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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. Articles in this series:
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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
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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
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61----4------+-------4----+-------il-_,-....,__ _ _ _ _ _ _ _--t
=
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:::,
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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