This is only a preview of the April 2023 issue of Practical Electronics. You can view 0 of the 72 pages in the full issue. Articles in this series:
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500 POWER
WATTS AMPLIFIER
PART 1
BY JOHN CLARKE
This large power amplifier produces big, clear sound with low noise and
distortion. It delivers 500W RMS into a 4Ω load and 270W into an 8Ω load.
It has been designed to be very robust and includes load line protection
for the output transistors and speed-controlled fan cooling that remains
off until needed. With two of these, you could deliver 1000W into a single
8Ω loudspeaker. Good luck finding one that will handle that much power!
Features and Specifications
Output power
>500W into 4Ω, >270W into 8Ω – see Fig.3
Frequency response
+0, −0.1dB over 20Hz-20kHz (−3dB <at> 97kHz) – see Fig.1
Signal-to-noise ratio
112dB with respect to 500W into 4Ω or 250W into 8Ω
Total harmonic distortion (4Ω) <0.005% <at> 1kHz for 1.5-350W – see Fig.2 and Fig. 3
Total harmonic distortion (8Ω) <0.025% <at> 1kHz for 2-270W – see Fig.2 and Fig.3
Input impedance
10kΩ || 4.7nF
Input sensitivity
1.015V RMS for 500W into 4Ω, 1.055V RMS for 270W into 8Ω
Power supply
±80V nominal from an 800VA 55-0-55V transformer
Quiescent current/power
94mA, 15W
Protection
DC fuses, dual-slope thermal tracking, SOA current limiting, output clamping diodes
Other features
output offset nulling, blown-fuse indicators, onboard power indicator
Practical Electronics | April | 2023
17
O
ur 500W amplifier is big in several ways.
It is physically big, requiring two heatsinks stacked
end-to-end to keep the temperature under control. It
requires a significant power supply using an 800VA transformer, and the amplifier and power supply fit into a three
rack unit (3RU) rack case, again of rather large dimensions.
It does deliver a prodigious amount of power. It is ideal
for a public address system where high power can be necessary for sound reinforcement in a large venue. It is also
well-suited to driving inefficient loudspeakers. As noted
above, used in bridge mode, it could deliver just over
1000W per channel. Build two pairs for a sound system
so massive, it would need to be plugged into two different mains power points!
Two of these amplifiers could also be the basis of an
amazing stereo system for use in a large listening room.
You might think that a 500W per channel stereo system
is just too much power. Whether that is true depends on
what sort of music you like listening to and how efficient
your loudspeakers are. If you like rock music with its
somewhat limited dynamic range, then with this amplifier, you will be able to play it loud. That makes it ideal
for music that just has to be loud to be enjoyed.
But please don’t deafen yourself with the extreme sound
levels possible with such a large amplifier. You might also
need to provide ear protection for your neighbours!
It isn’t just for rockers, either. Classical music requires
lots of power as well. This is not because the performance
is necessarily loud, but it allows the wide dynamic range
in volume of concert hall performances to be replicated.
You want high power without distortion to produce the
high peak volume levels of the performance, like massive
kettle drum hits or pipe organ stings, with low noise from
the amplifier so that it does not drown out the whisper-
quiet passages.
Big power like this does not come easily. The amplifier
uses 12 output transistors and they are all mounted on a
400mm-wide heatsink. The main circuit board is also significant at 402 x 124mm. The final installation within the
3U rack enclosure measures 559mm x 432mm x 133.5mm
and weighs just over 12kg.
Fig.1: the frequency response of this amplifier is
exceptionally flat, varying by less than 1/20dB between
20Hz and 20kHz. The upper −3dB point is just short of
100kHz. While the lower −3dB point is not visible in this
plot, it’s likely around 1Hz. An active subsonic pre-filter
would be necessary to prevent over-extension if you’re
using this amp to drive a subwoofer directly.
18
This article will concentrate on describing the Amplifier
Module circuit. Over the next two months, we’ll also give the
full assembly details for this Module, plus describe a suitable power supply. Then we’ll show you how to build the
Module, power supply, speed-controlled fan cooling (which
switches off at light loads), speaker protector and clip detector – all into an aluminium 3RU rack-mountable chassis.
