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An audio volume unit table displays the amplitude of the audio associated with the peak to help accurately set the recording level or to display the operating status of the amplifier. A simple diode and capacitor network provides a peak-weighted response to the classic volume unit table, but the circuit typically limits the response to a display dynamic range of approximately 23 decibels, and the inertia and mechanical "shock" of the volume unit pointer Bring errors. Contemporary displays use arrays of lighting elements to form a bar graph, thereby eliminating inertia problems, but all of the shortcomings in response and accuracy characteristics are now shifting to the field of signal processing. You can use DSP technology and applied mathematics to reproduce the functionality of the instrument in firmware, but this method can be expensive if the instrument does not already include DSP functionality to save money.The weakness of the cheap analog meter is still its peak-retaining component, which must be charged quickly to accommodate large signals, and must be accurately charged for small signals – two mutually exclusive targets. In addition, if the full-wave rectification and peak-retention characteristics of the diode are not ideal, the dynamic range of the analog volume unit table is also limited. Maintaining a display dynamic range of 20 decibels and monitoring various signal levels that vary over a range of 40 decibels (typical in consumer electronics) requires both a circuit with a dynamic range of approximately 60 decibels.
In most cases, conventional circuits cannot provide both the expected accuracy and slew rate, especially at low signal levels over a wide dynamic range. The circuit in Figure 1 provides a simple configuration that provides high accuracy over a dynamic range of more than 60 dB and provides the fast/slow feature required for high quality displays.
At the heart of the circuit is Linear Technology's LT1011 comparator, IC 2 , which monitors the difference between the amplitude of the input signal and the peak detection output. In addition, as long as the state of charge of the holding capacitor C 6 of 4.7 mF is too low, the IC 2 also supplies a charging current to the capacitor. Unfortunately, the inherent input-to-output delay of the comparator and non-linear amplifier determines the minimum output pulse width. If the holding capacitor is quickly charged to track a larger input burst, the minimum charging step must greatly exceed the level of the small signal, thus limiting the dynamic range.
Inductor L 1 provides a charge current source that can be adaptively varied, thereby solving the capacitor response problem. If a 10 mH inductor is added, the maximum current rate is limited when the comparator produces a narrow pulse, so the minimum charge amplitude step is reduced to a lower 1 mV level or lower. For a wider charge pulse, the current is automatically raised to a higher level to provide the required high slew rate. The minimum charging step size is roughly proportional to the signal step size, thus ensuring a constant relative accuracy of better than 1 dB over the 60 dB signal range. The signal level of -59 dB corresponds to the input voltage of 13 mV, while the scale factor of 0 dB at 2V peak voltage corresponds to a typical gain of 20 audio power amplifier inputs 100 Wrms to 8Ω load (or approximately 40V peak output voltage) ) The required input level.
The circuit also includes two operational amplifier stages based on Linear Technology's high precision LT1469 dual operational amplifier. The first stage IC 1A provides 6 times the gain in this example, so the 2V input peak provides a 12V output voltage. The second operational amplifier stage IC 1B constitutes a precision inverter half-wave rectifier. The outputs of IC 1A and IC 1B and the positive peak detection voltage across C 6 are combined at the input of IC 2 to provide a zero crossing threshold to the comparator. When the input of IC 2 drops below 0V, its output switches to Q 1 and provides charging to C 6 until the voltage of C 6 reaches or slightly exceeds the amplified audio voltage. A feedback network consisting of R 8 and C 4 provides the best volume unit metering discharge.
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