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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electronic level controller. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment This relatively simple signal level control is made on discrete elements. It can be recommended to radio amateurs who want to introduce electronic regulators into their equipment, but cannot purchase the appropriate microcircuits. This regulator, subject to the selection of elements, allows you to obtain the parameters necessary for use in high-quality sound reproduction equipment. The proposed electronic level control, in contrast to the volume control, which can also be tone-compensated, is made according to the scheme of a dual differential cascade, in which the sound signal is fed to the emitter circuit, and the transfer coefficient varies over a wide range by means of control over the transistor base circuit. In microcircuits of electronic regulators based on transistors of the same structure (for example, K525PS1), the collectors of differential stages are loaded with resistors connected to the power bus + Upit (Fig. 1). The resistance of resistors R3 and R4 is much lower than the dynamic resistance of transistors VT1 - VT4, so noise and ripple from the power bus are output without attenuation. As a result, low ripple power supplies are required.
In addition, such a structure does not allow directly obtaining the maximum swing of the output signal of ±12 V at a supply voltage of ±15 V, and the coefficient of non-linear distortion turns out to be significant. These factors make it difficult to use such devices for volume controls in high-quality equipment. If the cascade is performed according to a symmetrical scheme (Fig. 2), then interference in the power circuit can be significantly reduced. In addition, here the signal always remains symmetrical, i.e. even harmonics are lower than in the original version. But the maximum output level of the regulator in such a connection of transistors is even more limited: it is only about 300 mV. To increase it, it is possible, of course, to "dilute" the voltage at the bases of transistors up to the value of ±(|Upit| -1 V), but this will require a noticeable complication of the device.
The problem can be solved in a simpler way - by connecting the output of the regulator to the inverse input of the op-amp covered by feedback in an inverting connection (acting as a current-voltage converter). Its output voltage depends on the ratio of the resistance of the feedback resistor to the resistance of the signal source (for an op-amp). The maximum signal amplitude in this case will be standard for a particular type of op-amp and will be at least 9 V, with virtually no increase in the level of harmonics. In this version of the regulator, the operating frequency band will be narrowed to the one that this op-amp is capable of providing, but for use in sound engineering devices, there are quite modern op-amps with excellent parameters. The complete circuit of the electronic regulator (Fig. 3) is somewhat more complicated than the functional one. Such a regulator is used to control the volume level as part of an audio complex amplifier. Transistors VT1 - VT4 represent the actual electronic regulator. The audio frequency input signal through resistors R4 and R5, which convert the input voltage into current, is fed to the connection points of the emitters VT1, VT2 and VT3, VT4, respectively. The bases of transistors VT2 and VT3 are connected to a common wire through a resistor R1, and a control voltage is applied to VT1 and VT4 in the range of -50 ... + 50 mV, which leads to the redistribution of the collector current VT1 - VT4 either to a common wire or to an inverse input OS DA1. The latter amplifies it in the ratio R10/[(R4 R5)/(R4 + R5)] for the maximum signal transfer coefficient.
For the above circuit, the maximum gain Kmax = 4,4. By changing the resistors R4, R5 and R10, it can be made almost anything allowed for the applied op-amp. With this construction, all regulator transistors operate at a practically constant voltage on the collectors, and therefore harmonic distortion does not occur. The main source of distortion remains the op amp, and it determines the quality of the regulator as a whole. The applied op amp can be replaced by another one with low harmonics at audio frequencies and corrected for unity gain. In an electronic controller, it is advisable to supplement some op amps with two additional transistors VT5, VT6 to reduce switching distortion of the output stage (transfer to class A operation mode when the output current decreases). But this is not at all necessary. Other transistors can also be used in the regulator: for example, complementary pairs of the KT3107 and KT3102, KT315 and KT361 series with any letter indices, provided that their base current transfer coefficient is more than 100 in the regulation current range. If it drops noticeably as the collector current decreases, then this creates additional distortion. The field effect transistor can be of the KP307 series. All MLT resistors - 0,125, electrolytic capacitors - K50-6 or similar, input capacitor - with a film dielectric (for example, the K73 series). In this regulator, the voltage on the transistors is almost constant, but the currents change significantly, and in order to reduce these changes, the bias current of the differential stages is chosen to be several times larger than the input one. It is also necessary to pay attention to the need to install additional resistors R1 and R3; without them, the amplifier is excited. It is possible that one R1 will be enough, it is also permissible to reduce their resistance, but not lower than 200 ohms. The power supply can be used with an unstabilized voltage, but with a fairly good ripple suppression (up to about 0,01 ... 0,1%). The controller setting is as follows. First, at the maximum transmission coefficient (Uypr \u50d +6 mV), a zero offset is set at the output of the op-amp by selecting resistor R7 (or R5). In the author's copy of the regulator, such a selection was not required (deviation of resistor resistances up to 10 - XNUMX% is permissible). This parameter is somewhat more affected by the difference in voltage UBE for the controller transistors (at the same current). After checking and, if necessary, zeroing the output of the op-amp at the maximum gain, the DC component is checked with a decrease in gain by 6 dB (Uypr = 0), when the deviation reaches a maximum. In the author's version, in each of the channels of the regulator, the deviation from zero reached 1 ... 3 V. This can be corrected either by selecting one of the regulator transistors (any), or by introducing a bias circuit into the gap between the bases of a pair of transistors (also any). You can, however, like the author, leave it without additional balancing or selection, since even in the worst case for an undistorted signal, a signal swing up to a voltage of ±5 V remains. The regulator differs from its analog counterparts by a lower harmonic coefficient (less than 0,2%), and from digital ones by the ability to process input signals with a voltage arbitrarily higher than the supply voltage, and the absence of signal modulation by the regulator. In addition, the level adjustment is smooth (if the control voltage changes without jumps). If the control voltage is created digitally, then discreteness will appear, but without modulation. Author: L.Levitsky, Mytishchi, Moscow region.
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