Wohler AMP2A User Manual page 18

AMP2A and AMP2A-2S User Manual P/N 821553 Rev-B Section 3: Technical Information
LED Bargraph Driver PCB (919030) Description
Input Stage and Level Calibration
The PPM-2 display driver PCB circuit (P/N 919030) provides a total of 44 dB of visual audio level indication via a 20-segment LED
bargraph.
The main PCB's input buffer amps pass unbalanced signals through a DIP switch selectable attenuator pad on each 919030 meter
driver PCB. A choice of four different reference levels, -6, 0, +4, and +8 dB for the 0 dB indication are available for selection by the
user. A chart showing these input gain calibration settings can be found on page 17 and on a label on the top cover of the AMP2A unit.
Each of the differential outputs from the meter driver's input buffer drives a half-wave rectifier circuit, IC6a for positive input
excursions and IC5b for negative ones. The outputs from the rectifier stages are combined and presented to two different filter
circuits that generate both PPM (IC3B) and VU (IC4) characteristic signals.
Display Mode (Meter Ballistics)
When DIP switch section 4 is open and 3 closed, the PPM signal drives the display circuitry. This signal is created by IC4B and
associated circuitry. A positive signal from the rectifiers causes the opamp output to go high turning on Q1. This charges C10
through a reverse protection diode (D2) and a resistor (R18). When the signal level decreases, Q3 turns off and C10 discharges
through R15 and R9.
Closing DIP switch section 4 and opening 3 allows the VU signal to drive the bargraph drivers. This signal is generated by IC3a and
associated components configured as an active filter to provide the proper VU characteristic.
Bargraph Driver
The signal (PPM or VU) directly drives the LM3916, which drives the upper bargraph display. IC4a provides an additional 16 dB of
gain to the lower bargraph display driver (LM3915). D1provides the reference supply to all opamps and both LM391x display
drivers. The LED drive current (brightness) is about 10x the current taken from pin 7. Pins 6 and 4 are the top and bottom connections
to the reference dividers inside the display drivers. The voltage across R1, P1, R2, and DP1 is the reference against which the
rectifier output applied to pin 5 is compared. The Meter Zero "0" Level is calibrated by adjusting P1. See page 19 for setting
instructions.
NOTE: The driver PCB connects the LEDs in each bargraph in two SERIES strings (one for the upper, and another for the lower). The
anode of the LED corresponding to the LOWEST input level in each string (-10 or -40) is connected to V+, with the other 9 LEDs (8
in the upper one) following; the outputs of the 3915/16 are connected to the node between each LED. Only ONE output of the 3915/
16 is active (LOW) at any one time ("dot" mode), so if the LED string is "broken" (open) at any point, the bargraph in that half of the
display (only) will go OFF whenever the input level is AT OR ABOVE the level corresponding to the break! IC5A is configured as a
comparator, nominally set at 6 dB above "0" to drive the topmost segment of the upper bargraph display as a peak indicator. The Peak
Segment Threshold is set by P2. Factory setting for Peak Segment Threshold is +9.5 dBu. See page 19 for setting instructions.
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