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A COMPACT 70W CLASS-H
AUDIO AMPLIFIER MODULE
Based on a Philips TDA1562Q IC, this compact audio
amplifier module can deliver up 70W into a 4-ohm load
when powered from a 12V car battery. It’s just the shot
for use in a portable PA, a tiny sub-woofer amplifier, a
busking amplifier or a car audio amplifier.
By RICK WALTERS
O
UR NEW MIGHTY MIDGET
Amplifier can really pack a
punch – around 36W RMS
continuous into a 4-ohm load when
using a 13.8V supply. However, it’s
the 70W of output power that it can
deliver during dynamic (music) signal
conditions that really make you sit up
and take notice.
As can be seen from the photos
and the circuit diagram, the Mighty
Midget uses just a handful of parts.
It’s built on a PC board that measures
just 104mm x 39mm but while its size
may be modest, these’s nothing at all
modest about its power output. And
the noise and distortion figures are
pretty good too.
At the heart of the circuit is the
TDA1562Q IC, described by Philips
as a “monolithic integrated BridgeTied Load (BTL) class-H high-efficiency power amplifier”. It comes
in a 17-pin “DIL-bent-SIL” plastic
package and is not only designed for
use in car audio and portable PA work
but for mains applications as well;
16 Silicon Chip
eg, mini/midi audio components and
TV sound.
Specifications
The specifications panel and the
accompanying graphs show the performance of our prototype, as measured
on our Audio Precision test gear. Note
that the total harmonic distortion
is typically less than 0.2% at 1kHz
for output powers up to about 16W
PERFORMANCE
Output power: 36W RMS into 4Ω
Music power: 70W into 4Ω
Frequency response: -1dB down
at 28Hz and 55kHz
Input sensitivity: 130mV RMS
(for 36W into 4Ω)
Harmonic distortion: typically
0.2% (see graphs)
Signal-to-noise ratio: >95dB
unweighted (22Hz to 22kHz)
RMS, while the signal-to-noise ratio
is better than 95dB unweighted (22Hz
to 22kHz).
The frequency response is virtually
ruler flat from 28Hz to 55kHz.
Pumping it out
So how does it achieve such high
output powers when powered from
a 13.8V rail? Well, it employs a few
clever tricks. Let’s take a closer look.
First, the TDA1562Q chip actually
incorporates two power amplifiers in
its package and these are operated in
bridge mode to boost the available
output power. Fig.1 – the block diagram of the TDA1562Q IC – shows the
general idea.
Normally, if we have just one amplifier stage operating from a 13.8V
supply, the maximum power that can
be delivered into a 4Ω load is about
6W. The reason for this is that the maximum voltage “swing” possible from
a 13.8V supply is 6.9V in the positive
direction and 6.9V in the negative
direction. This is equivalent to about
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C1+
5
C13
status I/O
mode
select
16
4
CLASS-B
CLASS-H
FAST MUTE
VP2
10
LOAD DUMP
PROTECTION
TEMPERATURE
SENSOR
disable
STANDBY
MUTE
ON
VP1
9
LIFT-SUPPLY
CURRENT
PROTECTION
VP*
IN+
1
PREAMP
7
POWERSTAGE
75
kΩ
FEEDBACK
CIRCUIT
TDA1562Q
PREAMP
POWERSTAGE
LOAD
DETECTOR
75
kΩ
IN-
Vref
2
DYNAMIC
DISTORTION
DETECTOR
8
11
diagnostic
OUT-
VP*
14
15 kΩ
signal 17
GND
OUT+
DIAGNOSTIC
INTERFACE
disable
LIFT-SUPPLY
TEMPERATURE
PROTECTION
reference
voltage
MGL264
15
C2-
13
C2+
6
PGND1
12
PGND2
Fig.1: block diagram of the TDA1562Q class-H audio amplifier IC. The
“Lift Suppy” stages drive external capacitors (C1 & C2) to boost the supply
rails when needed.
4.88V RMS (ie, 6.9/1.4142).
From there, the mathematics is
simple enough – the power output is
equal to the square of the RMS voltage
divided by the load resistance; ie, P =
V2/R. This means that we get 4.88 x
4.88/4, or about 5.95W RMS.
However, this is a theoretical maximum and is never realised in practice.
The actual output power is likely to
be closer to 4.5W RMS due to losses
in the output devices of the amplifier.
One way of obtaining more power
is to wire two identical power amplifiers in “bridge” mode, with each
amplifier essentially a “mirror” of the
other. One amplifier drives one side of
the loudspeaker in a positive voltage
direction, while the other drives the
other side of the loudspeaker in a
negative voltage direction. As a result,
the voltage across the loudspeaker is
effectively doubled compared to the
voltage delivered by a single power
amplifier.
This doesn’t just double the power
output, though. Instead, as shown
Fig.2: the circuit uses a phase splitter based on IC1a & IC1b to drive the inputs of IC2 in anti-phase.
www.siliconchip.com.au
March 2002 17
Fig.3: total harmonic distortion (THD) vs. frequency at
12W (measurement bandwidth 10Hz-80kHz). The dip at
100Hz is due to cancellation with supply ripple.
Fig.4: total harmonic distortion vs. frequency at 36W
(measurement bandwidth 10Hz-80kHz). It’s less than
0.5% for frequencies from 30Hz to 10kHz.
Fig.5: total harmonic distortion vs. power output at 1kHz
(measurement bandwidth 22Hz-22kHz).
Fig.6: the frequency response at 1W. It’s just 1dB down at
28Hz and 55kHz.
by the above formula, doubling the
voltage swing effectively quadruples
the output power! So if we use two
amplifiers which on their own can
deliver only about 4.5W into 4Ω, we
can expect to obtain about 18W RMS
into the same load when they are
connected in bridge mode.
And 18W RMS is a “helluva” lot
better than 4.5W RMS.
Jacking up the supply
That’s by no means the end of the
story, though. As previously stated,
the TDA1562Q is capable of delivering
36W RMS and up to 70W of music
power.
How does it do this? Well, according
to Philips, at low output powers, up to
18W, the device operates as a normal
BTL amplifier. However, when a larger
output voltage swing is required, the
18 Silicon Chip
internal supply voltage to the power
amplifiers is “jacked up” by using the
“Lift-Supply” stages to switch in two
external electrolytic capacitors – see
Fig.1.
There are no details in the specifications as to how the “Lift-Supply”
stages work but we assume it’s a type
of boot-strapping circuit whereby each
of the two power amplifiers in the
bridge circuit actually “jacks up” an
external 4700µF capacitor to increase
the effective supply voltage.
Normally, the external electrolytic
capacitors are switched across the
13.8V supply and charge to about
12.8V (ie, about 1V less than the
supply rail). When extra power is
required, these capacitors are boosted
up by the respective power amplifiers
so that ultimately, the supply voltage
is almost doubled.
As a result, the amplifier module
can briefly deliver much greater output
power – up to 70W of music power or
up to 36W RMS (ie, continuous power)
as previously mentioned. Of course, it
cannot maintain 70W of output power
for long. The two external capacitors
immediately begin to discharge when
the amplifier is delivering this sort of
power and so the supply rails quickly
falls again.
On practical music signals, however, this isn’t normally a problem, as
the electrolytic capacitors are quickly
switched out and charged again between the signal peaks.
Class-H or Class-G?
Philips refer to this scheme for operating the power amplifiers as class-H
operation. In class-H operation, the
input signal is monitored and the
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supply rail is constantly adjusted to
provide just enough voltage for optimum operation of the output devices.
By contrast, class-G operation involves monitoring the input signal and
switching the output stage between
two different supply rails, as required.
In this scheme, the (class-AB) output
stage is normally connected to the
lowest rail and automatically switches
to the higher rail for large signal peaks.
So which of the two schemes is
it? Philips label the TDA1562Q as a
class-H amplifier and we are inclined
to agree with this although we still
don’t know the exact mechanism of
the “Lift Supply” circuitry. No matter
– which ever label is used, it’s very
effective at boosting the output power.
Another advantage of class-H (or
class-G) operation is that it reduces
dissipation in the output stages by
about 50%. That’s because the output
stages operate at low voltage for most
of the time when the amplifier is
driven by music signals. This means
that the heatsink size can be greatly
reduced.
By the way, all switching from
class-AB to class-H opera
tion (and
vice versa) takes place at zero crossing
points of the input signal. This is done
to eliminate switching artefacts, which
could otherwise cause distortion in
the output signal. It’s a very effective
technique – we could find no traces of
switching noise whatsoever.
A rugged device
Another good thing about the
TDA1562Q is that it is virtually indestructible (within limits). It’s output
stage is short-circuit proof (either to
ground, the supply rail or across the
load) and it features thermal overload
protection, good supply ripple rejection and static discharge protection.
There are also no switch-on or
switch-off plops and the output is
automatically muted if the supply
voltage drops below the minimum
operating level. The thermal overload
protection works by automatically
switching the device from class-H to
class-AB operation if its case temperature exceeds 120°C. This basically
disables the high-voltage supply and
thus limits the output power to less
than 20W.
Circuit description
Refer now to Fig.2 for the complete
circuit details of the Mighty Midget
Amplifier. Apart from the TDA1562Q
itself (IC2), there is a dual op amp IC
(IC1), two air-cored inductors and a
few resistors and capacitors.
Op amps IC1a and IC1b together
function as a phase split
ter. These
stages are necessary to provide a
differential input to the amplifiers in
IC2, both of which have their inputs
at pins 1 & 2.
As shown, the input signal is fed
to pin 3 of IC1a. This functions as a
non-inverting amplifier with a gain of
2.4, as set by the 47kΩ and 33kΩ feedback resistors (ie, Gain = 1 + 47/33). Its
output is AC-coupled to pin 2 of IC2
via a 0.1µF capacitor and drives IC1b
which is wired as an inverting unity
gain amplifier.
IC1b in turn drives pin 1 of IC2 via a
0.1µF capacitor. As a result, the signal
on pin 1 is inverted (180° out of phase)
compared to the signal on pin 2 and
so we get true differ
ential drive to
IC2, with a gain of 2.4 for each input.
The input impedance for each pin is
75kΩ (with respect to Vref) and so the
low-frequency rolloff with 0.1µF input
capacitors is about 20Hz.
Bias for IC1a & IC1b is provided by a
voltage divider consisting of two 10kΩ
Parts List
1 PC board, code 01203021,
104 x 39mm
1 1-metre length of 1mm-dia.
enamelled copper wire
1 heatsink (eg, Jaycar HH-8566
& HH-8572; DSE H-3460;
Altronics H-0560 & H-0522)
2 2-way PC-mount screw
terminal block (5.08mm pitch)
4 PC stakes
2 15mm x 6BA machine screws
2 6BA nuts
4 6BA washers
Heatsink compound
Semiconductors
1 TL072 dual FET op amp (IC1)
1 TDA1562Q BTL power amplifier (IC2)
Capacitors
2 4700µF 16VW PC-mount
electrolytic
1 2200µF 25VW PC-mount
electrolytic
2 100µF 25VW PC-mount
electrolytic
1 0.33µF MKT polyester
2 0.22µF MKT polyester
4 0.1µF monolithic ceramic
Resistors (0.25W 1%)
2 47kΩ
4 10kΩ
1 33kΩ
2 2.2Ω 1W
resistors. The resulting half-supply
voltage (Vcc/2) is then filtered using
a 100µF capacitor and is directly connected to pin 5 of IC1b. It also biases
pin 3 of IC1a via a 47kΩ resistor and
this ensures that pin 1 swings symmetrically about Vcc/2.
IC2 operates with a nominal fixed
gain of 20 or 26dB. Its two internal
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March 2002 19
Table 2: Capacitor Codes
Value
IEC Code EIA Code
0.33µF 330n 334
0.22µF 220n 224
0.1µF 100n 104
(pin 16) is tied high to ensure that the
amplifier operates in class-H mode.
PC board assembly
Fig.7: follow this layout diagram when installing the parts on the PC
board. Make sure that all polarised parts are correctly oriented.
All the parts for the Mighty Midget
Amplifier are accommo
dated on a
PC board measuring 104mm x 39mm
and coded 01203021. Fig.7 shows the
assembly details.
Before mounting any parts, first
check your PC board for etching defects or undrilled holes by comparing
it with the published pattern (Fig.8).
This done, you can start the assembly
by fitting the wire links, the resistors
and the MKT capacitors. The two
100µF capacitors can then go in, taking
care with their polarity.
Table 1 shows the resistor colour
codes but it’s a good idea to also
check them using a multimeter. That’s
because some of the colours can be
difficult to decipher.
The next step is to wind the two
inductors that are used in the Zobel
networks. These are made by winding
20 turns of 1mm enamelled copper
wire onto a 5mm former (eg, a 5mm
or 3/16-inch drill). Note that you will
have to wind the last five or six turns
back over the winding, so that the
inductor leads line up with the board
mounting holes.
Because of space restrictions, the
inductors are mounted proud of the
PC board, so that they sit clear of
the 2.2Ω resistors. Clean and tin the
ends of the leads before soldering
them in position. That done, fit the
screw terminal block for the supply
connections, followed by PC stakes
for the signal input and loudspeaker
terminals.
Fig.8: this full-size etching pattern for the PC board (code 01203021).
amplifiers amplify the signals on pins
1 & 2 and in turn provide out-of-phase
(mirror image) signals to drive the
bridge tied load (BTL). This BTL consists of a loudspeaker which is fed via
two Zobel networks, each consisting
of a 2.2Ω resistor and a parallel 880nH
inductor.
The Zobel networks present a resistive load to the amplifier at high
frequencies and ensure stability. They
also help reduce transient and RF
interference, which can be picked up
by the loudspeaker leads, from being
fed back into the early stages of the
amplifiers via the feedback paths.
The two external capacitors for the
“Lift Supply” blocks each have a value
of 4700µF. This value determines the
low-frequency power roll-off.
Pins 4 and 16 of IC2 have been tied
high in this circuit. Pin 4 (Mode) must
be tied high for normal operation –
tying it low places IC2 into “standby”
mode (effectively switch
ing it off),
while leaving it open circuit mutes
the output by suppressing the input
signal.
Pins 8 (Diag) and 16 (Status) are
normally used in conjunc
tion with
a microcontroller to monitor various
parameters. For example, the “Dynamic Distortion Detector” inside the
TDA1562Q can detect the onset of
clipping and this information is fed
to the diagnostic output. It could then
be processed to drive a DC-volume
control to attenuate the input signal
accordingly and so limit the distortion.
Similarly, the diagnostic output
can indicate various short-circuit and
temperature conditions. It can either
be left open-circuit or tied to the +12V
rail via a 10kΩ resistor. The Status pin
Table 1: Resistor Colour Codes
No.
2
1
4
2
20 Silicon Chip
Value
47kΩ
33kΩ
10kΩ
2.2Ω
4-Band Code (1%)
yellow violet orange brown
orange orange orange brown
brown black orange brown
red red gold brown
5-Band Code (1%)
yellow violet black red brown
orange orange black red brown
brown black black red brown
red red black silver brown
www.siliconchip.com.au
The Tiger
comes to
Australia
The BASIC, Tiny and Economy
Tigers are sold in Australia by
JED, with W98/NT software and
local single board systems.
This close-up view shows the prototype PC board. The final version (Fig.7) has
been amended to include a screw-terminal block for the supply connections and
also features improved component spacing (especially near the inductors).
Tigers are modules running true compiled multitasking BASIC in a 16/32 bit core, with typically
512K bytes of FLASH (program and data)
memory and 32/128/512 K bytes of RAM. The
Tiny Tiger has four, 10 bit analog ins, lots of
2
digital I/O, two UARTs, SPI, I C, 1-wire, RTC and
has low cost W98/NT compile, debug and
download software.
JED makes four Australian boards with up to 64
screw-terminal I/O, more UARTs & LCD/keyboard support. See JED's www site for data.
TIG505 Single Board
Computer
The TIG505 is
an Australian
SBC using the
TCN1/4 or
TCN4/4 Tiger
processor with
512K FLASH
and 128/512K RAM. It has 50 I/O lines, 2
RS232/485 ports, SPI, RTC, LCD, 4 ADC, 4 (opt.)
DAC, and DataFLASH memory expansion.
Various Xilinx FPGAs can add 3x 32bit quad shaft
encoder, X10 or counter/timer functions. See
www site for data.
$330 PC-PROM Programmer
The TDA1562Q is secured to the heatsink using two 15mm x 6BA machine
screws, nuts & washers. This also provides sufficient support for the PC board.
IC1 & IC2 can now be installed and
the PC board fitted to the heatsink.
