Building a Hi-Fi
amp on the cheap
by Julian Edgar
How do you build a great amp on a low budget? Using a salvaged amplifier
as the starting point lowers costs a lot – so, rather than developing an audio
amplifier from scratch, this article assumes that you are going to use either
prebuilt or kit amplifier modules. Available amplifier modules span quite a
range – you can choose from cheap and nasty, to low cost and very good, to
high cost and excellent!
B
uilding your own quality Hi-Fi
amplifier sounds like a great idea,
but there’s a problem – these days,
the costs add up so quickly. A good quality
case – £50. Power transformer – £50. Rectifier/smoothing capacitors – £25. Heatsinks
– £50. Amplifier modules – £75. We haven’t even included hardware like switches,
sockets and nuts and bolts yet, but suddenly you’re into the cost realm of quite good
commercial amplifiers – and with those,
all you do is hand over the money… no
construction needed!
But there is a solution at hand – start
with a good quality salvaged amplifier.
At minimum you’ll get the case, heatsinks and a lot of hardware. And at maximum? The power supply transformer,
rectifiers and filter capacitors.
Suddenly, creating a new, quality
audio amplifier can be as simple as installing some new pre-built or kit amplifier modules!
Starting points
There are two directions from which you
can begin – and the first is to start with
the main power supply transformer in
the salvaged amplifier. Audio amplifiers typically require positive and negative supply rails, achieved by using a
centre-tapped transformer followed by a
rectifier and filter capacitors (these latter
parts are easily achieved by again buying
an off-the-shelf module).
If the discarded amplifier has a good
quality transformer (eg, a large toroidal
design), measure its AC outputs. Depending on the amplifier, the transformer
Practical Electronics | December | 2020
output might be anywhere in the range
from 18V to 43V per winding – or more.
The power capability of the transformer can be measured by adding loads (eg,
high-power resistors or incandescent
light bulbs) and monitoring the voltage
sag. Most transformers are specified for
their rated current at a 10% voltage drop.
However, rather than making measurements, it’s often easier to just guess the
VA rating of the transformer based on its
size. For example, a 300VA toroidal will
usually be about 110-120mm in diameter and about 50mm high, while a 500VA
toroidal will be similar in diameter but
about 65mm high. If in doubt, look at
a few online catalogues, the physics of
transformers means that sizes are pretty
consistent. In rare cases, the VA rating
of the transformer will be written on it.
With the transformer specs available,
you then have a starting point from which
you can select suitable amplifier modules
– either kit or prebuilt.
For example, the very well regarded
SC200 amplifier kit mono module (see PE,
January to March 2018) requires a 40-040V transformer. At the other end of the
pricing spectrum (and probably also audio quality spectrum – all things are relative!) is the two-channel 300W amplifier
board (available from Banggood – model
V-MOS300W) that requires a 24-0-24V
transformer. This module has a built-in
power supply, so no further electronics
are needed – just speaker and input connections. (But you would probably also
want to upgrade the provided heatsink.)
PE audio guru Jake Rothman suggests
a good rule of thumb is to select a transformer with a VA rating double the audio
power rating of the amplifier. That is, a
150W (total) amplifier would require a
300VA transformer. Obviously, the exact
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n Do ensure that the power transformer
output matches the amplifier board
requirements in both voltage output
and power
n If using a new power transformer and
salvaged filter capacitors, check that
the capacitors are still within their
voltage ratings, and the rectifier within its current and voltage ratings.
You will also need to fit everything in the
old case. However, unlike much electronics equipment, many amplifiers are relatively roomy inside, so this is not usually
a problem. If, for example, you’re aiming
to fit a four-channel amplifier into an enclosure that once had only two-channel
internals, do some careful measuring before buying any bits.
Doing it
Fig.1. The completed 400W, two-channel amplifier. By using parts salvaged from a
defective amplifier, construction cost was brought way down. With its heavy-duty case,
chassis and heatsinks, it weighs 12kg. All the external panels were freshly painted in
trademark Edgar red (see last month’s pedal power station!).
requirement depends on the efficiency
of the amplifier circuit design, and the
‘double’ rule might also stretch your budget a long way! (In the amplifier design
covered in a moment, I used one 300VA
transformer per nominal 200W module.)
So, unless the transformer in your salvaged amplifier is really unusual in its
output voltage, or too low in its power
capability, you should be able to find an
off-the-shelf prebuilt or kit amplifier module that suits it.
The other approach is to start with
the amplifier modules you intend using.
Unless you are lucky, that means in turn
you will probably need to buy the power transformer – but you may be able to
salvage the filter capacitors and rectifier
from your cast-off amplifier. There are
really lots of ways of going about it, but:
My starting point was an old commercial
amplifier, a two-channel design in a rackmount enclosure. What attracted me to it
were the very substantial heatsinks, one
each side of the case, that used vertical fins.
Because the natural convective airflow
past the heatsinks is vertical, having vertical fins is likely to provide much better
cooling than the more common horizontal
fins. The other element that attracted my
interest was that the amplifier was really
heavy – and invariably with amplifiers,
heavy = better! That might sound a bit
of a simplification, but a heavy amplifier usually has a large transformer as well
as a strong enclosure and big heatsinks.
I’d bought the amplifier – it came
from the local recycling shop for about
£10 – not expecting to use it as a salvage
Fig.2. The salvaged commercial PA amplifier from which the enclosure, heatsinks and other parts were taken. This amplifier kept
blowing an internal fuse – and without a circuit diagram, it wasn’t worth chasing-down the fault.
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my amplifier modules, power supplies
and transformers. These were:
n Two 200W mono LM3886 BTL
amplifier boards
n 40,000µF capacitance, 35A rectifier
power supply module
n 300VA, 25-0-25V toroidal transformer
Fig.3. Inside the new amplifier. It uses two new transformers, their associated rectifier/
capacitor modules, and new amplifier modules. The salvaged amplifier provided the
case, heatsinks and bits and pieces like the mains switch, two volume controls and the
input sockets.
amplifier; I thought in fact it might work!
However, testing showed that it repeatedly blew an internal fuse when powered-up. It wasn’t the main power fuse
(implying that the transformer was still
OK) but a fuse on the amplifier board itself. I could have tried fault-tracing, but
to be honest, finding the problem in an
amplifier I knew nothing about, and for
which I didn’t have a circuit diagram, was
a bit much for me. So instead I decided
to use it as the basis of a new amplifier.
The 300VA toroidal transformer had a
measured output of 43-0-43V (that is, 43V
measured across each winding, and 86V
across both) which would have made it
suitable for the aforesaid SC200 modules.
However, in this case I’d already bought
Furthermore, in addition to the two
LM3886 mono modules, I’d also bought
two of the transformers and two of the
power supplies. That is, I wanted to effectively build two completely separate
mono amplifiers in the same case. Taking this dual-transformer approach can
reduce costs over buying a single large
transformer – especially if you already
have one of the transformers.
But would all the parts fit? The first step
was to disassemble the salvaged amplifier. I removed the transformer and amplifier board (that incorporated the power
supply) and studied what space I now
had to work with.
Interestingly, the amplifier enclosure
consisted of four large heatsinks, two
joined along each side by heavy aluminium angle. In the original amplifier design, the output transistors bolted to this
aluminium angle, that in turn conducted the heat to the main heatsinks. Bridging the gap between the heatsink sides
of the amplifier was a folded aluminium
sheet chassis on which the transformer
sat. The main PCB just bridged the gap
under its own strength. The front panel
was a thick, machined aluminium sheet,
while the back panel was a thin folded
section, again made from aluminium.
Power supply for tone
controls or speaker
protection?
The amplifier, as shown here,
doesn’t use tone controls or external speaker protection. These extra
boards typically require an AC 12V
supply. Many amplifier transformers have an additional winding to
provide this lower voltage. However, the transformers I was using
didn’t have these windings.
