1
2π
S C1 (RIN)VR1)
7
32
VCC*VLED*Vsat (Q1*Q4) *VDL
ILED
667
Audio ICs BA6820F / BA6822S / BA6822F
FAttached components
S C1 and C2: 1µF input coupling capacitors (electrolytic)
for the square-root compression amplifier. The rectified
voltage output from pins 2 and 4 is attenuated by 1.5dB
at the frequency determined by the following formula (re-
fer to the data for the relationship between the pin 2 and
4 DC voltage frequency characteristics and the LED
lighting level).
fC = (R
IN = input resistance)
When the LEDs light due to low-frequency mechanical
noise or induced hum, reduce the values of C
1 and C2
to cut the cut the low-frequency frequency characteris-
tic.
S VR
1 and VR2: 5kΩ to 100kΩ potentiometers for sen-
sitivity and L/R balance adjustment (refer to item 2 of
Circuit operation).
S C
3 and C4: 22µF electrolytic capacitors.
S R
1 and R2: 47kΩ carbon-film resistors.
These resistors set the attack and release times (refer
to item 6 of the Circuit operation).
S C
5: 0.01µF mylar capacitor.
R
3: 39kΩ carbon-film resistor.
This resistor sets the oscillator frequency (refer to the
Operation notes).
S C
6: 100µF electrolytic capacitor. This capacitor is
used as a ripple filter. The positive lead should be con-
nected close to V
CC (pin 22), and the negative lead
close to the junction of pins 10 and 11.
S Q
1 to Q4: 2SA854S Transistors for display switching.
If the current for one LED is I
LED, the values of IC and PC
for each of Q1 to Q4 when all LEDs are lit are given by:
I
C = 6 ILED (for duty cycle of 7/32)
P
C = 6 ILED Vsat
Choose transistors that have sufficient margin for I
C and
P
C.
S R
4 to R7: 2.2kΩ carbon-film resistors.
These resistors limit the base current for transistors Q
1
to Q2. The base current is the collector current divided by
h
FE, but as the transistors are operated in the saturation
region, h
FE will drop considerably. Choose the resistors
so that sufficient base current flows.
S LED
1 to LED12: SLB-26
S R
8 to R13: 330Ω carbon-film resistors. These resistors
set the LED current. Four LEDs are connected to each
resistor, but as the LEDs light sequentially, only the cur-
rent for one LED flows in a resistor.
R8X13 =
V
DL = Display switch output “L” voltage.
FCircuit operation
(1) Function
There are 12 LEDs each for the left and right channels,
and these are divided into 4 groups of six. A dynamic-
drive technique is used to drive the LEDs in order, and
provide 12 display points for each channel. A 12-point
VU-scale bar display is produced over the display range
*38dB to +10dB. The top eight points have a peak hold
function which may be cancelled if required (there is no
peak hold function for DC mode).
Independent AC (two channels) and DC (one channel)
inputs are provided, and a control input pin is used to
switch between them.
In DC mode, only channel 2 operates.
A power-on mute circuit is provided, so no measures are
required to prevent erroneous lighting of the LEDs when
power is applied.
The attack and release times for the display can be set
using external components.
(2) Input block (pins 1, 3 and 5)
Pins 1 and 3 are the AC inputs, and pin 5 is the DC input.
Pin 6 is used to switch between AC and DC input. When
the pin 6 input is “H”, AC input is selected (pins 1 and 3),
and when it is “L”, DC input (pin 5) is selected. Therefore,
if pins 1 and 3 are used for audio input, and pin 5 as the
input for signal meter output (DC) from a tuner, the switch
input (pin 6) can be used to make LED display do two
jobs.
Pins 1 and 3 have low input impedance, so we recom-
mend that you connect potentiometers (VR
1 and VR2:
5kΩ to 100kΩ) in series with the inputs to adjust the bal-
ance between ch1 and ch2 and set the levels.
