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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING UMZCH with broadband OOS. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers "At one time, I tested many UMZCHs described in the Radio magazine in my work. Currently, I listen to music through UMZCHproposed by I. Akulinichev. I did not make any selection of parts (transistors), only reduced the capacitance of capacitor C2 from 5 to 1 uF. To be honest, I was "stunned" by this UMZCH - it has very big advantages, if you do not take into account the 24-watt output power. But she suits me. Many thanks to Ivan Timofeevich." The letter of thanks given here from a radio amateur from the city of Revda, Sverdlovsk Region, G. Khamatnurov, is not the only one after the publication of an article by a member of the editorial board of the journal I. Akulinichev "UMZCH with deep environmental protection" ("Radio", 1989, No. 10, pp. 56-58). At present, on the basis of this amplifier, Ivan Timofeevich has developed an even simpler UMZCH, the parameters of which are no worse than the prototype. He used this amplifier for a long time as a control in comparative tests of various UMZCH options. In the article published below, its description is offered to the attention of readers. The main feature of the new UMZCH is the use of a broadband OOS in it, the frequency response of which, unlike the OOS of conventional multi-stage UMZCHs, does not have a deep cut at higher audio frequencies. To implement the linearizing capabilities of broadband OOS, it was decided to abandon the multi-stage UMZCH and limit the number of its cascades to only those that are absolutely necessary. In addition, it was necessary to abandon the use of elements that create a delay in the amplified signal, which made it possible to use the OOS in the frequency spectrum of switching distortions. As a result, with the help of OOS operating in the range of 40...60 kHz, it was possible to reduce the coefficient of nonlinear distortion at a frequency of 20 kHz to 0,05...0,01% when using the output stage operation mode with zero quiescent current. This amplifier has been used for a long time as a control amplifier in comparative tests of non-inverting variants of the UMZCH. It was repeated by designers who became interested in its scheme and is currently working reliably in several stereo complexes. Schematic diagram of UMZCH with broadband OOS is shown in fig. one.
The terminal voltage amplifier is built on two transistors UT1 and VT2. Through the capacitor C1, the input signal enters the base of the transistor VT1, and through the resistors R3, R4, the balancing voltage of the power supply. To guarantee stable operation of the amplifier, the capacitances of capacitors C1, C6 and C8 should not differ from those indicated on the circuit diagram by more than ± 50%. In order to protect against accidental current overloads, a resistor R1 is included in the collector circuit of the transistor VT7. The cascade on the transistor VT2 provides the main signal amplification. The resistor chain R11R12 with a traditional voltage boost through the capacitor C8 gives an increase in the amplitude of the amplified signal by 10 ... 12%. The synchronism of functional processes in the arms of the amplifier is provided by capacitor C5. The final current amplifier is built on a complementary pair of transistors VT5-VT8, connected according to a common collector circuit. Transistors VT3, VT4 interconnected by emitters are connected by bases to the bases of transistors UT7, VT8, and by collectors to the bases of transistors VT5, VT6. Using the variable resistor R13 included in the current feedback circuit, the voltage at the bases of transistors VT3, VT4 is adjusted and, thus, the voltage at the bases of transistors VT7, VT8 is set to 0,1 ... 0,2 V lower than usual and the operation of the terminal transistors in gain mode with zero quiescent current. The UMZCH is powered by an autonomous rectifier without galvanic connection with a common wire. Thanks to this, it was possible to reliably protect the speakers from the direct current component of the terminal transistors, without introducing complex relay-transistor protection devices into the amplifier. UMZCH is made in a single unit with a rectifier. Its dimensions (135x90x60 mm) are determined by the dimensions of heat sinks and filter capacitors. The mass of the block is 560 g. The block is mounted on two plates measuring 130x58, between which heat sinks and filtering capacitors are clamped. On one of the plates there are rectifier diodes and output circuits, and on the other - all transistors, capacitors and resistors. Most of the connections are made by own conclusions of the components. Resistor R6, capacitors C11. With 12, the input circuits and the load circuits are connected to a common wire at one point. If the recommendation of a monoblock construction of the UMZCH is not used, then it will be necessary to block the supply circuits with capacitors with a capacity of 0,1 μF. In the author's version, the UMZCH block was installed in the AC and connected to the power transformer and the tone block with four wires.
Since the selector was adapted to control the operation of the UMZCH with a gain of 10 and with a minimum delay of the output signal, the limits of its setting are deliberately limited. Using it to control non-inverting variants of amplifiers with a gain of 15 ... 20 will require a constant or variable resistor with a resistance of 2 ... 1 kΩ to be connected in series with resistor R2. Multi-stage UMZCH usually create a significant delay in the output signal, and therefore in these cases it may be necessary to increase the capacitance of the capacitor C3 to 350 ... 500 pF or turn on a variable capacitor instead. And in conclusion, I would like to note: if UMZCH with broadband OOS is of interest to radio amateurs, then the author will consider it useful to contribute to overcoming their fear of gain mode with zero quiescent current.
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