Load equivalent for UMZCH. Scheme, description

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Load equivalent for UMZCH. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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Any newly assembled or refurbished audio power amplifier needs to be tested for performance, reliability, and performance. Debugging amplifiers both at the most fruitful night time for men, and at any other, no less fruitful, with speaker systems connected to them, is a considerable problem.

To understand the magnitude of these inconveniences that you can be the source of, remember your own annoyance that you experience if someone, while cleaning or repairing a car parked next to your windows, turns on the radio at full capacity, believing that his musical preferences will deliver limitless aesthetic pleasure by the will of circumstances to people who are nearby. If 100 years ago a crowd of idle people gathered at a singing gramophone put out by a gentleman on the street, then at present the situation has changed radically, and people who demonstrate their musical tastes to the whole district can be called at least ill-bred. In order not to cause great inconvenience to other people, it is advisable to pre-configure the UMZCH with a silent load equivalent. On fig. 1 shows a diagram of a simple device that can be used to tune audio frequency amplifiers with an output power of up to 20 ... 100 W or more.

Load equivalent for UMZCH, scheme
Fig. 1

Instead of a real acoustic system, 6 powerful wirewound resistors serve as the load for the amplifier. Powerful wirewound resistors, unlike conventional carbon and metal-film resistors, have a significantly higher self-inductance, which slightly approximates the operating conditions of the amplifier with a load equivalent to work in real conditions.

The dummy load has a resistance of about 5 ohms. The main part of the incoming power is dissipated in the form of thermal energy on wire resistors R1-R4. Wire resistor R5 - adjustable, has an open design, included as a trimmer. Depending on the position of the slip ring on its body, a greater or lesser part of the incoming power is supplied to the control dynamic head. The C1R6 circuit simulates the presence of a high-frequency dynamic head in the speakers. LEDs HL1, HL2 indicate the presence of a constant component at the output of the UMZCH, which usually indicates a serious malfunction of the amplifier. The HL3 LED, in the absence of a useful signal at the UMZCH input, signals with its glow that the amplifier is self-excited at high frequencies. LED indication of emergency operation modes allows you to notice the abnormal state of the UMZCH in time, which in many cases allows you to avoid the occurrence of more serious malfunctions, the cost of which can reach tens and hundreds of USD.

In place of the wire resistors R1-R4, you can use any "old" resistors with a power of 4 ... 100 W, for example, PEV, PEVR, PEVT, S5-35, S5-36, S5-37V. The use of modern imported wirewound resistors in a white ceramic case in their place is highly undesirable due to their low reliability in this design. Resistor R5 can be installed type PEVR or similar with a resistance of 200 ... 470 ohms. The power of the resistors R1-R4 will determine how much power can be supplied to the equivalent load.

Even if the total power of the resistors R1-R4 is greater than the output power of the amplifier, with its power of more than 20 W, these resistors can heat up significantly and heat the entire structure as a whole. Therefore, it is desirable to provide forced air cooling in the design, for example, using a 12 V computer fan. Resistor R6 can be installed with a power of 5 ... 20 W of any type already mentioned. If necessary, the number of resistors connected in parallel can be increased, for example, with 10 resistors connected in parallel with a power of 20 W each with a resistance of 40 Ohms, 200 W of power can be supplied to the device. Forced air cooling of the structure in this case will be mandatory, especially when powerful resistors are located close to one another. The dynamic head is used for control listening to the operation of the amplifier; it can be of any type with a coil resistance of 4 ... 16 ohms with a power of 2..5 W. Capacitor C1 type K73-17, K73-24, MBM for an operating voltage of at least 160 V. Diodes VD1-VD4 can be replaced by 1N4148, series KD521, KD522 or a KTs407A rectifier bridge. LEDs are suitable for any type, for example, from the AL307, KIPD40 or L-1503 series.

In addition to being used as a load equivalent for preliminary tests of the UMZCH, it can be used to set up power supplies, force the discharge of batteries, quickly dry the surfaces to be glued, and heat solutions to increase the rate of chemical reactions. When operating this device, it should be remembered that wire resistors can get very hot, and the amplitude of the output signal of powerful UMZCH can reach 100 ... 160 V or more - be careful.

Author: A.L. Butov, p. Kurba, Yaroslavl region, Radioamator No. 2, 2009; Publication: cxem.net

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