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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Amplifier-corrector for EPU ARKTUR-006-STEREO. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers Interest in recordings on vinyl records is still unabated, and articles are published on new circuits and designs of corrective amplifiers (UK). The articles give estimates of their quality either by actually achieved technical characteristics, or by subjective impressions of the sound; at the same time, in comparison of different ACs, preference is often given to the latter [1].
When upgrading old equipment, you can use a ready-made development, focusing on radio components that can be actually purchased, or implement your own by applying technical solutions that will certainly provide high quality playback. At the same time, it is not at all necessary to solve the problem by using modern ultra-low-noise and ultra-high-speed op-amps. For example, the overload capacity of the UK can be increased by connecting two stages in series to the OS. This solution makes it possible to reduce dynamic distortions and eliminate the influence of the noise of the second amplifier, and the rational use of active and passive correction circuits makes it possible to more accurately form the frequency response at high frequencies [2]. It is hardly worth striving to achieve an overload capacity of the UK up to 40 dB, since in my collection of 400 phonograph records there are not even ten recorded using DMM technology, which provides a low noise level and an increased level of high-frequency signal components. Using widely used externally corrected op amps and advanced compensation, it is possible to increase the output voltage slew rate up to 10 V/µs [3], which also helps to reduce dynamic distortions. In addition, if you load the output stage of the op-amp with a current generator, i.e. transfer it to class A amplification mode, you can significantly reduce harmonic distortion by narrowing their spectrum and increase the phase stability margin [4]. In [1] and [5], the authors point to a subjective assessment of the noise level of the AC in comparison with the noise level of the mute groove of a record. What signal-to-noise ratio is necessary so that when listening in a real environment and at real power, this noise does not irritate? According to GOST 5289-88/94, the signal-to-noise ratio of an unmodulated groove of a gramophone record should be at least 60 dB, and after 50 plays it should decrease by no more than 2 dB, therefore, this parameter of the UK can also be indirectly determined. The UK built into the EPU is made on the K157UD2 OS, the background level is indicated for it - 63 dB, the noise level and harmonic coefficient, unfortunately, are not indicated, obviously, the noise level is comparable to the background level. EPU "Arktur-006-stereo" of 1984 of release is used complete with the musical center SONY FH-B1200 and AS "Radio engineering S-90B". Here are some of the listed factors taken into account in the development of the proposed MC, the characteristics of which, according to the calculation, are much higher than those of the built-in regular one. Schematic diagram of one channel (for the DA1 chip, the pin numbers of the second channel are indicated in brackets) of the developed UK is shown in fig. 1. Based on the preamplifier circuit described in [3], and it was redesigned taking into account the above. The ratings of passive elements are calculated taking into account the following conditions - minimum ratings of capacitors; sufficient overload capacity and minimal non-linear distortion at a frequency of 20 kHz. The presence of high-quality K71-7 capacitors (polystyrene, soldered from the ZUSTST TV board) with a capacity of 4700 pF ± 1% gave rise to the calculation of the parameters of the remaining frequency correction elements. The corrector amplifier contains two stages. The first one is made on a low-noise two-channel op-amp KM551UD2A, in which the level of intrinsic noise, reduced to the input, does not exceed 1 μV in the frequency band of 20 Hz ... 20 kHz. Actually, it was developed for use in the UK (for the K157UD2 op-amp, this indicator reaches 1,6 μV). This cascade has a uniform frequency response and gain Ku=16. The correction circuits C2R4 and C3R5 provide the correction of the frequency response of the op amp DA1, which is necessary for stability and a sufficient slew rate of the output voltage. The output of the op-amp DA1 is loaded with a current generator on the transistor VT1, which ensures the operation of the output stage of the op-amp in mode A with a quiescent current of about 4 mA (in this case, the microcircuit case heats up to 40 ° C). The circuit R7,C5,R9 forms a passive correction with a time constant (R7IIR9)C5 = 75 µs, where the resistance of parallel connected resistors R7IIR9 = R7-R9(R7+R9). The load resistance of the op-amp DA1, taking into account the parallel circuits R3 + R4 and R7C5 at a frequency of 20 kHz, slightly exceeds 600 ohms, which corresponds to an overload capacity of the input stage of 15 dB at a frequency of 20 kHz at Kg = 0,015%. The second stage of the UK is made on the OS K553UD2 (DA2) in an inverting connection (Ku = 10 at a frequency of 1 kHz), and due to the use of advance correction through capacitor 09, the output voltage rise rate is increased. The main time constants (t = RC) of the corrective RC circuits included