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.
TL/H/7380 – 10
*
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 .
TL/H/7380 – 11
*
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.
V
OUT
e
50 V
IN
#
1
b
150
c
10
3
k
1
R
V
a
150
c
10
3
J
0
s
k1
s
1
(1)
TL/H/7380 – 12
*
For Stability with High Current Loads
**
Audio Tape Potentiometer (10% of R
T
at 50% Rotation)
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 .
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
V
IN
e
50
K
1
b
150k
150k
a
k
1
R
T
k
2
R
V
a
k
2
R
V
j2
q
fc
1
k
1
R
T
a
k
2
R
V
a
1
j2
q
fc
1
L
0
s
k
1
s
1
0
s
k
2
s
1
(2)
Figure 11 shows the response of the circuit of Figure 10 .
TL/H/7380 – 13
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 .
TL/H/7380 – 14
FIGURE 12. RIAA Playback Equalization
This response is achieved with the circuit of
Figure 13 .
The mid-band gain, between frequencies f
2
and f
3
,
Figure
12 , is established by the ratio of R
1
to the input resistance
of the amplifier (150 k
X
).
4
