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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Tube stereo pre-amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Preamplifiers Schematic diagram and design The amplifier described here is a system that is quite close in function to the current amplifying-switching devices.
This common denominator means both the leveling of their levels and the frequency correction, the need for which arises when using long shielded lines from sources located in different places in the room. Before proceeding to the description of the amplifier, let's make a reservation that all of the above applies only to one of the two channels of the stereo amplifier, therefore, when assembling the amplifier, arranging its components, manufacturing printed circuit boards or choosing switching nodes, you need to remember that there will be two channels, and appropriate decisions should be made with this in mind. This must be taken into account when choosing or manufacturing a power transformer, as well as rectifier elements. In addition, it is completely unacceptable that after the end of the adjustment, without exception, all parameters of one channel differ from similar parameters of another channel by more than 1 ... 2%. So, the amplifier starts with an 8-position switch, assembled on a P2K type switch and designed to switch the following audio signal sources: 1. Dynamic microphone 2. Dynamic stereo head 3. Piezo head of a stereo player 4. Laser disc player 5. Stereo recorder 6. Stereo VHF tuner or AM/FM receiver 7. Television 8. Three-program radio broadcasting network.
Connection of sources to the switch is carried out by means of standard 5-pin cylindrical connectors. Each of the signals (except the signal from the microphone) falls on its own resistive divider, the lower arm of which is made variable. The high-side resistor is blocked by a capacitor, the purpose of which is to compensate for the attenuation of the high-frequency part of the spectrum in a long line. The nominal value of this capacitance is selected empirically, since the losses in the line cannot be accurately determined. How this is done will be discussed later. The corrected signal is fed through another group of contacts to the grid of the lamp of the first stage of the two-stage voltage preamplifier. Here, at the input of the amplifier, there is a thinly compensated volume control. Between the first and second cascades, a two-band tone control is included, which regulates separately sections of the spectrum above and below the crossover frequency - 1000 Hz. This two-band regulator, without any changes in the circuit, can be replaced by a clang register and a four-band regulator used in the high-end amplifier described earlier. The signal from the microphone, before reaching the input of the first stage, is pre-amplified by an additional microphone stage. The cascade is assembled on a low-noise pentode of the EF-86 type (full domestic analogue - 6Zh32P). This lamp was once used in some domestic tape recorders (for example, "Yauza"). The features of the installation of this cascade will be discussed in more detail later. After amplification by the second stage, the signal taken from the anode of the second triode is divided into two: one goes to the grid of the first terminal stage - the cathode follower, assembled on one half of the 6N6P double triode (VLZ in the diagram in Fig. 34), the other - to the grid of the additional amplifier lamp voltage VL2 6C3P or 6S4P, after amplification of which the signal enters the input of the second terminal triode - cathode follower VLZ 6N6P. To save the total number of tubes in the amplifier, instead of two 6S3P (or 6S4P) tubes, it is permissible to use one dual 6N1P type triode in two channels - one triode for each channel. In this case, it is necessary to apply an antiphonal circuit for supplying the filament of this lamp with a constant voltage with additional recharge (+15 ... 25 V), as was done in a high-end amplifier. Thus, any input signal, before reaching the grid of one of the terminal cathode repeaters, is amplified in one case by a two-stage, in the other - by a three-stage preamplifier. This is done in order to be able to change the overall gain of our UZCH by n times by simply pressing the output switch button, where n is the actual gain of the additional stage on the VL2 lamp. In the process of adjusting the amplifier, its value is chosen equal to 10, 20 or 50 and, accordingly, the two buttons of the switch n mark "x1" and "x10" (or 20 or 50).
The output stages are assembled according to the cathode follower circuit, which has a very low output impedance. This is necessary so that when the signal passes from the output of the preliminary UZCH to the input of a powerful final amplifier, there are no additional losses and distortions of the high-frequency part of the spectrum, especially if the connecting lines are long enough. Let's return to the additional microphone amplifier. It was introduced into the UHF circuit so that, if desired, it was possible to implement a rather fashionable "karaoke" function, which allows solo accompaniment of any phonograms (from disks or magnetic media). Instead of one, three microphones can be turned on at the input, which will expand the solo possibilities to choir ones. The microphone cascade has its own independent volume control, which allows you to mix your own and accompanying musical signals in a wide range. The complete circuit of this cascade is shown in Fig. 35. Strictly speaking, the microphone cascade does not have to be a tube cascade. Today, there are circuits for many microphone amplifiers based on transistors and microcircuits, which have excellent characteristics, low noise and do not have, like a tube, a tendency to microphone effect. However, their use will entail the need to introduce an additional low-voltage rectifier with good filtering into the overall power circuit, so that as a result, the overall gain from using a transistor microphone amplifier may be negligible or even zero.
And one more caveat. The amplifier circuit provides an input from a radio transmission line, which today is available in almost every city and even a regional center. In big cities, this broadcast is multi-program and includes stereo broadcasts. If such wired broadcasting exists in your city, it is advisable to introduce an additional node into the amplifier circuit - a three-program broadcasting decoder with a stereo output. It makes no sense to describe the scheme of such a node and its design: it is standard and has been repeatedly published (for example, in the Radio magazine). We only note that if it is used, it is advisable to immediately replace the 8-position switch of the input switch with a 10-position one and switch the signals from each of the three broadcast channels according to the same principle as for one channel. To switch broadcast channels, you can also enter an additional three-position button or button switch. That, perhaps, is all that concerns the amplifier circuit. Its structural design depends 100% on where and how it will be placed - as part of a music center, as a separate device, on a separate table, on a cabinet shelf or next to the final amplifier and other equipment of the complex.
Here is one of the many possible options, in which the amplifier is designed as an autonomous control unit for all devices of the audio complex. Unlike the high-end amplifier described earlier, this amplifier is quite compact and lightweight. In this regard, it was necessary to abandon the horizontal placement of the switch-switch on the front front panel, since when you press the switch buttons, it is possible to move the entire amplifier unit around the table. The switch is located vertically and placed on the front of the top panel of the amplifier. All operational controls are also located there - volume, tone, stereo balance, microphone mixer controls. The appearance of the amplifier is shown in fig. 36, and the control panel - in fig. 37. The amplifier is placed on one common printed circuit board, shown in fig. 38, in fig. 39 shows the placement of parts and circuit elements on the board.
The power transformer and rectifier parts are arranged on a base-frame, the dimensions of which are not critical and must be determined by the designer himself based on the number of switched sources, the presence or absence of a microphone amplifier, a wired stereo broadcasting decoder unit and other factors. In the author’s design, holes are drilled above each button of the switch in the top panel, into which red LEDs are inserted from the inside, which are connected when the corresponding button is pressed to a 12 V voltage source and signal that one or another device is connected to the amplifier. This system is not displayed on the amplifier diagram, since formally it has nothing to do with it. If desired, any radio amateur can easily perform it on their own. Adjustment and adjustment Consider adjusting the amplifier. First, with the lamps removed, the operation of the rectifiers and the presence of voltage on the electrodes of all lamps, including the filament circuits, are checked. If everything is in order with this, all the lamps are put in place and after the lamps have warmed up (about 1 min), the steady-state voltage values \u5b\u10bare checked at the anodes and cathodes of all lamps, as well as on the screening grid of the microphone cascade lamp. These values should not differ from those indicated in the diagram by more than 1 ... 34%. After that, a signal with a frequency of 1000 Hz of a small level (20 ... 