An Audio Amplifier Power Supply Design

National Semiconductor
An Audio Amplifier Power Application Note 1849
Supply Design Troy Huebner/John DeCelles
March 15, 2010

Introduction Overview
This application note provides design information for a power This application note will cover the design of a ±72V unregu-
supply for use with National Semiconductor’s newest offering lated power supply designed specifically for the LME49810,
of high-performance, ultra high-fidelity audio amplifier input LME49811 and LME49830 high-fidelity audio amplifier mod-
stage ICs. ules. The output power of the modules are approximately
Analog audio circuit power supplies can have an audible ef- 220W to 250W into 8Ω and 350W to 400W into 4Ω. Complete
fect in listening test and quantifiable effect in bench measure- documentation for the amplifier modules can be found in the
ment results. Power supply designs that operate from the documents listed below.
power mains are of three common types: Switch mode AN-1850 LME49830TB Ultra-High Fidelity, High-Power
(SMPS), regulated, and unregulated power supplies. Amplifier Reference Design
Switch mode power supplies have become very popular, Although the power supply design is specific to the amplifier
common, inexpensive, and readily available. SMPS are used modules the concepts and circuit design may be used for any
extensively in computer hardware. They are well suited for power supply purpose.
such use providing good regulation with high efficiency in a The power supply is an unregulated design with an option to
small physical size. A drawback to SMPS is the switching na- allow connection to either 120V or 240V mains. The design
ture of the design which creates EMI and RFI plus electrical uses toroidal transformers, a fully integrated bridge, and var-
noise on the supply rails. Small signal analog circuits are more ious rail capacitors for ripple voltage reduction, noise sup-
susceptible to noise in the form of EMI or electrical noise on pression, and to act as high current reservoirs. Additional
the supply lines. Certain classes of amplifiers, namely Class circuitry to control inrush current on power up and power up/
G and Class H, may be more easily realized with SMPS that down Mute control are also included. A complete schematic,
are fast responding for full audio bandwidth signals. Using PCB views, and Bill of Materials are provided for the power
SMPS for audio circuits presents additional design chal- supply design.
lenges than when using a SMPS for non-audio circuits.
A regulated supply can be a simple linear regulator IC with Schematic and Design
the rectified voltage from the transformer as input and a hand-
ful of external components or any number of more complicat- POWER SUPPLY
ed and often higher performance designs. There are the Figure 1 shows the complete schematic of the power supply
tradeoffs of complexity, cost, space, thermal design, reliability design. The heart of the design is the basic power supply
and protection with any regulated design. It is common for consisting of the transformers, the bridge, and various ca-
regulated supplies to be used for the analog small signal por- pacitors. Many of the capacitors used may not be commer-
tions and other sensitive circuits for best performance. For an cially necessary or may have a minimal effect on perfor-
audio power amplifier, regulated supplies will need high band- mance. Because the design is not a commercial design where
width for good audio performance. The complexity and cost tight cost constraints must be taken into account, additional
for such a power supply design may not be acceptable. Most capacitors are freely used. For a commercial design, bench
linear regulator ICs do not have high bandwidth and are slow and listening test or some other test criteria is recommended
compared to audio signals which can result in reduced audio to determine the exact number, size, and type of external
performance. components required. A short explanation of the purpose of
For simplicity, good performance, and reasonable cost, an each capacitor at the primary side of the transformers, around
unregulated supply is the most common for an audio power the bridge and on the supply rails follows. Some capacitors
amplifier. An unregulated supply uses a transformer, a bridge are doubled up on the PCB for flexibility or to achieve the de-
rectifier, and various rail capacitors. A draw back to the un- sired total capacitance.
regulated supply is the voltage fluctuations with load and • C1, C2, C4 are to protect against turn on/off spikes caused
power mains fluctuations. A design should allow for a mini- when the power switch changes positions. C3 is not used
mum 10% high line condition on the power mains. Unregu- and is redundant.
lated supplies may have only a fuse in the power mains input • CS1, CS2 are low value, ceramic capacitors to filter higher
to protect against excessive current unlike more sophisticated frequency noise right at the DC output of the diode bridge.
regulated designs. Additionally, the power supply voltage rails
• CS3, CS4 are the large reservoir capacitors to supply large
may have inline fuses to add some additional protection.
current demands and stabilize the supply rails to minimize
The circuit and solution presented in this application note has low frequency fluctuations. These are very large value
not been tested to any industry standards. It is the responsi- electrolytic capacitors. Two capacitors are used to achieve
bility of the reader to perform standard industry testing to the desired 40,000μF capacitance per rail.
assure safety when using the solution in part or in whole in
• CS5, CS6 are high quality film capacitors to filter higher
any form. National Semiconductor does not provide any guar-
frequency noise. Two footprints are used on the PCB for
AN-1849

antees, written or implied, about the safety of the solution.

flexibility.
• CS7, CS8 act in conjunction with RS1 and RS2 to decouple
the large electrolytic capacitors and reduce impedance.

© 2010 National Semiconductor Corporation 300631 www.national.com

AN-1849 • CS9, CS10 are low value, ceramic capacitors to filter higher mine optimal values are beyond the scope of this application
frequency noise from the transformer secondary AC lines note.
at the diode bridge. Additionally, the supply rails have bleeder resistors, RBL1,
• CS11 - CS14 are in parallel with the bridge diodes to reduce RBL2, to drain the large reservoir capacitors (CS3, CS4). Two
high frequency noise and ringing of the diode. An footprints per rail were placed on the PCB to allow for lower
additional RC snubber in parallel with each diode of the power resistors to be used and a wide range of bleeder cur-
rectifier will further reduce noise and ringing. rent. More sophistication can be added by including an addi-
tional DPDT relay and controls to only connect the bleeder
The values for the different capacitors were not chosen based resistors below a set voltage and remain unconnected during
on extensive bench work or research. The values were cho- normal operation.
sen based on general guidelines and commonly used values. The fully integrated bridge has a peel & stick heat sink at-
Additional performance may be obtained through refinement tached. (See Table 1) for robustness in use and higher am-
of the capacitor values. The equations and methods to deter- bient temperature conditions.

30063118

FIGURE 1. Complete Power Supply Circuit

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allowing the option to put the primaries into series or parallel.
Additional Circuit The primary side of each transformer is connected in parallel
120V/240V SELECTION OPTION for 120V operation with series connection used for 240V op-
eration. The schematics below, Figures 2 and 3, show the
For multi-country operation a switch is included to select be-
different connections with the switch set for either 120V or
tween 120V or 240V input at the primary side of the trans-
240V input from the power lines.
formers. The transformers are dual primary with the switch

30063119

FIGURE 2. 120V Transformer Connections, Primaries in Parallel

30063120

FIGURE 3. 120V Transformer Connections, Primaries in Series

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AN-1849 INRUSH CURRENT CONTROL
A simple inrush circuit is used to limit the high current that
occurs at power up. The portion of the schematic that controls
inrush current is shown in Figure 4.

30063115

FIGURE 5. Supply Ramp at Power On

POWER UP/DOWN MUTE CONTROL

The Mute function of the audio amplifier input stage IC is used
for a completely quiet turn on and turn off. The amplifier is
held in Mute mode until the voltage supplies are nearly stable
and also goes into Mute mode once the supplies have col-
30063106
lapsed below a determined voltage. With 40,000μF of supply
reservoir capacitance per rail the amplifier can continue op-
FIGURE 4. Inrush Current Control eration for some time after the mains power has been re-
moved. The mute control circuit removes the drive signal for
The inrush circuit consist of three 68Ω/5W resistors (RIR1 - a quicker turn off well before the supplies have collapsed
RIR3, labeled just RIR in Figures 1 and 4) in parallel, a relay down below the minimal operating voltages. The amplifier will
and the relay controls. The RIR resistors limit transformer pri- turn off quietly and smoothly without any undesired noise. The
mary current flow and the resulting secondary current flow Mute control circuit portion is shown in Figure 6.
when the transformer is powered for a softer turn on. Once
the VCC rail voltage exceeds 33V the relay is activated short-
ing out the resistors. The relay is deactivated when the VCC
voltage falls below 10V resetting the circuit. The circuit is very
simple and does not limit inrush current if the mains power is
switched on before the VCC rail drops below 10V. The relay
control consists of the RZ1 and RZ2 resistors to limit current
through the voltage clamping DZ2 Zener diode. DZ2 limits the
relay voltage below the maximum 48V rating. The D1 diode is
for the relay coil EMF and CSR2 is to remove ripple and stabi-
lize the relay voltage. The oscilloscope photo in Figure 5
shows how the positive rail charges up with the increase in
charge rate once the relay is closed shorting out the inrush
current limiting resistors. The RIR resistors will get warm but
they are only conducting for 500ms each time the amplifier is
powered on keeping the power dissipation well within the 5W
rating.

30063107

FIGURE 6. Mute Control

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The voltage threshold is set by the value of the DZ1 Zener
diode, the current limiting RZ1 resistor and the forward voltage
on the LED. The circuit works by simply requiring a certain
positive supply rail voltage before the LED turns on and the
amplifier switches out of Mute mode. The DZ1 Zener diode will
begin to conduct once the positive supply rail exceeds it's rat-
ed voltage. At this point the LED will begin to develop voltage
across it. The LED's forward voltage (typically 2V ~ 4V) is
used as the amplifier's Mute voltage. Setting the Mute resistor
on the amplifier PCB module correctly allows the amplifier to
go out of Mute mode once the LED's forward voltage is high
enough to supply the needed Mute current. The LED is also
used as an indicator, lighting when the amplifier is in Play
mode. The values shown set the Mute voltage threshold to
57V on power up and 58V on power down. Because of com-
ponent tolerances the threshold voltages will vary. At power
down, the forward voltage of the LED will collapse quickly
putting the amplifier into Mute mode well before the supplies
are discharged for a quiet and relatively quick power off. Fig- 30063116

ures 7 and 8 show the Mute signal with supply voltage at

power on and power off. There is additional delay from when FIGURE 8. Mute at Power Off
the Mute signal reaches the Mute threshold (~1.80V for the
amplifier PCB) and when the amplifier enters PLAY mode as The RZM Zener diode is for protection in the event of LED
a result of the mute delay capacitor on the amplifier PCB. failure locking the Mute voltage so it will not exceed 4V. The
amplifier PCB module's Mute resistor is sized for a maximum
of 4V safely limiting Mute current. RPD is needed so DZ1 will
conduct and CSR1 is for a steady LED/Mute voltage.
A short coming of the simple Mute control circuit is the LED's
brightness will vary under heavy amplifier load with the circuit
values shows in Figure 6. Either the threshold of the Mute
circuit can be lowered by changing the value of DZ1 for more
consistent brightness in operation or a constant current circuit
may be used. Figure 9 shows a basic constant current (LED
brightness) circuit with similar threshold voltages as the Mute
control circuit.

30063117

FIGURE 7. Mute at Power On

30063101

FIGURE 9. Constant Brightness LED Circuit

The LED will first begin to light when the positive supply rail
voltage exceeds 45V. Once the positive rail reaches 60V the
LED will have 6.5mA of current and only increase to 6.7mA
at 80V with indiscernible change in brightness. Zener diode

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AN-1849 DZA sets the minimum threshold for first light of the LED. The circuit in Figure 10 will have the same threshold voltages
Combining the values of DZA, DZB, along with voltage drop as Figure 9 and similar Mute control thresholds as Figure 6
across R1 sets the voltage when the LED current reaches a but can also be used to control the Mute signal to the audio
constant value and constant brightness. R3 and DZC set the amplifier module. For a reduced supply voltage window from
LED current and R2 is used to bias QLED and limit current LED first light to constant brightness, DZA should be increased
through DZC. By using a 10V Zener diode (DZB) the power while DZB is reduced. This will increase the LED first light
dissipation in Q LED is kept very low so that a small transistor threshold while reducing the additional voltage needed to
can be used without power dissipation concerns. The trade- reach the constant brightness threshold. The value of DZC
off is that the DZA Zener diode is required to dissipation about may also be adjusted to achieve the designed circuit re-
1W when the supply reaches 80V. Figure 9 does not give both sponse.
constant LED current and the Mute signal control the same
as Figure 6 although the Mute control could be taken at the Summary
emitter of QLED. An alternate circuit to combine both Figure 6
The unregulated power supply presented will give very good
and 9 is shown in Figure 10.
performance while powering an audio amplifier. While circuit
modifications and additions can improve performance the so-
lution presented has a relatively low part count and simplicity
is maintained with all circuits. The power supply will provide
a ±70V to ±73V supply under quiescent conditions with full
load voltage dropping to ±59V to ±62V.

30063102

FIGURE 10. Constant Brightness LED and Mute Control

Circuit

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Board Layer Views

30063114

FIGURE 11. PCB Composite View From Top

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AN-1849

30063111

FIGURE 12. PCB Top Silkscreen View

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30063109

FIGURE 13. PCB Bottom Silkscreen View

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AN-1849

30063112

FIGURE 14. PCB Top Layer View

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30063108

FIGURE 15. PCB Bottom Layer View

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AN-1849
Bill Of Materials
TABLE 1. Bill Of Materials

Reference Value Tolerance Description Manufacturer Part Number

400V, metalized
polyester film,
C1, C2, C4 0.01µF 10% Panasonic ECQ-E4103KF
7.5mm lead
spacing
C3 Not Used
100V ceramic,
CS1, CS2, CS7,
0.1µF 10% X7R type, 200mil AVX Corporation SR211C104KAR
CS8, CS9, CS10,
lead spacing
250V, metalized
CS11, CS12, polyester film,
0.1µF 10% Panasonic ECQ-E2104KF
CS13, CS14 7.5mm lead
spacing
CS3A, CS3B, 100V electrolytic CDE Cornell DCMC203U100B
20,000µF 20%
CS4A, CS4B can Dubilier C2B
100V, metalized
CS5A, CS5B, polyester film,
1µF 10% Panasonic ECQ-E1105KF
CS6A, CS6B 10mm lead
spacing
63V electrolytic
CSR1, CSR2 1µF 20% radial, 2mm lead Panasonic EEU-EB1J1R0S
spacing
400V diode, Vishay
D1 1A 1N4004-E3/54
DO-41 Semiconductor
2W Zener diode, Microsemi
DZ1 51V 5% 2EZ51D5DO41
DO-41 Corporation
2W Zener diode, Microsemi
DZ2 43V 5% 2EZ43D5DO41
DO-41 Corporation
500mW Zener
DZM 3.9V 5% Diodes Inc. 1N5228B-T
diode, DO-35
International
RBLD1, RBLD2,
2kΩ 5% 5W metail oxide Yageo SQP500JB-2K0
RBLD3, RBLD4
Corporation
International
RFAN 1.2kΩ 5% 5W metail oxide Yageo SQP500JB-1K2
Corporation
5W wirewound Huntington
RIR1, RIR2, RIR3 68Ω 1% ALSR-5-68-1%
silicone Electric, Inc.
RS1, RS2 1Ω 5% ¼ Watt carbon film Panasonic ERD-S2TJ1R0V
International
MFR-25FBF-100
RG 100Ω 1% ¼ Watt metail film Yageo
R
Corporation
1 Watt metail
RZ1 560Ω 5% Panasonic ERG-1SJ561
oxide film
RZ2 390Ω 5% ½ Watt carbon film Panasonic ERD-S1TJ391V
RPD 10kΩ 5% ¼ Watt cardon film Panasonic ERD-S2TJ103V
48V, 400mW Panasonic Electric
RL1 16A ALE15B48
SPST, N.O., relay Works
700V bridge Fairchild
U1 35A GBPC3510W
rectifier Semiconductor

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 AN-1849
Reference Value Tolerance Description Manufacturer Part Number
DPDT PCB
S1 6A mount, mini slide C&K Components 1201M2S1CQE2
switch
3 pin 156mil
Molex/Waldom
J1, J5 header, right 26-60-5030
Electronics Corp.
angle, tin plating
2 pin 156mil
Molex/Waldom
J2, J9, J4A, J4B header, right 26-60-5020
Electronics Corp.
angle, tin plating
4 pin 156mil
Molex/Waldom
J3A, J3B header, right 26-60-5040
Electronics Corp.
angle, tin plating
2 pin 100mil
J7, J8, J11, J12, Molex/Waldom
header, right 22-05-3021
J13, J14, J15 Electronics Corp.
angle, tin plating
Dual primary, dual
Transformer1, Plitron
24V, 300VA secondary, torrid 77060201
Transformer2 Manufacturing Inc.
transformer
Peel & stick heat
sink for bridge, CTS Electronic
θCA = 16.5°C/W BDN12-5CB/A01
1.21" square, Components, Inc
0.55" tall
RZ3, RZ4, DZ3,
Option unused
DZ4, CSF1,
circuits
CSF2, CSF3

Revision History
Rev Date Description
1.0 06/03/08 Initial release.
1.01 03/15/10 Deleted all references to AN-1625.

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An Audio Amplifier Power Supply Design

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AN-1849

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