Performance
The main performance parameters are summarised in
the specification panel and Fig.1 to Fig.3. These indicate that just because a power amplifier delivers a lot of
power, that does not mean that it cannot deliver high performance as well.
For one, the frequency response is ruler-flat from 20Hz
to 20kHz, a mere 0.1dB down in response at 20kHz.
Power into 4W is a genuine 500W. At typical power levels, between 1.5W and 350W, the total harmonic distortion
plus noise (THD+N) is below 0.007% at 1kHz.
For an 8W load, maximum power is around 270W until
the onset of clipping, with <0.004% THD+N at 1kHz at
more typical power levels from 1W to 200W. Under ideal
conditions, it’s close to what we’d call ‘CD quality’ at
around 0.002% THD+N.
As you can see from Fig.2, distortion rises somewhat
with frequency; in fact, it’s considerably lower than quoted
above at more typical audio frequency ranges for most
instruments of around 100-500Hz. Above 1kHz, distortion rises modestly, although it’s still relatively low even
by 10kHz, above which the filters in our test equipment
start attenuating the harmonics.
The THD+N result of under 0.05% for 266W into 3W
shows that the performance of this amplifier does not
degrade significantly even under harsh conditions, driving lower load impedances than you’d expect to see with
most high-power 4W loudspeakers.
Perhaps the most important aspect of this high-power
amplifier is the very good signal-to-noise ratio of 112dB.
This means that you can get a very high output level,
including loud transients, without an annoying background hiss the rest of the time.
Fig.2: THD+N plots for 8W, 4W and 3W loads (two
different power levels are shown for 4W) with 20Hz-22kHz
bandwidth. You can see that the base distortion largely
depends on the load impedance, and it rises steadily with
frequency above about 100Hz. The 3W curve is mainly
presented as a ‘worst-case scenario’ and shows that it can
drive very low load impedances without too much difficulty.
Practical Electronics | April | 2023
Two of our other projects: the Cooling Fan and
Loudspeaker Protector (February 2023) and
Amplifier Clipping Indicator (this issue) are both
used in the 500W Amplifier.
Circuit details
The full circuit diagram is shown in Fig.4. Aside from the
large number of output transistors, the circuit is similar in
configuration to many of our previous amplifiers, including the Ultra-LD Mk.2 (August and September 2010).
One major difference is the addition of safe operating
area (SOA) protection for the output transistors. This
helps prevent damage to them if the amplifier is short-
circuited or presented with a load that exceeds their safe
operating area (SOA). This is not just protection against
a short circuit; it works over the entire operating range
of the amplifier.
We’ve heard it stated in the past that SOA protection
degrades the performance of an amplifier, but we tested this
one with it in-circuit and disconnected, and we couldn’t
measure any differences. So you don’t need to be concerned about its impact on sound quality.
The supply rails are ±80V or 160V in total. This high
voltage requires rugged transistors, particularly the output and driver transistors, which need a large SOA. We
could have used the NJL3281D/NJL3282D ThermalTrak
transistors as used in previous amplifiers. However, we
would have needed 12 of these transistors per side or 24
in total to ensure it was robust.
The ThermalTrak transistors have two main advantages:
good linearity and each device includes a separate diode
for biasing. The diode within the transistor package allows
the quiescent (idle) current to be controlled accurately with
temperature variations. Unfortunately, the sheer number
of these transistors required would make the amplifier
impractically large and expensive, so they are unsuitable.
Instead, we are using MJW21196/MJW21195 transistors, with only six required per side, thanks to their generous SOA curves.
The input signal is AC-coupled via a 47μF non-polarised electrolytic and high-frequency stopper components, ferrite bead FB1 and a 22W resistor to the base of
transistor Q1. The 22W input resistor and 4.7nF capacitor constitute a low-pass filter with a −6dB/octave rolloff above 1.5MHz.
Q1 is part of the input differential pair of Q1 and Q2,
which are Toshiba 2SA1312 PNP low-noise transistors.
Fig.3: THD+N vs power at 1kHz. Distortion starts to rise above
350W for 4W loads but it delivers 500W without gross distortion
(and even more on a short-term basis). The performance is
pretty good in the middle power range, from a few watts to a
couple of hundred watts; it will give ‘CD quality’ into 8W up to
about 200W. Double the numbers on the horizontal axis and
check the 4W curve for 8W bridged performance!
The finished
Amplifier Module
shown mounted in its 3RU
case with heatsink and fans. Note
the 120mm PWM fans attached to the
heatsink, as anything larger wouldn't fit in the
case with its lid on.
Practical Electronics | April | 2023
19
500W Amplifier
These are responsible for the very low
residual noise of the amplifier.
2SA1312 transistors are becoming
somewhat challenging to get, but we
have secured a good supply for our
readers as we couldn’t find any suitable alternatives – see the parts list for
our sourcing recommendation.
(Editor’s note – the practice of manufacturers discontinuing components
with no direct replacement is very
frustrating, and it has bitten us several times.)
The bias resistor for Q1 and the
series feedback resistor to the base of
Q2 are set to a relatively low value
of 10kW to minimise signal source
impedance and thereby reduce thermal noise. The 10kW input resistance
and the 47μF input capacitor provide
a low-frequency roll-off at 0.34Hz.
The amplifier gain is set by the
ratio of the 10kW and 220W feedback
resistors at the base of Q2. This gain
is 46 times (33dB), while the 2200μF
capacitor sets the low-frequency rolloff (−3dB point) in the feedback loop
20
Fig.4: the main difference between this amplifier and our last few designs is the
sheer number of output devices (six pairs) and the addition of SOA/load line
protection circuitry. This protection circuitry is based on voltage references REF1
and REF2, transistors Q25 and Q26 and the associated resistor network, including
the series of 3.3kW resistors connected to the emitter of each output transistor.
to 0.33Hz. The relatively high gain
helps to keep the amplifier stable
and makes the input sensitivity reasonable at around 1V RMS for fullpower output.
Coupling capacitors
The high-value electrolytic capacitors
for the input coupling (47μF) and feedback (2200μF) networks eliminate any
effects of capacitor distortion in the
audio pass-band and also minimise
the source impedance.
To explain, if we use a smaller input
capacitor at say 2.2μF, its impedance
will be 1447W at 50Hz. This will only
have a small effect on the audio frequency response but represents a substantial increase in the source impedance at low frequencies. By contrast,
the 47μF input capacitor we used has
an impedance of only 67.7W at 50Hz.
This also means that the voltage
across these capacitors is minimal
compared to the audio signals, so the
inherent non-linearity of electrolytic
capacitors does not matter.
Diodes D1 and D2 are included
across the 2200μF feedback capacitor as insurance against possible
damage if the amplifier suffers a
fault where the output is pulled to
the −80V rail. In this circumstance,
the capacitor would have a significant reverse voltage.
We use two diodes instead of one to
ensure that there is no audio distortion due to the non-linear effects of a
single diode junction at the maximum
feedback signal level of about 1V peak.
This prevents diode conduction under
normal operating conditions.
Voltage amplification stage
Most of the amplifier’s voltage gain is
provided by Q9, fed via emitter-follower Q8 from the collector of Q1.
Together, these transistors form the
voltage amplification stage (VAS). Q8
buffers the collector of Q1 to minimise
non-linearity.
Q9 is operated without an emitter
resistor to maximise gain and also
maximise its output voltage swing.
Practical Electronics | April | 2023
Maximum voltage swing is required
from the voltage amplifier stage to
obtain as much power as we can from
the output stages.
Current mirror
The collector loads of Q1 and Q2 are
NPN transistors Q3 and Q4 which
operate as a current mirror. Q4 acts
as a sharp cutoff diode, providing
a voltage at the base of Q3 equal to
the base-emitter voltage drop of Q4
(about 0.6V) plus the voltage drop
across its 68W emitter resistor.
If Q2 draws more than its share of
emitter current from Q5, the voltage at
the base of Q3 increases, so Q3’s collector current also rises. This forces Q1
to pull a bit more current and stops Q2
from taking more than its fair share.
As Q3 mirrors the current of Q4, Q1
is provided with a collector load that
has a higher impedance than would
otherwise be the case.
The result is increased gain and
improved linearity from the differential input stage.
Practical Electronics | April | 2023
Similarly, the collector load for Q9
is a constant-current load comprising transistors Q6 and Q7. Interestingly, the base bias voltage for constant current source Q5 is also set
by Q6. Q5 is the constant current
tail for the input differential pair of
Q1 and Q2, and it sets the current
through these transistors. LED1 is
connected to this circuit as a ‘free’
power-on indicator.
The reason for the somewhat complicated bias network for Q5, Q6 and
Q7 is to produce a major improvement
in the power supply rejection ratio
(PSRR) of the amplifier. Similarly, the
PSRR is improved by the bypass filter
network consisting of the 100W 1W
resistor and 470μF 100V capacitor in
the negative supply rail.
Why is PSRR so important? Because
this amplifier runs in class-AB, it
pulls large asymmetric currents
from the positive and negative supply rails. The currents are asymmetric in the sense that it’s pulling from
one or the other at any given time;
the waveforms will be a similar shape
for a sinewave, just time-shifted compared to each other.
So, for example, when the positive
half of the output stage (Q13 to Q18)
conducts, the current waveform is
effectively the positive half-wave of
the signal waveform; ie, rectification
occurs. Similarly, when the negative
half of the output stage (Q19 to Q24)
conducts, the current is the negative
half-wave of the signal.
So we have half-wave rectification
ripple of the signal superimposed on
the supply rails, as well as the 100Hz
ripple from the power supply itself.
And while the PSRR of an amplifier
can be very high at low frequencies, it
is always worse at high frequencies. If
these ripple voltages can get into the
earlier stages of the amplifier, they will
cause distortion, so we need to minimise them there.
Diode D3 is included to improve
recovery performance when the
amplifier is driven into hard clipping.
It makes the recovery from negative
21
K
Parts List – 500W Amplifier Module (to build one)
1 double-sided, plated-through PCB coded 01107021,
402 x 124mm – see bottom of page
2 200mm-wide heatsinks [Altronics H0536]
2 small PCB-mounting heatsinks [Jaycar HH8516]
12 TOP-3 silicone insulating washers
3 TO-220 silicone insulating washers
2 insulating bushes for the TO-220 transistors
4 M205 fuse clips (for F1 and F2)
2 fast-blow ceramic M205 fuses
(5A for 8W load, 10A for 4W load) (F1, F2)
1 ferrite bead (FB1) [Jaycar LF1250, Altronics L5250A]
1 6-way PCB-mount screw terminal with barriers
(CON2) [Altronics P2106]
1 2-way pluggable vertical terminal socket (CON3)
[Altronics P2572, Jaycar HM3112]
1 2-way pluggable screw terminal (CON3)
[Altronics P2512, Jaycar HM3122]
1 vertical PCB mount RCA (phono) socket (CON1)
[Altronics P0131]
1 pot core bobbin for L1
[Altronics L5305, Jaycar LF1062]
1 2m length of 1.25mm enamelled copper wire
(for winding L1)
1 60mm length of 0.7mm diameter tinned copper wire (links)
12 M3 x 20mm panhead machine screws
5 M3 x 15mm panhead machine screws
6 M3 x 6mm panhead machine screws
17 M3 hex nuts
12 M3 steel washers
6 M3 tapped 9mm spacers
2 transistor clamps [Altronics H7300, Jaycar HH8600]
1 15mm length of 25mm diameter heatshrink tubing (for L1)
1 60mm length of 1mm heatshrink tubing
(for the wire links)
1 small tube of heatsink compound/thermal paste
Semiconductors
6 MJW21196 250V 16A NPN transistors (Q13-Q18)
[element14 1700966] ●
6 MJW21195 250V,16A PNP transistors (Q19-Q24)
[RS 790-5410] ●
