The Class-A Amplifier Site
This page was last updated on 9 August 2001
Construction of a 20W
class A amplifier
2. Construction
Jean Hiraga
(l’Audiophile No. 11)
The first
part of the article on this amplifier is presented in the preceding issue. At
the present time it is difficult to find diagrams of high quality class A
amplifiers, moreover, the circuit described offers many benefits, in particular
a great simplicity of construction. So that the reader may be reassured, we
will describe in this article the various aspects of construction by attacking
the passive components to use, the heatsinks, the power supply and the chassis.
A
little history...
Before
returning to the point of the subject, we would specify to you that the circuit
described (fig. 8 Issue 10) is not actually a completely original circuit. It
is an improvement of a circuit going back nearly two years, and marketed as a
kit under licence in Japan. This amplifier met with a very great success for
its cost price, its ease of assembly and its reduced dimensions. For French
readers, we have introduced some improvement to the original diagram. This is
characterized by a symmetrical power supply with a slightly higher voltage of
+/-25 V and use of different complementary transistors. In fact, the value of
the voltage is not very critical since it can vary, for this diagram, between
19 and 26V, without modification. As regards the transistors, the original diagram
(fig. 1) employs a complementary pair 2SA539/2SC815 which are close to those used in our
design, the SA872A/2SC177A. The latter, more recent devices, have however
better performance in the parameters of noise, Cob and Ft. In addition, the higher supply
voltage offers a slight increase in power, to between 20 and 21 W without
distortion, whereas the improved circuit is limited to 18 W under these
conditions, 20 W at the maximum in extreme cases of the clipping, because of
the reduction in the power supply to + and - 18 V This reduction gives
advantages with many other criteria, it is for this reason that we retained it.
Fig. 1: Unmodified version of the circuit.
Upgrading capability of the
circuit
The
improved diagram can be modified so that the class of operation moves from mode
A to mode AB. The modification is simple and relates only to 12 kohm resistors,
which are replaced by higher values. The power available is thus largely
doubled, the supply voltage having however to be increased.
However,
in the case of using speakers with a relatively good output (as realised by Y.
Neveu and J Mahul) the amplifier as described is sufficient in many systems
intended for reproduction in an apartment, the noise level it makes can be very
high.
However,
the diagram can evolve with regard to the power. The basic circuit is not
called into question, the changes relate to the power supply, the heatsinks and
the transistors. The technology of the vertical field effect semiconductors,
power V FET, makes it possible to obtain nearly 60 W in pure class A. Figure 2
shows the modifications to be made; it will be noticed that the basic module,
the printed circuit, remains unchanged.
Fig. 2: Modified version, class A, 60 Watts, using V-Fet transistors at
the output .
Obtaining higher power passes
naturally to the choice of other power transistors, such as V FET, R.E.T. (Ring
Emitter Transistor), MOS FET, VMOS FET, and bipolar. It is necessary to
note that the paralleling of output stages is not possible if the preceding
stage does not have itself a parallel structure. Indeed, in the particular
layout of the output stage, reversed Darlington, the driver transistor and the
output transistor do not constitute, from a functional point of view, that of a
single transistor. However, an output stage consisting of paralleled power
transistors limits the performances as regards band-width and distortion,
compared to a stage consisting only of a suitably selected single transistor
with a higher Pc. In all the cases, the increase in the output power is accompanied by
an increase in capacitance Cob which is already
sufficiently significant to limit the performance.
This can
appear to be in contradiction with many " commercial " circuits. It
should well be seen that at the mass production level, economic considerations
are necessarily taken into account. In the catalogues of the American
manufacturers in particular, there are transistors of very high power, with a
Pc of more than 350 W, available in complementary pairs. In spite of this, in
the construction of high power amplifiers, one prefers to employ the parallel
layouts of transistors at a much lower cost price. The manufacturers can say
what they want, but the perfect complementary pairs do not exist yet, in power
transistors particularly. The use of a parallel arrangement inevitably results
in placing the power transistors on various points of the heatsink. Also, the
slightest variations in temperature immediately disperse the pairs, and
inevitably disturb the operation of the amplifier. This is why we wait before
publishing the description of an amplifier with high power and high quality.
The progress of solid state physics and of semiconductors in particular, is so
fast that we are persuaded that within one year, one will be able to find in
the power MOS FET and VMOS FET series,
transistors such that it will be possible to design a 100 W class A amplifier
using only four transistors in all. Because let us not forget that simplicity
is a decisive criterion. At which time the prepreamp/preamp/amp unit uses only
five or six stages in total?
Printed circuit
Figure 3 shows the printed circuit
of the unmodified version. In figure 4, one finds the printed circuit of the
improved version and the location of the components. The circuit is made out of
epoxy glass whose conducting tracks have a 70 microns thickness. These are
silver plated. The assembly after soldering is not covered with varnish.
The width of the printed tracks is
often the object of discussion. It should well be seen that if the linear
electrical resistance of the tracks were null, the best solution would be to
use tracks of very low width, rather than of the broader traces bringing higher
stray capacitances. The same observations are essential for the earth circuits.
As for the circuits of
preamplifiers, whether they are with valves or transistors, the construction of
an amplifier must observe the following conditions:
- Short connections
- Input far away from the output
- Distribution of the currents for
the various sections of the circuit starting from a given point, the power
supply for example. This distribution will be done preferably by separate
tracks when this is possible, if not it will be necessary to recourse to tracks
thickness from 70 to 120 microns or, to solder onto the circuit a copper wire
of 1 mm^2 diameter
- Use of epoxy glass (the
difference in price for circuits of small surface is far from important)
- Length of the equivalent tracks
in the case of a symmetrical circuit.
These various aspects of detail can
contribute largely to the subjective qualities. It should not be forgotten
either that a printed circuit is not an improvement compared to an assembly in
the air, it is simply an enormous practical advantage. However, degradations
are increasingly more significant in the case of a printed circuit. Thus for
some famous pieces of equipment, noticeable improvements were observed by the
simple act of doubling up certain printed tracks with copper wire. Naturally,
these considerations are only of a subjective nature, because the means of
investigation and analysis we have at the present time are not sufficiently
"refined" to explain such influences.
Fig. 3: Unmodified version, location of the components.
Fig. 4 Printed circuit, modified version,
rear view, solder side, and equivalent view component side (printed circuit seen in transparency).
Scale 1.
Following the same idea, the
loudspeaker output connectors used (figure 5) offer excellent characteristics.
They are made up, for the conducting part, from pure copper formed under
vacuum, called "oxygen free", with direct plating. They were designed
to be used with large section cables, up to 3,2 mm^2.
Fig. 5: Example of very high quality
loudspeaker terminals (Japanese
craftsman made), capable of receiving 3.2 mm^2 diameter cables.
Chassis
The design of the chassis for a
class A amplifier inevitably passes to the installation of the heatsinks. We
have indicated in the first part a possible solution. If this is satisfactory
from the thermal point of view, it is not so much so from the aesthetic point
of view. On the basis of a 19” rack, one can place two or four heatsinks, mono
or stereo version, in line on the back face. Below these heatsinks could be
located the inputs and the outputs, with the small disadvantage of
accessibility. There is, of course, the solution of placing the heatsinks
inside the rack. But this is relatively critical, taking into account the great
inherent thermal dissipation with class A operation. If, despite everything,
this solution is adopted, one will choose for the top and the bottom, a grid or
a very well ventilated panel.
Let us benefit this article by
examining the various layouts encountered in the amplifiers of this kind.
a. In the interior of the chassis:
either the heatsinks are larger, or the use of a fan proves to be necessary, if
not the temperature inside the chassis very quickly becomes very significant
and causes heating of the components which often tolerate this very badly:
electrolytic capacitors primarily, the mains transformer and the printed
circuits comprising the driver and input stages.
b. On the back of the chassis: the
ventilation is better but the surface is generally reduced to the dimensions of
the box. In addition, this arrangement limits the accessibility to the inputs
and the outputs.
c. On the front, this is a very
original solution which provides good ventilation. Moreover, even when placed
in a badly ventilated place, the front face is in general unrestricted.
d. On the sides, the power is
limited to 30 W class A for a stereo apparatus. For higher powers, it is
necessary to have recourse to other solutions.
e. On the sides and the back: the
dissipation surface is much larger, it is a solution adopted by many American
and Japanese manufacturers.
f. Laid out in a square with the
fins placed inside thus constituting a chimney inside which must be placed a
fan. This solution however is not the best.