Make sure that IC1 is installed the
right way around, with pin 1 adjacent
to the 47kΩ resistor. IC2 can only go
in one way, so there’s no chance of
confusion here.
The next job is to drill two holes in
the heatsink to match the mounting
holes at either end of the TDA1562Q.
That done, deburr the heatsink
mounting holes using an oversize drill
and smear the mating surface of the
TDA1562Q with heatsink compound.
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The assembly can then be bolted
together using two 15mm x 6BA machine screws, nuts and washers.
You can now complete the assembly
by fitting the three large electrolytic
capacitors but watch their polarity –
electro
lytic capacitors have a nasty
habit of exploding if installed the
wrong way around. These capacitors
are all left until last to avoid accidental damage and, in the case of the
centre 4700µF unit, to ensure access
to the lefthand mounting screw for the
TDA1562Q.
This programmer plugs into a PC printer port and
reads, writes and edits any 28 or 32-pin PROM.
Comes with plug-pack, cable and software.
Also available is a multi-PROM UV eraser with
timer, and a 32/32 PLCC converter.
JED Microprocessors Pty Ltd
173 Boronia Rd, Boronia, Victoria, 3155
Ph. 03 9762 3588, Fax 03 9762 5499
www.jedmicro.com.au
March 2002 21
Fig.9 (above left): the top trace shows the continuous output from the amplifier terminals at 36W into 4Ω. The lower two
traces show the individual amplifier outputs which add to give the final BTL output. Fig.10 (above right) shows the same
test conditions as for Fig.9 but with a music-power signal at 70W into 4Ω.
Note that pin 4 (Mode Select), pin
8 (Diag) and pin 16 (Status I/O) have
been brought out to vacant pads. These
are not used in this circuit.
Testing
To test the amplifier module, you
will need a regulated power supply
with an output of 12-15V DC (eg, a car
battery or a 12V SLA battery). Be sure
to connect it to the terminal block with
the correct polarity.
Note: a car battery charger can
NOT be used as a DC supply for this
amplifier module. That’s because
battery chargers deliver significantly
more than 12V (they have to in order
to charge) and also because they don’t
include any filtering.
Initially, you should connect your
multimeter – set to a DC current range
– in series with one of the supply leads.
This done, switch on and check that
the current consumption is some
where in the range of 110-150mA. If
you don’t get this, switch off immedi-
ately and check for wiring mistakes.
After that, it’s simply a matter of
connecting the loudspeaker and feeding in an audio signal to confirm that
it works.
Note that if you are using a mains
power supply to drive the module,
it may lack sufficient output current
capability for the amplifier to deliver
full power during transients. In fact,
a low-current supply could even
activate the muting circuit in the
TDA
1562Q, due to the supply rail
falling below about 8V. Insufficiently
thick supply wire can cause the same
problem - use 4mm (copper) diameter
automotive cable at a minimum.
In practice, to drive the amplifier
to full power, the power supply will
need to be able to deliver at least 6A.
Note too that car electrical systems
normally don’t run at 12V. Instead,
most run at 13.8-14.4V when the motor
is running. The Mighty Midget Amplifier is designed to handle this but
don’t push the supply voltage beyond
about 16V – you’ll exceed the ratings
of the 4700µF electrolytic capacitors
if you do.
Speaker requirements
You can use a huge variety of speakers with this module – even low-power
speakers can be used as long as long
you don’t wind the wick up to far!
The main thing to note is that the
TDA1562Q is designed for use with
4Ω speakers and will deliver maximum
power into 4Ω. Most car audio speakers
are 4Ω for this very reason. Of course,
the amplifier will also quite happily
drive an 8Ω loudspeaker. The drawback
is that you will only get half the power
output compared to a 4Ω speaker.
Finally, there is a common misconception that large speak
ers require
more power to drive than small speakers. This is not usually the case – large
speakers are usually more efficient
than small speakers of similar rating
and will therefore sound louder when
SC
driven by the same amplifier.
UM66 SERIES TO-92
SOUND GENERATOR.
THESE LOW COST IC’S
ARE USED IN MANY TOYS,
DOORBELLS AND NOVELTY
APPLICATIONS
1-9
$1.10
10-24 $0.99
25+
$0.88
EACH INC GST
22 Silicon Chip
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