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E
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FEAT ECT
PROJ
The PortaPAL
A State-Of-The-Art Portable
Public Address Amplifier
Features
d unit with safe plugpack
charger
• Portable, 12V battery-powere
• High power output
eaker with tweeter
ut
• Inbuilt 200mm (8") loudsp
s 6.35mm guitar or aux inp
m microphone inputs plu
• 2 combined XLR/ 6.35mwit
stereo to mono mixer
• Stereo RCA line inputs hRCA and 6.35mm jack outlets
• 2 line outputs with stereo ut
• Level control for each inp
• Bass and Treble controlsto extend battery charge with power-down indication
• Automatic power-down indication
arger on and charging
• Ch
14 S
ilicon
hip
re leads
ce forCspa
rage spa
Sto
•
and corner protectors
dle, speaker stand socket
han
ry
• Box includes car
siliconchip.com.au
Part 1: by JOHN CLARKE & LEO SIMPSON
T
HIS PORTABLE PA amplifier
can be powered from the 240VAC
mains or its inbuilt 12V SLA
battery. It delivers up to 70 watts and
pulls a number of tricks to keep battery
current low while still maintaining
very high performance.
Back in March 2002, we published
the “Mighty Midget”, a 70-watt class-H
audio amplifier module based on the
Philips TDA1562Q power IC.
This ground-breaking IC uses special techniques to deliver up to 70
watts from a 12V battery and does
away with the need for a DC-DC inverter.
At the time, we said the “Mighty
Midget” was ideal for use in a portable
PA system and now we have followed
up with the PortaPAL: a complete system, including mixing for two 600Ω
balanced or unbalanced microphones,
guitar input and line inputs for a CD
player or a cassette deck.
This new circuit makes use of the
TDA1562Q’s muting feature, to further
reduce battery drain and keep hum and
noise very low.
We’ve also taken advantage of the
inbuilt 12V SLA (sealed lead acid) battery to eliminate a heavy transformer
and large and expensive electrolytic
filter capacitors from the power supply
– leading to a considerable cost saving.
No DC-DC inverter needed
Up until the release of the Philips
TDA1562Q IC, if you wanted more
than about 16 watts from a 12V-powered PA system, you had to resort to
a DC-DC inverter to provide power
supply rails of, say, ±40V, to get around
50 watts into an 8-ohm load.
Not only are DC-DC inverters relatively complex but even the most
efficient designs inevitably lead to a
reduction in overall circuit efficiency.
With its special class-H operation
and bridged amplifier operation, the
TDA1562Q not only eliminates the
need for an inverter but its class-H operation is considerably more efficient
than a normal class-B amplifier which
is what would be normally used.
So enough of the rave about the
TDA1562Q – if you want more information, refer to the March 2002 issue
of SILICON CHIP. Now let us have a
look at the other features of this great
little (actually, not so little) portable
PA amplifier.
Features
Housed in a timber cabinet meas-
uring 450 x 280 x 240mm, the PortaPAL amplifier uses a coaxial 200mm
speaker which is rated at 50W and
has relatively high efficiency of 92dB/
1 watt <at> 1 metre. It is a 4Ω speaker
which incorporates a separate concentrically mounted miniature dome
tweeter; that’s where the “coaxial”
term comes from. The speaker is
specified as a 4Ω model because the
amplifier is designed to deliver maximum power into a 4Ω load.
All the controls are at the rear of the
cabinet. There are two XLR sockets for
connection of low impedance (600Ω)
balanced microphones. These special
XLR sockets also accept standard
6.5mm jack sockets so that unbalanced
microphones can be used as well.
There is also a pair of RCA phono
sockets for connection of a CD player
or cassette deck and RCA sockets are
also provided for line out signals to a
cassette deck, if the proceedings need
to be recorded, or to another PA system. This output is also duplicated at
a 6.35mm stereo jack socket.
There are four mixing knobs for the
microphones, guitar and line inputs
but there is no master level control,
to keep things simple. Bass and treble
tone controls are provided and automatic VOX is built in.
There are four LEDs on the panel,
PortaPAL Specifications
33W RMS into 4Ω (depending on battery voltage)
70W RMS into 4Ω (depending on battery voltage)
Line
-3dB at 26Hz and 40kHz
Guitar and microphone -3dB at 42Hz and 20kHz
Tone Controls
+13dB and -14dB at 100Hz
(see graphs)
+11dB and -13dB at 10kHz
Input Sensitivity:
Line
340mV RMS
(for 30W into 4Ω)
Guitar
16mV RMS (1.9V RMS overload)
Microphone
1.3mV RMS (130mV RMS overload)
Signal-to-Noise Ratio:
-83dB unweighted (20Hz to 20kHz); input level controls all off
(all figures with respect to 33W)
-71dB unweighted with microphone level set at maximum sensitivity
(-73dB A-weighted)
Muting
Threshold:
<7mW output power
Time:
15 seconds (typical) after signal drops below threshold
100ms (typical) unmute when signal applied
Battery Consumption
Standby (mute)
26mA
No signal (unmute)
160mA
Battery Charger
Charge rate:
1A maximum
Charge voltage:
13.8V maximum
Dimensions:
500 x 295 x 250mm (including handle and corners/feet)
Mass
13kg (including charger plugpack)
Output Power
Music Power
Frequency Response
siliconchip.com.au
February 2003 15
Fig.1: all of the functional areas of the PortaPAL are shown in this block diagram, with the exception of the mains
power supply/SLA battery charger.
two at the top righthand corner and
two lower down, adjacent to the plugpack charger input socket. At the top,
one of the LEDs flashes about twice a
second to indicate that the unit is on
while the other is the “Fault” indicator. We’ll talk more about this and the
charger LEDs later.
The only other control is the On/Off
switch. Also on the rear panel is the
lid for the 12V battery compartment
and the battery can be quickly changed
over if that is necessary. Power comes
from the battery or an external 16VAC
1.5A plugpack.
While the plugpack has relatively
low power rating (24VA) with respect
to the maximum output of the PA
amplifier, it is quite adequate to keep
the battery fully charged in normal PA
operation. In fact, the power supply is
really just a battery charger with the
battery permanently connected.
The inbuilt VOX operates to mute
the power amplifier if there is no signal
for more than 15 seconds, reducing
the standby battery consumption from
16 Silicon Chip
160mA to around 26mA.
While it is hard to be precise, we
estimate that the inbuilt 12V 7Ah battery should be good for about five to
six hours use. In practice, that means
you could typically use the PortaPAL
all day on battery power.
Circuit overview
Fig.1 shows the simplified block
diagram. Apart from the TDA1562Q
power amplifier IC, there are eight
low-cost ICs and not a lot else. The
microphone signals are amplified in op
amps IC1a and IC1b, while the guitar
signal is amplified by op amp IC3.
The microphone, guitar and line
signals levels are set by VR1, VR2, VR3
& VR4 and then mixed and fed to the
tone control stages and to the muting
circuit involving IC6, D1 & D2. This
circuit provides a mute signal to the
power amplifier when the audio signal
levels are below a certain threshold.
Circuit details
As already noted, the Portable PA
uses two special XLR sockets which
also accept stereo or mono 6.5mm
jacks. For simplicity, these sockets are
shown on the circuit of Fig.2 as separate XLR and jack sockets but remember that they are combined into what
look like ordinary 3-pin XLR sockets.
In normal operation, using a microphone with an XLR plug, the
balanced microphone signals are fed
to the inputs of op amps IC1a & IC1b.
These provide a gain of 22 from a 600Ω
microphone.
Both microphone preamps are identical except that the MIC1 preamp has
provision for a bias voltage (phantom
power) for electret microphones, if
required.
The use of 1% resistors in the balanced microphone circuits ensures
good rejection of common mode signals such as hum and hash.
High frequencies above 50kHz are
rolled off by the 150pF capacitors
across the 22kΩ feedback resistors.
The 390pF capacitors shunting the
balanced input lines, in conjunction
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with the microphone impedance, also
roll off the high frequencies.
Should you insert a 6.5mm stereo
jack plug from a balanced mike into the
XLR socket it will again be accepted
as a balanced signal and converted to
a single-ended output.
But here is the clever part. We have
wired it so that if you insert a mono
jack into the socket, the non-inverting
input (pin 3 of the XLR) is grounded
and IC1a (or IC1b) operates as a normal non-inverting amplifier with a
gain of 22.
Thus we cater for both balanced
and unbalanced low impedance microphones.
The unbalanced outputs of op amps
IC1a & IC1b are each fed to level potentiometers VR1 & VR2 via a 150Ω
resistor and 1µF capacitor. The signals
from VR1 and VR2 are then applied
to op amps IC2a and IC2b, both of
which have a gain of 11. This means
that maximum gain for microphone
signals is 242.
Guitar input
The guitar input stage involving the
TL071 Fet-input op amp IC3 looks
like a straightforward non-inverting
amplifier but there are a number of
interesting wrinkles.
First, the guitar signal is coupled in
via a relatively large value of capacitor,
47µF, especially when the input load
resistor is also high at 470kΩ. This is
because are aiming for two separate
outcomes. We have specified the high
load resistance of 470kΩ to ensure
optimum high frequency response
with the relatively high inductance
of typical guitar pickups.
With such a high load resistance,
you might wonder why we have used
such a large input coupling capacitor.
After all, to maintain a flat response
to below 20Hz, all you need is a 15nF
(0.15µF) input capacitor. Why use
47µF, 300 times bigger?
The answer is that the inductive
guitar pickup represents a low source
resistance at low frequencies. In order
to minimise noise, op amp IC3 needs
to see as low a source resistance as
possible. Ergo, we use a big capacitor.
IC3 is set for a gain of just two. This
is adequate for any guitar (when the
following gain is accounted for) but it
also means that this input can handle
line input signals of up to 1.9V before
overload occurs.
Following potentiometer VR3, the
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Construction will be fully described next month but to whet your appetites, at
top we show the completed amplifier assembly ready for mounting in the box,
while immediately above is the separate SLA charger board. By the way, this
could be built independently as a high performance SLA battery charger.
guitar signal is fed to op amp IC4a,
which has identical gain to IC2a &
IC2b.
Stereo line inputs
Stereo line inputs (eg, from a CD
player) are mixed to a mono signal
with 2.2kΩ resistors and fed to potentiometer VR4. All of the signals
from the four potentiometers are then
mixed in IC5a which has gain of unity.
Note that the input resistor from VR4
is 10kΩ rather than 15kΩ to make up
for a slight gain loss in the resistive
mixing of the stereo line inputs.
IC5a drives the tone control stage
involving IC5b and this has its output
signal fed to three places: the line outFebruary 2003 17
18 Silicon Chip
siliconchip.com.au
siliconchip.com.au
February 2003 19
Fig.3: total harmonic distortion versus frequency at 12W
using the microphone input.
put to RCA and 6.35mm jack sockets,
the muting stages involving IC6 and
the power amplifier involving IC8
and IC9.
All of the op amps in the circuit,
with the exception of IC4b and IC6,
have their non-inverting (+) inputs biased from the Vref line which is at +6V.
This is derived from the +12V line
by a voltage divider consisting of two
10kΩ resistors with the centre point
bypassed by a 100µF capacitor. The
bypassed supply is then buffered by
op amp IC4a to provide the Vref line.
This means that all op amps will
have symmetrical clipping at overload,
to maximise the output signal. All op
amp outputs, with the exception of
IC6b, will sit at +6V (or half the battery
voltage).
Muting stages
As noted above, we have incorpo-
rated VOX into the circuit to mute the
amplifier and cut current consumption
when no signal is present. This muting
function is provided by dual op amp
IC6. Op amp IC6a is a non-inverting
stage with a gain of 471 by virtue of
the 470kΩ and 1kΩ feedback resistors.
The 22pF capacitor rolls the gain off
above 15kHz, while the 10µF capacitor
in series with the 1kΩ resistor rolls off
signals below 16Hz.
The amplified signal from IC6a
is then fed to a diode pump circuit
consisting of diodes D1 & D2 and 1µF
& 10µF capacitors. Hence, the peak
level of the signal from IC6a will be
stored in the 10µF capacitor which is
continuously being discharged via the
1MΩ resistor across it.
The 10µF capacitor is monitored by
IC6b which is connected as a Schmitt
trigger. A 10MΩ resistor between pin 5
and pin 7 applies a degree of positive
Fig.5: distortion versus frequency at 30W but using the
line input.
20 Silicon Chip
Fig.4: total harmonic distortion versus frequency at 30W
using the microphone input.
feedback to give hysteresis. This makes
the comparator output switch cleanly
between high and low, to prevent the
possibility of parasitic oscillation at
the switching points.
The inverting input of IC6b is set
at +3.4V using the 100kΩ and 39kΩ
resistors across the 12V supply.
When power is first applied to the
circuit, the 10µF capacitor between the
12V supply and the inverting input
to IC6b is initially discharged and
therefore pulls pin 6 high, causing pin
7 to be low.
