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Items relevant to "The SC480 50W RMS Amplifier Module":
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E
R
U
FEATJECT
PRO
Build the
–– aa new,
new, high
high perform
perform
By LEO SIMPSON
& PETER SMITH
The transmogrification of an amplifier.
Many readers will recognise the
venerable (26-year-old!) ETI-480
top left. But the SC480 (bottom right)
is as modern as tomorrow –
with performance to match.
Performance
into 4Ω
50 watts into 8Ω; 70 watts
Output Power
tts into 4Ω
77 watts into 8Ω; 105 wa
Music Power
1W – see Fig.1)
B at 14Hz and 70kHz (at
Frequency response -1d
0.875V for 50W into 8Ω
Input sensitivity
typically <.003%
5% from 20Hz to 20kHz;
d,
Harmonic distortion <.0
22kHz); -119dB A-weighte
4dB unweighted (22Hz to
-11
io
Rat
ise
-No
-to
nal
Sig
into 8Ω
both with respect to 50W
t PTC thermistor
h respect to 8Ω and withou
wit
z,
1kH
&
Hz
100
at
>140dB
Damping factor
thermistor
es plus “Polyswitch” PTC
Protection fus
26 S
ilicon Chip onditional
Stability unc
www.siliconchip.com.au
SC
ETI-480
mance
mance amplifier
amplifier module
module
Have you built one – or more – of the popular ETI-480 power
amplifier modules over the years? Here is the module to
replace that old design. The SC480 produces a great deal
less distortion, is much quieter and has inbuilt protection.
It also sounds much better.
B
ack in the October 2002 issue we noted that we
intended to produce a replacement module for the
very popular ETI-480 amplifier module which was
published back in December 1976 – 26 years ago.
In the same note we stated, somewhat controversially,
that the ETI-480 was a dog of an amplifier and that it was
not a good performer, even by 1976 standards.
Having made that outrageous statement (to some readers,
at least), we had to come up with the goods. Fortunately,
we were pretty confident that we could, and we are pleased
to state that this new module is even better than we had
hoped. It uses the same power output transistors as in the
ETI-480 and just one more low-cost transistor has been
added to the overall component count. Kit cost should be
about the same as for the ETI-480.
When this project was first mooted, we decided to base
it on TIP3055 and TIP2955 plastic
power transistors. These are
60V 15A 90W
transistors in the
TO-218 (SOT-93)
encapsulation.
We i n t e n d e d
to produce a new
version of a 100W
module which was
published in the December 1987 issue of
SILICON CHIP. That design was based on a Hitachi
amplifier circuit and used
2N3055/MJ2955 power
www.siliconchip.com.au
transistors in TO-3 metal encapsulation.
Accordingly, we produced a PC board pattern for the new
module and while we waited for it to be produced by RCS
Radio Pty Ltd (thanks Bob), we realised that a substantial
number of readers who had built countless ETI-480 modules
would probably like to “graduate” to our new design but
would wish to at least reuse the TO-3 power transistors
from their ETI-480s on the new board.
Hence, the idea of a TO-3 version of the new module
also came to pass, as is featured here.
The plastic version of the module has the power transistors lined up along the back edge, making it easy to mount
them to the vertical surface of a finned heatsink.
The TO-3 version is larger and has the four power transistors mounted on the horizontal shelf of a cast heatsink
or on a rightangle bracket which can then be mounted on
a vertical heatsink.
Why publish both modules? The simple answer is that
we had produced them both, so why not? However, each
module has its own advantages.
Version 2, using the TO-3 transistors, is rugged but takes
up more space and is likely to be less convenient to mount
in a typical chassis. There is also more work in assembling
Version 2 with the TO-3 transistors.
Version 1, with the plastic power transistors is quite a
bit more compact and less trouble to mount in a typical
chassis but the module assembled onto a heatsink is not
quite as rugged to handle.
By the way, if you decide to build Version 1, don’t be
tempted to substitute the (usually) cheaper MJ“E”... versions of the transistors. These TO-220 transistors are rated
lower (only 75W) and will inevitably cause you great pain
and suffering.
January 2003 27
Which version to build?
Our preference is for Version 1 but we have a sneaking
suspicion that Version 2, with the TO-3 power transistors,
will be the more popular module (especially amongst
those looking for a somewhat look-alike ETI-480 substitute).
Depending on the particular brand of power and driver
transistors used, both modules will give virtually identical
performance.
Regardless of which version you decide to build, the
performance will be vastly better than the old ETI-480
design.
And that is as it should be. After all, we should have
learnt quite a bit about amplifier design in 26 years or so,
shouldn’t we?