1 MJE15035G 350V 4A PNP transistor (Q11)
[Mouser 863-MJE15035G] ●
1 MJE15034G 350V 4A NPN transistor (Q12)
[Mouser 863-MJE15034G] ●
1 FZT558TA 400V 300mA PNP transistor (Q7)
[RS 669-7388P] ●
1 FZT458TA 400V 300mA NPN transistor (Q9)
[RS 669-7326] ●
2 2SA1312 120V 100mA low-noise PNP transistors (Q1,Q2) ●
3 BC546 65V 100mA NPN transistors (Q3, Q4, Q25)
1 BC639 80V 500mA NPN transistor (Q8)
3 BC556 65V 100mA PNP transistors (Q5, Q6, Q26)
1 BD139 80V 1.5A NPN transistor (Q10)
2 1N4148 75V 200mA signal diodes (D1, D2)
4 UF4003 200V 1A ultra-fast switching diodes● (D4-D7)
1 BAV21 250V 250mA low-capacitance switching
diode● (D3) [RS 436-7846]
2 TL431 programmable voltage references, TO-92
(REF1, REF2) [element14 3009364] ●
1 5mm green LED (LED1)
2 5mm red LEDs (LED2, LED3)
Capacitors
1 2200μF 16V or low-ESR 10V electrolytic
3 470μF 100V electrolytic [element14 3464457]
1 47μF non-polarised (NP/BP) electrolytic
1 47μF 50V electrolytic
1 47μF 16V electrolytic
1 1μF 100V MKT polyester
1 470nF 100V MKT polyester
2 100nF 100V MKT polyester
1 100nF 250V AC metallised polypropylene X2-class
2 10nF 100V MKT polyester
1 4.7nF MKT polyester
1 1nF 100V MKT polyester
1 75pF 200V COG [Mouser 80-C315C750JCG or
80-C325C750KAG5TA] ●
Resistors (all 1/4W, 1% thin film unless specified)
1 1MW
2 35.7kW ● (or 2 82kW and 2 62kW)
1 33kW
2 33kW 1W 5% (carbon type OK)
1 22kW
2 18kW
5 10kW
1 10kW 1W 1% thin film [Yageo MFR1WSFTE52-10K] ●
2 8.2kW
2 4.7kW
14 3.3kW
3 2.2kW
2 470W
2 220W
2 205W ● (or 2 430W and 2 390W)
3 100W
1 100W 1W 5% (carbon type OK)
2 68W
2 68W 5W 5% wirewound (for testing purposes)
8 56W 1W 5% (carbon type OK)
2 47W
1 39W
1 22W
1 10W
12 0.47W 5W 5% wirewound
1 100W single-turn top-adjust trimpot (VR1)
[Altronics R2591]
1 200W multi-turn top-adjust trimpot (VR2)
[Altronics R2372A]
UK readers – kit of parts for the 500W Power Amplifier
This is a large project with hard-to-find parts, especially the
transistors. We normally supply the PCB and then readers
source components using the Parts List. However, for this
project constructors should buy kit SC6727 from Silicon
Chip in Australia: www.siliconchip.com.au/Shop/20/6727
project means this restriction does not apply. Note: UK
purchasers will be liable for import duty and VAT.
The UK’s 2021 VAT/import regulations mean it is not
worthwhile for Silicon Chip to sell to UK customers for
purchases under AUD250 (plus p&p), but the size of this
22
The parts list for the power supply, chassis, wiring etc
will be presented in an upcoming issue.
The parts supplied in the kit are detailed on the Silicon
Chip website and include: the 500W amplifier module
PCB; set of hard-to-get parts for the 500W amplifier
module, including most of the semiconductors (marked
with red dot above); Clipping Indicator PCB; Fan
Controller/Speaker Protector PCB with programmed
microcontroller, plus three 4-pin PWM fan headers.
Practical Electronics | April | 2023
voltage clipping as clean and fast as
that from positive voltage clipping,
improving signal symmetry and
reducing ringing under these conditions. For this role, we are using a
BAV21 diode with a low capacitance
of 2pF at 1MHz so that it doesn’t affect
sound quality.
Feedback and compensation
As mentioned, the feedback components at the base of Q2 set the closedloop gain of the amplifier. The bottom
end of the feedback network is connected to ground via a 2200μF electrolytic capacitor. As this reduces DC gain
to unity, the amplifier output offset voltage is dramatically lower than it would
otherwise be (by a factor of 38 times).
The 75pF compensation capacitor
connected between the collector of Q9
and the base of Q8 prevents oscillation
by limiting the slew rate.
The 22kW resistor in Q8’s collector
limits the current through Q9 under
fault conditions. Should the amplifier
output be shorted, it will try to pull
the output either up or down as hard
as possible, depending on the output
offset voltage polarity.
If it tries to pull it up, the output
current is inherently limited by the
15mA current source driving Q9 from
Q7. However, if it tries to pull down,
Q9 is capable of sinking much more
current. The 22kW resistor limits Q9’s
base current and therefore, its collector
current and dissipation. The 1nF parallel capacitor is required to keep its
AC collector impedance low, improving stability.
Driver stage
The output signal from the voltage amplifier stage Q9 is coupled to
driver transistors Q11 and Q12 via
47W resistors. The 47W resistors act
as stoppers to help prevent parasitic
oscillation in the output stage. They
are also needed to allow the load line
protection circuitry to override the
drive from the VAS.
Q10 sets the DC voltage between Q7
and Q9, and this determines the quiescent current and power in the output
stages. It provides a bias of about 2.3V
or so between the bases of Q7 and Q9
so that they are always slightly conducting, even without an input signal.
Q10 is a ‘Vbe multiplier’, multiplying the voltage between its base and
emitter by the ratio of its collector-
emitter and base-emitter resistances.
While trimpot VR2 varies the resulting
collector-emitter voltage, it is actually
adjusted to set the quiescent current
through the output transistors.
It is important that the bias voltage
produced by Q10 changes with the
Practical Electronics | April | 2023
The first part of our 500W Amplifier series focuses on describing how the
Amplifier Module works; assembly and testing will be handled in later parts.
temperature of the output stage transistors. As the output transistors become
hotter and their base-emitter voltages
reduce, Q10’s collector-emitter voltage
should also drop, so that the quiescent
current is the same or less as at lower
temperatures, averting the danger of
thermal runaway.
Output stage
The amplifier’s output stage is effectively a complementary symmetry
emitter follower comprising six NPN
transistors (Q13-18) and six PNP transistors (Q19-Q24).
Each output power transistor has a
0.47W emitter resistor, and this moreor-less forces the output transistors
to share the load current equally. The
emitter resistors also help to stabilise the quiescent current to a small
degree, and they slightly improve the
frequency response of the output stage
by providing current feedback.
Output offset adjustment
DC offset adjustment is provided by
the 100W trimpot (VR1) between the
emitters of the input pair, Q1 and
Q2. VR1 adjusts the current balance
between the input pair, and this causes
the DC offset at the output to vary. The
trimpot is set to make the DC offset as
close to 0V as possible; it should be
possible to keep this within ±5mV.
This is generally a good figure to
keep low, but it’s especially critical if
using the amplifier to drive a step-up
transformer for 100V line operation.
That’s because the DC resistance of
the transformer primary is much lower
than that of a loudspeaker voice coil,
so significant DC can otherwise flow
through it.
Load line protection
It is crucial to prevent the output transistors from operating beyond their
Safe Operating Area (SOA). A highpower amplifier like this is quite likely
to see abuse, being driven beyond its
limits at times.
Fig.5 shows plots of collector current versus collector-emitter voltage (Vce) for the six-per-side paralleled MJW21196 and MJW21195 output transistors. Of the two types, the
MJW21195 (PNP) has the lower SOA
curve, with a lower current allowed
beyond 150V than the complementary
MJW21196, so that is the curve we’ve
plotted (the solid green line).
The SOA curve is based on a transistor junction temperature of 150°C and
a case temperature of 25°C. That is not
a very practical case temperature to
maintain, especially when the transistors are dissipating significant power.
The actual transistor case temperature depends on the dissipation, the
thermal resistance of each transistor’s
junction to its case (0.7°C/W) and the
case-to-ambient thermal resistance,
which is determined by the heatsink
and fans. Having a large heatsink with
fan-forced air greatly helps to keep
transistor temperatures low.
