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DESIGN BY PHIL PROSSER
485W into 4Ω (single channel)
Operates with loads between 4-8Ω
Very high efficiency (typically >80%
at moderate power levels)
Very low in cost and easy-to-build
with minimal soldering required
Typically, 0.02% distortion
over most power levels at
1kHz
Frequency response from
<5Hz to 20kHz,
+0,-1.5dB
Built-in speaker protection
5 MONOBLOCK
0 class-d amplifier
0
If you need a serious amount of audio power, are on a budget, and
are not after ‘high fidelity’, this is for you! It uses two prebuilt
modules and not much else, mounted in a compact metal chassis,
WATT to deliver heaps of audio power all day long.
B
uilding a 500W+ amplifier is a
serious undertaking. To make
sense, a high-power Class-D
amplifier would need a switch-mode
power supply. After all, why bother
with a Class-D amplifier if you need a
1kVA transformer and bank of capacitors, making the thing half the weight
of a VW Beetle?
DANGER – LIVE COMPONENTS
Do not consider touching the
heatsinks or anything on the PCBs
when the amplifier is powered or for
several minutes afterwards. Assume
that contact will be lethal! Never, ever
touch the PCBs if the amplifier is even
plugged in. If you want to measure
the heatsink temperature, use a noncontact IR thermometer.
26
Silicon Chip
So we came up with the idea of using
some of the relatively cheap modules
available on sites like eBay and AliExpress.
There were three questions on our
minds: were they safe, would they
even work, and would the performance be acceptable? So we started
surfing online shops and came across
two promising modules (see the adjacent panel).
Deciding on the modules
All the modules we purchased for
evaluation have reasonably good availability and have been on sale for many
months. Some things that drove us to
choose them were:
Table 1 – Measured performance into a 4W
W resistive load
Voltage Freq
(RMS)
Load Power THD+N Notes
8V
1kHz
4W
16W
0.026% Warm up test, heatsinks 36°C
20V
1kHz
4W
100W
0.017% Heatsinks 40°C after a few minutes
30V
1kHz
4W
225W
0.019% Heatsinks 49°C after a few minutes
40V
1kHz
4W
400W
0.03%
44V
400Hz 4W
484W
Output started clipping
44V
1kHz
484W
Output started clipping
4W
Australia's electronics magazine
siliconchip.com.au
● The power supply modules have
decent mains-to-secondary isolation.
● They have decent heatsinking and
quality capacitors.
● They are common/available parts
sold in a range of voltages, ie, a moderately mature and supported design.
● The prices are neither too cheap
to be true nor overly expensive.
So we placed orders for one of each
to test out (plus the modules listed in
the panel overleaf that we didn’t end
up using). The cost of each module
was in the region of $100.
$200 for a power supply and amplifier module is bonkers for this sort of
power level. If you have built a 500W
amplifier using discrete parts and a
linear power supply, you will know
that this would barely pay for the transformer, let alone the rest.
So is this still too cheap to be true?
Our greatest concern with purchasing
this sort of equipment online is safety
and electrical standards. In choosing
these modules, we spent a lot of time
downloading photos and trying to
see how they were laid out, if there
were slots milled between feedback
opto-isolators and suchlike.
Once we had received them, we
inspected them to see if they matched
the pictures – they did. We then tested
them to the best of our ability using our
old-school megger (500V) and found
no measurable leakage from primary
to secondary on both power supplies
during a 60-second test.
We are not promoting these power
supplies as being compliant with any
standard, mind you! But there is visible isolation built into the design and
measurable isolation on test, which
was enough for us to work with them.
The power supplies purchased
both claim to be capable of “1000W”,
although the smaller of the two owns
up to being more like a 500W continuous unit. We think it reasonable to
rate both power supplies under 1kW
continuous, given the parts used, especially the smaller one.
Of the two sets of modules, we chose
to proceed with the larger, black modules. We have provided some information on the ‘also-ran’ modules for
interest but recommend that you stick
with the two shown opposite.
Performance
Table 1 shows some spot measurements of distortion at various power
levels. These agree with the claims
siliconchip.com.au
#1 Large Class-D Amplifier
IRS2092S 1000W Class-D Mono
Amplifier (see Photo 1):
siliconchip.au/link/abic
siliconchip.au/link/abid
siliconchip.au/link/abie
Claims
Speaker protection operating from an
independent power supply
Supply voltage: ±65V to ±80V
Photo 1: this
Maximum output power: 1000W
500W+ Class-D
amplifier module
Efficiency: ≥90%
was under $100 and includes a speaker
Signal-to-noise ratio (SNR): 90dB
protection relay. The control circuitry is
Dimensions: 157 × 101 × 44mm
mounted on a vertical sub-PCB.
