Again about the completion of tape recorders. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

 Comments on the article

The sound quality of a modern cassette recorder can be as good as that of an inexpensive CD player. How to improve the quality of some domestic-made tape recorders and bring them up to this level is described in this article.

With the advent of digital methods of recording sound in recent years, the demands made by amateurs on household magnetic recording equipment (BAMZ) have increased significantly. The loss of quality when re-recording from a compact disc (CD) to a cassette tape recorder produced in the eighties - early nineties turned out to be too great. However, switching exclusively to CD requires considerable material costs: their prices are quite high, and the cost of a mid-range player exceeds $150. A high-quality imported cassette tape recorder is even more expensive, and domestic devices are not able to compete in the market.

On the pages of "Radio" and other radio engineering literature, materials have been repeatedly published on the refinement of domestic cassette tape recorders, which make it possible to improve the quality of recording and playback [1]. However, the costs of implementing many of the recommendations did not always pay off: often a cardinal alteration of the tape recorder itself was required. In addition, not every radio amateur has the necessary instruments for tuning. The tuning methods proposed by the authors of a number of publications are often "vague" and do not contain specific advice on adjusting the equipment. Many of these shortcomings are taken into account in the published article.

The author's recommendations mainly relate to a cassette recorder, which has better operational conveniences than a reel-to-reel one. However, the proposed refinement will slightly increase the dynamic range in the high frequencies and reel-to-reel tape recorder.

So, what kind of tape recorder should be finalized?

First of all, you should evaluate the quality of performance and the operation of the tape drive mechanism (LPM) of the tape recorder. Its refinement is a separate topic: a cardinal improvement in the LPM is associated with the performance of precise turning work (which is not always possible) and is not considered in this article. It should be noted that in the domestic BAMZ produced in the 80s, the best CVLs are installed in Vilma prefix tape recorders of all models, Sanda MP-207S, Vega MP-120S, Vega MP-122S, Morion MP- 101S", "Yauza MP-220S", "Yauza MP-221S". As for the LPM tape recorders "Mayak" (almost all models), "Comet", "Note", they do not provide high stability of the tape feed and do not allow you to accurately determine the moments of winding and slowing down. Due to the use of asynchronous motors in them, it is practically impossible to accurately set the speed of the tape, and the DC motors that appeared in later models have low power and do not provide high stability of the tape movement, especially when switching the operating mode of another LPM (in two-cassette tape recorders). This applies to the Mayak MP-242S, Mayak MP-240S, Comet MP-225S-1 models. Refinement of the electronic components of tape recorders with low-quality CVL, the alteration of which is usually difficult, seems inappropriate.

Analyzing the circuit diagram of the tape recorder, it is necessary to pay special attention to the erasure-bias generator (GSP). If the GSP has a unipolar supply and the switching of the high-frequency bias current (HFB) is carried out by changing the supply voltage, then the refinement of such a GSP will not be difficult and will not require changes in the tape recorder circuit. In a recording amplifier (US), it is desirable that its frequency response at high frequencies can be adjusted with a tuning resistor. This eliminates the need for selection of capacitors that form the frequency response of ultrasound, since the selection of accurate capacitors is usually limited. The presence of a filter plug is mandatory, in extreme cases it will have to be made and installed independently. The reproduction amplifier (UV) remains standard, its completion is not provided. (In the case of replacing the head with a single-crystal ferrite one, refinement and UV are desirable. - Approx. ed.). It is enough that this amplifier has a standard frequency response and low noise level. I will only note that the K157UL1 chip in the standard inclusion suits many.

For a good setup of the tape recorder, a minimum set of measuring instruments is required. It's good to have a dual-beam oscilloscope, but you can get by with a regular one. In addition to it, you will need an audio frequency generator (GZCH), a swept frequency generator (GCh). The device described in [2] perfectly combines both functions. The tuning quality is improved by a white or pink noise generator and a spectrum analyzer [3]. Unfortunately, such devices are not available to most radio amateurs. Instead, it is permissible to use a home-made test signal generator (GIS), the description of which is given below.

Such an oscillator is a combination of a GKCH, three fixed-frequency generators and three active band-pass filters (PF) with detectors and arrow indicators, as well as a power supply. The oscillators and bandpass filters are tuned to 300, 3000 and 12 Hz. Thus, it becomes possible to take into account the magnetizing effect of high-frequency signals. It turns out a very simplified analogue of the noise generator and spectrum analyzer, which, although it has only three frequencies for analysis, nevertheless perfectly fulfills its task.

The generator circuit for fixed frequencies is shown in fig. 1, and the filter circuit - in fig. 2. GIS (Fig. 3) contains a three-frequency generator A1, a sweeping frequency generator A2 and a meter unit A3. The level switch SA2 of the three-frequency generator simultaneously changes the gain of the input amplifier to the op amp DA1 of the A3 unit: when the attenuator introduces attenuation, for example, 10 dB, the gain in the unit also increases by 10 dB.

The test signal generator is powered by a power supply unit with a balanced +12 V output (not shown in the diagram). You can use any block that provides a load current of at least 150 mA.

Setting up the GIS, connect the oscilloscope to the output of the generator (see Fig. 1) and rotate the resistor R6 to achieve the maximum symmetry of the sinusoidal signal. The same must be done with the rest of the generators of block A1. Then, one by one, the connections of the right (according to the scheme) ends of the resistors R4, R5, R6 are disconnected with the switch SA1 and the adjustment of the tuning resistors R1, R2, R3 sets a voltage of 200 mV on each of them.

After the restoration of broken circuits, switch SA2 to the "0 dB" position. By adjusting the resistor R7, they ensure that when SA1 is switched to the "Calibration" mode, the signal value at the output of the three-frequency generator does not change. Then the output of the block A1 is connected to the input of the filter block A3. The "Input level" regulator and the trimmer resistor R16 of block A3 are set to the middle position. Using trimmer resistors R22, R23, R24, the measuring instruments PA1-RA3 are calibrated at a level of 0 dB. Then the generator signal is attenuated by 10 dB (switch SA2 in the "-10 dB" position) and the trimming resistor R18 again sets the instrument arrows to 0 dB. A similar adjustment must be done in the switch position "-20 dB" with resistor R20. The test signal generator can now be considered configured.

In the frequency-setting circuits of generators and filters, as well as in the attenuators of blocks A1 and A2, it is desirable to use parts with a tolerance of no more than 5%, the rest - up to 20%. Operational amplifiers use any with appropriate correction circuits. Measuring instruments RA1 - RA3 - pointer indicators of the recording level from tape recorders of the M4761-M1 type.

The choice of a magnetic head is a responsible task: the results obtained after revision show that to a large extent everything depends on the quality of the head. Based on personal experience, I recommend universal magnetic heads (GU) 3D24.751 or 3D24.752 made of single-crystal ferrite, since they have a high stability of parameters over time and a long service life [4]. With success, you can use GU 3D24.080, 3D24.081 from sendust and similar ones. With an uncompromising approach to the choice of heads, the possibility of selecting from several copies one with a minimum difference in sensitivity and frequency response of the block heads is assumed.

To select the head, a tape recorder, an oscilloscope and a GKCH are needed. The playback amplifier (UV) must have a sufficiently wide frequency response band (at least 16 kHz) and the same gain across the channels. For such a check, the windings of the heads installed in the tape recorder connected in parallel are connected to the output of any of the ultrasound channels. Before measuring the GU and LPM, it is desirable to demagnetize.

Make a few test recordings of the GCCH signal, set to the maximum sweep range (20...20 Hz), with different levels, -000, -20 and 10 dB are sufficient. These levels do not need to be set with high precision. Then restore the normal connection of the HU with the HC and play back the recording, comparing the frequency response in the channels. If there are doubts about the quality of the work of the shock absorber, you can alternately connect different heads of the block to one of its channels, comparing the resulting frequency response with each other. In this situation, the shape of the frequency response plays a secondary role. Of greater importance is the identity of the characteristics of different block heads at all recording levels.

The spread of head parameters is very large. Thus, thirty sendast heads of types 3D24.080 and 3D24.081 were tested. Of these, two copies were selected that met my requirements. Of the three 3D24.752 that came across, one was chosen. Available one copy 3D24.751 was successful. I must say, the accuracy of the frequency response of the end-to-end recording-playback channel strongly depends on the thoroughness of the selection of heads.

After checking the effectiveness of several systems of dynamic bias, the author came to the conclusion that it is better to install SADP in a tape recorder [5]. (We draw the attention of readers to the latest publication about SADP with an optocoupler regulator in "Radio", 1998, No. 10. - Approx. ed.). When repeating the design, special attention should be paid to the manufacture of the transformer and tuning it in the resonant circuit to the GSP frequency. So, in the gap between the halves of the cups, it is better to place a thin layer of raw rubber. It is convenient to make a rough adjustment to the generator frequency by tightening the cups with a screw made of non-magnetic material (which is also the fastening of the transformer to the board), and fine tuning by capacitor C2. After completing the adjustment, fill the transformer with glue from the outside.

Instead of the foreign-made 2N2905 transistor used by the author, it is better to use KT626 with indices A, B, D - G. Install the SADP into the tape recorder according to the author's recommendations. Although this version of SADP is recommended for installation in the Yauza MP-220S tape recorder, it works fine in all models of Wilma, Sanda, Vega and Mayak tape recorders.

For the selected head, it is better to set the optimal bias current according to the criterion of the maximum return of the head-tape system at medium frequencies (300 - 400 Hz).

Now let's deal with the correction of their frequency response necessary for most ultrasounds. The current recommendations for raising the frequency response of US at high frequencies up to 20 dB seem outdated, as they were standardized when the quality of the carriers and the heads themselves was still quite low. This, in my opinion, explains the complaints about the "hardness" of sound when using ferrite heads, in which the HF losses are significantly lower, and the maximum magnetic induction in the core is noticeably limited. Under such conditions, the GU magnetic core is saturated much earlier than the carrier.

To eliminate this phenomenon, the following procedure is proposed. A signal voltage with a frequency of 300 Hz is set on the generator, corresponding to a recording level of -20 dB. Then the generator is rebuilt to the frequency at which the rise in the frequency response of the ultrasound is maximum; usually this frequency is not lower than 14 ... 16 kHz. Without changing the signal level, a recording is made, and during subsequent playback, its level is measured at the output of the SW. Then, gradually reducing each time the degree of HF correction by 1-2 dB, these operations are repeated until the signal level during playback begins to decrease. By returning the correction setting one step back, the optimum amount of pre-emphasis for a given head-tape system is reached. The decrease in the rise in the frequency response of ultrasound with a new head can reach 8 ... 14 dB. During this operation, the slider of the R24 SADP resistor must be in the leftmost position according to the diagram.

After that, you should check the unevenness of the frequency response in the operating frequency band. To do this, a signal with a frequency of 400 Hz is fed from the output of the GKCH (block A2, Fig. 3) to the recording input of the tape recorder. Turn it into recording mode and set the recording level to 0 dB on the indicator. The generator is switched to the frequency swing mode, and the "Attenuation" switch is switched to the "-20 dB" position. Record for one minute. After rewinding the tape to the beginning of the recorded phonogram, it is played back and the frequency response of the end-to-end recording-playback channel is controlled by an oscilloscope. With large, more than 3 dB, deviations from the linear resistors R4, R6 in the SADP, the VChP current is corrected: when the frequency response rises at high frequencies, the current must be increased, and when it decreases, it must be reduced.

