Ing . P. WEINBOYM, G. MIRONENKO, Leningrad
Analyzing old archives, I came across this article and decided to share it. Once upon a time, as a student of the Gorky Radio Engineering Technical School, this device was the subject of a graduation project of my fellow student and laboratory friend. The whole process of assembly and testing of the device took place before my eyes. The design of the assembled device was somewhat different - a high-quality coil for that time was used. record player (it seems "Jupiter 001-stereo") and the circuitry is slightly changed. The initial skepticism about this device was replaced by confidence in its effectiveness after the very first tests in the dentist’s office. In addition, we used this device just to relax in between work. Interested parties can make a similar device, based on the principle outlined in the article below. And, using modern means of sound reproduction, such as a CD player, and modern circuitry , you can create a compact device. So, the idea is - go ahead!
The device offered to the attention of readers is intended for anesthesia during dental treatment and prosthetics. Everyone knows what unpleasant omissions occur when a tooth is treated with a drill. It is therefore understandable, therefore, the interest of specialists to the problem of pain relief in the treatment of teeth. At one time, many methods were proposed, but none of them was effective enough. The most promising was the so-called method of sound analgesia. It lies in the fact that during treatment the patient listens to a musical program and white noise (a mixture of all components of the sound frequency spectrum) fed to the headphones. Music has a beneficial effect on the nervous system of the patient, and white noise dampens the focus of excitation in the cerebral cortex, caused by pain.
Device for pain relief (sound analgizator ) was designed by Leningrad engineer P. Weinboim in collaboration with the doctor of the Military Medical Academy. S.M. Kirov G. Mironenko. Technicians V. Kuznetsov and F. Guliancii provided great help in creating the device.
During the two-year operation in the dental department of the clinic of the Military Medical Academy, the sound analgesic has consistently received good patient feedback. The device was exhibited at the 17th Leningrad exhibition of creativity of radio amateurs-designers DOSAAF and was awarded the diploma of I degree.
The block diagram of the sound analogue is shown in fig. 1. The device consists of an LF gain unit, a mixer unit (patient console), a power unit and a tape drive mechanism with an endless loop of magnetic tape. Its purpose is to simultaneously play a music program and white noise recorded on four tracks of a magnetic tape. Low - impedance dual - channel reproducing heads with a working gap of 3 microns , magnetic tape of type 6, tape speed of 9.53 cm / sec were used in the device.
Correction of the frequency response in the LF preamplifier stage allows for high-quality reproduction in the frequency range up to 10 kHz .
In fig. 2 shows a schematic diagram of the sound analgesic . All four signals are removed by magnetic heads from the tracks of the magnetic tape, the signal voltage is supplied to the corresponding preamplifier of the LF, made on the transistors T1-T3. Two LF preamplifiers amplify the music channel signals ( Am and Bm ), while the other two amplify the noise channel signals ( Ash and Bsh ). At the input of the woofer amplifier with passive mixers (R10, R11, R23 and R24), music and noise are mixed in the required ratios. After mixing and amplification of the signal voltage by two LF (T4 – T9) amplifiers, the Al + Am signals are fed to one of the headphones, and Бш + Бм - to the other, as a result of which a binatural effect occurs. If necessary, the patient can regulate separately the amount of music and noise from zero to maximum.
Three - stage preamplifier LF is made on the transistors T1-T3 (P13B). The amplifier is covered by a frequency-dependent negative feedback of 19 dB in depth. The first amplifier stage is made according to the scheme with a common emitter. The playback head is connected to the input of the amplifier through the capacitor C 1 . The transistor used in the first stage must have a minimum level of noise. To match the first stage with the following, an emitter follower is used, which is galvanically connected with the first stage. The third cascade is similar to the first. The negative feedback voltage is supplied from the collector circuit of the last transistor (T3) to the emitter circuit of the first through the elements C4R4R6. The output stage is loaded on the level regulator, which is located in the remote unit - mixer.
The mixer unit, designed as a separate console, is a rheostat divider consisting of two paired potentiometers (R10 and R24) and decoupling resistors R11, R23. Potentiometers play the role of regulators of music and noise. Decoupling resistors are needed to eliminate the mutual influence of the preamplifier outputs. The resistances of these resistors are selected experimentally.
A final amplifier boosts the signal level to what is necessary for normal playback in headphones. This amplifier should have a small non-linear distortion, high input impedance, a slight voltage gain and an additional correction of the frequency response in the low frequency region (about 4 dB per octave).
The terminal amplifier is assembled on six transistors (T4 – T9), five of which are of the pn-p type , and the sixth (T 9 ) is of the n -pn type (P10).
The first and third stages, performed on transistors T 4 and T6 according to the circuit with a common emitter, amplify the signal voltage. The second and fourth cascades on transistors T5 and T 7 (according to the emitter follower circuit) are used to match the output resistances of the previous stages with the subsequent resistances of the subsequent ones.
The output stage is a push-pull power amplifier assembled according to a circuit with a common collector on transistors of different conductivity. This eliminates the need for a phase inversion cascade. The amplifier is covered by deep feedback (26 dB ), which dramatically reduces the nonlinear distortion coefficient. The feedback voltage depends on the frequency ( frequency dependent feedback), which results in an additional correction of the frequency response of the playback channel in the lower frequency range. In addition, the input impedance of the final amplifier covered by the feedback rises. The amplifier is not critical to the load.
The power supply unit of the device contains a voltage stabilizer on transistors T10, T11. The reference voltage is removed from the D811 silicon Zener diode. The rectifier is assembled according to a bridge circuit on four D7A germanium diodes. The entire device is powered by AC power through a power transformer.
The kinematic diagram of the tape drive mechanism is shown in fig. 3. The use of an endless loop of a magnetic tape placed in a special cassette made it possible to simplify the tape drive mechanism and use a low-power EDG-1M engine.
The device cassette is a plastic box with a roll of tape length
For reliable contact of the magnetic tape with the heads, a special felt clamp is used, which is retracted at the moment of switching on the device.
Structurally, the entire apparatus is designed as a separate unit (Fig. 4).
On the top panel of the device, a cassette with a device for installing and clamping a magnetic tape to the shaft, two stereo heads and an on / off toggle switch (Fig. 5) are fixed. The thermal heads are closed with a safety cap.
Under the panel is a plate of alloy B-95 thick
Pre-playback amplifiers are designed as a separate unit. It is mounted on a box-shaped steel chassis. Sizes of the chassis are 160Х80Х40 mm. All four amplifiers are assembled on textolite boards and separated from each other by screens (Fig. 5).
The final amplifiers are assembled on two textolite boards and are located on the other side of the installation of the preamplifiers. The control panel is made as a separate unit. The structure of this unit includes two dual potentiometers, which play the role of level controls, and output pads for connecting the doctor’s and patient’s headphones.
Fig. 3 Kinematic scheme of the tape drive mechanism: 1. Shaped disk, 2. Roll films, 3. Sleeve disk, 4. Base, 5. Pinch roller, 6. Guide column, 7. Drive shaft, 8. Magnetic reproducing stereo head , 9. Clamping strap, 10. Felt pad, 11. Loop magnetic tapes. 12. Flywheel. 13. Lavsan belt, 14. Engine axis, 15. Flywheel thrust bearing, 16. Electric motor.
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