© 2003 Wohler Technologies Inc. ALL rights reserved
18
AMP2A-4S
and
AMP2A-10S
User Manual P/N
821554
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
Two 10-segment bargraphs are stacked together (upper and lower) to create a 20-segment bargraph display. 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). D1 provides 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. Refer to National Semiconductor Linear data books for a detailed discussion of the LM3915 and 3916 bargraph display
drivers.
The driver PCB connects the LEDs in each bargraph in two SERIES strings (one SERIES for the upper bargraph, and another for the
lower bargraph). The anode of the LED corresponding to the LOWEST input level in each string (-10 or -40) is connected to V+, with
the other nine LEDs (eight LEDs in the upper bargraph) 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, nominall y set at 6 dB above “0” to drive the topmost segment of the upper bargraph
display as a peak indicator.
Bargraph Driver Adjustment Settings
The
Meter Zero "0" Level
adjustment sets the meter to show a zero level for a particular input level calibration and is set by
adjusting
P1
on the 919030 PCB. The Factory setting sets the "0" level for an input level gain of +4 dBu (see page 17 for setting the
Input Level Gain Calibration). To zero the meter for other input level gain selections, see page 19 for instructions.
The
Peak Segment Threshold
adjustment sets the meter to show a peak indication on the meter for a selected input level and is set
by
P2
on the 919030 PCB. Factory setting for
Peak Segment Threshold
is 9.5 dBu. To set this threshold for other input levels, see
page 19 for instructions.
Calibration Note:
Each bargraph segment will turn on at 0.5 dB before that segment’s silk-screened scale indication. Example:
When the gain calibration is set for +4 dBu, the zero indication on the level meter will light up at 3.5 dBu. Calibration is implemented
in this way to effect a “rounding” function for more accurate indication of signal levels which occur BETWEEN the thresholds of any
two bargraph segments.
