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A compact
audio
powerhouse
This rugged, single channel amplifier module comes
complete with power supply and a fan-cooled heatsink.
It is based on the once-popular “Pro Series One”
originally featured in “Electronics Australia” magazine
and now re-designed by Altronics.
200 Watt Mosfet
by LEO SIMPSON
W
e have had quite a few en- power output of 140W into 8Ω and TO-3 metal cases. These are no longer
quiries from readers who 200W into 4Ω. Frequency response available and their plastic equivalents
want to build a Mosfet am- is within 1dB from 20Hz to 80kHz are quite difficult to obtain as well.
(Fig.1). Total harmonic distortion is Altronics looked at this situation
plifier with a rating of about 200W.
We had not designed such a module rated at less than 0.1% up to full power and have used essentially the same
(Fig.2) and signal-to-noise ratio with circuit designed around some equivand as has been mentioned previalent plastic Mosfets made by Exicon
ously in SILICON CHIP magazine, our respect to 200W is better than 100dB
of the UK. This
preference has been to
has necessitated
design high performance
Performance of Prototype
a re-design of the
amplifier circuits around
PC board so that all
bipolar transistors rather
Output Power (RMS):.... 140W into 8 ohms; 200W into 4 ohms
the Mosfets and the
than Mosfets.
Frequency Response:.. 20Hz – 80kHz at -1dB points (see Fig.1)
driver stage tranHowever, many people
sistors all line up
prefer Mosfets because
Input Sensitivity:........... 830mV for 200W into 4 ohms
along one edge, alof their legendary rugHarmonic Distortion: .. <0.1% (20Hz – 20kHz) (see Fig.2)
lowing them to be
gedness. Altronics had a
mounted vertically
Mosfet amplifier module
Signal-to-Noise Ratio:.. >102dB unweighted; 105dB A-weighted
on the heat-sink.
which produced 200W
with respect to 200W into 4 ohms
Apart from usinto a 4Ω load and so we
Stability:........................ Unconditional
ing plastic power
decided to take a look it.
transistors which
It turned out to be based
greatly simplify
on the “Pro Series One” as mentioned
unweighted.
mounting compared to metal TO-3
above, although this version by AlOriginally, the “Pro Series One”
power transistors, Altronics have emtronics has been derated and adapted
was based on Hitachi Mosfets with ployed spring clips to mount adjacent
to different Mosfets. It has a rated
32 Silicon Chip
www.siliconchip.com.au
Amplifier Module
Ideal amplifier for: ✪ hifi ✪ subwoofer ✪ public address ✪ guitar
transistor pairs, to make things simpler
again. The spring clips apply just the
right amount of tension to the transistors and there is no danger of damaging
a transistor due to over-tightening a
mounting screw.
The heatsink is a black-anodised
aluminium extrusion with fins on one
side. It measures 300mm long and is
fitted with a cover which allows it to
be cooled by an 80mm 24V DC fan.
The fan runs continuously and this
means that the heatsink is always cool
(or at least, at little above ambient
temperature).
A really attractive feature of the
module is that it comes with its own
power supply, consisting of a 300VA
toroidal power transformer and accompanying power supply board. The
bridge rectifier is mounted on the same
vertical heatsink as the amplifier and
so it also gets the benefit of continuous
fan cooling.
The whole assembly is mounted on
www.siliconchip.com.au
a sheet of black enamelled steel measuring 300 x 214mm. This could be
built into a larger chassis for a PA system, stereo system, active sub-woofer
or whatever.
Easy connections
Both the amplifier and power
supply boards are designed for easy
connection to supply wires, input
and output lines and so on, by virtue
of the insulated terminal blocks lined
up along one edge. The particular advantage of this feature is that it makes
it very easy to connect and disconnect
the boards, without any need to resort
to the soldering iron.
Circuit details
The circuit of the amplifier appears
to be loosely based on an original Hitachi design produced about 20 years
ago but it still rates as a very good
design today. Fig.3 shows the circuit.
Q3 & Q4 are the input differential
pair with the input signal applied to
the base of Q3 via a 0.47µF capacitor
and 1kΩ resistor. Negative feedback
is applied to the base of Q4. Q3 and
Q4 amplify the “difference” between
the input and negative feedback signals and the output signals appear
at their collectors. Q1 & Q2 form a
constant current “tail” circuit for the
differential pair, ensuring that it has
high gain and good common mode
rejection ratio.
Following the input differential pair
of Q3 & Q4 is the voltage gain stage
involving differential pair Q5 & Q6
and their balanced current mirror load
of Q7 & Q8. The bases of Q5 and Q6
are driven from the collectors of Q4
& Q3 respectively and their collector
loads are provided by Q7 & Q8, the
current mirror.
Current mirror loads for differential
gain stages are widely used in op amps
as they give very good linearity (ie,
distortion free). In fact, most of the
August 2001 33
AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz)
10.000
06 MAY 100 05:15:08
AUDIO PRECISION SCTHD-W THD+N(%) vs measured LEVEL(W)
10
06 MAY 100 04:52:33
8.0000
6.0000
1
4.0000
2.0000
0.1
0.0
-2.000
0.010
-4.000
-6.000
-8.000
0.001
-10.00
.0005
10
100
1k
10k
100k 200k
Fig.1: as measured in the SILICON CHIP laboratory, frequency
response is a very wide 20Hz-80kHz (-1dB).
distortion in this amplifier will be
generated in the Mosfet output stage
although most of this is then corrected
by the negative feedback applied back
to the base of Q4.
The output of Q6 drives the gates of
Mosfets Q9, Q10, Q11 & Q12 which
0.5
1
10
100
300
Fig.2: maximum power output before it “hits the wall” is
about 220W into 4Ω at <0.1% distortion.
operate as complementary source
followers, ie, with a voltage gain of
a little less than unity. It may appear
that Q6 drives Q11 & Q12 while Q8
drives Q9 & Q10 but Q6 is the driver
while Q8 is merely part of the current
mirror load for the second differential
amplifier stage.
The gates of Q9 & Q10 are “separated” from the gates of Q11 & Q12 by
the 200Ω trimpot RV1. The trimpot
actually provides a small amount of
forward bias to the gates of the Mosfets
so that they are always conducting
Viewed from a steeper angle than the previous photo, this gives a good idea of the layout of all the major components in
the amplifier. Note the insulated cover over the fuse in the foreground: this is essential for your safety.
34 Silicon Chip
www.siliconchip.com.au
Fig.3: the circuit is relatively conventional, using four power Mosfets in the output stages. Q13 and 14 form an overload
indicator, detecting the large spikes generated at the onset of clipping.
to some extent. This is done so that
the effects of crossover distortion are
minimised.
Readers may wonder why trimpot
RV1 does not have an associated NPN
small signal transistor across it to
function as a “Vbe multiplier”. This
is required in Class A, AB and Class
B amplifiers using bipolar output
transistors to ensure that the quiescent
current is thermally stable.
Without such a transistor to monitor and continually adjust the bias
setting, the quiescent current just
www.siliconchip.com.au
continues to rise until eventually, the
output transistors get so hot that they
are destroyed. This is called “thermal
runaway”.
So why don’t Mosfet amplifiers need
this same transistor?
It is not true to say that Mosfet
circuits cannot experience thermal
runaway in particular circumstances
but generally the power Mosfets used
in high-quality audio power amplifiers
have a negative temperature coefficient for currents above a particular
level, typically 100mA.
Above that current, if the Mosfet
gets hotter, it tends to throttle back its
operating current and thus reduce its
dissipation and so there is no tendency to thermal runaway. We’ll discuss
the quiescent current setting later in
this article in the section on testing
and setup.
The drive signal to the gates of the
four Mosfets is limited to about 12.6V
peak-to-peak by the diode network
consisting of D1, D2, ZD1 & ZD2. This
limiting is included because Mosfets
can be destroyed if they have excessive
August 2001 35
Fig.4: there are two parts to the power supply; a centre-tapped bridge to give the main plus and minus rails, along with a
conventional bridge fed by a 6.8µF capacitor, stabilised with a zener diode for the 24V fan supply. Note that the voltages
are nominal, being “off load” and the type of figures you would expect to measure with a digital multimeter.
gate drive. In normal signal conditions
this does not occur but if the output
of the amplifier is shorted or fed into
a very low impedance, the negative
feedback action causes the amplifier
to compensate by increasing the gate
drive.
Also part of the circuit of each
Mosfet is a 220Ω resistor in series
with the gate. This is referred to as a
“stopper” resistor because it is there
to prevent (ie, to stop) spurious or
parasitic oscillation at very high frequencies.
Mosfets can oscillate at much higher
frequencies than bipolar transistors
and it is not unheard of for a badly
designed Mosfet amplifier to oscillate
at 100MHz or more (ie, in the middle
of the broadcast FM band). So those
stoppers are vital for stable amplifier
operation.
By the way, amplifiers with bipolar
transistors often have stopper resistors
as well but the potential frequency
of oscillation is much lower than for
power Mosfets.
Another interesting feature of the
Mosfet output stages in this amplifier
is the presence of 47pF capacitors
between Drain and Gate of Q9 & Q10.
These are incorporated to equalise the
gate capacitance of the N-channel Mos36 Silicon Chip
fets (ie, Q9 & Q10) to the higher gate
capacitance of the P-channel Mosfets
(Q11 & Q12).
Each Mosfet has a 0.22Ω 5W source
resistor. This is included to improve
current sharing in each pair of Mosfets and it also contributes to overall
thermal stability.
Right at the output of the amplifier,
at the junction of the four 0.22Ω source
resistors, is a Zobel network consisting of a .022µF capacitor and 6.8Ω
resistor. This is necessary in virtually
all solid-state amplifiers to ensure
that a predictable load impedance
(ie, around 6.8Ω) is presented at high
frequencies where the inductance of
typical speakers means their impedance is becoming very high.
The overall gain of the amplifier is
set by the negative feedback network
consisting of C4, C6, R8 & R12. R12 and
R8 set the overall gain to 34 while C4
sets the low frequency rolloff to 3.3Hz.
However, the main determinant of the
amplifier’s low frequency response is
the 0.47µF input capacitor C1 and the
33kΩ input resistor which together set
the -3dB point at 10Hz.
The high-frequency response of
the amplifier is mainly determined
by the 1kΩ input stopper resistor R2,
in conjunction with the .001µF input
shunt capacitor C2. They set the high
frequency -3dB point to about 160kHz.
Overload indicator
Finally, an interesting feature of
the amplifier is the overload indicator
involving transistors Q13 & Q14. It
depends on the fact that, at the onset
of clipping, large spike signals appear
at the collector of Q5. These are fed
via the voltage divider comprising
R25 & R26 to the base of Q13. Each
time a positive spike is fed to Q13, it
charges capacitor C20 in its collector
circuit. As the capacitor subsequently
discharges, it turns on Q14 and lights
LED1, the overload indicator. Thus
Q14 and C20 function as a pulse extender so that each momentary signal
overload is made visible on the overload indicator.
Power supply
The power supply (Fig.4) is fairly
conventional, employing a 300VA
toroidal transformer with 45V secondaries feeding a bridge rectifier, BR1.
Each diode in the bridge rectifier is
bypassed with a 0.22µF capacitor to
provide suppression of rectifier hash.
The bridge rectifier feeds four 4700µF
80VW capacitors to provide balanced
supply rails of ±67V (nominal). On
www.siliconchip.com.au
O P T O PA C K 1 0 4 D E V I C E S :
various colours & types. Top brands.
Siemens etc. just $10 VISIBLE
LEDs...5mm...14X Yellow clear, 6X Red
(clear) 24deg, 2X Yellow (clear) 24deg,
16X Red (clear) 24deg,38X Green (clear)
24deg.VISIBLE LEDs... 3mm...14X Red
diffused 70deg. 4X 3mm or rect. Yel.
diffused 70deg SPECIAL...1X 5mm IR,3X
3mm Clear Phototransistor, 3X 5mm
Phototransistor, 1X IR RX module. 2X DIL
rect. black PIN Photodiode.
PELTIER EFFECT DEVICES.
4A
T 65deg. Qmax 42W $24
6A
T 65deg. Qmax 60W $26
8A
T 65deg. Qmax 75W $28
Comes with info
to build cooler /
heater
All 40 X 40mm.
PELTIER CONTROLLER KIT
this kit is a switch mode
design and correctly
controls the temperature
of peltiers to 10A using
a very efficient design.
Inc PCB, all on-board
components . (k140) $19
(NEW) MULTI FUNCTION BATTERY
CHARGER / DISCHARGER:
New in original box with instructions. This
unit was designed to charge NI-CD & NIMH mobile phone batteries of 4.8V, 6.0V
and 7.2V. Operates from 12-24V DC input.
Features include processor control & multi
stage charge indicator. By changing the
value of one resistor it can charge higher
voltages, although a higher voltage
plugpack is required for 9.4V or higher.
Includes cigarette lighter lead, 12V / 1A DC
plugpack & instructions for modifications
for higher voltages. The unit has battery
charging terminals but the user will have to
make their own adaptor to interface to a
battery. The plugpack supplied alone is
worth around $30 retail. Weight is 0.9kg.
$29... 15V DC / 1A Plugpack for charging
batteries 9.4V or higher: (ZA0055) $6 If
you ask when ordering you will receive a
free 6-pack of batteries.
CFL INVERTER KIT
our very popular
inverter. Very
Efficient Driver kit
can drive a number
of CFL’s from 12vdc$25.
QUALITY AUSTRALIAN MADE
FEATURE PACKED MINI ALARM
SYSTEM CONTROL
Features inc. boot
release, central locking
output, imobiliser output,
indicator flash relay. With
2 key-fob transmitter keys. $99
12V AUTOMOTIVE RELAY:
Has 30A SPDT
Contacts with
73ohm relay
coil. These are
the standard
size and normally
retail for around
$7 each: (RL3) $3 each
SUBSCRIBE TO NEW KITS FROM “OATLEY’S”
We are constantly developing many electronic projects, but there is only a limited
amount of these that the electronics magazine can publish. If you wish to receive a
regular Email and be informed about these projects just send a blank Email with the
following text in the subject heading: newkits-subscribe<at>oatleyelectronics.com
Where possible our Emails will include descriptions, PCB overlays, parts lists and
pictures. We will also offer you regular kit specials and where necessary, additional
notes and or errata. In the future you will be able to access this same information at
www.newkits.com but for the moment the ONLY WAY you can do this is by
subscribing to the above Email address. As an example if you do it now you would be
Emailed the following two projects within the next few weeks.
MULTI PURPOSE INVERTER
This modified square wave inverter can
be used to convert 12-24V DC to 120V
AC or 240V, or any other voltage. Power
and voltage O/P’s depend on transformer. O/P freq. is adjustable between
50 and 60Hz and a beat indicator cct. is
included (LED) so you can easily adjust
the freq.. to be the same as the mains
freq..With one pair of MOSFETS and
no additional heat sinks 100W power O/P is possible, 200W with two pairs of
MOSFETS and no H/S’s, 400W+ with two pairs of MOSFETS and additional H/S’s,
etc…PCB plus all on-board components kit (No transformer):$18...Two additional
MOSFETS: $6...US Plugpacks with a 30VA transformer: $2.50Ea. We will include
notes on how these can be rewound for 120V O/P (1 needed) or 240V o/p (2 needed)
FINALLY IT'S HERE!!! THE RIGHT WAY TO DRIVE STEPPER MOTORS.
Now stepper motors can give high torque at high revs with our new 2 part kit driver
system K142C Constant Current Source and K142B New Stepper Motor Driver. As a
stepper motor's speed increases the current drawn and the power output slowly drop
until it reaches a certain speed (varies greatly with motor type) then suddenly drops to
almost nothing. Some drives like "Chopper drives" try to overcome this with a linear
response to a non-linear problem. Our new K142C Constant current source drive
senses the drop in current and increases the voltage to the motor and thus the current
as speed increases. This gave similar torque at around 290 RPM as at 1 or 2 RPM
(this is as high as we tested with a 200 step motor) Because of the wide voltage output
range of the constant current source we had to re-design our stepper motor driver to
cope. K142C: features easy construction, kit inc. PCB, heat-sink with fan and all
onboard components. K142B: features inc. 4 or 6 wire motor drive, Opto Isolation to
protect your computer, MOSFET placement for ease of heat-sink installation (if
required) (heat-sink not supplied). Kit inc. PCB and all onboard components inc. high
power MOSFET's. Both kits inc. full instructions, component ID and orientation
printed on PCB for easy assembly. All Circuit boards (PCBs) are solder masked for
easy soldering.
GENUINE MAGLITE
TORCHES
So new it’s hard to tell that
they are used, (during the
Olympics). The same type
as used by police, security
guards etc. Complete in
original box with booklet,
rechargeable batteries,
charger and charger clip
/wall bracket etc. $150
SOLAR PANELS: Quality SIEMENS
brand Polycrystalline cells. Open circuit
voltage 5.7V, Short circuit current 0.22A,
Peak power 1W <at> 100mW per square
cm. 4 panels req. to charge 12V batteries.
160 x 55 x 5mm. Terminated
with a 25cm
long
figure
eight cable.
$10 ea. or 4 for $36.
SONY UNIVERSAL CAMCORDER
BATTERY + CHARGER: Brand new in
original packing Less than 1yr. old. 7.2V
1500mAh lithium-ion As commonly used
with SONY digital cameras, camcorders,
SONY and some other brand products .
US made OPREX brand. Charger has an
unusual plug that is easy to adapt.
Requires power plug-pack (not supplied)
9V 1A (2A peak for 5 minutes)...$39.
SERIAL SERVO CONTROLLER KIT:
This kit is ideal for robotics kits etc, it
controls up to 5 servos via the serial port of
your computer. A lot of shareware and
support for this kit on the Internet.
Features inc. small kit size & hi servo
resolution. Kit inc. software, PCB & all
ELECTRONIC HOT WATER BOTTLE
This kit would be ideal for sports injuries, pets, home-brewing etc Features include onboard com-ponents.:$24
insulated heating wire & thermostatic control. Some suitable power supplies may be GEARED AC MOTORS
available, check when ordering. Kit inc. PCB, all onboard components & heating wire. Brand new small
mains operated
BOOK SHELF LIGHT SHOW K170
This 4 channel light controller is ideal for processional musicians or DJs. It is sound geared motors,
triggered with adjustable gain or it will change through lots of different patterns at very strong,
random by its self when its quiet. It is designed with 4 high powered MOSFETs to made for
generate minimal heat while switching high loads and easily switches 4 12/50W rotating
microwave
halogen down lights. Kit inc. PCB, all onboard components inc. 4 MOSFETS.
turntables, 240V/
Some suitable transformers may be available, check when ordering.
50Hz/3W/5RPM.,
SOOPER SNOOPER / STETHOSCOPE
$4Ea. or or 4 for $12.
This amazing parabolic microphone can listen in on all sorts of things from a distance,
like bird calls and wildlife sounds, etc. Or by attaching the microphone to a metal rod NEW 500W Tungsten Halogen Lamps
or screwdriver handle it can be used to listen to white ants chewing on your house! It is (All are new but packing may be shop
also ideal for detecting engine knocks and worn bearings etc. We even heard water soiled) Ideal replacement or spare bulbs
rushing through a radiator hose! Kit inc. PCB, all onboard components, stethoscope for yard and security lights. $2ea
pickup, electret microphone and 300mm parabolic dish.
SOLAR FURNACE /PARABOLIC REFLECTOR
This is the same 300mm dish as used in our Sooper Snooper project. It is mill finished
ie. unprotected aluminum and is reflective enough to ignite paper allmost instantly,
With the use of some automotive cutting compound / polish it could easily be made W e h a v e m o r e u s e d t e s t
equipment. we need to clear some
highly reflective:$25 ea.
VIDEO SYNC. STABILISERS
Various forms of copy protection are used
on video tapes & DVDs, that may cause
playback problems like the jitters. This
device removes the copy protection. thus
cleaning the
ONE / TWO CHANNEL UHF REMOTE picture.
It has
CONTROL On freq. of 304MHz,
been
transmitter is
suggested to us that
assembled,
receiver is a
these units could be used to copy commkit, inc. 2 12V/
ercial videos & DVDs but we do not
12A relays, 1Tx +
condone any breach of copyright. This
1Rx kit:$45, additional Tx: $15
item comes as a ready built PCB with a
I CHANNEL Kit just $25
new recycled metal case to suit. Just...$29
NEW 80mm 12V FANS
WE HAVE TOO MANY ITEMS TO
Ideal replacement for
ADVERTISE HERETHE ONLY WAY TO
computer power supply fans.
SEE IT ALL IS TO CHECK OUT OUR
12V <at> 0.15A..$4 or 4 for $12
WEB STIE oatleyelectronics.com
NEW SHIPMENT
to make way for the next lot. But
you may have already missed it.