I therefore decided to provide
an AC 12V power supply – just in
case I later decided to add some
more functions to the amp. The
easiest way of providing this was
to use the transformer from an old
9V DC plug-pack (wall wart). The
plastic case of a plug-pack can be
most easily opened by crushing
it a little in a vice until it cracks
open – there’s normally plenty
of room to squeeze the enclosure
before the transformer inside is
damaged. The small transformer
was held in place by a metal strap.
The small transformer mounted between the toroidal power supply transformers is a 12V
unit designed to power a speaker protection or tone control board, should one be added
in the future.
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Fig.4. The view of the new amplifier with the rear panel removed. Note the heavy
aluminium angle that connects the LM3886 modules to the original heatsinks, and
the aluminium angle used to heatsink the bridge rectifier – it’s thermally connected
to the aluminium chassis below the boards.
Two steel cover sheets fitted top and bottom, attached to the heatsinks via screws.
The reasons for this detailed description are as follows. First, I could see that
with some minor changes, almost the
whole amplifier enclosure could act as
a heatsink. To achieve that, all that was
needed was to thermally bond the various
aluminium parts together. Second, because the enclosure could be completely
disassembled, the panels could be used
as templates if I wanted to make any new
ones. For example, and jumping ahead a
little, the rear panel was full of holes for
connectors I no longer needed. But by
unscrewing the panel, it was fairly easy
to make a replacement – the pattern was
right in front of me!
it should still work fine. The two transformers could mount where the original
transformer had sat – there was enough
room on the original bridging chassis.
However, because this panel had holes
that were now in the wrong places, I made
another from aluminium sheet.
But what of the power supplies – the
fairy large boards containing the filter capacitors and rectifiers? The issue was heatsinking the rectifiers. I’d selected boards
that mount the rectifiers at the edge of the
PCB (many do not) so that I could attached
heatsinks – but how was this to occur?
There was no room to use the main heatsinks, and the very small heatsinks that
could be attached would likely be insufficient. So instead I used a variation on the
approach being taken with the main output transistors. I used heavy aluminium
angle to attach the rectifiers to a new folded
aluminium cross-chassis that supports the
amplifier and power supply boards. This
panel attaches to the heatsinks – so the rectifiers are effectively thermally connected
to the main heatsinks – and the aluminium
angle and bottom aluminium panel provide plenty of heatsinking, even if acting
alone. (As I said, almost all the enclosure
is a heatsink!)
So, let’s take stock. Using the old panels as a template, I’ve made new rear
and transformer support panels. I’ve also
made a new panel that supports the amplifier and power supply boards, and additionally acts as a heatsink and thermal
bridge for the rectifiers. From the original
amplifier’s enclosure, I am retaining the
Finding space
I moved my various newly bought components around in the space until I found
an arrangement I thought could work. The
output transistors of the new modules
would need to be bolted to a new piece
of aluminium angle that in turn could
be bolted to the amplifier’s original angle. This would add another step before
the heat could get to the heatsinks, but I
thought that if I used really heavy angle,
Fig.5. When applying heatsink compound, you should use sufficient that it just
squeezes out all around the components, as here. In this amplifier, all the aluminium
parts of the case are thermally connected using heatsink compound – the whole
enclosure therefore acts as the heatsink.
Fig.6. If you are using prebuilt or kit modules for the power
supply and amplifier boards, the circuit will look something
like this. Variations include the powering of two amplifier
boards (eg, two mono boards) from the one power supply, or
the use of two transformers and two power supply modules,
each powering one amplifier board. Important aspects to
take note of are the use of the fuse and a double-pole,
single-throw (DPST) switch on the mains input, the grounding
of the mains earth lead to the metal case, and the observing
of polarity with all the amplifier board connections.
