LM4865
LM4865 BOOMER 750 mW Audio Power Amplifier with DC Volume Controland
Headphone Switch
Literature Number: SNAS035F
LM4865
750 mW Audio Power Amplifier with DC Volume Control
and Headphone Switch
General Description
The LM4865 is a mono bridged audio power amplifier with
DC voltage volume control. The LM4865 is capable of deliv-
ering 750mW of continuous average power into an 8load
with less than 1% THD when powered by a 5V power supply.
Switching between bridged speaker mode and headphone
(single ended) mode is accomplished using the headphone
sense pin. To conserve power in portable applications, the
LM4865’s micropower shutdown mode (I
Q
= 0.7µA, typ) is
activated when less than 300mV is applied to the DC Vol/SD
pin.
Boomer audio power amplifiers are designed specifically to
provide high power audio output while maintaining high fidel-
ity. They require few external components and operate on
low supply voltages.
Applications
nGSM phones and accessories, DECT, office phones
nHand held radio
nOther portable audio devices
Key Specifications
jP
O
at 1.0% THD+N into 8
SO, micro SMD
750mW (typ)
jP
O
at 10% THD+N into 8
SO, micro SMD
1W (typ)
jShutdown current 0.7µA(typ)
jSupply voltage range 2.7V to 5.5V
Features
nDC voltage volume control
nHeadphone amplifier mode
n“Click and pop” suppression
nShutdown control when volume control pin is low
nThermal shutdown protection
Connection Diagrams
micro SMD Package
Small Outline Package (SO)
Mini Small Outline Package (MSOP)
10102536
Top View
Order Number LM4865IBP
See NS Package Number BPA08CFB
10102502
Top View
Order Number LM4865M, LM4865MM
See NS Package Number M08A, MUA08A
BOOMERis a trademark of National Semiconductor Corporation.
October 2002
LM4865 750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch
© 2002 National Semiconductor Corporation DS101025 www.national.com
Typical Application
10102501
FIGURE 1. Typical Audio Amplifier
Application Circuit
(Numbers in ( ) are specific to the micro SMD package)
LM4865
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Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage −0.3V to V
DD
+0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Soldering Information
Vapor Phase (60 sec.) 215˚C
Infrared (15 sec.) 220˚C
Thermal Resistanc
θ
JC
(SOP) 35˚C/W
θ
JA
(SOP) 150˚C/W
θ
JC
(MSOP) 56˚C/W
θ
JA
(MSOP) 190˚C/W
θ
JA
(micro SMD) 150˚C/W
Operating Ratings
Temperature Range
T
MIN
T
A
T
MAX
−40˚C T
A
+85˚C
Supply Voltage 2.7V V
DD
5.5V
See AN-450 “Surface Mounting and their Effects on Product
Reliability” for other methods of soldering surface mount
devices.
Electrical Characteristics (Notes 1, 2)
The following specifications apply for V
DD
= 5V, unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4865
Min
(Note 7)
Typical
(Note 6)
Max
(Note 7) Units
V
DD
Supply Voltage 2.7 5.5 V
I
DD
Quiescent Power Supply
Current
V
IN
= 0V, I
O
= 0A, HP Sense = 0V 4 7 mA
V
IN
= 0V, I
O
- 0A, HP Sense = 5V 3.5 6 mA
I
SD
Shutdown Current V
PIN4
0.3V 0.7 µA
V
OS
Output Offset Voltage V
IN
= 0V 5 50 mV
P
O
Output Power
THD = 1% (max), HP Sense <0.8V,
f = 1kHz, R
L
=8500 750 mW
THD = 10% (max), HP Sense <0.8V,
f = 1kHz, R
L
=81.0 W
THD+N=1%,HPSense >4V,
f = 1kHz, R
L
=3280 mW
THD = 10%, HP Sense >4V,
f = 1kHz, R
L
=32110 mW
THD+N Total Harmonic Distortion +
Noise
P
O
= 300 mWrms, f = 20Hz–20kHz,
R
L
=80.6 %
PSRR Power Supply Rejection Ratio V
RIPPLE
= 200mVrms, R
L
=8,C
B
=
1.0 µF, f = 1kHz 50 dB
Gain
RANGE
Single-Ended Gain Range Gain with V
PIN4
4.0V, (80% of V
DD
) 18.8 20 dB
Gain with V
PIN4
0.9V, (18% of V
DD
) −70 −72 dB
V
IH
HP Sense High Input Voltage 4 V
V
IL
HP Sense Low Input Voltage 0.8 V
Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur. “Operating Ratings” indicate conditions for which the device
is functional, but do not guarantee specific performance limits. “Electrical Characteristics” state DC and AC electrical specifications under particular test conditions
that guarantee specific performance limits. This assumes that the device operates within the Operating Ratings. Specifications are not guaranteed for parameters
where no limit is given. The typical value, however, is a good indication of device performance.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX,θJA, and the ambient temperature TA. The maximum
allowable power dissipation is PDMAX =(T
JMAX −T
A)/θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For the LM4865M, TJMAX =
150˚C.
Note 4: Human body model, 100pF discharged through a 1.5kresistor.
Note 5: Machine Model, 220pF–240pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
LM4865
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External Components Description
(Figure 1 )
Components Functional Description
1. C
i
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. It also creates a
highpass filter with the internal R
i
. The designer should note that10kOhm<(Ri)<110kOhm.Therefore f
c
=
1/(2πR
i
C
i
). Refer to the section, Proper Selection of External Components, for an explanation of how to
determine the value of C
i
.
2. C
S
Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing
section for information concerning proper placement and selection of the supply bypass capacitor.
3. C
B
Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of
External Components, for information concerning proper placement and selection of C
B
.
Typical Performance Characteristics
THD+N vs Frequency THD+N vs Frequency
10102505 10102506
THD+N vs Output Power THD+N vs Output Power
10102507
10102508
LM4865
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Typical Performance Characteristics (Continued)
THD+N vs Output Power THD+N vs Output Power
10102510 10102511
Power Dissipation vs Load Resistance Power Dissipation vs Output Power
10102512 10102513
Power Derating Curve Clipping Voltage vs RL
10102514 10102515
LM4865
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Typical Performance Characteristics (Continued)
Noise Floor
Frequency Response vs
Input Capacitor Size
10102516
10102517
Power Supply
Rejection Ratio
Attenuation Level vs
DC-Vol Amplitude
10102518 10102519
THD+N vs Frequency THD+N vs Frequency
10102520 10102521
LM4865
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Typical Performance Characteristics (Continued)
THD+N vs Frequency THD+N vs Output Power
10102522
10102523
THD+N vs Output Power THD+N vs Output Power
10102524 10102528
Output Power vs Load Resistance Clipping Voltage vs Supply Voltage
10102529 10102530
LM4865
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Typical Performance Characteristics (Continued)
Output Power vs Supply Voltage Output Power vs Supply Voltage
10102531 10102532
Supply Current vs Supply Voltage
10102533
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4865 consists of two opera-
tional amplifiers internally. An external DC voltage sets the
closed-loop gain of the first amplifier, whereas two internal
20kresistors set the second amplifier’s gain at -1. The
LM4865 can be used to drive a speaker connected between
the two amplifier outputs or a monaural headphone con-
nected between V
O
1 and GND.
Figure 1 shows that the output of Amp1 serves as the input
to Amp2. This results in both amplifiers producing signals
that are identical in magnitude, but 180˚ out of phase.
Taking advantage of this phase difference, a load placed
between V
O
1 and V
O
2 is driven differentially (commonly
referred to as “bridge mode“ ). This mode is different from
single-ended driven loads that are connected between a
single amplifier’s output and ground.
Bridge mode has a distinct advantage over the single-ended
configuration: its differential drive to the load doubles the
output swing for a specified supply voltage. This results in
four times the output power when compared to a
single-ended amplifier under the same conditions. This in-
crease in attainable output assumes that the amplifier is not
current limited or the output signal is not clipped. To ensure
minimum output signal clipping when choosing an amplifier’s
closed-loop gain, refer to the Audio Power Amplifier De-
sign section.
Another advantage of the differential bridge output is no net
DC voltage across load. This results from biasing V
O
1 and
V
O
2 at half-supply. This eliminates the coupling capacitor
that single supply, single-ended amplifiers require. Eliminat-
ing an output coupling capacitor in a single-ended configu-
ration forces a single supply amplifier’s half-supply bias volt-
age across the load. The current flow created by the
half-supply bias voltage increases internal IC power dissipa-
tion and may permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful bridged or single-ended amplifier. Equation (1)
states the maximum power dissipation point for a
single-ended amplifier operating at a given supply voltage
and driving a specified output load.
P
DMAX
=(V
DD
)
2
/(2π
2
R
L
) Single-Ended (1)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in
internal power dissipation point for a bridge amplifier oper-
ating at the same given conditions.
P
DMAX
= 4*(V
DD
)
2
/(2π
2
R
L
) Bridge Mode (2)
The LM4865 has two operational amplifiers in one package
and the maximum internal power dissipation is 4 times that
LM4865
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Application Information (Continued)
of a single-ended amplifier. However, even with this substan-
tial increase in power dissipation, the LM4865 does not
require heatsinking. From Equation (2), assuming a 5V
power supply and an 8load, the maximum power dissipa-
tion point is 633 mW. The maximum power dissipation point
obtained from Equation (2) must not be greater than the
power dissipation that results from Equation (3):
P
DMAX
=(T
JMAX
–T
A
)/θ
JA
(3)
For the micro SMD and SO packages, θ
JA
= 150˚C/W. The
MSO package has a 190˚C/W θ
JA
.T
JMAX
= 150˚C for the
LM4865. For a given ambient temperature T
A
, Equation (3)
can be used to find the maximum internal power dissipation
supported by the IC packaging. If the result of Equation (2) is
greater than that of Equation (3), then either decrease the
supply voltage, increase the load impedance, or reduce the
ambient temperature. For a typical application using the
micro SMD or SO packaged LM4865, a 5V power supply,
and an 8load, the maximum ambient temperature that
does not violate the maximum junction temperature is ap-
proximately 55˚C. The maximum ambient temperature for
the MSO package with the same conditions is approximately
30˚C. These results further assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power decreases. Refer to the Typical Performance Char-
acteristics curves for power dissipation information at lower
output power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitors connected to the bypass and power
supply pins should be placed as close to the LM4865 as
possible. The capacitor connected between the bypass pin
and ground improves the internal bias voltage’s stability,
producing improved PSRR. The improvements to PSRR
increase as the bypass pin capacitor value increases. Typi-
cal applications employ a 5V regulator with 10µF and a
0.1µF filter capacitors that aid in supply stability. Their pres-
ence, however does not eliminate the need for bypassing the
supply nodes of the LM4865. The selection of bypass ca-
pacitor values, especially C
B
, depends on desired PSRR
requirements, click and pop performance (as explained in
the section, Proper Selection of External Components),
system cost, and size constraints.
DC VOLTAGE VOLUME CONTROL
The LM4865 has internal volume control that is controlled by
the DC voltage applied its DC Vol/SD pin (pin 5 on the micro
SMD and pin 4 on the MSOP and SOP packages). The
volume control’s input range is from GND to V
DD
. A graph
showing a typical volume response versus input control
voltage is shown in the Typical Performance Characteris-
ticssection. The DC Vol/SD pin also functions as the control
pin for the LM4865’s micropower shutdown feature. See
theShutdown Function section for more information.
Like all volume controls, the LM4865’s internal volume con-
trol is set while listening to an amplified signal that is applied
to an external speaker. The actual voltage applied to the DC
Vol/SD pin is a result of the volume a listener desires. As
such, the volume control is designed for use in a feedback
system that includes human ears and preferences. This
feedback system operates quite well without the need for
accurate gain. The user simply sets the volume to the de-
sired level as determined by their ear, without regard to the
actual DC voltage that produces the volume. Therefore, the
accuracy of the volume control is not critical, as long as
volume changes monotonically and step size is small
enough to reach a desired volume that is not too loud or too
soft. Since gain accuracy is not critical, there will be volume
variation from part-to-part even with the same applied DC
control voltage. The gain of a given LM4865 can be set with
a fixed external voltage, but another LM4865 may require a
different control voltage to achieve the same gain. Figure 2 is
a curve showing the volume variation of twenty typical
LM4865s as the voltage applied to the DC Vol/SD pin is
varied. For gains greater than unity, the typical part-to-part
variation can be as large as 8dB for the same control volt-
age.
