This is only a preview of the February 1988 issue of Silicon Chip. You can view 35 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Studio 200 Stereo Power Amplifier":
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By LEO SIMPSON & BOB FLYNN
200
Power Amplifier
This rugged stereo amplifier puts out a
genuine 100 watts per channel into
8-ohm loads and more than 160 watts
per channel into 40.
Every now and again we have a
brainwave at SILICON CHIP. Truly!
We were considering the design of
a stereo power amplifier which had
to have a genuine power output of
at least 100 watts per channel into
8-ohm loads. It had to be rugged,
easy to build and reasonably priced. While we discussed the pros
and cons of various design approaches, we realised we already
had the basis of a suitable design,
the 50 and 100-watt modules
described in our December issue.
As it stands the circuit of the
module described in our December
issue cannot be pushed any harder
to deliver more power. The supply
rails cannot be increased because
the 2N3055/MJ2955 output transistors would not stand it. Nor could
the transistors for the first and se-
cond differential amplifier stages.
Then one of us had a flash of insight and after a minute or two of
checking the concept on a plastic
brain (calculator), we knew we had
a simple solution: increase the
power supply rails from ± 40 volts
DC to ± 50V and swap the
2N3055/MJ2955 output transistors
for the more highly rated
MJl 5003/4 transistors.
Naturally, we had to follow up
with a lot more calculations, plotting load lines for drivers and output transistors and so on, but the
concept was confirmed. We
modified a power module to put the
concept into practice, building a big
power supply with a well regulated
output of ± 50 volts DC and changing all the transistors which needed
higher voltage ratings. Now, after
considerable cooperation from
Altronics of Perth in supplying key
components, we present the
"Studio 200" stereo power
amplifier.
The Studio 200 is a rugged stereo
power amplifier housed in a large
rack mounting case. It is eminently
suitable for work in discos, bands
and in the home. It is rugged but
that does not mean it is not hifi. It
has very good performance figures
and excellent power output.
Music power output, as
measured by the IHF method
(Institute of High Fidelity), is 120
watts per channel for 80 loads and
190 watts per channel for 40 loads.
This gives a dynamic headroom
figure of 0.8dB for both 40 and 80
loads.
Typical harmonic distortion for
powers of less than 90 watts into 80
loads is less than .01 % . Frequency
response is 20Hz to lOOkHz ± ldB.
Signal to noise ratio is better than
105dB unweighted with respect to
100 watts into 80.
The Studio 200 stereo power
amplifier features protection
The Studio 200 stereo power amplifier is built into a large rack mounting case from Altronics. It has excellent specs
and can be used for home hifi, or put to work in discos and bands.
FEBRUARY1988
37
How the Circuit Works
Thirteen transistors and three
diodes make up each power
amplifier. The input signal is coupled via a 1 JLF capacitor and 2.2k0
resistor to the base of 02 which
together with 03 makes up a differential pair. 01 is a "constant
current tail" which sets the current
through 02 and 03 and renders
the amplifier insensitive to variations in its supply rails (this is
known as supply rejection).
Signals from the collectors of 02
and 03 drive another differential
pair, 04 and 05, which have a
"current mirror" as their load. The
current mirror, 06 and D3, does
not give this second stage a particularly high gain but it does make
it very linear (ie, relatively distortion free).
The output of 05 is then used to
drive the class-AS output stage
consisting of drivers 08 and 09
and power transistors O 1 o, O 11 ,
012 and 013 .
07 is a Vbe multiplier, so called
because it multiplies the voltage
between its base and emitter by
the ratio of the resistors between
its base and collector and base
and emitter, respectively. It effectively maintains a fixed voltage between its collector and emitter,
regardless of the drive current
delivered to the output stage by
05. The voltage is adjusted by
trimpot VR1.
The function of 07 is to set the
DC voltage applied between the
bases of 08 and 09 . By doing this
it sets the "quiescent current"
through the output stage (ie, the
current when no signal is present) .
This minimises crossover distortion
The complementary output transistors are connected in parallel to
give high current output capability.
Each output transistor has its own
0 .4 70 emitter resistor. These are
included to ensure that the output
current is shared more or less
equally between the output transistors and to help stabilise the
quiescent current.
Negative feedback is applied
from the output stage back to the
base of 03 via a 22k0, resistor.
The level of feedback, and
therefore the voltage gain , is set
by the ratio of the 22k0 resistor to
1 kO . The low frequency rolloff is·
set by the ratio of the impedance
of the 1 kO resistor to the impedance of the 4 7 µF capacitor.
This sets the - 3dB point at about
3Hz .
The 1 J,tF input capacitor and the
22k0 base bias resistor feeding
02 have a more important effect
and set a - 3dB point at about
?Hz . The two time-constants
together give an overall - 3dB
point at 1 OHz.
The 330pF capacitor and the
2 .2k0 resistor feeding 02 forms a
low pass filter which rolls off frequencies above 200kHz .
The 68pF capacitor between
base and collector of 05 rolls off
the open-loop gain to ensure its in-
against short circuits and against
damaging loudspeakers if it is
seriously overdriven (which often
blows tweeters) or develops an internal fault. This protection is provided by the same PTC (positive
temperature coefficient) thermistors used in our lOOW modules
featured in the December issue of
high power amplifiers used for
disco and rock bands is an internal
fault which not only blows the
amplifier 's output transistors but
burns out the speakers as well.
Repairing the amplifier is
relatively cheap but repairs to
loudspeakers can be very expensive. By incorporating the
Polyswitch PTC thermistors into
each channel of the amplifier, you
can effectively insure against
amplifier and loudspeaker damage.
SILICON CHIP.
Apart from its generous power
output capability and high fidelity
performance, we regard the simple
and effective protection incorporated in the Studio 200 as its
most important feature. An expensive and common occurrence in
38
SILICON CHIP
The circuit
The circuit is shown in Fig.1. This
shows the power supply and one
herent stability with feedback applied . Another contributor to the
amplifier's excellent stability is the
output network consisting of a
6 .8JLH air-cored choke, a 6 .80
resistor and 0.15J,tF capacitor.
Instead of using relays for
loudspeaker protection each
power amplifier uses a Polyswitch
(made by Raychem Corporation,
USA) . This device is a positive
temperature coefficient thermistor
with a very low resistance value,
under normal operating conditions.
When the current through a
Polyswitch goes above a set
value, it immediately switches to a
high resistance state and stays in
that state until the fault condition is
removed . It's like a fuse which can
repair itself.
The resistance of the Polyswitch
is so low (typically much less than
0.10) that it has a negligible effect
on amplifier performance. The
distortion figures we quote are applicable whether or not the
Polyswitch is used .
The Polyswitches give comprehensive protection . They allow
the amplifier to deliver full power
on program signals but the moment a short circuit is applied or
the amplifier is seriously overdriven, the Polyswitch goes high in
resistance to give protection .
If a transistor fails, and causes
the amplifier to deliver a large DC
voltage to the speaker, again the
Polyswitch goes high to give
protection .
channel, to save space. It is identical in configuration to the circuit
of the lOOW module referred to
previously but most of the semiconductors have been changed and so
have some of the resistors.
For those who have not read the
abovementioned article, a brief
description of the power amplifier
circuit is given in one of the accompanying panels. Most of the
changes we 've made relate to the
higher supply voltages necessary to
extract higher power from the
amplifier. We'll discuss the main
changes briefly.
