Using a tape recorder with a battery, you can record during the campaign, in the car, on the boat, record «talking message». This tape should be cost-effective, easy to transport, t. E. Have a low weight and dimensions.
To meet these requirements, then it is advisable to tape the amplifier instead of vacuum tubes to use planar transistors, but as a tape drive motor — spring drive. Application of transistors allows along with a decrease in weight and size to significantly reduce the power consumption of the power supply, and greatly increase the operational reliability of the device.
Below is a description of the portable recorder designed for eapisi speech amplifier is assembled on planar transistors.
Slow movement of the ferromagnetic tape (95.25 mm / s), along with two-track recording allows you to significantly reduce costs of tape and tape drive simplifies the design, since there is no rewind the tape.
Cassette tape can hold up to 100 m of the ferromagnetic tape. This is enough for continuous recording or playback for 35 minutes. Plant spring drive is targeted at four minutes of the work, so the longer recording must be winding.
The recorder has a universal amplifier is used as a recording, and playback. The output power of the amplifier is obtained about 0.25 volts, and the THD is less than 12%. Through channel bandwidth 200-2500 Hz. Application polished ferromagnetic ribbon and allows to reproduce a wider bandwidth. The recorder does not erase the generator, so the recording is made on a pre-demagnetized tape. However, you can perform erasure directly to the recorder by using a permanent magnet special design.
Switching from recording to reproduction by means of a switch.
The first stage of the amplifier transistor is made on the type of P1V (P1D) with grounded emitter. To increase the input impedance of the cascade, used negative current feedback. For this purpose the emitter circuit resistance R2 included. This stage has an input resistance of 5 com.
Subsequent stages will also perform on the circuit with a grounded emitter. With this inclusion transistors turns the smallest difference between the size of the input and output impedance of the cascade, and thus can be dispensed with a relatively expensive and cumbersome matching transformers. However, it should be noted that the parameters of the stage, collected on the circuit with a grounded emitter, are highly dependent on temperature. Therefore, for stabilization and to increase the linearity of the amplitude characteristic in the emitter circuit includes a negative feedback impedance (R2, R7, R12, R15, R13 and R19).
To reduce battery consumption initial current emitter and collector is selected within the 0.5-1.2 Ma. At lower currents, the operating point is on the lower fold of the emitter characteristics that will reduce the steepness of the transistor, t. E. Decrease the gain stage and increase the nonlinear distortions. The required initial value of the emitter current is provided by the selection of resistances rg, R8, R13 and R16.
The output stage is formed by a push-pull circuit in planar transistors such as P-2. Using the output stage a class B mode of operation allows to reduce the power consumption to increase battery life and power output compared with the mode of class A. In order to avoid occurrence of substantial distortion output impedance of the cascade transition is reduced by the transformer Tr1.
In the recording mode to the secondary winding of the output transformer Tr2 connected universal head (PG), the headphones to control the level of the recorded signal. In playback mode, the output is switched dynamic speaker (Gy).
To minimize distortion of the cascade covered the last two profound negative feedback. The voltage feedback from the output transformer secondary winding through the resistance R2o served on the base of the transistor driver stage KP4. To eliminate self-excitation voltage on the collectors of the transistors of the first two stages is fed through an isolation filter R9, C3.
Gain control is performed via the potentiometer R5, included in the base circuit of the transistor of the second stage KP2. Adjusting the tone produced resistance R17, which is connected via a capacitor C9 parallel with the primary winding of the transformer Tr1.
In recording mode, turn on the generator for the high frequency bias current. Generator assembled push-pull circuit of planar type transistors P2 transformer feedback. Oscillating circuit formed coil L1 and capacitor C13. Feedback coil L2 is included in the emitter circuit. The resistance R21 is used to supply a negative bias on the base of both transistors.
The generator produces a sinusoidal oscillations with a frequency of about 30 kHz. Bias current is supplied to the recording head sequentially energized with audio frequency. To obtain sufficient quantities bias current generator coupling coil L3 is tuned to resonance with a capacitor C11. Correction chain R22 C12 is used to lift the gain at high frequencies.
Structures and parts.
Power together with the generator mounted on the square, made of duralumin sheet thickness of 1.5 mm.
On the front panel are placed volume knob and tone controls, selector switch and two pairs of phone jacks to enable the microphone and speaker. Mounting resistors and capacitors (except for electrolytic capacitors), and the transistors made on getinaksovoy or textolite strip with metal blade.
