Peak Reading VU Meter
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The type of VU meter normally employed in tape decks and other items of audio equipment is the average reading type. These can give misleading results on signals that have a pulse-like waveform of relatively low average amplitude for the peak amplitudes involved. This can lead to overloading and consequent distortion on signals of this type, eg piano and percussion.
One way around this problem is to use a peak reading VU meter. This has a fast attack and slow decay time so that it responds properly to brief and intermittent signals. The normal response times for a unit of this type are 2.5ms attack and 1s decay. This unit roughly adheres to these figures.
Circuit in lines (Click to enlarge)
IC1 is an operational amplifier which is used in the non-inverting mode. R1,2 form a negative feedback network which sets the closed loop voltage gain of the circuit at a little under ten. D1 is included at the output so that IC1 can supply an output current, but a current cannot flow into the output of IC1.
The feedback is taken from the junction of D1, R2 etc., so that the input voltage appears here amplified by about ten times and the feedback overcomes the non-linearity of D1. C3 is rapidly charged to the peak output voltage as it is fed from the fairly low impedance of IC1 and D1.
Its only discharge paths are through the much higher impedances of R1-R2 and R3-M1. This gives the circuit the required fast attack and slow decay times. M1 responds to the voltage across C3, which is, of course, proportional to the peak positive input level (the circuit is a half wave type and does not respond to negative going inputs)
The VU meter movement used in the prototype had a FSD value of 130uA, but the circuit should work with any type having a sensitivity of between about 50 and 200uA.
R1 biases the non-inverting input of IC1 to the negative rail and also enables the sensitivity of the circuit to be adjusted to the correct level. At maximum sensitivity, less than 1V peak to peak is needed, for FSD of M1. Current consumption is only 400uA.