MUTE AND SHUTDOWN FUNCTION
The LM4865’s mute and shutdown functions are controlled
through the DC Vol/SD pin. Mute is activated by applying a
voltage in the range of 500mV to 1V. A typical attenuation of
75dB is achieved is while mute is active. The LM4865’s
micropower shutdown mode turns off the amplifier’s bias
circuitry. The micropower shutdown mode is activated by
applying less than 300mV
DC
to the DC Vol/SD pin. When
shutdown is active, they supply current is reduced to 0.7µA
(typ). A degree of uncertainty exists when the voltage applied
to the DC Vol/SD pin is in the range of 300mV to 500mV. The
LM4865 can be in mute, still fully powered, or in micropower
shutdown and fully muted. In mute mode, the LM4865 draws
the typical quiescent supply current. The DC Vol/SD pin
should be tied to GND for best shutdown mode performance.
As the DC Vol/SD is increased above 0.5V the amplifier will
follow the attenuation curve in Typical Performance Char-
acteristics.
HP-Sense FUNCTION
Applying a voltage between 4V and V
CC
to the LM4865’s
HP-Sense headphone control pin turns off Amp2 and mutes
a bridged-connected load. Quiescent current consumption is
reduced when the IC is in this single-ended mode.
Figure 3 shows the implementation of the LM4865’s head-
phone control function. With no headphones connected to
the headphone jack, the R1-R2 voltage divider sets the
10102547
FIGURE 2. Typical part-to-part gain variation as a
function of DC-Vol control voltage
LM4865
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Application Information (Continued)
voltage applied to the HP-Sense pin (pin 3) at approximately
50mV. This 50mV enables the LM4865 and places it in
bridged mode operation.
While the LM4865 operates in bridged mode, the DC poten-
tial across the load is essentially 0V. Since the HP-Sense
threshold is set at 4V, even in an ideal situation, the output
swing cannot cause a false single-ended trigger. Connecting
headphones to the headphone jack disconnects the head-
phone jack contact pin from V
O
1 and allows R1 to pull the
HP Sense pin up to V
CC
. This enables the headphone func-
tion, turns off Amp2, and mutes the bridged speaker. The
amplifier then drives the headphones, whose impedance is
in parallel with resistor R2. Resistor R2 has negligible effect
on output drive capability since the typical impedance of
headphones is 32. The output coupling capacitor blocks
the amplifier’s half supply DC voltage, protecting the head-
phones.
A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP Sense pin, a bridge-
connected speaker is muted and Amp1 drives the head-
phones.
PROPERLY SELECTING EXTERNAL COMPONENTS
Optimizing the LM4865’s performance requires properly se-
lecting external components. Though the LM4865 operates
well when using external components having wide toler-
ances, the best performance is achieved by optimizing com-
ponent values.
Input Capacitor Value Selection
Amplification of the lowest audio frequencies requires high
value input coupling capacitors. These high value capacitors
can be expensive and may compromise space efficiency in
portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. In application
5 using speakers with this limited frequency response, a
large input capacitor will offer little improvement in system
performance.
Figure 1 shows that the nominal input impedance (R
IN
)is
10kat maximum volume and 110kat minimum volume.
Together, the input capacitor, C
i
, and R
IN
, produce a -3dB
high pass filter cutoff frequency that is found using Equation
(4).
(4)
As the volume changes from minimum to maximum, R
IN
decrease from 110kto 10k. Equation (4) reveals that the
-3dB frequency will increase as the volume increases. The
nominal value of C
i
for lowest desired frequency response
should be calculated with R
IN
= 10k. As an example when
using a speaker with a low frequency limit of 150Hz, C
i
,
using Equation (4) is 0.1µF. The 0.22µF C
i
shown in Figure 1
is optimized for a speaker whose response extends down to
75Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of the bypass capacitor C
B
.
Since C
B
determines how fast the LM4865 turns on, its value
is the most critical when minimizing turn-on pops. The slower
the LM4865’s outputs ramp to their quiescent DC voltage
(nominally V
DD
/2), the smaller the turn-on pop. Choosing C
B
equal to 1.0µF, along with a small value of C
i
(in the range of
0.1µF to 0.39µF), produces a clickless and popless shut-
down function. Choosing C
i
as small as possible helps mini-
mize clicks and pops.