Cases of electrolytic capacitors, standing in the collector circuits are isolated from the chassis pressshpanovymi gaskets. Bottom mounted under the chassis all the other parts. Selector switch consists of three boards with three sections at three positions each.
The transformer cores are manufactured from transformer steel HVP 0.35 mm thick. Transition transformer Tr1 core is formed on the W-9 (gap 0.1 mm), the thickness of the pack 12 mm. Winding I have 4600 turns of PEL-1 0.06 winding II — 500 to + 500 turns of PEL-1 0.12.
The output transformer Tr2 is going at the core of plates W-9, assembled vperekryshku, package thickness of 16 mm. Winding I contains 1300 — [- 1 300 turns of PEL-1 0.12 winding II provides 80 turns of PEL-1 0.51. Winding III consists of 600 turns of PEL-1 0.29.
Generator coil L1, L2 and L3 are wound on the carcass diameter 15 mm, height 20 mm and consist of a pot-core type of carbonyl iron Sa-4a, which is put on top of aluminum or brass screen. Coil L1 contains 600 turns with a tap from the center, wire-1 PEL 0.12. The coil L2 has 30 turns with a center tapped, wire-PEL 1 0.12. Lk coil consists of 550 turns of PEL-1 0.12.
The tape is used a universal head for recording and playback. You can use the universal head of the tape machine or alter the typical head. For this it is necessary to reduce the thickness of the core pack to 2.7 mm. The winding section should contain 2 1500 turns of PEL-1 0.1.
When building in the working gap of the head is laid brass foil or foils of beryllium bronze thickness of 10-12 microns. The core is going back without a gap.
Power tape is made from a battery BAS-G-60, and the shift in the supply chain of emitters used by a flashlight battery type BSC-L-0.5. These batteries are sufficient to tape operation for about 100 hours.
Initially, you should make sure of the correctness of supplying negative feedback. For this kind of work the dial set to «play,» and to the output of the amplifier is connected speaker. If the amplifier will generate, should be reversed ends of winding I output transformer Tr2.
Adjustment of the amplifier is reduced to the selection mode emitter and collector circuits of transistors. It is convenient to carry with the device CT-1. The adjustment is necessary to start with the final stage. For maximum output power at the lowest harmonic distortion parameters of both transistors should be the same. Balancing work transistors made the selection of resistance in the circuits of emitters. To this end, alternately in each of the emitter circuit and the selection include milliamp resistance R18R (19) achieve the same magnitude of the emitter current of both transistors. Then, one by one, check the emitter current of other stages, and its value must be of the order of 1.2 mA. The same must be current in the collector circuit of the cascade.
After selection modes can proceed to check the amplifier to the speaker. The source voltage can be used sound sound generator or pickup.
After graduating from the amplifier test, the selector switch is set to «Record», and proceeds to establish a bias current generator. This amplifier is connected to the output of the universal head. Establishing generator conveniently made using an oscilloscope, which allows you to control not only the quantity but also the form of high-frequency current in the winding head, which is especially important.
For this purpose, the wire connecting the winding head chassis included a resistance of 500 ohms, which is connected to the vertical input of the oscilloscope amplifier.
When you turn on the generator does not work, swap the ends of the coil L2. By selecting the capacitor C11 to seek the necessary magnitude of the bias current in the winding head.
For a universal head, which is used in a tape recorder, a high-frequency bias current should be about 0.8 ma-I. To form a good quality recording of high frequency current in the winding head must be strictly symmetrical. This is achieved by the selection of the operating mode of the generator (resistance R21) and the parameters of transistors (they must be the same). For equal currents both transistors in the emitter circuit of the transistor having a large current value, it is possible to include a small resistance.
The final adjustment of the amplifier is made when checking the operation of the tape drive.
The disadvantage of the described amplifier circuit should include a higher level of noise compared with a tube amplifier, which is explained by the use of transistors. To reduce the number of parts required amount of the emitter current can be obtained by applying a small negative voltage to the base of the transistor (through a dropping resistor included to the source of the collector voltage). Thus no longer necessary for the second battery (3.5 in) and the electrolytic capacitors in the circuits emitters.
Application deeper correction at high frequencies together with recording tapes will expand polished band frequency response along with the recording and to record speech audio. Description tape recorder mechanism will be put in the following article.