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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Combined feedback in UMZCH. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers Experimenting with a combination of various types of general feedback in the UMZCH, the author of the article achieved a positive effect of combining a small output impedance at low frequencies and a large one at medium and high frequencies. A similar version of the frequency-dependent output impedance should be recommended, first of all, for amplifiers that work with wideband dynamic heads, for example, in TVs and simple sound reproducing equipment. The refinement of the amplifier is simple, and the improvement in sound is noticeable not only to the sophisticated audiophile. As shown in [1, 2], in order to sharply reduce the intermodulation distortion of a dynamic loudspeaker, especially noticeable at medium and high frequencies, the output impedance of the UMZCH should be much greater than the resistance of the sound head. On the other hand, the operation of speakers with such an amplifier often leads to a deterioration in sound quality at low frequencies due to uneven frequency response in the resonance region. The existing contradiction in the requirements for UMZCH for the entire frequency band can be overcome by applying combined feedback in the amplifier. It is known that in order to increase the output impedance in the UMZCH, the OOS for the load current (OOS) is used. As shown in [3], natural oscillations of the diffuser of the head are effectively suppressed by the UMZCH, in which two feedback circuits operate simultaneously: negative voltage (NOOS) on the load and positive - load flow (POST). This combination of two types of feedback in the UMZCH can create a negative output impedance. This mode is used, as a rule, at frequencies below 200 Hz, which, on the one hand, provides effective damping of the cone oscillations near the mechanical resonance frequency of most bass and midrange dynamic heads, and on the other hand, there are no problems with the stability of the UMZCH covered by such feedback . Based on this, a device diagram was developed that implements the idea of \u200b\u200ba frequency-dependent output impedance of the UMZCH. So, to improve the sound quality of a dynamic loudspeaker at frequencies in the region of the main resonance (usually below 3 Hz), the amplifier must have some negative output impedance, and at frequencies above 4 Hz, the output impedance must increase to units - tens of kilo-ohms. The required value of the parameter at low frequencies is selected depending on the resistance of the voice coil of the dynamic head and its acoustic design, based on the considerations and recommendations in [XNUMX, XNUMX], or focusing on hearing. The block diagram of the device is shown in fig. 1. Amplifier A1 - UMZCH without its own feedback circuit; A2 - differential amplifier on the op-amp; LPF and HPF - filters of low and high frequencies, respectively, with the same cutoff frequency (in our case, f = 200 Hz); R3 - current sensor resistor (R3RH/ten); R10 is a resistor that regulates the depth of the OOCH.
The device works as follows. Part of the UMZCH, consisting of amplifier A2, low-pass filter, resistors R3 and R4, is for amplifier A1 a combination of two types of feedback (OSN and POST), which implements a negative output impedance of the UMZCH at low frequencies (below 200 Hz). The principle of operation of UMZCH with OOSN and POST is described in detail in [3]. The part of the device, containing R2, R3 and HPF, forms in UMZCH A1 a parallel load CNF at frequencies above 200 Hz, which creates a high output impedance of the UMZCH at these frequencies (current source mode for load). To test the stated idea, a UMZCH layout was made, the schematic diagram of which is shown in Fig. 2.
As UMZCH A1 for the layout, an old (from amateur radio "bins") TESLA MDA2020 chip was used - an analogue of TDA2020 and domestic K174UN11, as well as a self-made loudspeaker with one head ZGD-38E (new designation - 5GDSh-1-4) with a resistance of 4 ohms, used in televisions. The low-pass filter is assembled on the elements R3 and C2; HPF - on elements R4, C4; current sensor - R8; OOSN circuit divider - resistors R10, R11. The tuned resistor R7, connected in parallel to the current sensor, serves to set the optimal negative output resistance of the UMZCH. The presence of resistor R1 in the circuit is mandatory to set the UMZCH mode for direct current. The device is set up in the following order. Instead of a loudspeaker, connect its resistive equivalent (Rн=4 ohm). The engines of the tuned resistors R7 and R10 (see Fig. 2) are set to the lower position according to the diagram. After turning on the power, a sinusoidal signal with a frequency of 50 Hz is fed to the input of the UMZCH at such a level that the voltage amplitude at the input DA1 is 1 V (monitored by an oscilloscope). By adjusting the resistor R7, the voltage at the DA1 output is increased by p times, where p is the increase factor determined from the following relationships: p=1/(1-RO/Rн) or RO= -Rн(1-1/p). In the layout assembled by the author, the value of P = 2, while the output impedance of the UMZCH DA1 at the head resonance frequency (about 70 Hz) became negative -2 Ohm, providing optimal (by ear) damping of the ZGD-38E head in the used acoustic design. Next, the trimming