Net weight: 0.45kg
This has four onboard 1000µF 100V supply bypass capacitors per rail labelled
Nichicon HE(M), 18mm in diameter and 42mm high. The Nichicon data sheet we
found did not list a 1000µF, 100V cap in this range, 820µF being the largest. The
size of this capacitor is consistent with the ratings.
The main switching transistors are both labelled IRFP4227. The output bobbin
is wound on a substantial toroid (35mm diameter) using 1.2mm enamelled copper
wire. This amplifier incorporates a speaker protection circuit with a substantial
relay. It is more of a high-power AC relay, rated at 30A, but the DC voltage rating
is only 30V. Still, we would rather have this in the circuit than not!
#2 Large Switch-Mode Supply
1000W LLC Soft-Switching Power
Supply (see Photo 2):
siliconchip.au/link/abif
siliconchip.au/link/abig
Claims
Output power: 1000W
Input voltage: 220V AC (nominal)
Output voltage options: ±24V, ±36V,
Photo 2: this inexpensive “1000W”
±48V, ±60V, ±70V or ±80V (±70V
switch-mode power supply seems to use
in our case)
reasonable quality components and, as far
Efficiency: 88-93.7%
as we can tell, is sufficiently safe. We were
Standby power: 2W
pleased that it passed a 500V insulation
Size: 156 × 100 × 50mm
breakdown test.
Net weight: 350g
It has four input filter capacitors rated at 180μF and 400V, which should provide
sufficient headroom at 220-240V AC. The CapXon brand capacitors have a ripple
current rating of 700mA each. When delivering 1kW, the ripple current will be
just over that. So their ratings are marginal if we use this to its full rated capacity.
The output capacitors are labelled Nichicon 1000μF 80V. These are low-
impedance capacitors made for switch-mode power supplies that are the right
size and look OK. The mains rectifier is a GBK2510, rated at 1000V & 25A. The
output diodes are MURF2040CT 20A ultrafast rectifiers.
The mains-side switching transistors have their part numbers ground off!
As shown in Photo 3, the clearance on this module between Neutral and the
mounting screw (which will be Earthed via the chassis) is just over the minimum
allowable. However, it is better than the other one we
bought and considered (see “The also-ran modules” panel
overleaf), so it is OK.
Photo 3: the distance between this component lead that
connects to the incoming mains Neutral and the mounting hole
is smaller than we would prefer, but is just enough to meet
separation standards if Neutral & Active are swapped. That is
more common than you might think, especially in old houses.
Australia's electronics magazine
April 2023 27
Fig.1: the frequency
response of the
500W Class-D
module is very flat,
dropping by only
0.4dB at 10kHz and
1.4dB by 20kHz. It’s
definitely suitable
for driving an LFE
(low-frequency
effects) channel,
given that there is
no such roll-off at
the low end.
made by the module suppliers. A distortion level of around 0.02% at 1kHz
is not exactly hifi, but it isn’t terrible
either. It is certainly acceptable for
many tasks, especially PA, sound reinforcement, or driving a subwoofer in a
hifi or home theatre system.
The frequency response into a 4W
load is shown in Fig.1. There is a bit
of a drop-off at the upper end, but it
isn’t terrible. It is, however, totally flat
down to 10Hz, making it perfect for
driving a subwoofer. The slight rise at
5Hz is irrelevant as it is minimal. LFE
(low-frequency effects) channel content might go down to 3Hz, at which
point it will still be very close to 0dB.
Maximum power testing
Scope 1: the amplifier output (yellow) into a 4W load near clipping, close
to 500W. As it approaches clipping, the Class-D switching frequency drops
from 225kHz to about 56kHz, allowing it to deliver a lot of power with some
distortion. The ‘choppy’ appearance of the waveform is normal for Class-D.
Scope 2: the amplifier pulsed output at around 1kW peak into a 2W load.
There’s something nasty going on near the zero-crossings that would lead to
very high distortion (if you can see it on a ‘scope, it’s bad!). Still, it is capable of
driving 2W as long as the signal dynamic range is high enough.
