Bargraph Driver Circuit Description (919017); Level Meter Specifications - Wohler AMP1A-4S User Manual

Amp1a series 1u multi-channel audio speaker monitors

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AMP1A-4S/-10S/-LP4S/-LP10S User Manual P/N 821527 Rev-B

The "PPM-1" version of the 919017 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 R1-R4, R11, R17, R18, and R19. The buffered signal (and its inversion) are

coupled to half-wave rectifiers IC2a and b.

The two half-wave rectifiers provide full-wave rectification of the AC signal from the input amplifier. IC2a provides output on

negative excursions; IC2b is for positive excusions. C8 is charged via R14 (or R26); the time constant of this resistor-capacitor

combination determines the attack time of the meter. When the input level decreases, C8 discharges through R14, R16, and D1 (R27,

R28, D3); the resulting time constant determines the decay time of the meter. By switching in additional RC (R21, C10) 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 of R21.

The popular LM3916 10-segment LED bargraph driver has internal voltage comparator references at VU meter intervals. R25 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 across R23, R24, and P1 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: