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Maxim > Design
support > App notes
> Audio
Circuits > APP 4320

Feb 27, 2009

Keywords: audio,class-d,classd,amplifier,power,active
filter,mp3,active eq,woofer,subwoofer,tweeter,2.1 channel,high
pass filter,parametric eq,sallen-key
filter,speaker,speakers
Related
parts

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REFERENCE DESIGN 4320

INCLUDES: Tested
circuit Schematic BOM Description Test
data Layout

Reference Design for a Class-D, 2.1 Channel, Audio
Amplifier for an MP3 Player Docking Station

Abstract: This
reference design demonstrates the use of the MAX9736 Class D
audio amplifier in a stereo audio, docking station
application. The MAX9736 2.1 demo box is a complete powered
speaker dock that uses two MAX9736 ICs to drive a
3-channel speaker system consisting of two 2in
satellite speakers and one 5in subwoofer. The reference design
is intended to be used with a portable audio player as its
main music source. The overall solution size is very compact
and features active equalization, power-supply monitoring, and
dynamic equalization for the subwoofer.

Attend
this brief webcast by Maxim on
TechOnline

Important
Design Features

Compact all-in-one design

Operates from a single 12V to 20V DC power supply

High SPL output from a small box

High-efficiency Class-D design

Active EQ, including dynamic bass equalization

Excellent sound quality from cost-effective drivers

Bill of Materials

Designator

Qty

Description

C1

1

470µF ±20% 25V aluminum electrolytic
capacitor
Panasonic® EEU-FM1E471

C2, C12, C310

3

0.1µF ±10% 50V X7R ceramic capacitor
(0603)
Murata® GRM188R71H104K
TDK
C1608X7R1H104K

C321

1

1µF (0603)

C16

1

10µF (1206)

C106, C109–111, C114, C206, C209–211, C214

10

NS (0603)

C3–5

3

0.1µF ±10% 16V X7R ceramic capacitor
(0603)
Murata GRM188R71C104K
TDK
C1608X7R1C104K

C13

1

0.1µF (0603)

C316–317

2

0.1µF (0805)

C303

1

0.33µF (0603)

C302

1

0.47µF (0603)

C305, C307

2

100nF (0603)

C103–104, C203–204, C301

5

100pF (0603)

C121, C220–221, C120, C314–315

6

100pF (0603)

C117–119, C217–219

6

10nF (0603)

C107, C207

2

150pF (0603)

C306, C308

2

15nF (0603)

C6–10, C15, C313, C318–320

10

1µF ±10% 16V X7R ceramic capacitor (0603)
Murata
GRM188R71C105K
TDK C1608X7R1C105K

C312, C14, C311

3

1µF (0603)

C216, C116

2

1µF (0805)

C100–101, C200–201

4

2.2µF (0805)

C304

1

2.2µF (0805)

C102, C108, C112, C115, C202, C215, C300

7

33pF (0603)

C208, C212

2

33pF (0603)

C322

1

33pF (0603)

C113, C213

2

4.7nF (0603)

C105, C205

2

68nF (0603)

C11, C309

2

220µF ±20% 35V aluminum electrolytic capacitor (10mm
x 12.5mm)
Panasonic EEUFM1V221

J30

1

4-pin header
HIROSE DF3A-4P

L100–101, L200–201, L300–301

6

Ferrite bead (1206)

FB1

1

Ferrite bead (1206)

J3

1

6-pin header

J10, J20

2

2-pin header
HIROSE DF3A-2P

J2

1

RCA® dual jack PCB
mount

D1

1

LED red (0603)

D5

1

LED blue (0603)

Q2

1

Transistor NPN (SOT23)
Fairchild MMBT3904

X1–2

2

Connector, screw mount

R7

1

Potentiometer 100K SMT

R303

1

Potentiometer 20K SMT

J1

1

Power jack, right angle
Switchcraft® 722RA

R1

1

10kΩ ±5% resistor (0805)

R110, R210

2

NS resistor (0603)

R111, R211

2

10.5kΩ ±5% resistor (0603)

R100, R200

2

100Ω ±5% resistor (0603)

R10

1

100kΩ ±1% resistor (0603)

R106, R108, R206, R208

4

10kΩ ±5% resistor (0603)

R300–301

2

10kΩ ±5% resistor (0603)

R2–3

2

10kΩ ±1% resistor (0603)

R113, R213

2

11kΩ ±5% resistor (0603)

R309, R311–312, R314

4

11kΩ ±5% resistor (0603)

R306

1

12kΩ ±5% resistor (0603)

R114, R214

2

137kΩ ±5% resistor (0603)

R305

1

14.7kΩ ±5% resistor (0603)

R112, R212

2

150Ω ±5% resistor (0603)

R317

1

1kΩ ±5% resistor (0603)

R107, R109, R207, R209

4

1MΩ ±5% resistor (0603)

R8–9, R315–316

4

2.2kΩ ±5% resistor (0603)

R101–102, R201–202

4

20kΩ ±5% resistor (0603)

R5

1

220Ω ±5% resistor (0603)

R310, R313

2

24.3kΩ ±5% resistor (0603)

R302

1

2kΩ ±5% resistor (0603)

R308

1

3.24kΩ ±5% resistor (0603)

R307

1

3.3kΩ ±5% resistor (0603)

R115, R215

2

30.1kΩ ±5% resistor (0603)

R4

1

4.22kΩ ±1% resistor (0603)

R103–104, R203–204

4

40kΩ ±5% resistor (0603)

R304

1

7.15kΩ ±5% resistor (0603)

R105, R205

2

0Ω resistor (0603)

D6

1

Diode, dual, common-cathode (SOT-23-3)
Fairchild
MMBD4148CC

D2–3

2

Zener diode
4.3V, 20mA (SOT-23)
Central CMPZ5229B

SPK1, SPK2

2

2in full-range loudspeaker
Tymphany® 830970

SPK3

1

5.25in subwoofer loudspeaker
Tymphany
830945

U1

1

Maxim® MAX9736BETJ+
(7mm x 7mm, 32-pin TQFN)

U2

1

Maxim MAX9736AETJ+
(7mm x 7mm, 32-pin
TQFN)

U3–5

3

Maxim MAX4234 (TSSOP-14)

U6

1

Maxim MAX809_EUR-T (SOT-23)

U7

1

Opto-FET (DIP6-SMT)
Fairchild™
H11F1SM

Detailed Design Description
The reference design
consists of a single, carefully tuned box enclosure containing
all of the electronics and speaker hardware. Only an external
power supply and a signal source
are required to complete the system.

The design
features two 2in loudspeaker drivers for the left and right
channels, and a 5in loudspeaker for the subwoofer. A single MAX9736B is used
in stereo mode for the left/right channels and a MAX9736A is used
in mono mode for the subwoofer channel. The detailed
description here is divided into two sections: electronic
circuit description and speakers/physical enclosure
description.

Electronic Circuit Description
The electronics of the
reference design comprise two sections: the left/right and
subwoofer. Each section has three stages: input, EQ, and
power. See Figure 1.

Figure
1. Electrical circuit block diagram features the MAX9736 Class
D audio amplifier. The design has an input, EQ, and power
stage.

Input Stage
The input stage is common to both the
left/right and subwoofer sections. A stereo, log-taper potentiometer
is used as a volume control to adjust the signal level to the
preamplifier. To avoid ground loops, the input is sensed
differentially and the RCA input connectors are not directly
connected to the system ground. A 100Ω resistor is used to
reduce the common-mode
voltage; U3-A and U3-D (see
schematics below) perform differential-to-single-ended
conversion with a gain of 2. The outputs of the input stage
are referred to as VREF, the reference voltage
of power
amplifier U2 which is buffered using U5-D.

EQ Stage
After the input stage, the left and right
signals are passed to a two-stage parametric EQ built around
U3-B (for left) and U3-C (for right). Each parametric EQ uses
an op-amp gyrator to simulate an inductor in an LC-series resonant
circuit. The series circuit has two access points for
either attenuation or boost, and the access points are
realized by two different capacitors. C105 and C106, for
example, are the boost and attenuation points for the first
parametric EQ band
for the left channel. If only C106 is used (and C105 is not
stuffed), the resonant circuit forms a voltage-divider in
combination with the series resistor R105 (10kΩ), and the
resonant frequency is attenuated. Conversely, if only C105 is
used, the feedback is reduced, resulting in a boost at the
resonant frequency. If neither of the capacitors is used, then
the band is disabled.

The resonant frequency,
f0, can be calculated using the equation:

f0 = 1/(2 × π
× √(L0 × C0))
Where:

L0 = R108 × C107 ×
R107
C0 = C105
And the Q is:

Q0 = √(L0/(C0 ×
R0²))
A third EQ band is added that
realizes a shelving filter and only needs an RC: one capacitor
(C113 for boost; C114 for attenuation) and one resistor
(R111).

As a stereo Class D IC, the MAX9736 also
features two integrated op amps
at the input. Available to use for general filtering, in this
reference design those op amps are used to build a
second-order highpass filter for the left/right channels. The
op amps have their negative inputs and outputs accessible, so
a multiple-feedback inverting filter configuration is used.
C116/C216 are for blocking the DC-input voltage to reduce
output offset.

Power Stage
The design features three channels of
speaker power amplification. A MAX9736B is configured in
stereo mode to drive the left and right speakers, and can
provide 2 × 11W into 4Ω. For the third channel, a MAX9736A is
configured as a mono subwoofer amplifier.

In mono mode,
the two Class D outputs of the MAX9736A are connected in parallel
to allow for higher output power. In mono mode the subwoofer
channel can deliver 30W into 4Ω (VDD = 19V). The
input stage for the subwoofer channel is the same as that used
for the left and right channels. After the input stage, the
left and right signals are then summed using U5-B to create
the mono subwoofer channel. R303 is used to set the subwoofer
channel gain at the summing amp.

To enable a maximally
flat response down to very low frequencies, the system is
configured with a sixth-order filtered alignment for the
subwoofer. This approach means that the fourth-order highpass
response of the vented speaker is supplemented with a
second-order electrical highpass in the active circuitry. U5-C
is used to realize the noninverting Sallen-Key, second-order
highpass filter for the subwoofer.

This system requires
a high Q, which leads to 13dB of boost at 40Hz. To avoid
overloading the speaker and amplifier, the Sallen-Key filter
is set up to act as a sliding highpass; the filter acts
dynamically as the amplifier output reaches its limit. The
value of input resistor R305 is designed to reduce
automatically (using the opto-coupled FET,
U7) to a value so that the filter Q is lowered to 0.5,
resulting in no boost at all.

The peak output voltage
of the subwoofer amplifier, U2, controls the signal to the
opto-FET. D6 and C16 form a peak detector, which senses the
output peak level with a fast attack time. The operating
threshold for the peak detector can be adjustable or fixed,
depending on whether R7 is stuffed or R8/R9 are stuffed. The
LED D5 turns on when this control circuit is
active.

Figure
2. Simulation results of the dynamic bass boost from the
subwoofer channel. The Q of the highpass filter is reduced
while the cutoff frequency is also increased, as the subwoofer
level approaches its limit.

The two op amps built
into the MAX9736A are configured into a fourth-order lowpass
subwoofer filter that complements the highpass filter used for
the left/right channels.

J1 is the power-supply input
connector chosen to be a standard laptop-type coaxial socket.
A typical laptop power supply has an average output voltage of
around 19V and serves as a good power source for the docking
system. After some filtering through C1 and C2, the voltage is
labeled PVDD and D1 is set to light blue when power
is attached.

A simple reset circuit (through U6) is
used to create a mute signal on power-up and power-down for
both amplifier chips. This step avoids output transient noises
due to input circuitry settling. The mute control threshold is
programmed to approximately 10V with the R3 and R4
resistor-divider.

The MAX9736 features a filter-less
modulation method which eliminates the need for bulky inductor-based
filters. Only simple ferrite beads (L100 to L301) are required
at the output.

Speakers and Physical Enclosure
The loudspeaker used
for the subwoofer is a 5in-diameter model from Tymphany, part
830945, with a 4Ω nominal impedance
and a resonance frequency of 47.4Hz. The 2in loudspeakers used
for the left and right channels are model 830970, also from
Tymphany. These loudspeakers have 4Ω nominal impedance and a
resonance frequency of 147.5Hz.

A low f3 of
35Hz is targeted for the acoustic system, so a sixth-order
alignment is used. That alignment includes a second-order
highpass filter within the active crossover.
The overall system is housed in a single box enclosure with
the three loudspeakers, tuned port, and circuit PCB. The
subwoofer is mounted as down-firing to minimize the box size
and to help increase efficiency by compression loading. The
resulting box volume is approximately 3.79l and the vented
port is tuned to 54Hz. The outside overall box dimensions are
approximately 355mm x 180mm x 120mm. The speaker crossover
equalization is designed as a fourth-order Linkwitz-Riley at
250Hz; the speaker equalization for the satellites consists of
a parametric EQ with f = 500Hz, gain = +6dB, Q = 0.5, and a
shelving filter with f3 = 3.8kHz and a gain of
+5.8dB. Figure 3 shows the system's
response.

Figure
3. Total system simulated response shows maximally flat down
to 40Hz.

The mechanical drawings included in this
document are copies of those used to construct the prototype
model. The prototype was built using 1/4in clear acrylic
plastic and assembled with drilled/tapped screws and acrylic
adhesive. A consumer product version might be constructed
using a more cost-effective material such as wood, MDF, ABS,
etc.

Circuit Schematics

More
detailed image (PDF, 80.4kB)

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detailed image (PDF, 84.4kB)

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detailed image (PDF, 92kB)

PCB Layout

More
detailed image of the Top Silkscreen (PDF,
277kB)

Mechanical Drawings

More
detailed image (PDF, 72kB)

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detailed image (PDF, 56kB)

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detailed image (PDF, 52kB)

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detailed image (PDF, 48kB)

Picture of System

Fairchild
is a trademark of Fairchild Semiconductor Corporation.

Maxim is a registered trademark of Maxim
Integrated Products, Inc.

Panasonic is a registered trademark and
registered service mark of Panasonic Corporation.

RCA is a registered trademark of RCA Trademark
Management.

Switchcraft is a registered trademark of
Switchcraft, Inc.

Tymphany is a registered trademark of Tymphany
Corporation.

Related parts

MAX9736A

Mono/Stereo High-Power
Class D Amplifier

Free
samples

MAX9736B

Mono/Stereo High-Power
Class D Amplifier

Free
samples

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Download,
PDF format (310kB)
APP 4320: Feb 27, 2009

AN4320, AN 4320, APP4320,
Appnote4320, Appnote 4320

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