The only way to make sure you
don’t is to subscribe to our
bargain corner and receive
advanced notice of what’s
comming...
Just send us a blank E-Mail to....
bargaincorner-subscribe
MONOCHROME CCD VIDEO CAMERA <at> o a t l e y e l e c t r o n i c s . c o m
VIDEO CAMERAS
The output of these cameras below is std
video & can be plugged into the "VIDEO
IN" socket of any Australian std VCR,
video monitor or TV, or via an RF
Modulator to an Ant. Input. The B/W
cameras are Infra Red responsive & can
be used in total darkness with IR
Illumination.
B&W Camera built on a PCB with auto iris.
(0.1 lux). Can be focused sharply down to
a few mm(useful for people
with visual impairment). Spec.:
Power req.: 10V to
12V <at> approx.
50mA.CCD: 1/3",
30grams: $89, with 92° lens:
www.oatleyelectronics.com Orders: Ph ( 02 ) 9584 3563, Fax 9584 3561, sales<at>oatleyelectronics.com, PO Box 89AOatley
NSW
2223
www.siliconchip.com.au
ugust
2001 37
major cards with ph. & fax orders, Post & Pack typically $7 Prices subject to change without notice ACN 068 740 081 ABN18068 740 081
SC_AUG_01
Fig.5: the component side of the PC board.
The eight large empty holes are for the 5W
resistors – see below.
Fig.6: the four 5W resistors mount on the underside (copper side) of the PC board and should be 3-4mm away
from the surface to avoid heat damage to the tracks or board.
the amplifier board, further filtering
is provided for the early stages via
diodes D3 & D4 and associated 100µF
capacitors.
Another bridge rectifier, BR2, is fed
from the two 45V secondaries (90V
total) via a 6.8µF capacitor to supply
a zener stabilised 24V for the fan. The
high impedance of the 6.8µF capacitor
throttles back the voltage to the bridge
rectifier so that little power is wasted
in zener diode ZD1.
Assembly details
Three PC boards need to be assembled: one for the amplifier, one
for the power supply and one for
the bridge rectifier. The amplifier PC
38 Silicon Chip
board measures 159 x 65mm while
the power supply board measures 122
x 65mm. We suggest you start on the
amplifier board first, placing all the
small components, then the insulated
terminal blocks, followed by the power
transistors and Mosfets along one edge.
Use the component overlay diagram of
Fig.5 as a guide to the assembly.
When soldering the transistors along
the edge, you will need to make sure
that each Mosfet is pushed right down
onto the board while the four other
transistors are mounted with a lead
length of about 11mm. This is not
really critical though because spring
clips are used to mount these transistor
packages to the heatsink.
The four 5W source resistors are
mounted underneath the PC board,
as shown in Fig.6. They should be
mounted so that the resistor bodies
are clear of the copper tracks by about
3 or 4mm.
The rectifier is attached to the heatsink with a long screw and nut and a
small PC board is fitted over the four
rectifier lugs and soldered to them, as
shown in the diagram of Fig.7. Before
you do that though, make sure you
mount and solder the four 0.22µF
250VW capacitors onto the rectifier
board.
The power supply board is also
quite straightforward – see Fig.8.
Mount and solder the insulated terwww.siliconchip.com.au
Fig.7: the bridge rectifier
and its associated capacitors are mounted on this
small PC board which
mates with the board at right.
minal blocks first, followed by the
small components, the 6.8µF capacitor
and finally, the four 4700µF capacitors. Make sure that all electrolytic
capacitors, diodes and zener diodes
are mounted the right way around,
as shown on the component overlay
diagrams of Figs.5 & 8. Note that the
2A fuse is a slow-blow type.
When all the boards are finished,
check your work carefully and make
Fig.8: the main power supply board holds the rest of the power supply components.
sure that you have not missed any
solder connections or have made
any solder splashes to short between
tracks.
The next step is to mount the power
transistors and Mosfets to the heatsink.
Line up the board so that its lower edge
is about 4mm above the lower edge of
the heatsink channel. Each transistor
is mounted using an appropriately
sized mica heatsink washer and this
is coated on both sides with heatsink
compound. Then it is a matter of attaching the heatsink clips to secure
each pair of transistors in place.
The heatsink is then attached to the
chassis plate with two self-tapping
screws and the forward edge of the
amplifier board is secured with metal
spacers and screws. Fit the tunnel
plate and the 24V fan to the heatsink
at the same time.