resistor R10 achieves the previous value (1 V) of a 50 Hz signal at the output of the UMZCH DA1. Instead of a resistive equivalent, a loudspeaker is connected to the UMZCH. This completes the setup. Tests of the layout showed its undoubted superiority (noticeable not only to audiophile friends) over the same UMZCH with a conventional OOSN in terms of "transparency", intelligibility and enrichment of medium and high frequencies. When reproducing low frequencies, the characteristic overtones of an undamped diffuser were not observed. In the mock-up for sound comparison, it is easy to implement the "clean" OOST mode in the entire audio frequency band. To do this, simply (with the power off, of course) bridge the capacitors C2 and C4 (see Fig. 2) with jumpers. In this case, the electrical damping of the loudspeaker is eliminated, which becomes immediately noticeable to the ear. For those wishing to repeat or modify the UMZCH scheme, the following comments will be useful: If instead of DA1 you use UMZCH on discrete elements, it must be pre-configured outside the structure under consideration with its OOSN circuit according to the usual method (setting the quiescent current, "zero" at the output, selection of the correction circuit). Further, its OOCH circuit is excluded, and UMZCH is used in the structure shown in Fig. 2, possibly with its own power supply. If the original UMZCH does not have a high input impedance at the differential input, you can reduce the resistance of resistors R2, R3 and R4, proportionally increasing the capacitance C2, C4 to maintain a cutoff frequency of about 200 Hz). However, the resistance R3 should not be less than 2 kOhm. With all changes in the ratings in the circuit, it is necessary that the following relations are fulfilled: 1+R10/R11=Rн/R8;
Here under Rн the passport value of the head resistance at a frequency of f = 1000 Hz is understood. The considered structure of the amplifier fundamentally works either with a single dynamic loudspeaker or with a group radiator assembled from the same type of wideband heads connected in parallel or in series or in combination to obtain the required resistance and power. In loudspeakers that incorporate passive crossover filters with such an UMZCH, frequency response distortion in terms of sound pressure is likely to occur, since most filters require a low output impedance of the signal source in the entire audio frequency range [1]. UMZCH with a frequency-dependent output impedance, in my opinion, is applicable, first of all, in radio equipment with a single head built into the case, or a separate loudspeaker with a wideband head. Such an amplifier will also work effectively in the midrange band of three-way loudspeakers (with a crossover filter at the input and amplifiers for each band), where it will successfully "combat" parasitic overtones that occur despite acoustic damping and a high order of active crossover filters. At the same time, the "transparency" and "air" of the sound will be preserved. Inherent amplifier with OOST. This UMZCH can be recommended for homemade audiophile beginners who want to feel the "old tube" sound, but do not want to bother with winding output transformers (and it's hard to find old books on tube amplifier calculations). But this, of course, provided that a "decent" UM1CH with an initially low level of distortion, but not necessarily with a high output power, is used as DA3 - 3-15 W is quite enough (with a supply voltage of ± 15 ... 17,6) . The power supply for such amplifiers may be common. Literature
Specialist commentary The main advantage of the UMZCH proposed by the author lies in the simplicity of additional feedback circuits for "traditional" UMZCH. When implementing the proposed idea, some of its features should be taken into account. Firstly, an improvement in the sound reproduction of an UMZCH operating on an electrodynamic loudspeaker (EDG) through the use of a negative output impedance is achieved at low frequencies only for certain ratios of the sound head parameters with its acoustic design. Secondly, the possibilities for improving sound reproduction in the UMZCH - EDG complex at medium and high frequencies are limited by the applied method for achieving a high output impedance of the UMZCH - due to the OOST in the "traditional" UMZCH with a low output impedance. With this solution, in fact, there is a decrease in intermodulation distortions caused by a change in the impedance of the EDG when the voice coil is heated and nonlinearity for a large amplitude of its oscillations in the magnetic system, as well as a decrease in distortions of the EDG during electroacoustic conversion. However, the reduction of distortion due to the EDH response occurs most effectively only when using UMZCH with an initially large (without OOS) output impedance. In the proposed UMZCH, additional distortion of the frequency response may be due to inaccurate matching of the frequency response in the filters LPF (R2, C4) and HPF (C4, RXNUMX). The structure considered in the article is applicable to the UMZCH operating on the EDD with one or more broadband heads (without crossover filters). In multiband active EDGs, such an UMZCH is not needed, since there are no contradictions in the value of the output impedance of the UMZCH. Author: L. Syritso, Moscow; Publication: radioradar.net
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