50 mV) is supplied to the grid of the VL0,1 lamp (Fig. 0,5) from a sound generator. This is done so that any voltage convenient for reading is set at the output of the first cathode follower (for example, 1 or 10 or 20 V). Then the voltmeter is switched from the output of the first cathode follower to the output of the second, the decade switch at the output of the sound generator reduces the output voltage by 50, 20 or XNUMX times, without touching the knob of the smooth output voltage regulator, and by rotating the setting potentiometer RXNUMX at the output of the second follower, one achieves the same output voltage as the output of the first follower. P After adjustment, without changing the input signal level, make sure that the output signals on both repeaters differ exactly by the number of times you have chosen (10, 20 or 50), the designation of which is applied with paint, engraving or decal on the output switch buttons: "x1" and " x10" (or respectively "x20" or "x50"). Having finished with this, proceed to the main part of the adjustment - leveling the levels of signals from various sources and correcting the frequency response of the connecting lines. How you do this depends to a large extent on whether you can get (purchase, rent, rewrite) standardized audio sources for the duration of this work. Such sources at enterprises engaged in the production, repair or operation of sound reproducing equipment, as well as at radio centers and in recording houses (studios) are test plates and magnetic test films (on cassettes), on which, instead of musical programs, in compliance with the requirements of GOST, pure tone of the full frequency range of the audio spectrum from 20 Hz to 20 kHz. Each of these frequencies is reproduced by a real source for 20...30 s. During this time, it is necessary to have time to measure the voltages at the output (or input) of the amplifier and record these values, and then plot the frequency response graph based on them. This method is the most accurate and reliable, since it takes into account the degree of influence on the overall characteristics of all elements of the sound reproduction path. If you can't get test plates or test films, you will have to use the second method, although not as accurate, but quite affordable. It consists in the fact that instead of test plates and test films, the same sound generator is used. Before starting the adjustment, you need to set the tone controls to the position corresponding to the linear frequency response. To do this, first set both tone controls to approximately the middle position. The volume control for this and all subsequent operations should be in the maximum volume position (fully clockwise), and the stereo balance control should be in the middle position. A low-level signal with a frequency of 1000 Hz is applied to the amplifier input so that a voltage convenient for measuring (for example, 0,5 V) is set at the amplifier output. Then, keeping the generator voltage unchanged, the frequency is switched to 100 Hz and by turning the low-frequency regulator, the output is the same voltage as at a frequency of 1000 Hz. After that, the position of the high-frequency regulator is specified in a similar way, but already at a frequency of 10000 Hz. Finally, it is desirable to "walk" the entire spectrum from 20 Hz to 20 kHz to ensure that the output voltage is kept relatively equal at all frequencies within the spectrum. Having set all the controls to the desired position, they begin to adjust the switching part of the amplifier, which is best started from the source with the lowest output voltage (excluding the microphone). In our list, such a source is most likely the electrodynamic pickup head of a conventional (non-laser) disc player. Press the "Dynamic head" button on the signal switch and take the sound generator to where the record player is located. The signal from the generator must be fed directly to the beginning of the cable or shielded line connecting the player to our amplifier. We emphasize once again: not to the input of the amplifier, but to the output of the pickup, so that the entire connecting cable is between the generator and the amplifier. And one more very important reminder: the output resistance of the generator must be equal to (or have one order of magnitude) the internal resistance of the source. This means that if the internal resistance of the dynamic cartridge is several hundred ohms, then the oscillator output impedance switch should be set to the position closest to the internal source impedance. If the signal source is a piezo pickup having an internal resistance of approximately 0,5 MΩ, then a constant resistor of the same resistance must be connected in series between the generator output and the beginning of the connecting line. To make it easier to navigate the output impedances of various signal sources, in Table. 2 shows their generally accepted standardized values. It also gives the average values of the output voltages of these sources at a frequency of 1000 Hz. Now apply a signal with a frequency of 1000 Hz to the input of the connecting line (with the pickup off!) at a level that is nominal for this source (Table 2), connect a tube voltmeter to the output of the first cathode follower ("x1") and rotate the setting potentiometer slider K 16 (in the diagram of Fig. 33) until a certain voltage is obtained at the output, taken as nominal, say, 0,5 or 1 V. After that, with the signal level from the generator unchanged, switch the frequency equal to 10 kHz. This will necessarily lead to some decrease in the output signal level, if, of course, you have correctly set the tone and volume controls. In order to restore the signal with a frequency of 10 kHz to the previous level, it will be necessary to experimentally select the capacitance of the SI capacitor connected in parallel with the K 15 resistor. On this, the adjustment of the first of eight (or ten) lines can be considered complete. The next channel (in our case, a piezo pickup) is regulated similarly, but now a different signal level and a different series terminating resistor are set at the line input in accordance with the table for this source. At the same time, the output signal level at the first cathode follower must remain unchanged for all sources, which is achieved by adjusting the setting potentiometers and selecting the capacitances of the compensation capacitors.