During the tuning process, it is necessary to achieve the most uniform frequency response of the through channel over the entire operating frequency range. To do this, a signal from a three-frequency generator (block A1, Fig. 3) switched on in the "Calibration" mode is fed to the input of the tape recorder, and the linear output of the tape recorder is connected to the input of the meter unit. The level switch is in the "0 dB" position. Turn on the tape recorder in the "Record" mode, and set the recorder's indicators to 0 dB with the recording level controls. After recording a short duration and rewinding the tape to the beginning of the recorded section, it is played back. Regulator "Level" - R11 (Fig. 3) set the arrow PA1 to 0 dB. Then the "Calibration" mode is turned off and the level switch is moved to the "-20 dB" position. Now record a three-frequency signal. When playing it, watch the measuring instruments. Their arrows should oscillate approximately at the same level (at high frequencies, the oscillations are greater due to parasitic amplitude modulation in the tape and CVL). A small spread in readings is best corrected by changing the VChP current.

Next, move the level switch to the "-10 dB" position and repeat the recording of the three-frequency signal. But this time, the spread of readings, most often due to a drop in the frequency response at high frequencies, compensate for an increase in the resistance R24 SADP.

With the level switch set to the "0 dB" position, use the recorder's recording level controls to set the recorder's indicator to 0 dB and record again. Repeat the adjustment of the depth of operation of the SADP with resistor R24. It is possible that in this case it will not be possible to equalize the readings of the instruments and a decline at high frequencies may be present. By recording the signal several times with the same level, each time the depth of operation of the SADP is changed. If, after the next step, the filter indicator for a frequency of 12,5 kHz did not change the readings, then the setting of the resistor R24 ​​in the SADP is returned one step back. It must be remembered that for the normal transmission of high levels, low and medium level signals, i.e. -20, -10 dB, are more important than high level signals (acting for a short time).

Return the recording level control and the "Level" switch to the position of maximum level and attenuation, respectively. Repeat all operations from the very beginning, since all adjustments are interdependent.

Having achieved the maximum linearity of the end-to-end recording-playback channel in one channel of the tape recorder, move the channel input switch (SA3) to another position and adjust the other channel of the tape recorder.

The SADP setting consists in using two current regulators of the HPV R4, R6 and the coefficient "K" - R24, in the unit it will achieve the maximum linearity of the frequency response of the end-to-end recording-playback channel at all levels, giving preference to levels from low to "-10dB ". The task of the SADP is not to compensate for the influence of the component signals of higher frequencies on lower ones. The time required to adjust the tape recorder reaches one hour for the first time, with the accumulation of experience it is reduced to 15-20 minutes.

Even better results can be obtained when using a specialized recording head 3A24.750 (also ferrite monocrystalline). However, its use is possible only in two-cassette tape recorders when using one LPM exclusively for the recording mode. In this case, it is advisable to introduce a voltage-to-current converter into the ultrasound without an AFC driver, as described in [6].

The author also tested ultrasonic devices that record using the pulse-width modulation method. The accompanying problems arising from the implementation of this method of solving related problems are associated with such hardware costs that it was decided to abandon this very promising method.

Literature

1. Lukovnikov A. Bias and recording current optimizer. Radio Yearbook-85. - M.: Ed. DOSAAF USSR, p. 81 - 90.
2. Anufriev L. GKCH universal. - Radio, 1991, No. 2, p. 58 - 63.
3. Biryukov S., Frolov V. Spectrum analyzer for an amateur radio complex. Radio Yearbook-85. - M.: Ed. DOSAAF USSR, p. 96-111.
4. Sachkovsky V. Ferrite magnetic heads for sound recording and features of their application. - Radio, 1998, No. 3, pp. 16 - 18, No. 4; With. 20 - 22; No. 5, p. 16 - 18.
5. Ivanov A. SADP in tape recorders "Yauza MP-221S". - Radio, 1995, No. 5, p. 17.
6. Shurgalin M. Cassette recorder recording amplifier. - Radio, 1990, No. 2, p. 72, 73.

