Sub-Oscillator - Yamaha CS-50 Servicing Manual

~ SUB-OSCillATOR
The low frequency modulation signals (sine and sawtooth waves) of 0.8 to 50 Hz are produced by use of SPEED
lever.
[Sub-oscillator and wave shaper converter]
T
SPEED+lO
lever
{a) Sub-oscillator
330K
SUB-oscillator
PITCH
lOOK
OV /1
-vT13Vv-P
so
so
SIO
OV
I
~-1
3Vp-p
ovN-i
3Vp-p
OV -fb-·13Vp-p
OV ffil3Vp-p
Wave Shaper converter
NOTE: OP
1
and OP
2
are paired in
IC (NJM4558)
The low frequency modulation signals (sine and sawtooth waves) of 0.8 to 50Hz_are produced by use of SPEED lever.
The sub-oscillator is a sort of voltage-controlled oscillator (VCO II). The SPEED lever applies 0 to IOV to VC
2
terminal. This voltage controls the frequency of VCO II oscillation. The higher the voltage is, the higher becomes
the frequency. (The level of the signal is fixed at 3Vp-p-)
vc2
terminal voltage
OV
5V
lOV
VCO II oscillation frequency
0.8±0.16Hz
6.7±1.3 Hz
50± 10 Hz
In this circuit, the VCO II frequency should be adjusted
at VR7 (B-500k) in relation to the voltage at the VC2
terminal, as shown at the left.
The VCO II produces the sawtooth wave in its IC, and shapes it into the sine wave, to be output at PIN 7 and PIN
6, respectiely.
(b) Wave Shaper Converter
This circuit converts the sine wave created by the VCO II into the square wave, and the sawtooth wave into an
inverted sawtooth wave by use of the operational amplifier.
{Sine wave
-7
Square wave)
For converting the sine wave into a square wave, the OP amplifier is used as a zero cross detector or comparator:
The OP amplifier does not include a feedback loop and operates with gain Ao. If the required voltage is 5V and Ao
=
50,000
Vo/Ao
=
5/50,000
=
100 (J1V)
Accordingly, the balance point of OP amplifier input is set at OV, and an alternating voltage is applied. Then plus
and minus saturations at ±1 00 11V are obtained around OV.
In this circuit, the plus input is compared with the minus input. If the gain of the OP amplifier is infmitely great,
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