LED VU METER  (277-0116)          Operation            Faxback Doc. # 15157

The VU METER consists of circuits which convert the audio signal to a DC
voltage.  This voltage is compared to a reference voltage.  As the audio
voltage becomes equal to the reference voltage, the LED's will be
illuminated.  The circuits used to convert the audio signal to DC voltages
consist of the amplifier (Z3, pins 8, 9 and 10), the active rectifier (Z3
pins 5, 6 and 7) and the active low pass filter (Z3 pins 1, 2, and 3).
The reference voltage is produced by the high compliance voltage reference
(Z3 pins 12, 13 and 14).  The ladder network provides an accurate voltage
divider network which will provide different reference voltages in 3 dB
steps to each of the eight comparator stages.  The comparator stages are
provided in Z2 and Z1 (quad comparators).

The comparator IC operates as follows:  When the minus input (-) is equal
to or has a more positive signal than the plus input (+), the output is
caused to switch towards ground.  This ground signal causes the LED to be
illuminated.  If the comparator (-) input is more negative than the (+)
input, the output is positive (Vcc) and the LED is off.

AUDIO CONVERSION CIRCUITS

Amplifier:         The input amplifier stage allows the sensitivity of the
                   VU METER to be set and prevents excessive loading of the
                   signal source by offering a 1 Meg impedance at IN1 (IN2
                   provides a 100K Ohm impedance).  It also assures that
                   the reminder of the circuit has enough drive voltage to
                   operate properly.  The VU METER is DC-isolated from the
                   signal source with a 0.22microF, 250V capacitor (C3).

Active Rectifier:  An active rectifier stage follows the input amplifier.
                   The difference between an active rectifier and an
                   ordinary diode is that the active rectifier utilizes an
                   op-amp to amplify the signal across the diode about
                   100,000 times.  A feedback circuit keeps the output
                   voltage equal to the input.  The active rectifier thus
                   effectively reduces the normal 0.6 volt diode drop to 6
                   microvolts which necessary if the VUMETER is to maintain
                   linearity and accuracy for small signals (i.e., the
                   center and bottom of the scale).  This is achieved by
                   separating the feedback of the rectifier stage which is
                   an inverting-follower circuit.  This separation is
                   accomplished as follows:  If the op-amp output is
                   positive, the signal and feedback go through CR2 and R36
                   and the final output appearing at R35 is equal to the
                   input.  If the op-amp output is negative, the feedback
                   is through CR1 and the final output at R35 is zero. 
                   Therefore, the final output can only be zero or
                   positive.

Active Low Pass:   An active filter, made up of the op-amp stage, R32-35,
Filter:            R26, C5 and C6 takes the rectified signal and smooths
                   it into a DC voltage proportional to the input.  This
                   is an active two-pole Butterworth low pass filter with
                   a cutoff of 7.25 Hz.  This filter was chosen because:
                   a) audible frequencies are attenuated to the extent
                   that they do not affect the LED's (no flicker is visible
                   due to steady bass tones), b) the response time is about
                   1/10th second, and c) overshoot, equivalent to meter
                   swing, is negligible, amounting to about 1.5 dB.


(lb/all-05/08/95)