As already noted, the main
01
1N4002
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680
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48.1V
8
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INPUT
22k
cr22k
1k
2x8F469
04
2.2k
~
1.9V 2.2k
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8
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48.3V
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l
1.3V
t
- - 4 - - - ' - - + - - - - - - - - - -.....- - - 4 9 .5V
0.ll
* ROE245A (80 LOAD)
POWER
RDE390A (4!1 LOAD)
240VAC
- - - - - - - - - - - - - - - - - + 4 9.5V
8000
63VW
N
+
8000
_ 63VW
+
-
10k
1W
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0
ELJc
0.22
100V
0
8
VIEWED FROM BELOW
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PLASTIC-0
CASE
8000
63VW
-
8000
63VW
-
10k
1W
moE
0.22
100V
LED1
111
ECB
STUDIO 200 STEREO POWER AMPLIFIER
SC1·1 -288
Fig.1: this diagram shows one powe.r amplifier and the power supply for the Studio 200. The circuit is essentially the
same as the 100W module published in our December issue. Note that two of the 8000µF 63VW filter capacitors can be
regarded as optional although they do give a worthwhile reduction in hum and a small increase in continuous power
output.
change is to the power output transistors which are now MJ15003
(NPN type) and MJ15004 (PNP
type). These rugged bipolar transistors have a maximum power
dissipation rating of 250 watts (at a
case temperature of 25°C), a collector current rating of 20 amps and a
collector voltage rating of 140 volts.
So they are considerably more rugged than the MJ2955/2N3055s that
they replace.
They also have better high frequency gain which means that it is
possible to obtain better high fre-
Performance of Prototype
Output Power (RMS)
Music Power
Frequency Response
Input Sensitivity
Harmonic Distortion
Signal to Noise Ratio
Protection
Damping Factor
Stability
1 00W into 8 ohms; 160W into 4 ohms
120W into 8 ohms; 190W into 4 ohms
20Hz-1 00kHz ± 1dB
1.25V
< 0 .01 % (20Hz-20kHz)
>105dB
5A fuses plus optional Polyswitch
> 100
Unconditional
FE BRUARY1988
39
600..----.---~-----.-------.---,----r------.----.-----,
i
<
300
i:l
The input transistor pair is now 2
x BC556 instead of BC557s but
apart from a higher collector
voltage rating, these transistors
(Vceo 65V instead of 45V for
BC557) are otherwise identical. The
second transistor pair and current
mirror are now 2 x BF469 and
BF470 instead of BC639s and
BC640. The substituted transistors
are normally used in high-voltage
video output stages in TV sets and
consequently have a very high collector voltage rating of 250 volts
and excellent beta linearity which
again improves performance.
Power supply
10
20
30
60
40
70
80
90
VOLTAGE (VOLTS)
Fig.2: this is the load line diagram for the MJE340/350 driver transistors. The
straight line shows the resistive load reflected by the output transistors while
the arched line shows a severe reactive load. The concave line to the right is
the maximum power dissipation (20W) hyperbola of the transistors with SOAR
derating included.
14--------.---.. . . . .-..-----,,---,-----,------r----.-----,
f
:E
~
i
61----4------+-------4----+-------il-_,-....,__ _ _ _ _ _ _ _--t
=
..,
:::,
The Studio 200 stereo amplifier
has a very good power supply and
this is the main reason it is able to
deliver so much power. It uses a
large toroidal power transformer
with a centre tapped secondary
winding of 35 volts a side, with a
nominal rating of 300VA. This
transformer has excellent regulation and very low hum radiation. It
is also a lot more compact and
weighs less than a conventional
transformer of equivalent rating.
Teamed with the transformer is a
chassis mounting bridge rectifier
and four B000µF 63VW electrolytic
capacitors. These deliver balanced
supply rails of ± 50V DC. Really,
we are gilding the lily by putting in
four B000µF electrolytics as it only
makes a relatively small difference
to the maximum continuous and
music power capability. You can
save about $40 by just using two
B000µF 63V capacitors.
