The Humm
Audio A
Like a hummingbird, this miniature
amplifier is nimble, small in size
but strong, delivering up to 60W
into 8Ω or 100W into 4Ω. It is ideal for
R
eaders frequently ask us for
advice on building amplifiers
with more than two channels.
We’ve published many Hi-Fi amplifier module designs over the years, but
mainly they have been designed for
maximum power and minimum distortion, resulting in modules that will
only fit one or two per case (unless you
use a huge case!).
We have published amplifier designs
using all-in-one IC ‘chip’ amps like the
LM1875T. They are always quite compromised, both in terms of maximum
power output (typically topping out
at around 30-40W) and performance,
with distortion and noise figures far
worse than a discrete amplifier.
This design offers an excellent
compromise between the two. It’s
cheaper and easier to build than
our best Hi-Fi amplifiers while still
delivering plenty of power with very
good performance. And because it’s
so compact and has modest power
supply requirements, you can quite
easily jam half a dozen (or more!) of
them into a reasonably sized chassis.
We designed these for driving
multi-way loudspeaker systems using
an active crossover to split the signals into frequency ranges to suit
each driver. This approach needs one
amplifier per driver (woofer, tweeter
and so on) but you generally don’t
need as much power per amplifier,
since they are working together.
Initially, we looked at using small,
low-cost Class-D amplifier modules
which could deliver 30-50W. After
quite a bit of searching, we concluded
that there was nothing readily available with distortion performance
within an order of magnitude of what
we’d call ‘Hi-Fi’. Many smaller Class-D
amplifiers exhibit high-frequency distortion above 0.5%, worse than many
decent loudspeaker drivers!
In the end, we looked at larger
high-quality amplifiers and shrunk the
design. The result is the Hummingbird Amplifier Module, which packs
a surprising punch for its size, while
keeping many of the low-distortion
characteristics of the larger amplifiers
from which it takes inspiration. It can
achieve up to 60W into 8W or 100W
into 4W with distortion below 0.0008%
at 1kHz and less than 0.008% all the
way up to 20kHz. That’s way better
than ‘CD quality’.
Design
If you are familiar with larger amplifier topology, then an examination of
the circuit diagram (Fig.7) will show
many similarities between the Hummingbird and larger siblings. On the
other hand, for compactness, the principal differences are:
n
We opted for one pair of output transistors, rather than two
Features
Specifications
● Low distortion and noise
● Extremely compact PCB
● Fits vertically on a 75mm heatsink and can be stacked in a 2RU case
● Produces specified power output continuously with passive cooling
● All through-hole parts
● Low in cost, simple to build
● Onboard DC fuses
● Output over-current and short-circuit protection
● Clean overload recovery with low ringing
● Clean square wave response with minimal ringing
● Tolerant of hum and EMI fields
● Quiescent current adjustment with temperature compensation
● Output power (±32V rails): 100W RMS into 4W, 60W RMS into 8W
● Frequency response (−3dB): 1Hz to 150kHz
● Signal-to-noise ratio: 118dB with respect to 50W into 4W
● Input sensitivity: 1.2V RMS for 60W into 8W; 1.04V RMS for 100W into 4W
● Input impedance: 22kW || 1nF
● Total Harmonic Distortion (8W, ±32V): <0.008%, 20Hz-20kHz,
50kHz bandwidth 32W (see Fig.2 and Fig.6)
● Stability: unconditionally stable with any nominal speaker load ≥4W
● Power supply: ±20-40V DC, ideally ±34V DC from a 25-0-25 transformer
● Quiescent current: 53mA nominal
● Quiescent power: 4W nominal
● Output offset: typically <20mV (measured)
16
Practical Electronics | December | 2022
mingbird
Amplifier
building multi-channel amplifiers for applications like surround sound
or when using an active crossover. Despite its compact size, only a few
compromises were made compared to much larger designs – it even
has output protection!
Image Source: https://pixabay.com/photos/hummingbird-bird-flight-wings-2139279/
n
We chose inexpensive NJW21193/4
output transistors
n
The maximum supply rail voltages
are just ±40V (larger amplifiers often
go to ±60V).
It’s also worth noting that the PCB is
not large – just 64mm wide – and circuit simplification lets us use throughhole components exclusively.
The width of the PCB is defined
by the two output devices and thermal compensation transistor. This is
also a neat fit for the emitter resistors required for a stable operating
bias point.
Despite their relatively large size, we
have used DC rail fuses in this design,
as they form an important protective
layer for the amplifier in case something goes wrong in use.
By Phil Prosser
The SOA protection is tightly coupled with the output stage and sits
between this and the voltage amplifier
stage (VAS). The VAS and driver come
next, and sit between the fuses, again
with little room to spare. At the front
end of the board is the input stage.
