Yamaha CS-50 Servicing Manual page 55

EXPRESSION CIRCUIT
EXPRESSION CIRCUIT
The expression pedal control circuit consists, as shown below, of expression pedal circuit, voltage-current (V-I)
converter including an operational amplifier and photo-coupler including light-emitting diode (LED) to change light
output in accordance with the current value and cadmium sulfide (CdS) photo conductor.
PRA circuit board
from -15 terminal
[Outline of operation]
Cdsl
Checkpoint
foot controller
~
TPI
jack
t
Input signal
o+o----,
MI
lOOK
Constant current inverting buffer
amplifier
V-I Converter and Photo Coupler
o+
Output signal
MO
The photo coupler contains a LED, and its gain is controlled by the current flowing through the LED. The voltage
proportional to the degree of treading in the expression pedal must be converted to the current. The output
current (Io) of V-I converter which is based on the inverting operational amplifier is given by the following
equation:
Io= Rr.
~
Rs R3
where Vs: input voltage from the expression circuit,
Rf = Rss=
100 kD and
R
3
=
820D
1
Hence, Io
=
820 Vs
Thus, the output current of V-I converter is proportional to the voltage controlled by the expression pedal. The
output current controls the light output of the LED, which changes the conductance of the CdS conductor in the
photo coupler to control the tone volume.
e EXP Pedal and Foot-Controller .lack
~
cv
~!_Fe®v
(EXP Pedal Circtiit)
1
~PRAsheet
~l
3- " ,
27K
(Foot Controller Jack)
EI terminal
-15V terminal
E terminal
*
When the expression pedal is not connected, terminals 2 and 3, and terminals 5 and 1 are in contact,
respectively, and--15V is applied to the EI terminal through the 27 k.\1 resistor. That is, if the expression pedal
is not connected, the maximum volume is output.
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