Chassis
10
30
40
60
70
90
VOLTAGE (VOLTS)
Fig.3: load line diagram for the MJ15003/4 output transistors. The straight line
is for a 40 resistive load while the arched line is for a reactive load of 2.75
± j2.750. The concave curves to the right are for the transistors' maximum
power hyperbola (2 x 250W) with SOAR derating included.
quency distortion performance
from them.
The driver transistors are unchanged but we have plotted load
lines for both the drivers and output
40
SILICON CHIP
transistors to check that they can
safely deliver the much higher
power output of this amplifier. The
load line graphs are shown in Fig.2
and Fig.3.
The chassis is a nicely finished
'three unit high' rack mounting case
from Altronics (Cat No H-0418).
This has heatsink extrusions at the
sides on which we mounted the
amplifier modules, one each side.
To save chassis space we
mounted the toroidal transformer
directly to the front panel. This is
more or less standard practice with
rack amplifiers as the front panel is
usually much thicker and stronger
than the top or base panels.
Order of assembly
If you are buying a complete kit
for this amplifier, no drilling of
metalwork will be necessary but if
mF and Music Power
The music power rating of the
Studio 200 was measured according to the method outlined in the
Institute of High Fidelity's
specification IHF-A-202. This
uses a pulsed 1kHz waveform
with a duration of 20 milliseconds . To conveniently
measure power according to this
method and those which use
longer pulse durations, we have
designed a self-contained test instrument. It will be described in a
coming issue of SILICON CHIP.
'·
.
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.
.
l's::d.f.i:t.s:Ll2:JEY.2:2:L:1t:i:J!S<at>E2:JER~D![Jl" M'f\$'. ,:,··. ''f&WtW<at>/;"»~f?&~:
-v
INPUT
- - - T03 DEVICE
.
:__ ~ .i:?-
+V
Fig.4: here's how to assemble the power amplifier modules. Be sure to use the
correct transistor at each location and take care with component polarity. For
80 loads, use the RDE245A thermistor; for 40 loads, use the RDE390A.
0
!r
-INSULATING
SLEEVES
-~O-~
~ II
HEATSINK
PCB
I
SHAKE-PROOF
·--~-WASHERS
~--
9-NUTS
Fig.5: mounting details for the TO-3
transistors. Trim the mica washers so
that they do not overlap.
The power amplifier modules are screwed to the heavy heatsink extrusions
that make up the sides of the case. Use PC stakes to terminate all external
wiring to the module. Note: the PTC thermistor has been replaced here by a
wire link but should be included in all kit versions.
you are working from scratch, any
drilling of the case, heatsink
brackets or whatever should be
done before any assembly work
gets under way.
Kits for the Studio 200 stereo
amplifier will be available shortly
after this issue goes on sale from
Altronics of Perth at $399.
The first step in putting the
amplifier together is to assemble
the power supply into the chassis.
You can fit all the other hardware
at the same time. To prevent marking your workbench or table, we
suggest that the case be fitted with
four large rubber feet (32mm in
diameter). The ones we used are
sold in hardware stores as rubber
door stops.
Mount all the hardware bits on
the rear panel and then mount the
transformer and hardware on the
FEBRUARY1988
41
All the power supply and amplifier output wiring should be run using heavy duty hookup wire. Take care when putting
the lid on the case so as not to damage the outer insulation of the transformer. Note: some of the internal wiring in this
photo differs slightly from the wiring diagram. The PTC protection thermistors are also not shown.
front panel. We used a bolt with a
blackened head to mount the
transformer so that the bolt head
would not be so obvious on the front
panel. You can obtain one from a
hardware store.
The mains cord should be
J-se•~
WASHER
-10126
~~~::;~.
0
-HEATSINK
PCB
..l_
- - SHAKE-PROOF
WASHER
(8-NUT
Fig.6: mounting details for the T0-126
transistors. Note that heatsink
compound should be lightly smeared
on the mounting surfaces.
