CIRCUIT DESCRIPTION
Design Philosophy on 430A and 410A
When using ahigh gain wide band open loop operational
amplifiers, it is possible to design apassive network
providing anegative feedback to control the amplifier gain
and frequency response. In fact it makes easy the
reproduction and repititivity of the wanted results.
We can define the gain of such an amplifier as follows.
Zm
o'VW-
Vln
Or
OP
AMP
3PHONO IN RCH C4
[PHONO AMPLtFIER USING OP AMP]
1) Vo =-Vin-
2) Rin =
Zf
Zin (Inverting Amp.)
OOUT
3) Vo =Vin •1+
Zinc—Rin ^
Zf
Zl
Zino •Gopenloop
Gclosedloop
(Non Inverting)
Where Rinis the input load resistor. Zmis the open loop
input impedance multiplied by open loop gain divided by
closed loop gain.
Example: If open loop gain is 10,000 (i.e) 80dB) and the
closed loop gain is 100 (i.e 40dB). For an
amplifier having Zin =10K ohm, the
equivalent Zinc=Rin // 1000K ohm.
Phono Equalizer: The phono non inverting amplifier
equalizer consists of an operational amplifier with afeedback
network to fullfil the RIAA equalizing requirements.
Zf =R8 // XC6+RlO »*c8
Zl =Ri2 +Xci2
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where Xc=2TTf-C 6.28 f-c
f=frequency
c=capacitance in Farads
R=Resistance in Ohms
The above network provides again of 60 at 1kHz and again
variation as function to the value of Zf and Z\ at any given
frequency. For example at 100Hz the gain is 265 or
+12.9dB referred to 1kHz gain, at 10kHz gain is about 13
or -l3.7dB referred to 1kHz gain.
Premain Amplifier
The premain amplifier is an operational amplifier built by
discrete components. It is directly coupled to the load (i.e
speakers). The high gain open loop is provided by the dual
differential amplifiers and the boostrap capacitor. The
complementary output drivers/buffers provide asymmetrical
output drive.
The amplifier is controlled by multiple feedback networks:
DC feedback (R93 +R95) on one channel (R94 +R96) on
the other are directly coupled between output and negative
input. The effect of the DC feedback is for longterm
stability and unity DC gain. The effect of this feedback is
minor at frequencies over 5Hz. The multipole AC
feedback network (with the bass and treble potentiometers
incorporated) provides aconstant gain with no effective
gain variations of tone potentiometers. At 100Hz the bass
potentiometer allows gain control of ±10dB. At 10,000Hz
the treble potentiometer allows gain control of ±!OdB.
The amplifier has acurrent limit network that limits
excessive current loading. Afuse is provided to prevent
damage to speakers if the amplifier fails.
Tone Control
The tone control is anegative feedback type whicli uses the
power amplifier stage as the active element. That is, the
gain of the power amplifier stage is controlled by the tone
controls circuitry. At 1kHz, the position of the tone
controls has little effect on the gain, as C5 3impedance is
high, removing VR2 from the circuit, and CI, C3
impedance is low, effectively short circuiting VR1.
Bass Control: As the frequency decreases below 1kHz,
the impedance of CI and C3 increases proportionately.
Thus at very low frequencies, the gain is mainly determined
by the position of the bass control VR1. Rotating VR1
toward R3 will boost the low frequencies, while turning it
towards C4 will cut the bass.
Treble Control: At high frequencies, as at 1kHz, \/Rl is
effectively short circuited. At these frequencies, \owever
C53 and C55 impdeance decreases, so that VR2 Is comes the
the main control of the amplifier gain. Rotating VR2
towards C53 cuts the treble response.
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