Performance
Power output is 50 watts RMS into a 8Ω load and 70
watts into 4Ω load, before the onset of clipping. Music
power is around 77 watts into 8Ω and 105 watts into 4Ω.
Hang on a minute! Wasn’t the ETI-480 claimed to be 100W
into 4Ω? Well, it was but the distortion graph published by
ETI back in December 1976 shows the amplifier heading well
into clipping at around 70W RMS. This is to be expected
since both the ETI-480 and the new SC480 use the same
voltage rails and the same output transistors.
A particular feature of the SC480 is low distortion.
Distortion for all power conditions, up to clipping, into
an 8-ohm load, is less than .05% for the full range of frequencies from 20Hz to 20kHz.
Similarly, with a 4-ohm load, total harmonic distortion
is less than .07% for the full audio frequency range.
In reality, this is a very conservative rating as the distortion will typically be .003% or less for both load conditions.
And for very lower power levels, less than 100mW, where
noise becomes a significant part of the measurement, the
distortion is really low, down to as low as .0005%. This is
two orders of magnitude better than the ETI-480!
Signal to noise ratio is better than -114dB (unweighted,
22Hz to 22kHz) with respect to full power into an 8Ω load.
Frequency response is just 1dB down at 14Hz and 70kHz
(see Fig.1).
Fig.1: this is the frequency response of both versions of the
new amplifier, taken at a power level of 1W into an 8Ω load.
Transistor quality
As in most things, you get what you pay for and it is no
different with these modules.
The plastic version (Version 1) of the amplifier was built
with the output and driver transistors in what we would
call the premium brands: Philips, Motorola (On Semi) and
ST Micro.
Version 2 was built with second rank power and driver
transistors (Mospec). We did this to compare performance
and we are pleased to report that although the premium
branded transistors do give slightly better performance,
there is a not a lot in it.
Refer to the distortion graphs of Figs.2-9 to make the
comparisons.
Either way, the performance of these modules is very
good, especially considering that we are not using expensive transistors such as Motorola MJL21193/4 or the even
more expensive MJL1302A & MJL3281A.
In fact, in some respects the measured performance challenges that of our popular and more powerful Plastic Power
module published in the April 1996 issue. Interestingly, a
These two oscilloscope screen grabs show just how clean this new amplifier is. The first screen (left) shows a 1kHz output
waveform at a level of 40W into 8Ω at top. The lower trace is the distortion waveform which has been “averaged” by the
scope to remove noise. Note that it is mostly second harmonic distortion. The same process has been applied to the screen
shot at right except that it is a 10kHz signal. Again, the distortion is mainly second harmonic.
28 Silicon Chip
www.siliconchip.com.au
key part of that performance standard comes about because
of improved PC board and wiring layout.
We’ll discuss these vitally important aspects in more
detail later in this article.
Oh, and we should state that the SC480 Version 1 and
Version 2 modules are a drop-in replacement for the ETI480 modules but will sound a great deal better.
While nominally of the same rating, they will deliver
more power, they’re quieter and as already detailed, much
lower in distortion.
By the way, these modules are not suitable for driving 2Ω
loudspeakers as used in car sound systems. We do not have
space to publish the load/line curves in this article but suffice
to say that attempting to drive 2Ω loads will blow the fuses
and may blow the output transistors as well.
Protection
The trouble with all high-power amplifiers is that, if a
transistor fails, there is a big chance that the loudspeaker
system could be damaged, despite having fuses in the
power supply.
The problem is that the fault condition may place a large
DC voltage across the speaker’s voice coil and the resulting
current may not blow the fuses. The speaker’s voice coil
then gets red hot and may actually set the speaker cone on
fire! Once that happens and if you’re not there to kill the
power to the amplifier, you can have a raging fire in your
home and enormous amounts of smoke being generated by
the burning of the filling material in the cabinet.
Our normal approach to this problem is to incorporate
relay protection which will disconnect the loudspeaker in the
event of a large DC fault condition occurring in the amplifier.
Relay protection works as far as the speaker is concerned
but it doesn’t protect the amplifier itself if the loudspeaker
leads are shorted. Here again the fuses may not blow before
the output transistors are damaged.
Neither do fuses protect the speakers if you seriously
over-drive the amplifier. This is a particular risk for tweeters
but even woofers can have voice coil damage by serious
over-drive.
Complete protection
The method we have used to provide protection to both
the loudspeaker and amplifier is to connect a high current
positive temperature coefficient (PTC) thermistor (known
commercially as a “Polyswitch”) in series with the output
circuit. This is the same method of protection as we used
in the original module published in December 1987.
