Bargraph Driver Circuit Description (919234); Level Meter Specifications - Wohler AMP1A User Manual

1u stereo audio speaker monitors with two or optional four analog audio inputs, stereo analog outputs, and optional dual 10-segment level meters and phase indication amp1a series

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User manual (28 pages)

Table of Contents

AMP1A/-2S/-LP/-LP2S User Manual, P/N 821526, Rev-D

The "PPM" version of the 919234 meter driver input is driven single-ended from the main circuit board. Gain selection via S1 and S2 of the

DIP switch module (page 17) allow the circuit to be configured for an input level of -6, 0, +4, or +8 dB to indicate "0" on the actual display.

The input attenuator network consists of eight resisters. The buffered signal (and its inversion) are coupled to two half-wave rectifiers.

The two half-wave rectifiers provide full-wave rectification of the AC signal from the input amplifier. The first provides output on negative

excursions; the second is for positive excusions. A capacitor is charged via one of two resisters; the time constant of this resistor-capacitor

combination determines the attack time of the meter. When the input level decreases, the capacitor discharges; the resulting time constant

determines the decay time of the meter. By switching in additional RC with S3, the attack time is increased and the decay decreased, to

approximate VU characteristics. S4 is closed along with S3 to adjust the reference voltage to compensate for the voltage dividing effects.

The popular LM3916 10-segment LED bargraph driver has internal voltage comparator references at VU meter intervals. A resister sets the

current for the external reference network and the LED drive current, which is about 10 times the current taken from pin 7. The top of the

internal comparator resistor ladder is connected to pin 6, the bottom to pin 4. The voltage is the reference against which the rectifier output

applied to pin 5 is compared. The meter is calibrated by adjusting P1 (PPM) or P2 (VU). ZD1 sets the voltage at the bottom of the resistor

ladder and the bias for the rectifier circuit with C11 as a bypass. NOTE: The driver PCB connects the LEDs in the bargraph in a SERIES

string. The anode of the LED corresponding to the LOWEST input level (-20) is connected to V+, with the other 9 LEDs following, and the

outputs of the 3916 connected to the node between each LED. Only ONE output of the 3916 is active (LOW) at any one time ("dot" mode),

so if the LED string is "broken" (open) at any point, the bargraph will go OFF whenever the input level is AT OR ABOVE the level

corresponding to the break.

Level Calibration:

Frequency Response:

Level Meter Type:

LED Colors:

Metering Range:

Display Mode (Ballistics):

VU Characteristics:

Rise Time =

Decay Time =

PPM Characteristics:

Attack Time =

Decay Time =

Bargraph Length:

LED Segment Size:

LED Segment Pitch:

Segment Brightness, (I f = 20 mA):

Segment Brightness, Uniformity:

Adjacent Segment "Off" Brightness:

Peak Emission Wavelength: