Signal level indicators are used to visually assess the changing parameter at intermediate points in the device circuit. According to their testimony, one can judge the operation of individual functional modules. The use of indicators in audio signal amplifiers allows you to set the level sufficient for comfortable listening to musical compositions, while preventing the amplifier from operating beyond its permissible values.
Main types of indicators
Indicators are an integral part of sound amplification devices. They allow you to get a visual assessment of the composition in the range of audio frequencies. To monitor the signal level, both pointer instruments and devices made in the form of LED columns are used, changing their geometric dimensions as the sound signal rises or falls in the selected frequency range. It is possible to distinguish the main typesindicators:
- Scale, which are devices of varying degrees of complexity, in which pointer microammeters are used to assess the strength of the sound signal.
- Peak (LED) signal level indicators, which can use both single elements and LED strips.
- Peak fluorescent.
Modern systems for reproducing sound information use electronic devices that reflect a number of necessary parameters. Their schemes use one or another of the main types of indicators listed above.
Simple scales
Indicators of this type contain an electromechanical microammeter with a full deflection current of the arrow up to 500 μA. The device works when a direct current flows through the winding of its coil. Therefore, the time-varying audio output needs to be pre-converted with a diode circuit.
By changing the value of the resistance of the resistor that limits the current flowing through the microammeter, you can achieve full deflection of the needle for the maximum level of the audio signal. The scale is graduated as a percentage of the maximum level or in decibels (dB) of its attenuation.
Scale on bipolar transistors
Microammeters in the circuits of these devices are included in the collector circuit of the output stages of transistor current amplifiers, made according to the common emitter (CE) circuit. The number of amplification stages is determined by the minimum level at whichwhich the scale of the dial indicator of the sound signal level should respond. The magnitude of the full deflection current of the pointer can be set by the elements of the AC voltage divider supplied to the input of the pointer indicator circuit for subsequent amplification.
In their composition, the circuits contain rectifiers for a constantly changing signal of an audio amplifier to direct current to create a more comfortable visual control of the volume of the composition being listened to. The scale is performed with the digitization of the percentage of the current signal level in relation to its maximum value. For the value of the maximum value, the volume level is selected, the coefficient of non-linear distortion of which does not exceed the permissible value and is determined by international quality standards.
Scale on operational amplifiers
Operational amplifiers (op amps) with high input impedance introduce minimal distortion into the measurement circuit. Amplifier signal level meters allow you to visually control the minimum levels that are inaccessible to the simplest meters and circuits with OE.
Op-amps are used as voltage/current converters or emitter followers. The electromechanical head of the microammeter has a scale that reflects (as in previous cases) the attenuation in decibels of the level of the measured signal relative to its maximum value.
Peak indicators
LED indicators of thistypes are performed on the basis of input signal level voltage comparators. The voltage at their outputs appears at the moment when the input signal exceeds a certain value of the input signal value, pre-set by the circuit elements. At the same time, the voltage level that occurs at the output of the comparator is sufficient to light up the LED of the indicator line.
The more threshold devices the signal level indicator circuit contains, the less jumpy movement of the LED bar on the scale will be noticeable, the more natural the observed picture will be.
Indicators using logical elements
These devices are used in circuits in which an LED or LED signaling device is used as an element signaling that the input signal has reached a level sufficient to trigger a logic component. It will glow for as long as the voltage level at the input of the circuit is sufficient for the open state of the logic circuit and, accordingly, the flow of current through the LED and for it to glow.
The Schmidt (Schmitt) trigger property is used in these circuits of LED signal level indicators - to keep their stable states. In the first of them, there is a positive voltage of the power source at the output of the power element. Another situation corresponds to the case of its closed state and in the absence of a positive voltage at the output. So the trigger can serveindicator of the signal level present at the input of the circuit.
The load of the logic circuit is a bipolar transistor n-p-n conductivity, connected according to the amplifier circuit with a common emitter (CE). An LED is included in its collector circuit, which signals the excess of the input signal level, set by the circuit elements.
The number of triggers used determines the number of audio signal levels to be controlled. 2 or 3 microcircuits, having 4 logical elements in one package, allow you to create an indicator with your own hands, in which there is practically no step dependence of the change in readings.
Level indicators on specialized microcircuits
The LM 3915 integrated circuit is manufactured by Texas Instruments. It has become widespread in the creation of signal level indicators for an amplifier. It controls 10 levels of changing audio signal based on built-in comparators. At the same time, it causes the LED output elements to light up according to a logarithmic law. This allows you to correct the perception of the output level of the amplifier in accordance with the property of the human ear.
Low levels are often not audible. The logarithmic law makes it possible to achieve a linear perception of the volume of the musical composition being listened to when its intensity changes over a wide range. In the case of using two chips, it becomes possible to create LM3915 signal level indicators for stereo sound systems.
Fluorescent
These indicators are equipped with high-quality audio playback devices. They are made in the form of ready-made panels, which include a set of specialized circuits controlled by microcontrollers. Their scales reflect the change in many parameters. Often they are indicators of bandpass equalizers, which allow you to correct the amplitude-frequency response of sound amplifiers over a wide range.
With a sufficient level of experience in the manufacture of amateur radio structures, such do-it-yourself signal level indicators can be created independently. Keep in mind that circuits using stylish fluorescent lights often require multiple power supplies.
Conclusion
The material presented in the article will help the reader to find out the device and the purpose of different types of level indicators. It should be borne in mind that many of them can be made on their own from the available construction kits. Switch-type devices are still widely used in high-quality sound reproduction devices today.
