The Class-B Power Amplifier.
THE DOUGLAS SELF CLASS-B POWER AMPLIFIER PCB.
6 APR 94 PCB001.DOC
This power amplifier is probably the lowest-distortion design
ever made available for construction. The design is explained in
detail in the Electronics World series "Distortion in Power
Amplifiers" by Douglas Self; This was published in eight parts
between August 1993 and February 1994. The essential philosophy is
to linearise each stage with a high degree of local negative
feedback before closing the global feedback loop; this allows the
global feedback factor to be kept relatively low to maximise
stability, while still showing exceptionally good linearity.
This PCB follows the design published in Electronics World for
February 1994, with the addition of SOAR (safe operating area)
protection against output short-circuits, and the inclusion of rail
fuses. The SOAR chosen allows output power to be increased up to
100W/8-Ohm without premature protection coming in.
The board is of high-quality fibreglass construction with a full
silk-screen component ident and a solder-mask to minimise the
possibility of solder shorts.
CIRCUIT NOTES.
The amplifier design as published yields 50W rms into 8 Ohms, but
may be configured for powers of between 20 and 100W by appropriate
choice of supply rail voltage; no component value changes are
required. The design has excellent supply-rail rejection, and so a
simple unregulated supply is possible. The details of this are the
responsibility of the constructor; as a guideline 4700uF reservoir
capacitors are suitable for most applications.
The published circuit is followed closely, and only the new SOAR
protection system is described. This is a fairly conventional two-
slope SOAR system. In the positive half, TR15 monitors the current
through TR7 (via R33) and also the voltage across it. (via R25,26)
This effectively draws a straight line across the Vce-Ic graph,
which is much better than simple current-limiting, but still fails
to follow the SOAR area, which is bounded by a curve; the problem
is that at high Vces the current capability of TR7 is excessively
restricted. The combination of D4,R31 prevents this by making the
protection characteristic a combination of two lines that fits the
SOAR curve much more closely. D2,3 prevent spurious operation of
the system on large negative output excursions.
TR17 is a current limiter for the VAS transistor TR4; when
protection transistor TR16 is conducting large currents can
potentially be drawn through TR4. Tr17 monitors the VAS current
through R35, and turns on to shunt base drive away from the VAS
when required.
NB: Two errors crept into the published circuit; there are two R9's
and two C7's. The capacitor C7 in the feedback arm has been
designated C12, and the resistor R9 in TR12 emitter has been made
R38.
OTHER PROTECTION ISSUES.
The published design was not intended as a complete cookbook
project- such are not the domain of Electronics World- and
therefore omitted important ancillaries such as relay protection
against DC offsets appearing at the output under fault conditions.
It is recommended that a DC protection system is added, such as the
Maplin version, which is known as Velleman Kit K4700 (Order Code
VE24B)
Please note that the rail fuses are intended only to minimise
amplifier damage in the event of output device failure. They should
not be relied on for speaker protection. For various 8-Ohm power
outputs, the required HT rails and fuses are as follows:
Power rms TX secondary HT rails Rail Fuse value
50W 25V-0-25V +/-33V 2A
75W 30V-0-30V +/-40V 3.15A
100W 34V-0-34V +/-46V 5A
COMPONENTS.
The board has been configured for easily obtainable components, in
particular the following, which are all available from Maplin
Electronics. The Maplin order codes are given for reference:
1) Driver heatsinks. The PCB has mounting holes suitable for
heatsink Type-SW38-2 (Order Code JW28F)
2) Fuseholder clips. 20mm Fuse Clip Type 1, (Order Code WH49D)
3) Quiescent-adjust preset. Cermet preset 1K, (Order Code WR40T)
4) Wirewound resistors. 3W "WW Min" (Order Code W+value)
5) Non-electrolytic capacitors; Polyester. (eg Order Code WW41U)
6) Output inductor; 18 swg enamelled copper wire.(Order code BL25C)
CONSTRUCTION.
The performance of a power amplifier depends as much on the
topology and layout of the power and ground wiring as on the
subtleties of the circuit design. This has been taken into account
in the PCB layout, but the external wiring is the responsibility of
the constructor. We therefore give a recommended wiring scheme that
has been approved by the designer. (The assumption is made that a
simple unregulated supply is used; a regulated supply is quite
unnecessary and may well cause unforeseen complications)
1) There are several important points about the wiring for any
power amplifier; see the attached wiring diagram:
Keep the + and - supply wires to the amplifiers close together.
This minimises the generation of distorted magnetic fields which
may otherwise couple into the signal wiring and degrade linearity.
The rectifier connections should go direct to the reservoir
capacitor terminals, and then away again to the amplifiers. Common
impedance in these connections superimposes charging pulses on the
rail ripple waveform, which may degrade amplifier PSRR.