Pin 7 of IC6b is connected to the
mute (mode) input, pin 4, of the power
amplifier, IC9. So at power-up, the
amplifier is muted.
Once the audio signal monitored by
IC6a is of sufficient level, IC6b’s output
will go high and the power amplifier
will be unmuted.
Muting indication is provided by
Fig.6: power versus distortion when driven by the mic
input. Maximum power here is 42W at 10% distortion.
siliconchip.com.au
Fig.7: power versus distortion when driven by the line
input. The distortion is lower than Fig.6 because of the
lower gain from the line input.
IC7, a CMOS 7555 timer, which drives LED1, the power/
standby indicator. Initially when power is switched on,
transistor Q1 is off and so pin 4 of IC7 is pulled high via
the 10kΩ resistor connecting to the 12V supply. This allows the 555 timer to run and it flashes LED1 on and off.
The rate of flashing is set by the 10µF capacitor connected to pins 2 & 6 and the associated 100kΩ and 10kΩ
resistors.
Note that the 10µF capacitor is tied to the +12V supply
rather than 0V, as in a normal 7555 timer setup. The reason
for doing this is so that pin 3 of the 7555 will be low when
power is first applied and the LED will light immediately
and then flash. If the capacitor was connected to 0V (as
in a conventional 7555 circuit), the LED would be off for
almost one second before flashing on.
In effect, the 10µF capacitor is charged via the 10kΩ
and pin 7 and then discharged to +12V via the 10kΩ and
100kΩ resistors. Since the ratio of the charge/discharge
resistances is 1:11, the LED flashes with about the same
duty cycle (on for 70ms, off for 740ms) and thereby keeps
current drain to a minimum when the amplifier is muted;
LED1 draws about 400mA.
Fig.8: the tone control action in the “flat”, “max boost” and
“max cut” settings.
siliconchip.com.au
February 2003 21
The main PC board includes most of the
electronics: the power amplifier (the large
IC attached to the heatsink), the mixer, tone
controls and so on. Input “daughter boards”
attach to this main board. The power supply
is also on a separate board.
When pin 7 of IC6b’s output goes
high to unmute the power amplifier,
transistor Q1 is switched on and it
pulls pin 4 of IC7 low. This forces
the pin 3 output low and LED1 is lit
continuously.
So LED1 is on continuously in normal operation and it flashes when the
amplifier is muted.
Power amplifier
IC9 is the TDA1562Q power amplifi-
The specified 200mm 4Ω woofer from
Altronics has a separate concentrically mounted plastic dome tweeter and
has quite a wide overall frequency
response.
22 Silicon Chip
er which can deliver up to 70W under
music power conditions, depending
also on the state of battery charge. The
circuit presented here is very similar
to that first presented in the March
2002 issue of SILICON CHIP. The main
difference is that here we are using
the mute (pin 4, Mode input) facility,
as described above and the diagnostic
output at pin 8. This is used to drive
LED2.
It will show when the amplifier is
clipping, if there is a short at the output, if there is an open circuit load and
if the amplifier has gone into thermal
shutdown.
If you want a full description of the
TDA1562Q, you will need to refer back
to the March 2002 issue. For those
readers who have not seen that issue,
we will briefly the describe the circuit
operation.
The TDA1562Q actually contains
two power amplifiers which drive the
4Ω speaker in bridge configuration and
its inputs are balanced. So we drive
these balanced inputs (pins 1 & 2)
with signals that are 180° out of phase.
Hence, pin 1 of IC9 is driven directly
from the output of IC5b (albeit via two
capacitors) while pin 2 is driven from
the output of IC8, a TL071 op amp
connected as a unity gain inverter.
The two 4700µF capacitors at pins
3 & 5 and pins 13 & 15 of IC9 are “lift
supply” reserves for when momentary
high power levels are required.
Both amplifier output terminals
pass through Zobel networks, each
comprising a 2.2Ω resistor and parallel
inductor shunted via a 220nF capacitor. The components are included
to guarantee stability (ie, stop any
tendency to supersonic oscillation)
when driving reactive loads.
Power for the circuit comes from a
12V 7 amp-hour battery which is fed
via switch S1 and a 7.5A fuse. Diode
D3 is included should the battery be
connected the wrong way around. If
that happens, the diode will conduct
and blow the fuse.
Next month
In March, we will present the charger circuit for the PortaPAL. This can
be built as a general-purpose charger,
as well as the power supply for this
amplifier.
We will also present the full construction details of the PortaPAL PA
SC
amplifier.
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