At elevated temperatures, it is
essential to ensure the transistors are
not operated beyond their maximum
power rating, 200W at 25°C, reducing
by 1.43W per °C. This power rating
curve can further reduce the power
they can handle beyond that imposed
by the SOA secondary breakdown area.
We plotted both the 25°C case temperature power curve (green curve)
and the 50°C case temperature power
curve (mauve curve). While a total of
23
1200W is available with the six 200W
transistors at 25°C, only 985W is allowable with a 50°C case temperature.
The curves assume that each of
the six parallel transistors share the
current equally, a fair assumption
since each has a relatively high-value
emitter resistor. If one of the power
transistors tends to take more than
its share of load current, the voltage
drop across its emitter resistor will
be proportionately higher. This will
throttle the transistor back until its
current comes back into line with
the others.
The blue and red curves show resistive 8W and 4W loads (straight lines)
that assume the load is purely resistive. In practice, this is not true for
loudspeakers as there is a considerable reactive impedance in a practical
loudspeaker that causes its resistance
to vary with frequency.
The curved blue and red lines show
the load impedance curves assuming that the resistive and reactive
A close-up of the front-end circuitry of the 500W Amplifier Module.
impedances are equal. The plots show
the worst-case impedance that occurs
over the operating frequency range.
For example, for a 4W speaker, we
plot the curve with a 2.83W resistance
and 2.83W reactive impedance that’s
90° out of phase with it (‘j’ is like ‘i’
in mathematics, the imaginary unit of
value √-1, forming a complex impedance value).
Fig.5: here are the load lines for 4W and 8W operation. The straight
lines are for resistive loads, while the arched lines are for reactive
4W (2.83W + j × 2.83W) and 8W (5.65W + j × 5.65W) loads.
The green and mauve lines are the power limit hyperbola at
25°C and 50°C, while the orange line is the one-second SOA
curve for six MJW21195/6 power transistors.
The dashed green and mauve lines are the dual-slope load
line protection curves at 25°C and 50°C.
24
Calculation of the total impedance
can be visualised as the two impedances forming two sides of a right-
angle triangle with the hypotenuse
length equalling the total, which in
this case is either 4W or 8W.
These plots are for a rather severe
amplifier load. Typically, a loudspeaker will not exhibit such a load,
but we want to ensure the amplifier
will not be damaged by designing for
worst-case loads.
Note how the curved impedance
plots encroach quite a bit closer to the
SOA curve than the purely resistive
loads. Note also that at elevated temperatures, the allowable dissipation
curve comes close to the 4W reactive
impedance plot, especially around
the 60V to 100V Vce region. At case
temperatures above 50°C, the allowable transistor dissipation could possibly be exceeded.
The two protection lines on the
graph prevent this. The dashed green
line is for a transistor case temperature of 25°C, while the dashed mauve
line is for a 50°C case temperature.
The lines show the points on the
graph where the output transistors
are protected by reducing their base
drive should the load reach the protection line.
The protection lines shift closer to
the 4W impedance curve with increasing temperature. Also, the protection lines have a dual slope with one
straight line between the Y-axis and
the small circle (dot), and the second
line between that dot and the X-axis.
Note that where the line meets the
X-axis, it must be at least the total supply voltage (160V) to prevent spurious limiting near zero output current.
As the temperature rises, the voltage at the zero current axis reduces.
However, even the 50°C curve meets
the axis above 160V, at 165V. If the
amplifier gets significantly hotter,
perhaps beyond 60°C, the output will
probably get cut off, but maybe that
is not a bad thing, as it’s a sign that
the cooling system might have failed.
Practical Electronics | April | 2023
While the difference between the
two slopes in the protection curve
is subtle, this is necessary to more
closely follow the power rating
curve and hence prevent the protection curve at 50°C and beyond from
encroaching on the 4W impedance
curve at a Vce of around 70V.
SOA protection circuitry
This dual-slope foldback protection scheme is based on the research
paper titled, The Safe Operating Area
(SOA) Protection of Linear Audio
Power Amplifiers by Michael Kiwanuka, BSc (Hons) Electronic Engineering, which you can view at:
https://bit.ly/pe-apr23-soa
The supply voltage, output voltage and current through the output
transistors are all monitored to provide load-line protection over the
entire voltage and current ranges of
the amplifier.