Author: A.Mokhov, Kstovo, Nizhny Novgorod region

See other articles Section Audio equipment.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Machine for thinning flowers in gardens 02.05.2024

In modern agriculture, technological progress is developing aimed at increasing the efficiency of plant care processes. The innovative Florix flower thinning machine was presented in Italy, designed to optimize the harvesting stage. This tool is equipped with mobile arms, allowing it to be easily adapted to the needs of the garden. The operator can adjust the speed of the thin wires by controlling them from the tractor cab using a joystick. This approach significantly increases the efficiency of the flower thinning process, providing the possibility of individual adjustment to the specific conditions of the garden, as well as the variety and type of fruit grown in it. After testing the Florix machine for two years on various types of fruit, the results were very encouraging. Farmers such as Filiberto Montanari, who has used a Florix machine for several years, have reported a significant reduction in the time and labor required to thin flowers. ... >>

Advanced Infrared Microscope 02.05.2024

Microscopes play an important role in scientific research, allowing scientists to delve into structures and processes invisible to the eye. However, various microscopy methods have their limitations, and among them was the limitation of resolution when using the infrared range. But the latest achievements of Japanese researchers from the University of Tokyo open up new prospects for studying the microworld. Scientists from the University of Tokyo have unveiled a new microscope that will revolutionize the capabilities of infrared microscopy. This advanced instrument allows you to see the internal structures of living bacteria with amazing clarity on the nanometer scale. Typically, mid-infrared microscopes are limited by low resolution, but the latest development from Japanese researchers overcomes these limitations. According to scientists, the developed microscope allows creating images with a resolution of up to 120 nanometers, which is 30 times higher than the resolution of traditional microscopes. ... >>

Air trap for insects 01.05.2024

Agriculture is one of the key sectors of the economy, and pest control is an integral part of this process. A team of scientists from the Indian Council of Agricultural Research-Central Potato Research Institute (ICAR-CPRI), Shimla, has come up with an innovative solution to this problem - a wind-powered insect air trap. This device addresses the shortcomings of traditional pest control methods by providing real-time insect population data. The trap is powered entirely by wind energy, making it an environmentally friendly solution that requires no power. Its unique design allows monitoring of both harmful and beneficial insects, providing a complete overview of the population in any agricultural area. “By assessing target pests at the right time, we can take necessary measures to control both pests and diseases,” says Kapil ... >>

Random news from the Archive Radar sees through walls 18.06.2022 The Israeli company Camero-Tech has created a specialized radar Xaver 1000, designed for intelligence units, rescuers and police. The radar is able to "see" objects behind walls. The device recognizes targets by type, can determine the position in which the person is currently located. The target tracking radar has an artificial intelligence algorithm and a function that allows you to visualize static objects behind walls. The device also records incoming data, which allows you to analyze the actions of the unit. The radar is equipped with a tripod for mounting the system and a touch screen with a diagonal of 10,1 inches.

Other interesting news:

▪ Acer SpatialLabs View Pro 3 27D monitor

▪ The world's largest wave power plant will be built

▪ Hangover cactus, taken orally

▪ Slugs helped create surgical glue

▪ NASA will build a supersonic aircraft

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ site section Lighting. Article selection

▪ article Digital video camera as a web-camera. video art

▪ article Why is the poker combination of ace and king sometimes called Anna Kournikova? Detailed answer

▪ article Cord twister. Standard instruction on labor protection

▪ article Metals. Simple recipes and tips

▪ article Power supply for a sick battery. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages ​​of this page

Arabic	Hebrew	Polish
Bengali	Hindi	Portuguese
Chinese	Italian	Spanish
English	Japanese	Turkish
French	Korean	Ukrainian
German	Malay	Vietnamese

Home page | Library | Articles | Website map | Site Reviews

www.diagram.com.ua
2000-2024