How the various sections of the amplifier fit on the PCB is shown in Fig.1.
We are only using one output device
per side, so we have chosen a robust
device with a generous safe operating
area (SOA). Few devices are sturdier
than the NJW21193G/NJW21194G
(or their beefier MJL21193/4 siblings). These are rated at 16A, 250V
and 200W.
We decided to add output SOA
protection to the amplifier that monitors the output current and voltage
and shuts off the output in case of a
short circuit or severe overload. This
protects the amplifier from all but the
worst abuse.
Calculations confirmed that using
a mains transformer with a 25-30V
AC secondary providing rail voltages
of ±35-42V would be safe with a single pair of output devices into 4W, 6W
or 8W, delivering 60W into 8W loads
and 100W into 4W loads. With a 25V
transformer, that’s reduced slightly to
50W for 8W.
For those of you who follow audio
amplifier design, the topology here is
basically the ‘blameless’ amplifier (as it
is dubbed by Douglas Self), which just
works. The innovation in this project
is more about our aproach to simplification and minimisation.
No doubt using SMDs would have let
us make the PCB less, err, packed.
Fig.1: this depiction of the Hummingbird PCB is at 90% of life-size and shows the purpose of each set of components.
The input stage is responsible for setting the gain and distortion cancellation while the VAS and drivers buffer the
signal from the input stage to provide suitable drive for the output transistors. The SOA protection circuitry keeps the
output transistors within their ‘safe operating areas’.
Practical Electronics | December | 2022
17
Still, we managed to fit all the required
through-hole components into an area
of just 88 by 64mm. That will easily fit
standing on its side in a standard two
rack unit (2RU) high case, and assembly is not especially difficult.
Performance
We took total harmonic distortion
(THD) measurements of the prototypes
at 10W and 35W into 8W by powering it from a bench supply, shown in
Fig.2. The 35W measurement required
using a 40dB attenuator with our test
equipment, while the 10W level only
needed a 20dB attenuator. That is why
the distortion results at 10W look so
much better than at 35W.
Given that the shapes of the two
curves are very similar, it’s likely that
the actual performance of the amplifier is closer to the 10W figures, even
up to its maximum 60W power output. We can confidently say that this
amplifier generates very low distortion
levels, and at 10W, is below 0.002%
THD over much of the audio range.
Note that Fig.2 also shows partial
distortion curves for various alternative output/driver/VAS transistors,
and we will explain those options a
bit later.
The amplifier behaves well at clipping. The most common problem is
the output ‘sticking’ as the amplifier
exits clipping from the negative rail,
when the VAS transistor comes out of
saturation. The Hummingbird behaves
well coming out of clipping, as shown
in Fig.3 and Fig.4.
We also tested with a square wave
signal, and the result is in Fig.5. There
is not a lot to show here; it generates
a bandwidth-limited square wave output as shown, with no overshoot and
minimal undershoot.
Finally, Fig.6 shows one of the
spectral plots taken while gathering
the measurements for Fig.2. The left
channel is connected to the output of
Fig.3: a scope plot of the amplifier’s
output waveform into an 8W resistive
load, driven into clipping. You can
see there’s a tiny bit of ‘sticking’
to the negative rail as it comes out
of clipping, but not enough to be
concerned about.
18
Fig.2: total harmonic distortion (minus noise) plots for the Hummingbird at two
different power levels: 36W (red) and 10W (blue). The other curves show the test
results with various combinations of output transistors, driver transistors and,
in one case, a different VAS transistor (BD139, pink curve). Regardless of which
devices you choose, the performance is pretty good.
Circuit description
Fig.7 shows the Hummingbird circuit.
A 220kW resistor biases the input signal at CON2 to 0V DC. The input signal
passes through a 10μF bipolar capacitor and then a 100W resistor shunted by
1nF and 22kW to the low-noise signal
ground. This connects to the output
ground via a 10W resistor. The 10μF
and 22kW combination at the input
sets the −3dB low-frequency cutoff
point below 1Hz.
The 22kW input resistor is selected
to match the 22kW feedback resistor so that each side of the differential amplifier formed by PNP transistors Q7 and Q8 has matched DC
input impedances. Assuming that
these transistors have equal current
through each leg and similar hfe, the
offset voltages at the bases of Q7 and
Q8 will be about the same.
This should ensure a low output offset voltage on the amplifier. We measured less than 20mV on our prototypes.