42
SILICON CHIP
secured in the rear panel of the
chassis with a cord grip grommet. It
is also anchored along the
righthand side of the chassis (looking from the front) by two cord
clamps. Wire up the transformer
exactly as shown in the wiring
diagram.
Both the active and neutral wires
from the power cord are terminated at the insulated terminal
strip, as is the blue primary wire
from the transformer (joined to the
incoming neutral wire). Cut a short
length off the brown wire from the
transformer and strip both ends.
One end goes to the incoming active
wire while the other end goes to the
power switch. The brown primary
wire from the transformer also goes
to the power switch.
Before soldering these wires to
the switch, slip a length of
shrinkable sleeving over the pair.
After soldering, push the tubing up
over the switch and then apply heat
from a hairdryer to shink it on.
Wire up the bridge rectifier and
filter capacitors using heavy duty
hookup wire. Do not omit the lOkO
1W resistors wired across each
supply rail. These are there to safely discharge the capacitors in the
event the fuses to the amplifiers
blow.
Check all your wiring carefully at
this stage and then apply power.
The supply rails should be close to
± 50V DC. Switch off and allow all
the capacitors to discharge.
Now you can assemble both the
amplifier modules. We suggest you
assemble one module and test it
before doing the other. That way, if
you make a mistake on the first, you
won't do it on the second. The procedure for each board is as follows.
First mount all the small components leaving the power transistors and heatsink till last. The
component overlay for the modules
is shown in Fig.4.
Note that the 68pF compensation
capacitor associated with Q5
should have a voltage rating of at
least 100 volts and so should the
0.15µF capacitor in the output filter
network.
The 6.BµH choke is wound with
24.5 turns of 0.8mm enamelled cop-
HEATSINK
HEATSINK BRACKET
LEFT AMPUAER
-V
r-l-.1
.
I
I
Q 22
0.22 ''
'
POWER TRANSFORMER
(MOUNTED ON FRONT PANEL)
1
I/I
-V
+v
RIGHT AMPLIFIER
v. /1
....
LED1~
2
. <at>t - .- -.- - ,. - ~
\
HEATSINK BRACKET
'
POWER CORD
HEATSINK
Fig.4: this diagram shows the wiring details of the amplifier and power supply. Note that for minimum hum output from
both channels of the amplifier, the transformer must be rotated so that its leads exit as close as possible to the base of
the case. Note the single point earth wiring which is essential for low hum output.
per wire on a 13mm diameter
plastic former . Alternatively,
Jaycar Electronics supply the choke
ready wound (Cat No EE-4030).
Mount the four 0.470 5W wirewound resistors so that they are off
ihe board by about 1mm or so. This
aids power dissipation.
Now mount the heatsink bracket.
It is secured to the board by the
mounting screws for the four output
transistors and the driver transistors. Mount the power transistors first.
These must all be isolated from
the heatsink by using mica washers
and insulating bushes, as depicted
in Fig.5. Smear all mounting surfaces with heatsink compound
before assembly. Solder the mounting nuts to the PCB pattern after
FEBRUA RY1988
43
PARTS LIST
1 rack-mounting case with side
heatsink extrusions (Altronics
H-0418)
1 70V centre-tapped 300VA
toroidal transformer (Altronics
M-3092)
1 push on/push off mains
switch with black button
(Altronics S-1 090)
1 red LED and bezel
1 3-core mains cord and
moulded 3 -pin plug
1 cord-grip grommet
2 plastic cord clamps
1 3 -way mains insulated
terminal block
4 binding post terminals, 2 red,
2 black
1 2-way RCA panel socket
3 plastic cable ties
2 solder lugs
4 large rubber feet (as sold by
hardware stores as door
stops)
1 50mm of twin shielded cable
500mm red heavy duty hookup
wire
500mm black heavy duty
hookup wire
2 8000µF 63VW chassis
mount capacitors (optional)
1 BR104 400V 10A bridge
rectifier or equivalent
2 0 .33µF metallised polyester
capacitors
2 1OkO 1W resistors
1 5 .6k0 1W resistor
4
2
2
2
4
Amplifier modules
Capacitors
2 4 7 µF 16VW PC electrolytic
2 1µF metallised polyester
(greencap or miniature)
2 0 .15µF metallised polyester
10 0.1 µF metallised polyester
2 0.012µF metallised polyester
2 330pF ceramic or miniature
metallised polyester
2 68pF 1OOVW ceramic
Power supply
Semiconductors
2 B000µF 63VW chassis
mount capacitors
6 BC556 PNP transistors
2 BF4 70 PNP transistors
assembly to ensure reliable contact. Alternatively, if the nuts are
nickel plated or stainless steel, use
lockwashers.
