Lamp-semiconductor UMZCH. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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In a number of articles on the pages of magazines devoted to audio technology, the question is often discussed: what is better for amplifiers - tubes or transistors? At the same time, both tube designs (in which rectifiers are sometimes made on kenotrons) and transistor UMZCH are described. The editors decided to acquaint readers with a brief description of the features of the combined UMZCH, made on amplifying devices of different generations: lamps, transistors, integrated circuits.

The parameters of the combined amplifier are very unusual for tube UMZCH. Suffice it to point out that for a push-pull transformer UMZCH on two output pentodes EL84, an output power of 32 W was achieved with a full power band of 5 ... 55 Hz (at -000 dB). The harmonic coefficient at a frequency of 3 Hz with an output power of 1000 W does not exceed 20%. The output impedance of the amplifier is 0,07 ohms. How are such parameters achieved if the device (Fig. 0,6) contains, in addition to output lamps, only two high-voltage transistors and a pair of operational amplifiers?

(click to enlarge)

Unlike transistor power amplifiers, tube UMZCHs require an output matching transformer that optimizes the load characteristic of the tubes to obtain maximum output power. In this case, the resistance of the loudspeaker is transformed into the load resistance of the lamps.

The use of a transformer inevitably limits the band of effectively amplified frequencies, and the low-frequency frequency response is due to the high-frequency filter formed by the output impedance of the cascade lamps and the inductance of the primary winding shunting the load. The high frequency cutoff is defined by a low pass filter consisting of the same output impedance and parasitic leakage inductance of the primary and secondary windings, which also results in a reduction in load signals. Therefore, the lower the output impedance of the lamps of the cascade, the wider the range of transmitted signal frequencies.

Negative feedback (NFB), used to reduce harmonic and frequency distortion of both lamps and transformers, is of limited use in such cascades due to the complex phase response of the transformer. Phase shifts generated by its parasitic leakage inductances and winding capacitances, and partly by the through capacitance of the lamps themselves, lead to the fact that at high frequencies the connection becomes positive and deterioration of parameters or even self-excitation occurs. To obtain a wide frequency band, in addition to using a high-quality transformer, it is necessary to reduce the output impedance of the lamps. This can be achieved by connecting lamps in parallel, using lamp triodes (or multi-grid lamps connected by a triode). The use of lamps in the cathode follower mode is not very economical due to the low voltage transfer coefficient (less than one).

The best way to reduce the output resistance of the lamps is to use a parallel negative voltage feedback in the cascade, which forms a current-controlled voltage source (in the domestic literature, the abbreviation INUT is used - ed.), And it is advisable to use a transistor cascade that is effective in this mode as a signal source for it. in the form of a voltage controlled current source (ITUN). Such a device is a transistor stage (Tri, Tr2) controlled by an operational amplifier (A1, A2) with their common series current feedback covering them. As a result, without a general feedback, high linearity was obtained and the output resistance of the lamps was significantly reduced: reduced to the secondary winding, it is 0,6 Ohm! The anti-phase control of a push-pull tube stage is achieved by using a feedback signal to excite the other arm of the amplifier, made with the phase inversion of the signal.

Due to the complete symmetry of the arms, the amplifier is insensitive to supply voltage ripples, so the op-amps are powered by half-wave rectifiers: the power supply circuit of the amplifier is shown in fig. 2. Here, the voltage for these rectifiers (D1C7, D2C8) is obtained from two six-volt windings of the power transformer for lamp equipment. The anode winding of this transformer should provide a voltage of about 280 V.

(click to enlarge)

In the design of the UMZCH, a matching output transformer with a transformation ratio of 20: 1 is used, the inductance of its primary winding is at least 8 H with a leakage inductance of not more than 10 mH. Permissible deviations of the resistor values ​​- no more than ± 1%, the power of the resistors, if it is not indicated on the diagram, is 0,5 W.

Editorial note. In this combined UMZCH, as in many transistor amplifiers, the output stage is covered by a sufficiently deep voltage feedback, therefore, when the output stage is overloaded, the signal cutoff is relatively sharp, it gives harmonics of a higher order than in tube amplifiers without feedback. In addition, if the grid current of the output stage lamp appears during overload, it leads to overcharging of the coupling capacitor (C1, C2) in the grid circuit and, consequently, to dynamic non-linearity. The best solutions to this problem may be to eliminate the decoupling capacitor and ensure that the lamp is in constant current mode by appropriately offsetting the operating voltages of the transistor part of the amplifier.

The disadvantage of the proposed version of the circuit is the use of an oxide capacitor, which requires a polarizing voltage. In this case, polarization is achieved by dividing the resistance of the resistor R11 into two halves and back-to-back (with a common minus) two identical double-capacity capacitors (C4) with this common lining connected to the -Ve power bus through a resistor with a resistance of several hundred kilo-ohms.

Domestic elements can be used in the UMZCH design: 6P14P lamps, KD226G, KD226D diodes (for the BR1 bridge), capacitors C1, C2 - K73-17, K78-2, C3, C4 - K50-16, K50-35 or oxide-semiconductor ( for C4 - for example K53-18). Transistors 2SC2547E are interchangeable with KT605A, KT605B. KT604, KT969. It is permissible to replace the op-amp TL072 microcircuits with domestic K140UD25, K140UD26, K140UD18, K544UD1 with any letter index.

Resistors - MLT of the appropriate power. Due to the significant voltage across the resistors R7, R8, a resistance of 220 kΩ is obtained by connecting two resistors with a resistance of 110 kΩ each (MLT-0,5) in series.

Literature

"Electronics Word + Wireless Word", 1995, No. 10, p. 856

Publication: cxem.net

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