28
Silicon Chip
Australia's electronics magazine
Using a 1kHz waveform, the amplifier ran for an extended period delivering 500W into a 4W resistive load.
When loaded, the 15V rail voltage
increases, almost certainly a result of
this rail being an unregulated winding on the switch-mode transformer.
The dummy load was a set of 1W
resistors made from very heavy duty
Nichrome wire. At full load, they
were just short of red hot, and the
heat generated was enough to make
it uncomfortable to hold your hand
20cm above the dummy load. The
amplifier sustained this on a continuous basis throughout a 20 minute test
– see Scope 1.
Reducing the output to about 30V
RMS and the load to 2W, the protection relay immediately switched off.
Assuming this was overload protection, we switched to using a pulsed signal that is more typical of music, with
six cycles at 1000Hz followed by 100
cycles of silence and then it repeats.
The amplifier was able to generate
this waveform at clipping into 2W. The
output voltage was about 60V peak,
consistent with a claim of close to 1kW
– noting that they specify 10% distortion and the tests here were below
clipping. The fact that the amplifier
shut down for continuous duty but
was capable of brief bursts of output
is important.
We doubt this amplifier would drive
a 2W subwoofer with modern music,
which can have significant content at
low frequencies. The amplifier was
happy with a continuous waveform
into 4W, though. The distortion into
2W was visible on the scope (see Scope
2), so we would dread to think of the
actual distortion level.
siliconchip.com.au
The ‘also-ran’ modules___________________________________________________
We considered other amplifier & power supply modules when designing this amplifier. The following modules looked OK, but
we decided they were not as good as the ones we went with. Some readers might still be interested in using them in different scenarios, although note that the safety of the alternative switch-mode supply is concerning.
#3 Small Class-D amplifier
IRS2092S 1000W Mono Digital Amplifier (see Photo 4)
siliconchip.au/link/abih
Claims
Supply voltage: ±58V to ±70V
Output power: 1000W (±70V power supply, 2Ω load, 10% THD)
Efficiency: ≥90%
SNR: 90dB
THD+N (±70V, 2Ω): 1% <at> 900W, 0.1% <at> 750W
Frequency response: 20Hz ~ 20KHz
Speaker load impedance: 2-8Ω
Voltage gain: 36 times
Input Sensitivity: 1.5V RMS
Protection: output short circuit, speaker protection (no relay, though!),
over-temperature
Dimensions: 132 × 68 × 45mm
Weight: 260g
The output filter capacitors are two 470μF 100V units per rail, labelled Fulkon
CD288H. Data sheets were not obvious on the internet, but they look about the
right size for the job. The main switching transistors are both labelled IRFP4227,
but the labelling is quite different between them. The output bobbin is wound on an
E-core using Litz wire, which is reassuring.
Photo 4:
we also
tested this
Class-D amplifier
module which
could deliver a similar
amount of power. We didn’t choose this
one because we’d be running it right at the
upper limit of its specified voltage range,
whereas the other module has another
10V of headroom and also seems a bit
better designed.
#4 Small Switch-Mode Power Supply
LLC Soft-Switching 1000W Power Supply (see Photo 5)
http://siliconchip.au/link/abii
Claims
Input voltage: 200-240V AC
Output voltage: ±35 to ±80V (±70V in our case)
Other output voltages: independent 12V, auxiliary ±12V
Voltage regulation: main ±3% with no load or ±10% with load; independent,
±15% with no load
Output current/power: 880W for main, 0.5A each for independent and auxiliary
Continuous power: 500W <at> 25°C
Rated power: 880W for about 5 minutes at 25°C. A cooling fan should be added
for long-term operation.
Peak power: 1200W (less than 100ms)
Efficiency: up to 95%
Weight: 400g
There are four input filter capacitors rated at 120μF and 400V, sufficient for running this from 220-240V AC with headroom. The input capacitors are smaller both
physically and in capacitance than the preferred unit. At 1kW, their ripple current
will be more than 800mA. The data sheet on the installed parts does not specify
this parameter, but looking at similar parts, this will likely exceed their rating.
The output capacitors are labelled SLF 1000μF, 100V in the CD288H range, specified for high-frequency and low-impedance. These look right for the job. The mains
rectifier is a KBL608 unit rated at 800V, 6A unit. That is marginal.
Somewhat disconcertingly, the clearance from the mounting hole (to an Earthed
standoff) and Neutral on this PCB is closer than desirable – see Photo 6. With a
shakeproof washer, it is a touch over 2.5mm, right on the edge of acceptability. A
solution might be to use no washer or a smaller washer.
Photo 5: the
alternative power
supply. It can’t
deliver quite as much
continuous power as the one
we ended up using and seemed to use
inferior components that are operated
too close to their ratings for our liking
(in some cases, beyond!).
Photo 6: the power supply shown in Photo 5 also has too little clearance between the
Earthed mounting hole and the nearest Neutral conductor.
siliconchip.com.au
Australia's electronics magazine
April 2023 29
So in summary, the amplifier ‘does
what it says on the box’ aside from
delivering that kilowatt into 2W.
Design
So, let’s look at what it takes to turn
these into a very powerful amplifier.
The basic arrangement is shown in
Fig.2. It is very much about the appropriate connection of the modules and
the provision of some cooling.
This is a ‘monoblock’ amplifier
with no volume control. We expect
you would feed it from a preamplifier that provides volume control,
input switching etc. For stereo use,
you would need to build two of these,
although if you want to power a subwoofer, one should be fine by itself.
In terms of a preamp as part of a stereo system, you could use our Digital
Preamp with Tone Controls from September & October 2021 (siliconchip.
au/Series/370) or our Ultra Low Distortion Preamplifier with Tone Controls
from March & April 2019 (siliconchip.
au/Series/333).
You could, in theory, add a volume/
level control pot on the front panel and
route the signal wiring to the amplifier module via that pot. We’ll leave
that as an exercise for our readers as
we expect most constructors will use
a separate preamp.
Build and testing
We first had to work out how to
house this safely and at a reasonable
cost. We chose the Jaycar HB5556 chassis as it is just right in size, of good
Fig.2: thanks to the prebuilt modules, the ‘circuit’ of this amplifier is dead
simple. The power supply generates three rails: -70V, +70V and +15V, which are
fed to the amplifier module. The 15V rail also powers the 12V fan via a 39W 1W
dropper resistor.
build quality and at a great price. This
case also lent itself to us implementing some forced air cooling.
There are three main baffles to keep
things cool, as shown in Fig.3.
We are striving to achieve forced airflow over the heatsinks for the power
supply and Class-D amplifier. Even
though these are better than 90% efficient, if you are driving 1000W into
Speaker power handling
Speaker power ratings are a bit of a vexing topic. Those who were around in
the 1980s and 1990s will have seen the outlandish Peak Music Power Output
or “PMPO” numbers that ran into the thousands of watts, often from a 10W
IC amplifier chip!
At a more pragmatic level, the power rating of a loudspeaker is primarily
defined by the capacity of the voice coil to dissipate energy and, at a mechanical level, the excursion limit of the cone.
For example, a tweeter typically has a 25mm coil weighing a small fraction of
a gram. Many are rated at 100W or more, but the actual continuous power they
can handle is only a couple of watts. They rely on the crossover and the nature
of music signals to reduce “100W” to only a few watts seen by the tweeter.
Woofers have a much tougher life. AES2-1984 defines the power handling
test. Power handling is measured with pink noise with a 6dB peak-to-RMS
ratio. For example, the BEYMA 21LEX1600Nd driver has a 3200W “program
power” rating and a 1600W continuous power rating, equating to a 400W RMS
sinewave power rating.
Be warned that this amplifier could be very bad for the health of your domestic speakers! We have not recently produced a speaker design that can handle
500W continuously, although the Majestic Loudspeakers (June & September
2014 issues; siliconchip.au/Series/275) are somewhat close, at 300W (tested).
30
Silicon Chip
Australia's electronics magazine
a load, that is 50-100W being dissipated in each module, mainly via
their heatsinks. They will get very hot
running this way without air moving
over them.
Of course, this will not normally be
the case. Typical music has a crest factor over 10dB (depending heavily on
the type of music), which means that
on average, with full-range music not
being driven heavily into clipping, the
output power would rarely be over
100W for very long.
But consider the realistic use
case for a 500W (or 1000W) amplifier; its niche is in subwoofer duty,
where, with modern music, all bets
are off. Modern music has periods of
close-to-continuous bass output. So
keeping everything cool is essential.
With modest output, say, averaging up to 100W or so, these amplifier
modules are fine in a case with passive
cooling. If that is your application, you
can avoid manufacturing the plenum
presented here. If you intend to play
loud music for extended periods, you
need to bolster the cooling.
Our plenum is made from three
folded sheets of aluminium and uses
the case’s lid as the top. This allows
us to add a fan and force air over the
heatsinks, increasing their efficiency.
siliconchip.com.au
Fig.3: the case is reasonably compact yet more than large enough to fit the two modules.
A series of baffles direct air sucked in through the rear panel (by an 80mm fan) across
the heatsinks of the amplifier module and power supply, then out through vents on
the left side. The top vents are blocked off to prevent air from escaping before it has
completed this route.
Without getting too much into the
details of heat removal, consider that
heatsinks dissipate energy through
convection (hot air rising from the
heatsink being replaced by cooler
air), radiation (mainly IR energy being
emitted) and conduction from the
heatsink into thermally connected
materials. Without running our amp
so hot that it’s about to melt, radiation
is not a significant factor.
Convection is an important means of
heat removal, but the case stifles this
somewhat, and even in free air, heat
will only be removed by convection
siliconchip.com.au
so fast. By forcing air from outside
through the case, over the heatsinks
and then exhausting it from the case,
we can increase the transfer rate
between the heatsinks and the air,
picking the heat up off the heatsink
and dumping it outside the case.
Making the baffles
We folded aluminium sheets to form
a labyrinth, with a fan forcing air in
from the rear of the enclosure and
using the perforations along the sides
of the case for exhaust. The panels are
all securely Earthed for safety. The
Australia's electronics magazine
cutting and folding details are shown
in Figs.4-6, with instructions to follow. We made ours from three sheets
of 1.2mm-thick aluminium, although a
thickness between 1.0mm and 1.5mm
will be fine.
You could alternatively use polycarbonate sheets and glue or tap and
screw them, or if you have the gear,
3D print it. Use our plan as a guide
and follow the principles of forcing air
across the heatsinks and out of the box.
Assuming you’re making the panels
as we did, first cut the metal sheets
to size. We used a jigsaw. An angle
April 2023 31
Fig.4: the plenum baseplate is bent up on either side to form the ends of the chamber. The cut-out in the upper left corner
is for air to exit into the left-hand side of the case, where it escapes via side vents.
Fig.5: this panel, also made from a bent aluminium plate, seals off the section of the plenum chamber closest to the case’s
front panel.
grinder with 1.6mm metal cutting
discs also works but requires caution. Drill the holes as shown before
bending.
If you do not have a pan brake,
1.2mm aluminium can be successfully bent by clamping it to a workbench with a tight 90° edge and using
a hammer and piece of timber to ‘panel
beat’ the corners into the metal sheet.
Go slowly and gently.
Make sure the end panel is a good
fit for the base. We achieved this by
32
Silicon Chip
making the base piece first, then, once
it was folded, adjusting the folded ends
of the rear panel to achieve an acceptable fit. This does not need to be perfect; there will be a fair bit of airflow,
so a leak here and there really does
not matter.
If you choose to paint your metalwork, make sure to mask off around
the Earth lug, as you need a good electrical connection there.
With the baffles made, cut the
holes in the rear panel for the fan,
Australia's electronics magazine
input, output and power connectors,
as shown in Fig.7. This is an inside
view, so if you are cutting from the
outside, make sure to mirror it. The
final result from the outside (once
all the components are mounted) is
shown in Fig.8.
Cutting the fan hole is a bit fiddly.
We used the ‘drill and file’ method, in
which you drill many 4-5mm holes
around the inside of the final cut line
to remove the bulk of the material,
then use a file to smooth the edges. An
siliconchip.com.au
Fig.6: this baffle
divides the plenum
chamber into two
halves, one side for
the power supply
and one for the
amplifier module.
The rectangular
cut-out allows air to
pass from one side
to the other.
Fig.7: this shows the cut-outs needed in the rear panel but note that the large hole at bottom centre, with two smaller holes
near it, is for the Speakon terminal that constructors might opt to leave out. The RCA socket hole has been moved since
we built the prototype, as it interfered with the fan.
Fig.8: this shows how the rear panel should look once completed. The Speakon terminal is wired in parallel with the
binding posts; only one is required, depending on the speaker connector you plan to use.
alternative method is to use a jigsaw
with a metal cutting blade.
To make the holes in the base of
the case, present the plenum base
to the rear panel with the rear panel
in the case, then mark the mounting
holes. These are shown marked on the
siliconchip.com.au
drawing; there are six of them between
the folds. This will ensure these are in
exactly the right spot.
Once marked, drill these, then the
mounting holes for the PSU and amplifier modules. These holes need to be
countersunk on the underside. Photo
Australia's electronics magazine
7 shows how we aligned the plenum
in the case to drill the mounting holes.
Mount the amplifier and PSU modules now, as shown in Photo 9. Use
countersunk M3 machine screws to
secure the eight 15mm threaded PCB
standoffs to the base. This will allow
April 2023 33
Photo 7: once you’ve made the plenum
base, you can fiddle with the baffle
separating the two halves, so it’s a
good fit and not too much air will leak
past.
Fig.9: cut a sheet of Presspahn or similar insulating material (thick cardboard will do) and mount it on the power supply
to ensure sufficient airflow over both the heatsinks and transformer.
the plenum assembly to sit flat in the
case when assembled. Then use 6mm
M3 machine screws and star shakeproof washers to secure the boards.
Optimising the airflow
We made an extra baffle for the
power supply module to force more
air over the heatsinks, shown in Fig.9,
made from Presspahn. Unfortunately,
Presspahn insulating card is becoming
hard to get, although we did find an
equivalent material (see the parts list).
If you can’t get that, use thick
cardboard, as we are not relying on its
insulating properties too heavily here.
Under no circumstances use metal.
This is secured with two M3
machine screws and star shakeproof
washers to the tapped holes in the top
of the heatsink. Use Loctite to ensure
these screws do not come loose over
time. You should also stick a piece of
card to the inside the top panel to cover
the vent holes over the plenum. This
way, the air does not escape through
there and has to flow past all the heatsinks on the way out.
Once that’s in place, cut and stick
lengths of weather-stripping foam
along all the top edges of the plenum
chamber and baffles, as shown in the
photos. This will make a seal with the
case’s lid so that too much air doesn’t
flow over the panels and mess up the
airflow pattern.
Wiring it up
With the modules installed, mount
the internal baffle. This is important as
it controls airflow, as shown in Fig.3.
You can see how this sits in Photo 8.
Photo 8: the rear view
of the 500W Class-D
Amplifier’s chassis.
34
Silicon Chip
Australia's electronics magazine
siliconchip.com.au
SILICONE SEALANT
OVER EXPOSED
METAL
HEATSHRINK SLEEVES OVER
SPADE LUGS & CONNECTORS
N
L
+15V
GND
CABLE
TIES
12V FAN
V+
GND
V–
OUT GND
39W 1W RESISTOR
POWER SUPPLY
MODULE
AMPLIFIER
MODULE
IN
GND
+15V
GND
V–
GND
V+
PRESSPAHN BAFFLE
(NOT FULL HEIGHT)
HEATSHRINK SLEEVES OVER ALL SPADE LUGS & CONNECTORS
Fig.10: all the wiring for the amplifier is shown here, except that the Speakon connector has been left off. If fitting it, wire
it in parallel with the binding posts. You could use the spare output terminals on the amplifier module for that if you
wanted to. Don’t leave off the insulation or cable ties for the mains wiring (also see the photos) and ensure the Earth lug
makes good contact with the chassis base.
Once it is screwed in, install 10A
mains-rated red, green and black wire
between the ±70V outputs from the
PSU to the amplifier module’s power
inputs, referring to Fig.10. This rating is essential as there is 140V DC
between these conductors and they
can carry significant current.
Use medium-duty hookup wire to
connect the independent 15V power
rail to the amplifier module.
Add lengths of 6mm heatshrink
siliconchip.com.au
tubing over much of these two sets of
wires because we will run these cables
through the hole in the internal baffle,
and we will be tying these to the very
top of this opening. This will control
where these cables sit, and the heatshrink adds a level of protection and
ruggedness to this cabling.
Fan connection
Next, connect the power to the fan.
The fan is a 12V type, but the closest
Australia's electronics magazine
rail we have is 15V DC, so a 39W 1W
resistor connected in series with the
fan drops about 3V. The fan is wired
to the 15V connector on the amplifier
board. Use light-duty or medium-duty
hookup wire.
Mains wiring
The mains wiring is also shown in
Fig.10. There is not a lot of it; however,
you must take caution with all wiring
as most is either mains potential or
April 2023 35
Photo 9: this shows how the two modules fit inside the plenum chamber within the case. The wiring between the two
modules has been run along with the input and fan wiring, but the output and mains connections have not been made yet.
high voltage DC or AC (the output).
Ensure all wiring is secured with zip
ties to keep it tidy and controlled if
anything comes loose.
Install the power switch on the front
panel as shown in Fig.11. Then take
two lengths of brown and blue mainsrated 10A wire and connect from the
IEC mains connector to the switch as
shown.
Connect the topmost terminals on
the switch to the IEC mains input, and
then run a second pair of wires from
the central switch terminals back to
36
Silicon Chip
the mains input on the power supply.
We used insulated crimp connectors on the IEC connector and switch.
If you wish to solder these connections instead, insulate the joints with
10mm diameter heatshrink tubing.
Keep these wires twisted and tidy,
and zip-tie them such that they cannot come loose in the case. We found
it handy to label the unswitched and
switched input wires.
Using a length of yellow/green
striped 10A mains-rated wire, connect
the Earth pin on the IEC connector to
Australia's electronics magazine
the M3 Earth screw that runs through
the case and plenum metalwork.
Before assembling this, take a utility
knife and scrape the paint from the
case around this bolt. Use a star shakeproof washer on the bottom and top
of the case and attach a 3.2mm solder
lug to this.
Connect the Earth wiring and
check continuity with a multimeter.
Install an 8A or 10A ceramic fuse
in the IEC mains input/fuseholder
assembly. Remember to insulate the
exposed metal strip on the back of this
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Fig.11: just one hole is needed in the front panel for the power toggle switch. That is unless you elect to add a volume
control pot or a power-on indicator (an illuminated switch could be used instead).
connector with neutral-cure silicone
sealant, as it will otherwise be live
whenever the mains cord is plugged in.
this to the top of the plenum with a
cable tie and want this as extra abrasion protection.
Output wiring
Input wiring
Use mains-rated 10A rated wire for
the amplifier output connections. We
used 400mm of green and red wire
twisted together from the amplifier
output to the output connectors. We
included both Speakon and binding
posts outputs; you may only need the
binding posts.
We sleeved the output wiring in a
250mm length of 6mm diameter heatshrink tubing. We did this firstly to
ensure there could be no confusion
between this and the power wiring
and also because we will be securing
Take 300mm of shielded cable
and connect the RCA connector on
the rear panel to the screw terminal
header on the amplifier board. Use
a short length of sleeving to insulate
the exposed ground braid and 20mm
of 3mm diameter heatshrink to form
nice terminations.
Caution
At this point, you should have a
standalone chassis with the amplifier
modules installed and wired up.
First and foremost: safety. If you are
not totally comfortable working with
high voltages then do not proceed
without help. It’s also safest to do the
first power up with the lid secured.
This amplifier can generate a lot of
power. To do this, it uses high supply
rails of ±70V DC. It could easily stop
your heart if you make contact with
these two rails. Also, the switch-mode
power supply operates from the mains
and has close to 400V DC in parts of
the circuit. This is also lethal.
Second: danger to your possessions.
The amplifier generates 44V RMS continuously into a 4W load. This is close
to 500W. If you feed this into your
speakers as a sinewave, we can guarantee you will destroy them. See the
panel on “Speaker power handling”.
This view shows how we wired up the output connectors and gives you a good view of the Presspahn baffle that optimises
airflow over the power supply module. You can also see how the mains input wiring has been insulated. Also note how
the output wiring and ±70V rail wiring is cable tied to the top of the plenum, just behind the Presspahn baffle.
siliconchip.com.au
Australia's electronics magazine
April 2023 37
Similar cautions apply for test
equipment; make sure that if you connect this to a distortion analyser, it is
on a 50V or 100V RMS range.
Testing
First, check that the mains power
switch is on, then with it unplugged,
do a final check with a DVM on its
20MW range (or similar) and check for
any measurable resistance between
the Active and Neutral inputs and
the output ground connector. If there
is, then you need to stop and find the
problem.
