Playback amplifier on the K157UL1 chip. The art of audio

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Playback amplifier on the K157UL1 chip

The art of audio

Currently, radio amateurs know many circuits of high-quality playback amplifiers (HF) for tape recorders, but most of them are quite difficult to repeat, use a scarce element base, and require selection of elements and careful tuning. Almost all high-quality CF circuits require a bipolar supply voltage, which makes it difficult to use them in self-powered equipment.

It is much easier to perform SW on a K157UL1 chip specially designed for this purpose. But among the designers there is an opinion that it is of little use for building a high-quality shock wave, since it has a high, by today's standards, harmonic coefficient and noise level. However, these shortcomings are largely caused by its incorrect use. When designing a microcircuit, special measures have already been taken [1] to reduce the noise level of its first and second stages - the NF signal is fed into the emitter circuit of the first stage, which contributes to an increase in the linearity and overload capacity and dynamic range of the shock wave as a whole, but when the microcircuit is turned on according to a typical scheme, its capabilities are not fully realized.

REPRODUCTION AMPLIFIER ON THE K157UL1 CHIP

The main disadvantage of a typical switching circuit is the presence of a separating oxide capacitor in the playback magnetic head (MG) circuit, which significantly increases the noise level of the SW. Attempts were made by radio amateurs to eliminate this drawback [2,3], but in all cases the exclusion of an isolation capacitor required a bipolar supply.

Another disadvantage of a typical switching circuit (as well as most other well-known construction circuits) is that the output signal after the amplifier on the K157UL1 chip reaches 180 ... 200 mV. To obtain the required SW transfer coefficient (50 ... 6 0 dB at a frequency of 400 Hz), it is necessary to reduce the depth of the OOS. Since the output stage is made according to the scheme of an emitter follower with a current generator in the load [1] and does not have a high linearity, this leads to an increase in the harmonic coefficient of the SW to 0,3 ... 0%.

Based on this, it is easy to conclude that in order to improve the characteristics of the SW on K157UL1, it is necessary to increase the depth of the OOS, covering the amplifier, and to exclude the isolation capacitor from the MG circuit. In order to avoid the use of a bipolar power supply in this case, it is necessary to use the inclusion of the MG in the circuit for supplying a bias to the base of the transistor of the first stage of the SW [4].

Designed with these requirements in mind and manufactured by the author, the UV on K157UL1 was compared in sound with the tape recorder-prefix "Radio Engineering MP-201" (UV on K157UL1 according to a typical switching scheme). In both cases permalloy magnetic heads 3D24N.1Y produced by the Republic of Hungary were used. The subjective examination confidently noted a "lighter" and "transparent" sound of the proposed version compared to the standard one, better reproduction of low and high frequencies.

The circuit design of the SW is quite simple and does not require special explanations. The MG is connected to the input of the SW in parallel with the element of the circuit for setting the mode of the first stage without an isolation capacitor. Capacitor C2 eliminates the OOS for alternating current, its purpose is the same as with the typical switching circuit K157UL1. High-frequency correction is carried out in the MG circuit by a resonant oscillatory circuit, composed by the capacitance of the capacitor C1 and the inductance of the magnetic head B1.

This circuit is tuned to the upper frequency of the operating range. Low-frequency correction is carried out in the frequency-dependent OOS circuit, covering the amplifier. Circuit time constants:

T1 = R3C3 = 3300mks, T2 = (R1+R2) x C3 = 122mks

A signal of about 30 mV from the SW output (pins 9 and 13 for different channels) is fed to a linear amplifier with a gain of 20 ... 25 dB, which can be made according to any scheme, for example, on the K157UD2 op-amp. Regulators of the rated output voltage can be turned on at the input of a linear amplifier or in its OOS circuit.

The reproduction amplifier is not critical to the types of parts used. Establishing an error-free mounted SW is reduced to tuning to the required frequency of the resonant oscillatory circuit. Using the proposed switching option, it is not difficult to upgrade industrial tape recorders that use this microcircuit in HC.

Literature

Ataev D., Bolotnikov V. Analog circuits for household radio equipment. Directory. - M, MPEI Publishing House, 1992, pp. 81-84.
Bereznyuk N. Playback amplifier - Radio, 1987, No. 9, p.42
Kuznetsov A. Improvement of Mayak-231. - Radio, 1987, No. 8 p.57.
US patent No. 4041538 dated 12.04.76/XNUMX/XNUMX

Publication: www.bluesmobil.com/shikhman

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