Do not use the connection between the two reservoir capacitors as
any form of star point. It carries heavy charging pulses that
generate a significant voltage drop even if thick wire is used. As
the drawing shows, the "star-point" is teed off from this
connection. This is a star-point only insofar as the amplifier
ground connections split off from here, so do not connect the input
grounds to it, as distortion performance is likely to suffer.
2) Driver transistor installation. These should be mounted onto
their small heatsinks with thermal washers, to ensure good thermal
contact. Electrical isolation between device and heatsink is not
necessary.
3) TO3 power transistor installation. The PCB layout allows the
TO3s to be mounted on an aluminium thermal-coupling flange which is
bolted to the PCB. It is recommended that the flange is drilled
with suitable holes to allow bolts to pass through the TO3 fixing
holes, through the flange, and then be secured by nuts and crinkly
washers which will ensure good contact with the PCB mounting pads.
Insulating sleeves are required around these bolts where they pass
through the flange. Depending on the size of the holes drilled for
the two TO3 package pins, these may also require sleeving.
An insulating thermal washer must be used between TO3 and
flange; these tend to be delicate and the bolts must not be over-
tightened. Do not solder the two pins until the TO3 is firmly and
correctly mounted and checked for isolation from the heatsink.
Soldering these pins and then tightening the fixing bolts is likely
to force the pads from the PCB. If this should happen then it is
quite in order to repair the relevant track or pad with a small
length of stranded wire to the pin; 7/02 size is recommended.
4) Bias-generator transistor TR13 mounting. As explained in the EW
series, the optimal place to mount TR13 for effective thermal
compensation is the top of the TO3 cans. The best solution is to
improvise a small spring clip which can be fixed under one of the
TO3 mounting bolts. A small piece of thermal pad material between
the bias-generator device and the TO3 top will improve sensing
accuracy.
A transistor outline is placed on the PCB to identify the
connections; it is not intended that TR13 be soldered in here as
without thermal compensation the quiescent stability will be poor.
TESTING AND FAULT-FINDING.
1) The most important step to successful operation is a careful
visual inspection before switch-on. As in all power amplifier
designs, a wrongly-installed component may easily cause the
immediate failure of others, making fault-finding difficult. It is
therefore advisable to meticulously check:
That the supply and ground wiring is correct.
That all transistors are installed in the correct positions.
That the drivers and TO3 output devices are not shorted to their
respective heatsinks through faulty insulating washers.
That the circuitry around the bias generator TR13 in particular
is correctly built. An error here that leaves TR13 turned off will
cause large currents to flow through the output devices and may
damage them before the rail fuses can act.
2) The second stage is to obtain a good sinewave output with no
load connected. A fault may cause the output to sit hard up against
either rail; this should not in itself cause any damage to
components. Since a power amp consists of one big feedback loop,
localising a problem can be difficult. The best approach is to take
a copy of the circuit diagram and mark on it the DC voltage present
at every major point. It should then be straightforward to find the
place where two voltages fail to agree; for example, a transistor
installed backwards usually turns fully on, so the feedback loop
will try to correct the output voltage by removing all drive from
the base. The contradiction between "full-on" and "no base-drive"
clearly indicates the error.
When pursuing voltages around the circuit, bear in mind that C2
is protected against reverse voltage by D1, which will conduct if
the amplifier saturates negatively.
This approach may fail if the amplifier is subject to high-
frequency oscillation, as this tends to cause apparently anomalous
DC voltages. In this situation the use of an oscilloscope is
essential.
3) The final step is to obtain a good sinewave into a suitable
high-wattage load resistor. It is possible for faults to become
evident under load that are not shown up in Step 2 above.
Setting the quiescent current for any Class-B amplifier can only
be done accurately by using a distortion analyser. if you do not
have access to one, the best compromise is to set the quiescent to
55mA when the amplifier is at working temperature. This should be
close to the correct value, and the inherent distortion of this
design is so very low that minor deviations from perfection are not
likely to be significant.
4) It can simplify faultfinding if D2, D3 are not installed until
the basic amplifier is working correctly, as errors in the SOAR
protection cannot then confuse the issue. This naturally demands
care in testing, as there is then no short-circuit protection.
INSTALLATION IN CHASSIS.
F1,2,5,7 are mounting holes for the PCB on the chassis.
F3,4,6,8 are fixing holes joining the PCB to the heatsink.
PLEASE NOTE.
Since the component selection, construction, and usage of this PCB
is outside our control, we can accept no responsibility for the
functioning or performance of amplifiers constructed with it. We
are therefore unable to enter into correspondence regarding
faultfinding, substitute components, etc.
to The Institute