Depending on whether you intend
to use the amplifier with 40 or 80
loudspeakers, you have a choice of
PTC thermistors for protection. ,For
80 loads, use RDE245As. For 40
loads, use the higher current
RDE390As.
The two driver transistors and
the Vbe multiplier (Q7) are bent
over and also attached to the heatsink bracket using T0-126 mounting
kits (see Fig.6).
When the whole assembly is completed, the heatsink bracket should
be attached to one of the heatsink
extrusions in the chassis. Heatsink
compound should be used between
the bracket and the heatsink to improve heat transfer.
Powering up
Before applying power remove
44
SILICON CHIP
2 printed circuit boards, code
SC11-1287, 121 x 133mm
2 heatsink brackets
8 3AG fuse clips
4 5A 3 AG fuses
12 PC pins
2 plastic formers, 13mm dia x
10mm, plus 1-metre 0 .8mm
enamelled copper wire
(ECW) ; or 2 x 6.8µH aircored chokes
2 Raychem RDE245A
polyswitch PTC thermistors
for 80 loads or RDE390A for
40 loads
8 T0-3 transistor mounting kits
6 T0-126 transistor mounting
kits
the two fuses from the board clips
and set VRl fully anticlockwise.
This gives the setting for minimum
quiescent current through the output transistors. Solder a 5600 5W
wirewound resistor across each
fuseholder . Set your multimeter to
the ZOOVDC range (or no lower than
50V DC if an analog meter).
Now apply power and measure
BF469 NPN transistors
8D139 NPN transistors
MJE340 NPN transistors
MJE350 PNP transistors
MJ 15003 NPN power
transistors
4 MJ 1 5004 PNP power
transistors
6 1 N4002 diodes
Resistors (0.25W, 5%)
2 x 56k0, 4 x 22k0, 2 x 18k0, 2
x 8.2k0 1W, 6 x 2.2k0, 2 X 1 kO,
2 x 4700, 2 x 2700, 6 x 1 ooo,
4 X 680, 2 X 6 .80 1 W, 8 X 0.470
5W wirewound, 2 x 5000
trimpots (Bourns cermet
horizontal mount, 0 .2 x 0 .4-inch)
Miscellaneous
Screws, nuts, washers, solder,
heatshrink tubing for mains
switch insulation .
the positive and negative supply
rails. Again, they should be within a
few volts of ± 50V. Now measure
the other volta ges on the circuit.
They should all be within ± 10% of
the nominal values. The voltage at
the output should be within ± 30mV
of OV. No load should be connected
at this stage, by the way.
continued on page 95
The rear panel carries a 2-way RCA panel socket for the inputs and two sets
of binding post terminals for loudspeaker connections.
deliver about three times the maximum current capability of the 7812
regulator. The power transistor
should be mounted on a heatsink.
+15
>'-+-----OUTPUT TO
MODULE 1
0.1
Bridging 100W
amplifier modules
I am interested in the 100W
amplifier module described in the
second issue of SILICON CHIP. Over
the next six months I propose to
build at least six of these modules,
incorporated into three stereo slave
amplifiers, for public address
applications.
