Signal level indicator - stereo balance indicator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Signal level indicator - stereo balance indicator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The device brought to the attention of radio amateurs can be used as an indicator of stereo balance and output signal level. It can be connected both to the outputs of AF power amplifiers and to the line outputs of any sound reproducing devices. phase pulses, which then fall on the indicator grids in time with the connection of the inputs of stereo signals. As a result, the signals from the left and right channels alternately (and synchronously with the voltage on the grids) arrive at the transistor switches A1-A14 (the figure shows the first A1.

The schematic diagram of the indicator is shown in fig. one.

Fig.1 (click to enlarge)

Main technical characteristics:

Dynamic range at a frequency of 1 kHz, dB.......25
Integration time, ms.......10
Reverse time, ms.......150
Number of scale segments.......14
Supply voltage, V.......27
Current consumption, A ....... 0,2

Rectifiers on diodes VD1, VD3 and VD2, VD4 are installed at its input, to which signals from the outputs of the left and right channels of radio devices are received through input dividers R5, R6 and separating capacitors C3, C4. The signals rectified and filtered by capacitors C5, C6 are fed to the transistor switch VT3, VT4.

The operation of the vacuum luminescent indicator (VLI) HQ1 is controlled by a multivibrator on transistors VT1, VT2. It generates paraphase pulses, which then fall on the indicator grids in time with the connection of stereo signal inputs. As a result, the signals from the left and right channels alternately (and synchronously with the voltage on the grids) arrive at the transistor switches A1-A14 (the figure shows the first A1 and the last A14), each of which is triggered at a certain signal voltage based on its transistor. The key operation threshold is determined by the voltage at the emitter of its transistor and depends on the resistance of the resistors 1R2-14R2. When the signal voltage exceeds the voltage at the emitter of the transistor (by about 0,6 V), it will open and the corresponding VLI segment will begin to glow.

The resistances of resistors 1R2-14R2 (in kiloohms), at which the segments indicate a certain recorded output power at a constant load resistance, can be calculated by the formula: R2 = (27,6-(PRн)0,5)/((PRн)0,5, 0,6-XNUMX), where P is the recorded output power, W, Rl is the load resistance, Ohm.

When connecting the indicator to the linear outputs of radio devices with a low output signal voltage, it is necessary to increase its sensitivity. To do this, you should connect a voltage amplifier to two op amps DA1 - DA2 (Fig. 2).

Ris.2

The indicator uses MLT-0,125 resistors (R9 is made up of several resistors connected in parallel). Diodes VD5, VD6 D220, KD503 and the like. Transistors 1VT1 - 14VT1 are any of the KT502 series (V, G), with a current transfer coefficient h21 > 60.

The indicator is assembled on a board made of one-sided foil fiberglass with dimensions of 130x80 mm. The board with a preamplifier for op amps DA1, DA2 is mounted separately. To increase the contrast and facilitate the reading of VLI information, it is recommended to close it with a neutral light filter (GOST 9411-81) with a light flux transmittance of 0,2 ... 0,3.

Since the indicator can cause interference, it is not recommended to place it near magnetic heads, microphone amplifiers and other devices sensitive to interference and is best placed in the screen.

A drawing of the circuit board and the location of the elements are shown in Fig.3.

Fig.3 (click to enlarge)

The establishment of the indicator begins with the selection of such a resistor R9, at which the voltage on the VLI filament would be in the range of 3 ... 3,8 V. Then, a signal with a frequency of 1000 Hz and a voltage corresponding to the developed amplifier at nominal output power and a given load resistance (U=(PRn)0,5, where P is the rated output power, W, Rn is the load resistance, Ohm). After that, trimmer resistors R5, R6 achieve the glow of all green segments HG1 to the level of "0 dB", respectively, for the left and right channels. So, with an output power of 50 W and a load resistance of 4 ohms. corresponding to the level "0 dB". the input voltage is 14,1 V. And finally, a resistor R10 is selected of such resistance at which the brightness of the scale, the inscriptions "left" and "right", as well as the first segments of the channels, was the same as the brightness of the glow of the remaining segments.

The amplifier, made according to the scheme of Fig. 2, will amplify the ripple of the power supply by 50 times, therefore it is advisable to connect the inputs 3 DA1, DA2 to a common wire through capacitors with a capacity of 10 microfarads for a voltage of 15 V. One divider and one capacitor for two op-amps are quite enough. In this case, the polarity of the capacitors C3, C4 of the indicator (Fig. 1) should be reversed.

Author: O. Zhelyuk

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