A heatsink-side view of the amplifier
showing the fan and the way the heatsink is
assembled and all mounted on the “chassis”.
www.siliconchip.com.au
August 2001 39
Fig.9: follow this inter-board wiring diagram when you’re assembling the amplifier
and you shouldn’t have any problems. The
chassis is designed to be mounted “as is”
into other equipment,
such as a PA or guitar amplifier.
40 Silicon Chip
www.siliconchip.com.au
The power supply board is also
mounted on the chassis plate with
four metal spacers and screws. Next
the power transformer can be mounted
using a large bolt, nut, flange washer
and rubber washer.
All the transformer leads, with the
exception of those to the main bridge
rectifier (BR1) are connected to the
insulated terminal block along one
end of the power supply board. All
the inter-board wiring is shown in
Fig.9.
Solder the two 45V windings to the
AC inputs on the bridge rectifier on
the heatsink, together with two white
wires which become the AC input to
the bridge rectifier (BR2) on the power
supply board. The white wires are connected to the relevant 45V terminals
on the power board.
Don’t forget to scrape the varnish
(enamel) off the ends of all the transformer wires, to make sure you make
good connections before soldering or
terminating them. Make all the input
connections to the power board but
don’t connect any of its DC outputs
to the amplifier. Do connect the 24V
output to the fan.
The green/yellow wire of the 3-core
mains flex is terminated to the solder
lug on the chassis adjacent to the
transformer. Also connected to this
lug is a 0.1µF capacitor which connects to the 0V line and centre-tap of
the transformer. This is done instead
of directly earthing the amplifier as it
can help to avoid hum and buzz due
to earth loops.
The Active and Neutral wires in the
mains cord connect to the relevant
insulated terminals on the power
supply board. Note that the slow-blow
mains fuse should be fitted with a
plastic shroud to stop any possibility
of accidental contact.
Initial power up
Before connecting the amplifier supply rails, it is a good idea to check out
the power supply operation. So carefully check out all your power supply
connections and then apply power.
The fan should immediately run and
you should be able to measure around
±70V at the DC outputs of the board.
Turn off the power and discharge
the main filter capacitors with a 5W
resistor of 100Ω or higher connected
across the DC outputs.
Now connect the DC supply rails
Winning Gold . . .
. . .After the Games
EX OLYMPIC GENUINE
MAG LITE TORCHES
Made in USA, complete with
240V battery charger kit and
in car battery charger kit.
As used by Police, Navy & RTA
from the power board to the amplifier, each via a 10Ω 5W resistor. Rotate
RV1 fully anticlockwise – this is to set
minimum quiescent current. Do not
connect any signal source or a speaker
load. Then apply power. If the fuses go
up in smoke or get red hot, turn off the
power and recheck your work. If all is
well, check the ±67V rails and the DC
voltage at the speaker output; it should
be within ±30mV of 0V.
To set the quiescent current, monitor the voltage across one of the 10Ω
5W resistors and adjust RV1 for a
reading of 0.8V. Leave the amplifier
running for a period of ten minutes
and if necessary, readjust RV1 for the
correct reading.
Then turn off the power, remove the
10Ω resistors and directly connect the
supply rails from the power supply
board to the amplifier.
The signal leads and loudspeaker
can be connected to the relevant terminals on the connector strip and then
you are ready to roll.
Where to buy the kit
This 200W amplifier is exclusive to
Altronics (Cat K-5172) and is priced at
SC
$349. Phone 1 800 999 007.
HURRY! This is your LAST CHANCE to
grab some of the equipment left over
from the Sydney 2000 Olympic Games
at never-to-be-repeated prices!
CALL NOW!
PH: (02) 9879 6782
FAX: (02) 9879 6993
180 ea
$
EX OLYMPIC SECURITY
DURACELL 9 VOLT BATTERIES
Brand new (expiry dates 2004) Boxed lots of 48
1
00
$
79 ea
$ 80
ea
EX OLYMPIC TOA MEGAPHONES
Complete with shoulder harness
and alkaline batteries
www.siliconchip.com.au
ALL
PRICE
INCLUDS
E
GST
®
DON’T MISS OUT!
®
Registered Trade Mark Australian Video Systems
August 2001 41
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