When a meter needle is sitting at -20 Vu the input of a negative going signal as would be applied by a AC signal source can not really be displayed. So basically you tap off a 8 ohm load and take that AC voltage sample and first rectify it so that it is a 1/2 wave AC signal and then limit the current so that it stays withing bounds of the scale and then add weighting to the level ballistics that you desire. This is why LED meters are better for fast reacting type signal displays.or what is called Peak meters or dB meters. This is why they are incorporated as the meter can never indicate a +8dB signal transient as the meter ballistics and weighting have not allowed the meter to indicate that. This is why some meters also have LED peak indicators to signify when a certain peak level has been hit even though the meter has never reached that position. Keep in mind that this is hard to do to a real fast rate as meters themselves have a damping and are slow to change due to the physics of the movement. A very small cap would allow for peak meters or dB meters. The larger the capacitor within reason the more damping or averaging there is to take place. This is usually done by placing a capacitor in parallel with the meter after the diode. The reaction type of meter has to do with the damping of the meter drive. Rather than go through all the calculations of making a VU meter which is an average meter, you could consult some amplifiers that have meters that have already done the calculation and designs for you- this would be equivalent to saying why reinvent the wheel? This is not to be confused with hitting the peg which is a overload condition and something the meter does not want to have happen a lot. The AC source signal needs to be rectified and then limited by resistors to establish a scale that would refer to the end point of the meter scale known as full scale deflection. The way you have it noted in the schematic is that you have no provision for this property. Meters of all kinds only sense or read a DC voltage.
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