The PTC thermistor normally has a very low resistance
but when the current through it rises to high value, it immediately switches to a high resistance state and stays in
that condition until the fault is fixed or power is removed.
The resistance of the PTC thermistor is so low (typically
0.1Ω or less), it has a negligible effect on amplifier performance, apart from the fact that it does cause a reduction
in damping factor.
In practice, it works extremely well. It allows you to
drive the amplifier to full power on program signals but
the moment a short circuit is applied or the amplifier is
seriously over-driven, the PTC thermistor goes high in
resistance to cut off the fault current.
After the protection thermistor has switched to its high
state, it takes some time to revert to its low resistance condition, after the fault current has ceased. This depends on
how much current is passing through it. If the drive level
is maintained after a fault has occurred, the protection
thermistor will stay high in resistance.
Circuit description
Now let’s have a look at the circuit of Fig.10. 13 transistors
and three diodes make up the semiconductor complement.
The input signal is coupled via a 1µF bipolar electrolytic
capacitor and 2.2kΩ resistor to the base of Q2. Q2 & Q3
make up a differential pair. Q1 is a constant current source
which sets the current through Q2 & Q3 and renders the
amplifier largely insensitive to variations in its supply rails
Just to confuse you, Version 2 of the SC480 amplifier (with TO-3 transistors) is on the left, while Version 1 (with TO-218
transistors) is on the right. There is only a small difference in performance between the two versions.
www.siliconchip.com.au
January 2003 29
Fig.2: THD versus power at 1kHz into an 8Ω load for
Version 1 (TO-218).
Fig.3: THD versus power at 1kHz into a 4Ω load for
Version 1 (TO-218).
(power supply rejection).
Signals from the collectors of Q2 & Q3 drive another
differential pair, Q4 & Q5 which have a “current mirror”
as their collector loads. The current mirror, comprising D3
and Q6, ensure that this second differential stage has high
linearity (ie, low distortion).
The output of Q5 is then used to drive class-AB output
stage consisting of drivers Q8 & Q9 and power transistors
Q10, Q11, Q12 & Q13.
Q7 is a Vbe multiplier, so-called because it multiplies
the voltage between its base and emitter to provide a fixed
voltage between its collector and emitter, regardless of
the drive current delivered to the output stage by Q5. The
voltage is adjusted by trimpot VR1.
The function of Q7 is to set the DC voltage applied
between the bases of Q8 & Q9. By doing this it sets the
“quiescent current” in the output stage (ie, the current
when no signal is present). This is to minimise crossover
distortion. In fact, our tests did not reveal any signs of
crossover distortion.
The complementary output transistors are connected in
parallel to give high output current capability. Each transistor has its own 0.22Ω emitter resistor. These are included
to ensure that the output current is shared reasonably well
between the output transistors.
Negative feedback is applied from the output stage back
to the base of Q3 via a 22kΩ resistor. The level of feedback, and therefore the voltage gain, is set by the ratio of
the 22kΩ resistor to the 1kΩ at the base of Q2. The 47µF
bipolar capacitor in series with the 1kΩ sets the DC gain
to unity and sets the -3dB point of the frequency response
to about 3Hz. The other determinant of the amplifier’s low
frequency response is the 1µF input capacitor and the
22kΩ base bias resistor feeding Q1 and these set a -3dB
point at about 7Hz.
The 330pF capacitor together with the 2.2kΩ resistor
feeding Q2 form a low pass filter to roll off frequencies
above 200kHz.
The 68pF capacitor between the base and collector of
Q5 and the 10pF capacitor between base and collector of
Q2 roll off the open-loop gain of the amplifier to ensure
stability with feedback applied. Note that the 68pF capacitor can be a ceramic or polystyrene type and must a have
a voltage rating of 100V or more. Other capacitor types are
not recommended.
Another important factor in the amplifier’s excellent
Fig.4: THD versus power at 1kHz into an 8Ω load for
Version 2 (TO-3).
Fig.5: THD versus power at 1kHz into a 4Ω load for
Version 2 (TO-3).
30 Silicon Chip
www.siliconchip.com.au
Fig.6: THD versus frequency at 40W into an 8Ω load
(Version 1).
Fig.7: THD versus frequency at 60W into an 4Ω load
(Version 1).
stability is the output RLC network consisting of the 6.8µH
choke, a 6.8Ω resistor and a 150nF capacitor. Not only does
this network ensure stability but the capacitor is an effective
killer of any RF and mains-interference signals which can
be picked up by long loudspeaker leads.