in the OS circuit are equal to R11 C12 = 318 μs, R12 C12 = 3180 μs. Another time constant is determined by the values of the elements R7, R9, C7 and, in accordance with the recommendations of the RIAA-78, is (R7 + R9) -C7 = 7950 μs. The output of the op amp DA2 is also loaded with a current generator on the transistor VT2, which ensures the operation of the output stage of the op amp in mode A with a quiescent current of 6 mA. The C13R16 circuit, together with a subsequent load with a resistance of 47 kOhm, forms a high-pass filter with a cutoff frequency of 12,5 Hz to attenuate infra-low-frequency interference from the operation of the EDA engine and when playing warped records, and the R14C16 low-pass filter with a cutoff frequency of 72 kHz attenuates noise and high-frequency interference. Capacitors C4, C6, C8, SYU, C14, C15 - filtering by power circuits, common to both channels of the Criminal Code. The gain in the CC channels at a frequency of 1 kHz is 103. In the UK, MLT-0,25 resistors (MLT-0,125 can be used) with a tolerance of ± 5%, marked with an asterisk in the diagram, are made up of two matched and connected in series; capacitor C5 - K73-17 with a tolerance of ± 5% (from ZUSTST), capacitors C7, C13 - K10-47 (TKE - ISO), but imported film ones are better (for example, small-sized HITANO from the MET, MEV groups at 100 V) with such same acceptable deviation. These capacitors were selected using a multimeter in pairs for each of the channels, and then the resistances of the corresponding resistors were calculated using the above formulas. The remaining capacitors are K10-47, KM-5, KM-6, KT, KD and imported oxide (Jamicon). Transistors VT1, VT2 - KT315, KT3102 with any letter index (under CONDITION Uke max> 25 V). Diodes VD1-VD4 - low-power silicon series KD521, KD522, KD503 and similar. Instead of OU K553UD2, you can use LM301.
All parts of the UK are placed on a printed circuit board measuring 115x77 mm from double-sided foil fiberglass 2 mm thick. The PCB drawing is shown in fig. 2. For ease of installation, unused leads of microcircuits (or panels) are bent at an angle or broken off. First of all, radio elements are mounted, the soldering of which is carried out from the side where the elements are located. The numbering and arrangement of the details of the left channel correspond to those shown in Fig. 2b below the middle line of the board to the details of the right channel, with the exception of those common to both channels. A properly assembled device does not need to be adjusted. It is only necessary, in the presence of a measuring record, to select the capacitance of the capacitor C1, achieving the minimum uneven frequency response of the UK in the frequency band of 10 ... 20 kHz (playing recording zones No. 3-6). As input shielded conductors, two pieces of a coaxial cable RK-75-2-13 m (linear capacitance 68 pF / m) 88 cm long (C = 60 pF) were used, which, together with capacitor C1 and mounting capacitance Cm = 4 pF make up the load capacitance recommended for the MF-104 head of 82 pF. However, for a new corrector, if desired, you can choose a head of higher quality. The UK is powered by a stabilized power supply (in the author's version on integrated voltage stabilizers 7815 and 7915). The supply voltage is supplied by two short twisted pairs. It is advisable to place the UK board in a shield that protects against electromagnetic fields. When setting up and operating, an effect appeared - the background level depends on the inclusion of the player's network plug: in one position there is a background, and in the other it is not; the reason is the presence of a capacitive coupling between the windings of the network transformer in the absence of a real ground and a third wire in the power cord. With the available measuring equipment, it was possible to objectively evaluate only the slew rate of the output voltage of the AC. To do this, a sinusoidal signal with a frequency of 5 kHz was applied from the function generator to the input of the UK (with the capacitor C20 disconnected) before the signal was limited at the output of the op-amp DA2. Then, instead of a sinusoidal signal, a pulse signal (meander) of a slightly smaller amplitude was applied. The signal was monitored by an oscilloscope at the input of the UK, the output of the op-amp DA1 and the output of the op-amp DA2. The waveform at the output of the op-amp DA1 practically corresponds to the shape of the input, and at the output of the op-amp DA2 the signal has a slight overshoot. When the amplitude of the input signal is reduced by half, the surge practically disappears. A subjective assessment of the signal-to-noise ratio showed that at a given maximum volume level, the noise level is barely noticeable and much lower compared to the noise level of a mute groove of a record. A subjective assessment of the quality of sound reproduction was carried out without a reference UK. I was pleased with the result: when listening to classical music, I noted the purity of sound throughout the entire sound spectrum. The following discs were used: Bach K. F. E. Concerto for harpsichord and string orchestra (С10-12417-8, Riga Record Factory, 1978), Rybnikov A. "Juno" and "Avos" (С60-18627-30, Leningrad Record Factory, 1980), Pink Floyd "Delicate Sound of Thunder" Live (F60-00543-007, digital, Aprelevsk Record Factory, 1989). Literature
Author: S. Semikhatsky
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