Also perform a final check of your
Parts List – 500W Monoblock Amplifier
1 1000W Class-D amplifier module (see links at the start of the article)
1 1000W 70V split-rail switch-mode power supply (see above)
1 vented metal bench enclosure, 304 × 279 × 88mm [Jaycar HB5556]
1 dual binding post for speakers [Altronics P9257A]
1 panel-mount insulated RCA socket [Altronics P0220]
1 fused IEC mains input socket [Altronics P8324]
1 10A+ mains-rated chassis-mount DPST/DPDT toggle switch
[Altronics S1052 or Jaycar ST0585]
1 8-10A fast blow sand-filled or ceramic M205 fuse [Altronics S5934]
1 Speakon chassis-mount speaker connector (optional) [Altronics P0792]
1 quiet 80mm 12V fan [Altronics F1150]
1 80mm fan guard [Altronics F1022]
1 2-way 2.54mm-pitch vertical polarised header
[Altronics P5472, Jaycar HM3412]
1 2-way 2.54mm-pitch polarised header plug
[Altronics P5492 + 2 × P5470A, Jaycar HM3402]
1 39W 1W resistor
Hardware
1 428 × 225 × 1.0-1.5mm aluminium sheet (for base)
1 225 × 103 × 1.0-1.5mm aluminium sheet (for baffle)
1 259 × 100 × 1.0-1.5mm aluminium sheet (for plenum end)
1 115 × 65mm sheet of Presspahn or similar insulating card
[www.ebay.com.au/itm/293254125529]
9 M3 × 16mm panhead machine screws
12 M3 × 10mm countersunk head machine screws
16 M3 × 6mm panhead machine screws
12 M3 hex nuts
32 M3 star shakeproof washers
8 15mm M3-tapped spacers
1 3.2mm solder lug [Altronics H1503]
9 blue insulated 6.3mm female spade crimp lugs for 1.5-2.5mm2 wire
[Altronics H2006B]
1 1.2m length of 9-10mm wide adhesive foam weather stripping
[Bunnings 3970353]
1 1.5m length of 5-10mm wide adhesive foam weather stripping
[Bunnings 3970353]
1 pack of small Nylon cable ties
Wire & cable
1 1.5m length of brown mains-rated 10A hookup wire
1 1.5m length of blue mains-rated 10A hookup wire
1 0.5m length of green/yellow striped mains-rated 10A hookup wire
(eg, stripped from a length of 10A three-wire mains flex)
1 1m length of red mains-rated 10A hookup wire
1 1m length of green mains-rated 10A hookup wire
1 1m length of black mains-rated 10A hookup wire
1 0.5m length of red medium-duty hookup wire
1 0.5m length of black medium-duty hookup wire
1 300mm length of single-core shielded audio cable [Altronics W3010]
1 1m length of 6mm diameter clear heatshrink tubing
1 200mm length of 3mm diameter clear heatshrink tubing
38
Silicon Chip
Australia's electronics magazine
wiring. A fault here will be both spectacular and dangerous.
Plug the amplifier in, switch it on
and listen for the speaker protection
relay switching in after a couple of
seconds.
Carefully measure the voltage
between ground, V+ and V− on the
power supply output using some properly insulated DMM probes and a suitably rated meter. The rails should both
be within 5V of 70V but with different polarities.
Carefully measure the voltage on the
+15V input to the amplifier and ensure
it is close to expected.
If any of the above fails, unplug the
amplifier and leave it off for 10 minutes. After verifying that the mains
plug is still out, disconnect the power
amplifier from the power supply so
you can check the PSU by itself. If
you can’t see the right voltages at
its outputs with no load, you have a
faulty PSU.
If the PSU measures OK, rebuild it
and check your wiring carefully.
Now plug in a signal generator
to the input and a CRO with a 10:1
probe set to measure up to 70V peak
to the output. Power up and look for
the sinewave on the output. Increase
the signal level until you see clipping; check that this is about 40-44V
RMS.
Connect a load and start the input
signal at a low volume level, increasing to a manageable level. Only use
a loudspeaker for this if you have no
other choice and are happy to test at
moderate levels only. If you have a
dummy load, run the amplifier at as
high a power as is safe for your load
for 5-10 minutes. If you are using a
speaker for the test, play some moderately loud music.
At this point, we are really just
checking that nothing goes wrong – no
puff of magic smoke etc. After testing
as hard as you feel safe, unplug everything and open the amplifier. Use an
IR thermometer to measure the temperature of the PSU heatsinks, the
E-core transformer on the PSU (in our
tests, this was the hottest part) and the
amplifier heatsink.
If these are all below 65°C, everything is fine and you are all set! Otherwise, check the airflow management
components (baffles, seals etc) to verify that there are no massive air leaks
and confirm that you haven’t skipped
any of the steps listed above.
SC
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