CLICK AND POP CIRCUITRY
The LM4865 contains circuitry that minimizes turn-on and
shutdown transients or ’clicks and pops’. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4865’s internal
amplifiers are configured as unity gain buffers. An internal
current source changes the voltage of the bypass pin in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the bypass pin. The gain of the internal
amplifiers remains unity until the voltage on the bypass pin
reaches 1/2 V
DD
. As soon as the voltage on the bypass pin
is stable, the device becomes fully operational and the gain
is set by the external voltage applied to the DC Vol/SD pin.
Although the bypass pin current cannot be modified, chang-
ing the size of C
B
alters the device’s turn-on time and the
magnitude of ’clicks and pops’. Increasing the value of C
B
reduces the magnitude of turn-on pops. However, this pre-
sents a tradeoff: as the size of C
B
increases, the turn-on time
increases. There is a linear relationship between the size of
CB and the turn-on time. Shown below are some typical
turn-on times for various values of C
B
:
C
B
T
ON
0.01µF 20ms
0.1µF 200ms
0.22µF 420ms
0.47µF 840ms
1.0µF 2sec
In order eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
DD
may not
allow the capacitors to fully discharge, which may cause
’clicks and pops’. In a single-ended configuration, the output
coupling capacitor, C
OUT
, is of particular concern. This ca-
pacitor discharges through an internal 20kresistor. De-
pending on the size of C
OUT
, the time constant can be
relatively large. To reduce transients in single-ended mode,
10102534
FIGURE 3. Headphone Circuit
LM4865
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Application Information (Continued)
an external 1k-5kresistor can be placed in parallel with
the internal 20kresistor. The tradeoff for using this resistor
is increased quiescent current.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figure 4 through Figure 6 show the recommended two-layer
PC board layout that is optimized for the SO-8 packaged
LM4865 and associated external components. Figure 7
through Figure 11 show the recommended four-layer PC
board layout for the micro SMD packaged LM4865. A
four-layer board is recommended when using the micro
SMD packaged LM4865: the two inner layers, one con-
nected to the GND pin, the other to the V
DD
pin, provide
heatsinking. Both layouts are designed for use with an ex-
ternal 5V supply, 8speakers, and 32headphones. The
schematic for both recommended PC board layouts is Figure
1.
Both circuit boards are easy to use. Apply a 5V supply
voltage and ground to the board’s V
DD
and GND pads,
respectively. Connect a speaker with an 8minimum imped-
ance between the board’s -OUT and +OUT pads. For head-
phone use, the layout has provisions for a headphone jack,
J1. When a jack is connected as shown, inserting a head-
phone plug automatically switches off the external speaker.
10102538
FIGURE 4. Recommended SO PC board layout:
component side silkscreen
10102539
FIGURE 5. Recommended SO PC board layout:
component side layout
10102540
FIGURE 6. Recommended SO PC board layout:
bottom side layout
LM4865
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Application Information (Continued)
10102541
FIGURE 7. Recommended micro SMD PC board layout:
component side silkscreen
10102542
FIGURE 8. Recommended micro SMD PC board layout:
component side layout
10102543
FIGURE 9. Recommended micro SMD PC board layout:
Inner layer V
CC
layout
10102544
FIGURE 10. Recommended micro SMD PC board
layout:
Inner layer ground layout
LM4865
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Application Information (Continued)
10102545
FIGURE 11. Recommended micro SMD PC board
layout:
bottom side layout
LM4865
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Physical Dimensions inches (millimeters) unless otherwise noted
Order Number LM4865M
NS Package Number M08A
LM4865
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
8-Lead (0.118’ Wide) Molded Mini Small Outline Package
Order Number LM4865MM
NS Package Number MUAO8A
LM4865
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
8-Bump micro SMD
Order Number LM4865IBP, LM4865IBPX
NS Package Number BPA08CFB
X1 = 1.336±0.03 X2 = 1.412±0.03 X3 = 0.850±0.10
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whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
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LM4865 750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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