There are a couple of issues I
would like to raise. Assuming PA
applications, is it possible to easily
switch such a stereo amplifier into
a bridged mono mode?
Second, is it proposed to develop
a project along similar lines to this
100W project but with an output of
the order of 300 to 500 watts into
40, so that it too can be incorporated into a stereo slave
amplifier, with a bridging operation
as well?
Lastly, how about a versatile,
high quality 8 - (or 12-) 4-2-1 mixer
and a simpler 6-2 mixer, to complement the power amplifiers described above? (R.W., Scoresby, Vic).
• Yes, it is quite easy to use the
100W power modules in bridge
mode but there are a number of
points to consider before you rush
off to buy the parts. First, if you
wish to use the 100W modules in
bridge mode, you will only be able
to do so if you use the bridged
amplifier to drive an 80 load. You
could not use it with a 40 load.
The reason is that each amplifier
in a bridge configuration "sees"
half the real load impedance.
Therefore, if you were to use a 40
load, each amplifier module would
"see" a 20 load and would be
overloaded. If you were using the
PTC thermistors we specified for
protection, they would operate
prematurely and the effective
power output would be low. Bridged power output into an 80 load
would be close to 200 watts, depending on the regulation of the power
supply.
This general rule about bridging
applies to all amplifiers, by the
way, so there is no way around it. If
INPUT
C>-:-11--e--""
220k
10k
1'/o
-15V
10k
1%
...
> - - - - OUTPUT TO
MODULE 2
Fig. 2: this simple circuit allows two power amplifier modules to be
driven in bridge mode. Be sure to use 8-ohm loads.
we had been designing the amp
modules to drive 40 loads in bridge
mode we would have had to make
sure that each module could safely
handle a 20 load. That would have
made them much more expensive.
If you want more power, and you
want to drive a 40 load, you would
be better off considering the Studio
200 stereo power amplifier
presented in this issue. It is essentially the same design but upgraded
with a bigger power supply and
higher-rated output transistors. By
using one of these upgraded
modules you get a lot more bang for
your buck. With one module driving
40 you could get close to 200 watts,
if you had a very well regulated
power supply. That would mean using a transformer rated at around
400V A or more.
Again, you could use the upgraded modules in bridge mode and probably get somewhere around 350
watts into an 80 load, depending on
how big and how well regulated
your power supply was. Again, you
would need a big transformer,
rated at 600VA or more.
A suitable circuit to enable the
modules to be driven in bridge mode
is shown in Fig.2. This uses a dual
op amp, type TL072.
There is one more point to consider. You mention using the
amplifiers for public address applications but you have not
specified if you will be using them
to drive line output transformers. If
that is what you intend, you are
suggesting a whole new ball game
and one for which these modules
were not designed.
We'd be wary about using them
for driving line transformers
without at least incorporating
flyback diodes across each of the
paralleled output transistors and
without a means of adjusting the DC
output offset voltage close to zero.
The offset voltage needs to be very
low othenvise substantial DC will
flow in the transformer. This would
cause problems in the transformer
and could cause excessive dissipation in one half of the power
amplifier.
We have not done any work along
these lines and so we hesitate to
recommend them in this more stringent application - they may
work OK but we couldn't guarantee
it.
We'll have a look at your suggestion for a mixer project but it is likely to take some time to develop. In
the meantime, have a look at the
Jaycar 8002 8-channel mixer. It
would take a lot of careful design to
better its excellent performance. ~
Stereo Amplifier
continued from page 44
Now switch your multimeter
back to the 200V DC range and connect it across one of the 5600
resistors. Adjust VRl for a reading
of 28 volts. This gives a total quiescent current of 50 milliamps.
After five minutes or so, check
the quiescent current and readjust
VR 1 if necessary to get the correct
voltage across the 5600 resistor.
Now switch off, remove the two
5600 resistors and insert the fuses.
If all is well, you can now assemble the second module and go
through the same procedure.
ic
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