Certain manufacturers, like
Mitsubishi or Sony have resorted to solutions much more elegant such as fins
cooled with freon or the "Heat pipe". The latter technique, developed
by Sony, consists of a copper tube, filled with freon vapour, one end comprises
cooling ribs; on the other end are fixed the power transistors which are thus
located very close to each other, and are thus placed under excellent
conditions from the thermal variation point of view.
Another solution consists of using
copper heatsinks, or even aluminium with copper to improve the thermal
conductivity, but this then poses problems of manufacture.
To remain practical, one must
therefore gain surface. In a normal rectangular chassis, the sides and the back
offer only a small part of the total surface. The top and the underside in fact
are only very seldom used, however if one wants to release much heat, it is obvious
that one must bring together the devices designed for this purpose, the
heatsinks! It is indeed more astute to lay out the walls with larger surfaces
vertically, so as to dissipate the maximum amount of heat. This is most
probably the solution which will be adopted for the construction of the 50 W.
The advantages are as follows: heatsinks with a large surface, significant
chimney effect, reduced side area, possibility of thermally separating the
input stage and the power supply from the power stage (figure 6). Naturally,
this elegant solution can be adapted for the present construction. The only
difficulty being obtaining a suitable aluminium section. Fortunately, there are
many shapes and sizes of heatsinks available in the trade. If however, this poses
problems, it is possible to make an assembly of heatsinks on two copper or
aluminium plates, by using silicone grease or certain new synthetic products
that are even more effective, among which one can even find special adhesives
for heatsinks.
Fig. 6: Example of the configuration of the
chassis and the heatsinks allowing a maximum thermal dissipation, while
thermally isolating the other components.
Passive components
The
passive component count being very reduced, there are few things to say on
these. The resistors are a 1 %, 1/2 W tantalum type, of small series Japanese
manufacture. From the point of view of subjective influences, it is these which
give less significant colouration and defects. The lead-out wires are of tinned
copper and are connected to the body of resistor by caps also made out of
copper. Only the 0.47 ohm power resistors are of the cement type, this because
of the question of room and obstruction. The solder used is Multicore Savbit or
Multicore LMP (figure 7).
The
electrolytic capacitors used on the prototypes are models with very low series
resistance, of Japanese origin and 150 000 uF value. Two other 39 000
uF capacitors were put in parallel to give 189 000 uF for each polarity of
the power supply. These values are not very available in France.
Conclusion
This
amplifier with its simple circuit is easy to adjust. It can evolve into a more
powerful amplifier without major modifications. It presents, moreover, a very
interesting quality-price ratio. With the exception of the output transistors
and their driver transistors, this diagram can be " remade " starting
with other transistors of European references, without much risk of failure. To
preserve the maximum of its subjective qualities and a maximum reliability, its
power is voluntarily limited to 18 W without audible distortions. This power is
largely sufficient for speakers of average or rather good output. One should not
lose sight of the fact either that any piece of equipment is only a compromise,
which in this case is able to be improved. It is possible to fine-tune it, in
particular at the power supply level: supplying power to the input stages by a
separate circuit from that supplying the power stages, separation of the left
and right power supplies... or from the example of experimental preamplifiers,
the low symmetrical voltage of +/- 18 V can be provided by 6V car batteries,
that is to say 6 in total, this being reserved for the fanatics.
Fig, 7: Tantalum resistors, of Japanese
origin, type 1/2 W, 1 % tolerance; thermal stability 50 PPM. It is these which
gave from any point of view the best results; when compared with more than
twenty other types of resistors, including the famous ultrastable Vishay (+/- 5
PPM).
HISTORY: Page created 09/08/2001