If all adjustments are made in accordance with the above recommendations and the obtained data matched the nominal ones, the adjustment of one channel can be considered complete. The easiest way to verify this is by connecting the output of our ultrasonic frequency converter to the input of any final amplifier with a speaker system (up to the "adapter" input of a conventional radio receiver, if any), and at a certain average sound volume, alternately using the switch switch, feeding real phonograms to the input from all switched sources. At the same time, the loudness of sounding by ear should be perceived as relatively the same, with minor deviations determined by the plot of the phonograms. If the signal of one of the sources differs in sound volume from the others or reveals a clear “blockage” of the characteristics from the high frequencies, you should once again return to adjusting this particular channel. It is possible that during the adjustment process you missed this particular channel or gave a signal from this source "not to your" line. Let's get back to the microphone cascade. If it is made on a lamp, try, if possible, to purchase an EF-86 lamp made in any European country (Germany, Czechoslovakia, Poland) or the USA. It was produced by many companies under various trade names: EF-86, E-7027, E-7108, EF-806S, EF-866, Z-729, 6BK8, 5928, 6267. As for the domestic analogue 6Zh32P, it is significantly inferior to Western lamps, at least in two very significant parameters: the level of its own background from the filament circuit and the tendency to the microphone effect. And if "the first one can still be eliminated by supplying the lamp filament with a well-filtered constant voltage, then in order to prevent the microphone effect, one cannot do without a" soft "suspension of the lamp (together with the socket) on an annular rubber damper gasket. In order to minimize the possibility of a background from the filament circuit, the microphone amplifier is usually made with a grounded cathode, and automatic bias in this case is achieved due to a slight grid current in the presence of a signal. It is for this that the resistance of the grid leakage resistor is chosen to be very large (in our case, 5,1 MΩ). This does not lead to noticeable non-linear distortion if the input signal level is low enough. The electrical mode of the microphone stage lamp is the least critical, since the levels of input signals from the microphone are very low, and the anode current under any circumstances does not go beyond the linear section of the anode-grid characteristic in its upper part. However, if, when setting up the amplifier, you hear distortions when working from the microphone, it does not hurt to take the dynamic response of the cascade “point by point” and, if necessary, change the position of the operating point by selecting the resistance of the grid leakage resistor or the resistor in the screening grid circuit. Since domestic resistors of large denominations tend to “lose” their resistance almost to infinity over time, we recommend that instead of one 5,1 MΩ resistor in the lamp grid circuit, install two resistors connected in parallel with resistances of 10 MΩ each. And finally, about communications. This question is quite serious, since we are talking about long connecting lines subject to various external pickups (for example, from a 220 V power network running in parallel). In addition, we are dealing with the transmission of signals of a very low level (5 ... 200 mV) and, moreover, from sources with high internal resistance (up to hundreds of kiloohms). These two factors require the use of special measures to prevent pickups and superimpositions on the useful signal from the outside and to exclude the mutual influence of lines from different sources. The situation is aggravated by the fact that different signal sources require different circuit solutions. We will try to give recommendations for each individual case. Three lines are most vulnerable: from the dynamic pickup head, the piezo pickup and the microphone. For these three sources, one general solution can be proposed: take a thin coaxial cable (for example, types PK-50-2-13 (old name PK-19), PK-50-3-13 (PK-55), PK-50- 2-21 (RKTF-91) or RK-75-2-21 with an outer diameter of 4...5 mm and linear capacitance of 70...115 pF/m) of double length (for each of the switched sources, except for the microphone) and place two pieces of cable of the desired length in one common metal braid, as shown in the figure. It is desirable that this common braid is also insulated, for which it is best to stretch the entire workpiece into a vinyl chloride tube. To facilitate this process as much as possible, the tube can be cut into several parts 0,5 ... 