We have specified BC556 transistors
for Q7 and Q8, although you could use
low-noise BC560 devices if you can
find them. These are commonly available and perform well in this application. 100W emitter degeneration resistors are used for Q7 and Q8. These
Fig.4: this time, the amplifier has been
driven into clipping with a 3W resistive
load, representing pretty much the
worst-case situation it will have to deal
with when driving a real 4W (nominal)
loudspeaker. Once again, the recovery
from clipping is fine.
Fig.5: we fed a square wave signal
(orange) into the Hummingbird and
connected its output to a 3W resistive
load (harsh, we know). It handled this
very well, with no sign of overshoot or
undershoot; clearly, it’s a very wellbehaved amplifier.
the amplifier via an attenuator, while
the right channel is monitoring the signal into the amplifier. As you can see,
the distortion at the output is hardly
any higher than the input signal, and
the second and third harmonics are
roughly equal at around −110dB.
Practical Electronics | December | 2022
Fig.6: one of the many spectral plots we produced as part of the performance tests. You can see the THD readings of the input
(red) and output (blue) signals towards the bottom. You can also see all the harmonics of both signals in the central area. The
test signal is at 1kHz, so the first harmonic is at 2kHz, third at 3kHz and so on. The amp’s output was passed through a 40dB
attenuator, reducing the fundamental to −15dB and dropping the measured noise floor to that of the instrument.
assist with achieving balance and linearity in the differential amplifier. This
reduces its sensitivity to transistor and
temperature variations.
The input stage operates with 3mA
of bias current. This is set by the 220W
resistor in the emitter leg of PNP
transistor Q3, which serves as a constant-current source.
The keen-eyed will note that we
have omitted a resistor from between
the constant-current source and the
differential amplifier. Our lower
voltage rails mean this is not necessary, as Q3 can handle the resulting
100mW dissipation.
The collector legs of the differential amplifier feed into a current mirror made using NPN transistors Q15
and Q16. A current mirror works by
exploiting the fact that with a matched
set of transistors at the same temperature, the Vbe (base-emitter voltage) relationship vs current will be the same.
So by connecting the bases of Q15
and Q16, and putting the same resistance in their emitter circuits, if we
drive 1.5mA through Q16, Q15 will
similarly seek to conduct 1.5mA as
it has the same base-emitter voltage.
This ensures that the differential
pair of Q7 and Q8 operates with the
same current in each leg, which means
it operates optimally as a linear differential amplifier.
The output of the differential amplifier is a current that flows into the base
of NPN transistor Q13. If the amplifier
output is higher than the input, the
Practical Electronics | December | 2022
input to Q8 increases, which reduces
the current into Q16. Because the current mirror ‘tries’ to keep the current
through Q15 and Q16 the same, this
excess current flows into Q13’s base.
Q13 forms part of a quasi Darlington transistor pair with Q14, which
ultimately drives the amplifier output. These transistors together form
the voltage amplifier stage (VAS). It
transforms the current from the front
end into a voltage.
Q14 is a KSC3503DS transistor,
which is specialised for this sort of
application. These are available from
Mouser, Digi-Key, element14 and RS.
The VAS transistor needs to have a
very low Cob or output capacitance.
There are not many really suitable devices being made these days,
most likely as the best VAS transistors were also video amplifier transistors for cathode ray tube (CRT)
monitors, which have gone the way
of the dodo! We used the BF469 video
transistor here in the past, but they
are now obsolete.
The load on the VAS is the constant
current from PNP transistors Q1 and
Q2, which is set to about 8mA, plus
the current required to drive the output stage.
The Hummingbird Amplifier is built on a
PCB measuring 64 x 88mm. It can be built
with multiple configurations of transistors.
For example, this photo uses MJE15032/3
transistors for Q4 and Q12. These could
be replaced with BD139/140 transistors
respectively. See Tables 1-3
for more detail.
19
Fig.7: the Hummingbird amplifier circuit is
pretty standard if a bit minimalist. It has a lot
in common with our previously published,
higher-power amplifiers like the SC200. Note
NPN transistor Q17, which has been added to
protect Q14 during negative clipping excursions
and the SOA protection transistors, Q6 and
Q10, with three resistors each to set the I/V
limit slope and intercept.
20
NPN
Class-A
NPN
NPN
Class-C
PNP
by MJE15032/33 driver devices, as
there is not enough current available
from the VAS to drive them directly.
The output devices both have 0.22W
resistors in series with their emitters,
Output Device Conduction
Output Device Conduction
Our output stage is a single pair
of transistors, Q5 and Q11. The
NJW21193/4 types, as stated earlier,
have been selected for their SOA large
(safe operating area). These are driven
Output Device Conduction
Fig.8: four common amplifier classes;
Class-C is mainly used for RF, not
audio, where distortion is less of
a concern. Class-A has a single
transistor that varies its conduction
over the whole cycle, while the other
three classes use complementary
pairs. In Class-B, one device conducts
for the positive half of the cycle; the
other conducts during the negative
half. Class-AB is like Class-B except
that both devices conduct when the
output is near 0V (the purple area
is where they overlap), while for
Class-C, neither device conducts in the
crossover zone.
