no. 102, p. 75-81
3. Tanaka S. New Biasing Circuit far Class B Operation. - Jaes. vol. 29, No. 3, 1981, March, p. 148-152.
4. Reshetnikov O. Distortion reduction in power amplifiers. - Radio, 1979, No. 12, p. 40-42.
5. Solntsev Yu. High-quality power amplifier. - Radio, 1984, No. 5, p. 29-34.
6. Russi O. UMZCH with distortion subtraction feedback. - Radio, 1997, No. 3, p. 12-14.
7. Cherry E., Combell G. Output Resistance and Intermolulalion Distortion of Feedback Amplifiers. - Jaes. vol. 30, 1982, no. 4, p. 178-191.
8. Otala M.. Lammasniemi J. Intermociulation Distortion in the Loudspeaker-Amplifiers Interface. 59 Convention of the AES, Hamburg, 1978, preprint. No. 1336.
9. Bengamin E. Audio Power Amplifiers lot Loudspeaker Loads. - Jaes. vol. 42, No. 9, 1994, p. 670-683.
10. Syritso A. Work of UMZCH on a complex load - Radio, 1994, No. 1, p. 17-19.
11. Ageev S. Should the UMZCH have a low output impedance? - Radio, 1997, No. 4, p. 14-16.
12. Mills PGL, Hawksford M. O. J. Distortion Reduction in Moving-Coil Loudspeaker Systems Using Current-Drive Technology. - Jaes. vol. 37, No. 3, 1989, March, p. 129-148.
13. Mills PGL, Hawksford MOJ Transconduciance Power Amplifier Systems for Current-Driven Loudspeakers. - Jaes. vol 37, no. 10, 1989, Oct, p. 809-822.
14. Kunafin R., Sokolov A, "Russian Hi-End'99". - Radio, 1999, No. 8, p. 28-30.
15. Cherry EM Nested Differentiating Feedback Loops in Simple Audio Power Amplifiers. - JAES, vol. 30, 1982, No. 5, p. 295-305.
16. Hawksford MOJ Distortion Correction in Audio Power Amplifiers - JAES. vol.29, 1981, no. 1/2, p. 27-30.
17. Margon E. Crossover distortion in class В amplifiers - Electronics & Wireless World. July 1987, p. 739-742.
18. Mcloughlin M. Reducing crossover distortion. - Electronics & Wi/eless World. 1999, Oct.. p. 879-882.
19. Cherevan Yu. UMZCH with dynamic response correction. - Radio, 1990, No. 2, p. 62-68.
20. Petrt-Larmi M., Otala M., Lammasniemi J. Psychoacoustic Detection Threshold of Transient Intermodulation on Distortion. - JAES, 1980. vol. 28. No. 3. p. 98-104.
21. Syritso A. Measurement of nonlinear distortions on a noise signal. - Radio, 1999. No. 4. With. 29.

Authors: A. Aleynov, Kharkov, A. Syritso, Moscow

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