Frequency Equalization; Circuit Description - ADC Sound Shaper Three Service Manual

With no signal, (-) input level [No.2 (L) or No.6 (R)
1
is kept to 0 V DC.
(+) input level [Pin NO.3 (L) or Pin No.5 (R)l of IC814 is kept to minus DC voltage (about
-45 mV) by half-wave rectifiers through VR801 (L) [VR802 (R)l and VR805 (L) [VR806
mn
Thus, the output voltage of IC814 [Pin No.1 (L) or No.7 (R)
1
is kept with minus DC voltage.
When minus DC voltage is applied to (-) input, and causes the (-) input voltage to be greater (or
equal) to the (+) input voltage, the output appears as plus DC voltage. (V3:s. V2)
Thus LED D925 (L) and D926 (R) are lit at -12 dB points. But D901 through D924 are not lit
because the input voltage is too low.
Each LED conducts with a (-) input level which is determined by VR801 (L), VR802 (R) (IC802),
R813 (L), R814 (R) (IC803), R817 (L). R818 (R) (IC804), R821 (L), R822(R) (IC805), R825(L),
R826 (R) (IC806), R829 (L), R830 (R) (IC807), R833 (L), R834 (R) (IC808), VR803 (L), VR804
(R) (IC809), R839 (L), R840 (R) (IC810), R843 (L), R844 (R) (IC811), R847 (L), R848 (R)
(IC812), R851 (L), R852 (R) (IC813), R855 (L), R856 (R) (IC814).
With an increase in minus DC voltage, provided to the (-) input, each LED is lit in sequence from
D923, D924 to D901, D902.
CIRCUIT DESCRIPTION
FREQUENCY EQUALIZATION
The input signal is fed into TR101 base. TR101, an emitter follower, provides the high input im-
pedance required by the signal source. The low output impedance of TR101 is required to drive
a voltage divider formed by R 111 and the sections of the frequency control pots between the cut
end of each control and its wiper. The wiper of each control effectively qrounds only those free
quencies resonated by the series traps (coil, capacitor and resistor from wiper to common). Thus,
the voltage division which occurs can be different for each frequency and depends on the frequency
control settings.
TR103, TR105 and TR107 are connected so that the voltage at TR105 base always follows the
voltage at T R 103 base. For example: An increase in voltage at TR 103 base causes the conduction of
TR 103 and TR 107 to increase, and feedback base drive of TR 105 to increase, until the voltage at
TR105 base is equal to that at TR103 base. Conduction cannot increase beyond this point since
the emitter voltage supplied by TR105 then tends to make TR103 conduct less. Conversely, a de-
crease in voltage at T R 103 base causes a corresponding change at T R 105 base.
The output voltage is taken from TR107 collector and is also fed back to TR105 base through the
voltage divider formed by R 115 and the sections of the frequency control pots between the boost
end of each control and its wiper.
For each frequency, moving the control pot off center towards boost causes an increase in the
amplitude of the signal presented to TR103 base and a decrease in the negative feedback to TR105
base. The gain is thus increased. Since the signal amplitudes at the bases of T R 103 and T R 105 are
always equal, the output signal divided by the feedback attenuation (R 115 and boost sections) is
approximately equal to the input signal amplitude divided by the input attenuation (R 111 and
"CUT" sections). By following a similar line of thought, it will be seen that when the controls are
centered the gain is unity; and when the controls are towards cut, the gain is less than one.
The resistors in series with the series resonant circu its are used to increase the bandwidth or lower
the
"Q",
of each circuit so that the effect of controls of adjacent frequencies overlap, thus pro-
viding a smooth overall response. The resistors are of different values so that the total series
resistance of each circuit (coil and resistor) is approximately the same. The ratio of the total series
resistance of the resonant circuit to the base resistor of TR 103 and TR 105 determines the maximum
boost or cut obtainable.
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