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APPLICATIONS
With the internal biasing and compensation of the LM380
the simplest and most basic circuit configuration requires
only an output coupling capacitor as seen in Figure 7
An application of this basic configuration is the phonograph
amplifier where the addition of volume and tone controls is
required Figure 8 shows the LM380 with a voltage divider
volume control and high frequency roll-off tone control
For Stability with High Current Loads
FIGURE 8 Phono Amp
When maximum input impedance is required or the signal
attenuation of the voltage divider volume control is undesir-
able a ''common mode'' volume control may be used as
seen in Figure 9
For Stability with High Current Loads
FIGURE 9 ''Common Mode'' Volume Control
With this volume control the source loading impedance is
only the input impedance of the amplifier when in the full-
volume position This reduces to one-half the amplifier input
impedance at the zero volume position Equation 1 de-
scribes the output voltage as a function of the potentiome-
ter setting
150
V
50 V
1
e
b
OUT
IN
k
R
a
1
V
For Stability with High Current Loads
Audio Tape Potentiometer (10% of R
FIGURE 10 ''Common Mode'' Volume and Tone Control
This ''common mode'' volume control can be combined with
a ''common mode'' tone control as seen in Figure 10
TL H 7380–10
TL H 7380–11
3
10
c
3 0
150
10
(1)
c
k 1
1
s
s
TL H 7380–12
at 50% Rotation)
T
This circuit has a distinct advantage over the circuit of Fig-
ure 7 when transducers of high source impedance are used
in that the full input impedance of the amplifier is realized It
also has an advantage with transducers of low source im-
pedance since the signal attenuation of the input voltage
divider is eliminated The transfer function of the circuit of
Figure 10 is given by
V
OUT
50
1
e
b
V
IN
k
150k
a
k
1
Figure 11 shows the response of the circuit of Figure 10
FIGURE 11 Tone Control Response
Most phonograph applications require frequency response
shaping to provide the RIAA equalization characteristic
When recording the low frequencies are attenuated to pre-
vent large undulations from destroying the record groove
walls (Bass tones have higher energy content than high
frequency tones) Conversely the high frequencies are em-
phasized to achieve greater signal-to-noise ratio Therefore
when played back the phono amplifier should have the in-
verse frequency response as shown in Figure 12
FIGURE 12 RIAA Playback Equalization
This response is achieved with the circuit of Figure 13
The mid-band gain between frequencies f
12 is established by the ratio of R
of the amplifier (150 kX)
4
150k
k
R
2
V
R
k
R
a
1
T
2
V
q
j2
fc
(2)
1
1
R
k
R
a
a
0
k
1
T
2
V
s
1 s
j2
q
fc
1
0
k
1
s
2 s
TL H 7380 – 13
TL H 7380 – 14
Figure
and f
2
3
to the input resistance
1
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