1 m long and put on them alternately. The wiring of the cables from the source side and from the input side of the amplifier must be done as shown in Fig. 41. For a microphone, since it will most likely be monophonic, there is no need for two separate cables, however, using the cable braid as another (neutral) wire is not allowed here due to the inevitable occurrence of hum. For a microphone line, if it is more than 1 m, you will have to make a home-made cable from two separate wires - signal and zero, which should be placed in a common shielding braid. The connection of both wires and the braid can be seen in fig. 40. Connecting lines for a stereo tuner, stereo tape recorder and stereo laser disc player can also be made the same type, but somewhat different. Here, three multi-colored wires must be stretched into one common shielding braid: two signal wires for the left and right channels (for example, green and blue) and one thicker one (black or white) for a common ground. This cable, together with the braid, is preferably placed in a PVC stocking. The signal from the TV can be transmitted over a conventional standard single coaxial cable, using its braid as a neutral wire, since the level of the TV's own background does not allow us to talk about really high-quality sound reproduction. It should be borne in mind that the audio signal can be taken both from the output of the UZCH TV (from the loudspeaker terminals) and from the load of the frequency detector. In the first case, we will deal with a low-resistance output (units of ohms), and, therefore, the connecting cable will practically not be affected by external pickups and will not create additional losses in the high-frequency part of the spectrum.
In this case, firstly, the output signal level will completely depend on the position of the TV volume control and, secondly, it will not be possible to reproduce sound only through the amplifier, without the obligatory sound of the TV itself. In addition, in this case we will receive a signal already pre-distorted by the TV's low-frequency amplifier, which, as a rule, does not differ in a high class. It is better to use the second method and take the signal directly from the output of the frequency detector. To do this, you will have to bring the signal from the detector to an additional connector, which can be installed on the carrier frame of the TV or, in extreme cases, on a removable back wall. Connect the connecting line to this socket using a plug. In this case, the connecting line must also be made shielded, with two separate wires.
And finally, about the last connecting line - from the radio broadcasting network. The features of this line are determined by two factors. The first is that inside the dwelling, none of the two wires is "zero" - they are both equivalent and each of them can be considered signal. Therefore, in the switch, in the circuit of each of the two wires (including the one that we have grounded), ballast resistors are connected in series (R1 and R2 in the diagram in Fig. 33). In this case, the signal loss can be neglected, since the signal level in the line is one or two orders of magnitude higher than from other sources. That is why the switch has an additional group of contacts that "ground" the signal from the broadcast line in all positions except the last eighth (or the last three, if there are ten in total), in order to avoid noticeable interference of the broadcast program when working from other sources. The second consideration is only relevant if the broadcast line is multi-programmed. As you know, the signals of additional channels are transmitted at sufficiently high ultrasonic frequencies (19 and 38 kHz), which makes the capacitive losses in the additional trunk very significant. That is why it is better to make the broadcast line not shielded, but to use for it the usual thin double network wire in vinyl chloride insulation or a telephone wire (but only a stranded one, since a single-core one easily and quickly breaks off). In order to exclude noticeable pickups of this line on all the others, it is desirable to conduct it not in a common bundle with the rest of the lines, but separately and at some distance from the others. Publication: cxem.net
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