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M ains
supply F use
L ive
D P S T
switch
T ransform er
+
+ V
0 V
P ower
supply
board
0 V
N eutral
– V
–
E arth
B olt to all parts
of m etal ca se
–
A m plifier
board
+
+
–
+
–
S peakers
A udio
inputs
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Fig.7. An infrared thermal image of the amplifier after about an hour playing music
at full volume in 20°C ambient conditions. (It was so loud I needed to wear ear
protectors.) The LM3886 ICs are running at just under 78°C. Their specified
maximum junction temperature before auto-shutdown is 165°C.
large heatsinks and their joining aluminium angles, the front panel and the top
and bottom cover panels. It may sound a
bit like ‘Grandpa’s axe’, but in fact it was
much easier (and cheaper) taking this
approach than starting with a new general-purpose (eg, rack-mount) enclosure
and new heatsinks. The main benefit was
that the big original heatsinks actually
form the sides of the original amplifier
enclosure, allowing direct access.
Wiring
The wiring is fairly straightforward, but
as with any electronic project, you should
test what you are doing, step-by-step. A
typical overview is shown in Fig.6 which,
for simplicity, shows a generic wiring diagram for an audio amplifier using a single
transformer and power supply board, and
a two-channel amplifier board.
The first thing I did was arrange the
mains power wiring. Note the use in Fig.6
of the double-pole, single-throw (DPST)
mains-rated switch. I used the one from
the salvaged amp – a hefty unit with an
inbuilt neon, rated at 20A. (That should
last, even with the turn-on gulp of the capacitors.) The live (hot) lead should have
a fuse holder inserted in it, immediately
the cable enters the case. I used a 10A fuse
– a 5A fuse is typically recommended for
each 300VA transformer. Don’t forget to
securely anchor the mains cable (eg, with
a clamp) so it cannot come loose. Cover
all the exposed mains power connections
with heat-shrink.
On the other side of the switch are the
connections to the transformer. Multiple
transformers are wired in the same way
– ie, in parallel.
The earth (ground) connection should
be made from the mains cable to a
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metallic part of the chassis eg, by an eye
terminal and screw and nut. Use the continuity function on a multimeter to ensure that all metal parts of the amplifier
are also connected to the ground terminal. If you find some panels are not connected, you must add some additional
earthing wires.
Once you have the mains power connected and made safe, switch on and
measure the outputs of the transformer
(or in my case, transformers). The measured voltages should be near to the transformer specs (or, where you are using the
transformer from the salvaged amplifier,
as was previously measured).
Switch off power and now make the
connections to the rectifier/capacitor
module(s). This should be as simple as
the wiring diagram shows – these boards
are always well-labelled. Once you have
done this, switch on and ensure you have
the required plus/minus DC voltages on
the outputs. Then, switch off power again
and make the power connections to the
amplifier modules, being careful to observe the correct polarities.
The connections to the speaker terminals can next be made. Again, be careful
to observe polarity. Finally, wire-in the
audio inputs. You can either use chassis-mount sockets (as I did, using the RCA
sockets salvaged from the old amplifier)
or use flying leads eg, cut-down ‘extension’ type RCA leads that have a female
socket at one end. To reduce noise, use
screened (shielded) cable for the inputs,
connecting the screen to the negative terminals. Try to keep the input leads as far
away from the transformer, power supply
and speaker leads as possible.
Outcome
Over the years I have built many amplifiers – and listened to a great deal more.
Perhaps I am a philistine, but with mega-dollar amplifiers I usually find it pretty hard to hear what some others rave
about. For me, if an amplifier has flat frequency response, low background noise
at high volume and no audible distortion at low or high listening levels, it’s
a good amp. And this one has all those
characteristics – I am very happy with it.
Fig.8. The rear view. From left, mains cable and fuse, speaker terminals and RCA
inputs. As the donor amplifier’s rear panel had many unwanted holes, a new rear panel
was folded from aluminium sheet. Ensure you use rubber feet under the amplifier so
that the heatsinks are raised off the ground, allowing better airflow circulation.
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