As noted earlier, the design of the PC board is a very
critical part of the overall circuit. The placement of the
components and the way that heavy currents flow in the
tracks is all arranged to minimise the radiation of harmonics
into the input stage involving Q1 & Q2.
This board is yet a further refinement of the topology
we first introduced in the Ultra-LD amplifier featured in
March, May & August 2000 and then again in November
& December 2001. The PC board for version 2 and the
component placement is shown in Fig.12.
It incorporates “star earthing” whereby all earth currents
come back to a central point on the board, thereby avoiding
any flow of output, supply and bypass currents flowing in
the signal earths.
Furthermore, placement of the copper tracks to the output
stages is arranged, as far as possible, to cancel the magnetic
fields produced by the asymmetric currents drawn by each
half of the output stage.
By way of explanation, when the positive half of the
output stage (Q10 & Q12) conducts, the DC current drawn
is effectively a positive half wave (ie, rectification takes
place) of the signal waveform. And when the negative
half conducts (Q11 & Q13), the DC current is the negative
half wave.
A major cause of harmonic distortion in class-B amplifiers is the magnetic fields produced by these asymmetric
Fig.8: THD versus frequency at 40W into an 8Ω load
(Version 2).
Fig.9: THD versus frequency at 60W into an 4Ω load
(Version 2)
Power supply
The power supply circuit is shown in Fig.11. This uses
a centre-tapped 56V transformer driving a bridge rectifier
comprising four 1N5404 diodes and two 4700µF 50V filter capacitors. This produces unregulated supply rails of
about ±40V.
Depending on the mains AC voltage, the rails will drop
to around ±32V or less when the amplifier module is delivering full power into a 4Ω load.
We have also provided a ±15V DC supply for a preamplifier. This is derived with 2.2kΩ resistors and two 15V
1W zener diodes.
PC board topology
www.siliconchip.com.au
January 2003 31
Fig.10: this direct-coupled amplifier module uses a differential input stage (Q2,Q3) with a constant current tail (Q1). This
drives another differential amplifier (Q4,Q5) with current mirror load (D3,Q6). Quiescent current in the output stage is set
by VR1 and Q7. The output stage is a complementary class-AB configuration using Q8 & Q9 as drivers and Q10 to Q13 as
the output devices. Voltage readings are taken with no signal applied.
currents inducing unwanted signals into the input stages,
in this case involving Q1 & Q2.
So we have tried to cancel these fields as much as possible
(in a single sided PC board).
For example, notice how the positive fuseholder (F1) is
placed close and parallel to the emitter resistors for Q10 &
Q12. So what happens is that the magnetic field produced by
the asymmetric current in fuse F1 is more or less cancelled
as the same current flows back in the emitter resistors. This
is the main reason why the layouts for these two modules
is much tighter than our designs of recent years.
You will see the same method employed in the Version
1 of the board, with the heavy collector and emitter tracks
32 Silicon Chip
placed close together but we think this has been more
fortuitous on Version 2 than on Version 1.
It is then most important to arrange the DC supply cables
to the amplifier to further this cancellation process. We’ll
detail this in the construction description.
To make the input stage less vulnerable to spurious magnetic fields from the output stage, we have concentrated it
into as small an area of the PC board as possible.
Another trick is the location of the takeoff point for the
22kΩ resistor and its orientation at rightangles to the output
stage emitter resistors.
Finally, the signal earth for the input stage is separated
from the main amplifier earth by a 10Ω resistor.
www.siliconchip.com.au
Fig.11: the power supply is very simple but adequate. The ±15V
preamplifier supply is optional.
This is not so important when a single module is in use
but it is most important when two modules are used in a
stereo system.
In that situation, the joining of the two signal earths back
via the input cables to a single program source such as a
CD player will cause an earth loop and a resulting major
degradation in the separation between channels and lesser
degradation in the distortion performance.
Well, that’s probably enough discussion of the PC board
but suffice to say that the overall design has been carefully
arranged to minimise distortion and leave as little to chance
in the wiring layout so that constructors are certain to get
excellent results.
Next month, we’ll give the full details of assembly, wiring and setup of both versions, the parts list and the PC
SC
board patterns.
Fig.12: version
2 of the SC480
amplifier with
the TO-3 (steel)
transistors.
We’ll be
presenting this
again next month
as part of the
constructional
details but it is
reproduced here
to demonstrate the
attention we have
paid to the PC
board design
to achieve the
exceptional
performance
figures depicted
on earlier pages.
www.siliconchip.com.au
January 2003 33
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