Transistors Q25 and Q26 and
diodes D6 and D7 provide the protection feature. Q25 (NPN) can shut
off the MJW21196 transistors, while
Q26 (PNP) acts on the MJW21195
transistors. The diodes are included
to prevent Q25 and Q26 from shunting the drive signal when they are
reverse-biased. This happens for
every half-cycle of the signal to the
driver transistors.
The circuits around Q25 and Q26
are essentially identical.
Normally, Q25 and Q26 are biased
off and play no part in the amplifier’s operation. However, if the load
encroaches upon the protection
curve, Q25 and/or Q26 switch on to
throttle back drive to the output transistors, limiting the output current
and protecting the transistors. This
also protects against short circuits.
Transistor Q25 and Q26 are
mounted on the amplifier’s heatsink
so that the protection circuit curves
shift with temperature as required.
In more detail, the voltage across
each 0.47W output stage emitter resistor is monitored via a set of 3.3kW
resistors. These voltages are averaged (equivalent to being summed)
at the base of Q25 or Q26. Resistive
dividers formed from pairs of paralleled resistors provide output voltage and supply voltage monitoring
by feeding extra current into these
summing points.
Effectively, what these dividers
do is make it so that as the voltage across a set of output resistors
reduces (either due to reduced supply voltage, or the output swinging
closer to that rail), the protection
circuitry becomes more insensitive
and requires a higher output current
Practical Electronics | April | 2023
The finished case is simple, with only a power button and clipping indicator
LED on the front and audio input/output and power socket on the back.
to be triggered. Similarly, as the Vce
increases, the trip current decreases,
forming the ‘curves’ shown in Fig.5.
The dual slope in the protection
circuit is created by voltage reference
REF1 for the positive half of the circuit and REF2 for the negative half.
The bias current to operate these
devices comes via 18kW series resistors. REF1 and REF2 are adjustable
voltage references, with the 10kW
and 3.3kW resistors setting the voltage across them to 10V.
The protection circuit relies on the
base-emitter voltage of Q25/Q26 being
around 0.6V at 25°C. This voltage
drops to 0.55V at 50°C, so these transistors switch on with less applied
voltage at higher temperatures. This
shifts the protection line downwards
with elevated temperature, following
the downward movement of the output transistors’ power rating curve.
Diodes D4 and D5 between the
amplifier output and supply rails are
also part of the protection circuitry.
They absorb any large spikes generated by the loudspeaker’s inductance
when the protection circuit cuts the
drive to the output transistors. D4 and
D5 are fast recovery diodes, included
to ensure their operation at high frequencies and high power.
These diodes are even more critical if driving a line transformer as
its primary inductance is likely to be
significantly higher than any loudspeaker load.
Output RLC filter
The remaining circuit feature is the output RLC (resistor-inductor-capacitor)
filter, comprising a 2.2μH air-cored
choke, eight paralleled 56W resistors
(giving 7W) and a 100nF capacitor.
This output filter effectively isolates
the amplifier from any large capacitive reactance in the load, thereby
ensuring unconditional stability.
It also helps attenuate any RF signals
picked up by the loudspeaker leads
and stops them from being fed back to
the early stages of the amplifier, where
they could cause RF breakthrough.
Fuse protection
The output stage supply rails are fed
via fuses F1 and F2 from the +80V
and −80V main power supply rails.
These provide ‘last-ditch’ protection
to the amplifier, limiting the damage
in the case of a severe fault. The recommended fuses are ceramic types.
LED2 is a blown-fuse indicator for F1
and LED3 for F2. They light up if the
fuse is blown as it isn’t always obvious, especially with ceramic types.
Next month
The following article next month
will have the full module construction details, including the heatsink
drilling and instructions for winding
inductor L1.
In the June issue, we’ll show you
how to build a suitable power supply,
mount it and the Amplifier Module in
the chassis, and wire it all up along
with the Fan Controller, fans, Speaker
Protector and Clipping Indicator.
Reproduced by arrangement with
SILICON CHIP magazine 2023.
www.siliconchip.com.au
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