About the lamp-field-bipolar-microcircuit UMZCH. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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I saw an article describing this amplifier in Radiohobby magazine a few years ago [1]. The simplicity of the circuit (we repeat it in Fig. 1 - Ed.). We also recommend reducing the resistance of R2 and R3 to 270 ohms) and the absence of an output transformer immediately interested. However, as almost always happens, current affairs did not allow me to immediately try to assemble this amplifier. In addition, there were doubts whether this amplifier will drive the 60 W speakers I have. Looking ahead, I’ll say, I shook it. Rocked and four-hundred-watt loudspeakers! Therefore, I want to share the experience of assembling, tuning and testing this excellent amplifier. So, in order.

Fig. 1

First, the output part of the amplifier was assembled. In order not to burn the transistors when you first turn on, the amplifier should be powered from a source with output current limitation. The current is set to 1.7 ... 2.0 A both in positive and negative circuits. If the parts are in good condition and there are no installation errors, then both ammeters in the positive and negative circuits will show the same current value. We check whether the current consumption is regulated by the potentiometer R9. If everything is fine, then a voltage of 2-9 V (amplitude) with a frequency of 10 kHz is applied to the left side of the circuit of capacitor C1 from the audio frequency generator. An undistorted sinusoid of approximately the same amplitude as at the input should be observed at the output. You can not connect the load to the amplifier yet. If the heat dissipation from the transistors is not very good, all actions should be done fairly quickly.

When checking the frequency response of the source follower, it turned out that at frequencies above 4 - 5 kHz there are significant non-linear distortions. Distortions of the positive half-wave of the output signal are clearly visible on the oscilloscope screen. Reducing the resistance of R4 in the gate circuit from 680 ohms to 33 ohms solved the problem, but it is not necessary to expect that the output resistance of the tube stage will be as small as the output resistance of the generator (50 ohms). The measured output resistance of the tube stage with resistors R2 and R3 in cathodes of 560 Ohm is 3,6 kOhm for the ECC88 lamp and 6,8 kOhm for the ECC85 lamp. Of the available field-effect transistors, I used 2SK2134. According to passport data, they have an input capacitance of 500 pF. With these FETs, distortion is observed when the signal source resistance is greater than 1 kΩ. To reduce the output impedance of the tube stage, the resistances of the cathode resistors were reduced to 120 ohms. To increase the gain, resistor R3 is shunted with a 1000 uF capacitor. Although the output impedance of the tube stage did not become less than a kilo-ohm, the distortions were noticeably reduced.

A modified power supply from a computer was used to power the amplifier. The fan, located on the top cover of the power supply, serves to cool the radiators on which the transistors are fixed. Refinement of the power supply is to get the anode voltage, minus 12 V at a current of at least 4 A and minimal supply voltage ripple.

The anode voltage is taken from an additional winding (approximately 30-35 turns) wound on the transformer. The voltage from this winding is rectified by a full-wave rectifier with a smoothing capacitor with a capacity of 47 microfarads for a voltage of 200 V. Do not forget to use pulse diodes in the rectifier. To eliminate ripples with a frequency of 100 Hz, the voltage to the anode of the lamp is supplied through an L-shaped RC filter - a 560 Ohm resistor and a 100 microfarad capacitor at 200 V. To obtain the required current from a minus 12 V source, low-power rectifier diodes were replaced with more powerful ones that are installed to the heat sink to the place vacated after the dismantling of the 3,3 V channel. Capacitors at the output of the minus 12 V source were replaced with larger capacitors with low ESR, which reduced the ripple with (frequency of operation of the power supply converter (about 12 kHz) by an order of magnitude. By changing the resistance of the resistor in the divider circuit of the TL12 microcircuit, the voltage of the main source was set to 5 V. The incandescent lamps are powered from it. The voltages of 2,2-volt sources have risen up to 25 V The output power of the amplifier at a load of 494 ohms -6 watts.

Setting up the amplifier is very simple Since the amplifier was designed to work with loudspeakers having an impedance of 8 ohms, the quiescent current was set to symmetrically limit the half-waves of the sinusoid at the output, loaded by a 6 ohm resistor (with a margin). 1,7 ... 1.8 amperes per channel is the optimal current value.

Fig. 2

Here is what the rice amplifier I assembled looks like. 2, and this is a view of the installation - fig. 3.

Now, listening. Three-way DENON speakers are connected. They say - Digital Sound D-3 8 ohm, 91 dB. There is no click when turned on, there is complete silence in the speakers. Signal sources are CD player SONY CDP-411. MP 3 DVD and AKAI AP-D40 player. head SONY XL-20. A SANSUl RZ-5000 amplifier corrector was used as an amplifier-corrector. For comparison, the loudspeakers were connected to SANSUl RZ-5000 and DENON RMA-360. Neither me nor them. who listened, we do not consider ourselves experts in sound. Therefore, all judgments are subjective. Apart from the volume level. When the volume control is turned two-thirds clockwise, the level is higher than needed for listening in a 40 square meter room. meters. Exactly the same volume level when connecting Peerless speakers, six heads, three bass, one midrange and two tweeters. 400 W, 92 dB.

Fig. 3

The sound, subjectively, seemed to everyone who listened much more pleasant than that of branded amplifiers. It can be noted that the vocals and accompaniment are much more clearly separated, and the bass sounds more natural and softer at the same volume level. Better with DENON speakers than with Peerless.

They also listened to the Three Tenors, DECCA. helicon records, and CHICAGO 7. Columbia rec, both pop and trance. From my point of view, this simple amplifier is worth it to build and listen to.

Literature

1. Bondarenko Igor. Lamp-field-bipolar-micro-circuit transformerless UMZCH without environmental protection. - "Radiohobby" No. 5 2000, p.66.

Author: Ilya Lipavsky, Kiryat-Ata (Israel); Publication: radioradar.net

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