Hummingbird 100W Amplifier
Output Device Conduction
Between Q2 and Q14, we have
NPN transistor Q9 and its base-biasing resistors. This forms a simple ‘Vbe multiplier’ that allows us to
set the voltage between the bases of
the output stage and driver transistors Q4, Q5, Q11 and Q12. These are
arranged in standard emitter-follower
connected pairs.
The amplifier must operate in
Class-AB for good performance, where
both the positive (NPN) and negative
(PNP) output devices are conducting for output voltages around the 0V
crossover point, as shown in Fig.8.
We want to bias the amplifier to draw
about 50mA in the quiescent state as
this gives the best output stage linearity around the crossover point.
To achieve this, we need to set a
‘constant’ voltage to bias the four
base-emitter junctions at just over
their turn-on voltage (about 0.6V
each), for a total of around 2.4V.
But the base-emitter threshold voltages of Q4, Q5, Q11 and Q12 all vary
with temperature, so Q9 is mounted
on the same heatsink as Q5 and Q11,
and Q9 is used to multiply its own Vbe
voltage using a 390W fixed resistor and
potentiometer VR1. This way, the bias
voltage will track the Vbe voltages of
those two transistors, giving a mostly
constant bias current.
When properly adjusted, VR1 will
be about 130W. Q9’s base-emitter voltage is across this resistance, around
0.6V, giving about 4.6mA through VR1
and also the 390W resistor. That gives
1.8V (390W × 4.6mA) between Q9’s
base and collector, for a total of 2.4V
(0.6V + 1.8V).
NPN
Class-B
PNP
NPN
Class-AB
PNP
Practical Electronics | December | 2022
providing a small amount of negative
feedback for their bias currents.
The driver devices are capable of
much higher current and dissipation
than demanded in this application.
However, they are widely available
and reasonably priced, so they suit
this application well. They do not dissipate enough power in this application to require heatsinking.
However, suppose you are pushing
your luck by increasing the rail voltage or driving very low impedances
with continuous waveforms, or you
wish to achieve ideal bias tracking. In
that case, you might benefit from fitting them to the heatsink (or the back
of the output devices) on flying leads.
Ideally, we would have mounted
them on the main heatsink so that their
Vbe voltages track those of the output
devices, as Q9 will multiply its own
Vbe changes by a factor of four. We
did not do that, to keep this module
as compact as possible.
The driver transistors still heat up
and cool down as the load changes,
which provides some thermal tracking, but it won’t be exact.
The result is that under transient
application of a heavy load, the output
stage bias will tend to decrease slightly
as the module gets hot delivering a significant amount of power. It does not
Practical Electronics | December | 2022
experience thermal runaway, nor does
the performance change due to this
change in bias, so it is a worthwhile compromise to keep the module compact.
SOA protection
We are using a single pair of output
devices so we feel it prudent to protect
them against unexpected overload or
short circuits. Shorting the output of
a typical amplifier often leads to the
failure of output devices, driver transistors and ultimately the fuse, often
in that order. We get around that by
adding some basic safe operating area
(SOA) limiting components.
The SOA curves for each pair of recommended output devices (taken from
their data sheets) are plotted in Fig.9
and Fig.10, along with curves representing the maximum specified output
power being delivered into purely resistive and reactive loads, the latter represents worst-case loudspeaker load.
As you can see, except for the
TIP35/36 pair, all devices will be
within their SOAs under these conditions. However, some loudspeakers can have significant impedance
dips at specific frequencies that could
cause the transistors to operate outside their safe areas, and also accidents can happen with the wiring
(accidental shorting together).
Fig.11 shows the same SOA curves
as Fig.9 and Fig.10, but also adds
dashed ‘SOA protection’ lines. These
are the limits we’ve chosen to ‘program
in’ for each pair of output devices to
ensure they stay within their SOAs.
The effect of driving the Hummingbird into a 1W load is shown in Fig.12.
The input signal is at the top, while
the ‘clipped’ output waveform below
shows the protection kicking in. This
will not save you from ultimately overheating the output transistors, but it
will prevent the immediate loss of
‘magic smoke’.
Some people claim that this type
of protection degrades the amplifier’s
performance, but the measured specifications speak for themselves.
To understand how the SOA protection works, consider the top half, based
on NPN transistor Q6 and diode D1 plus
three resistors: 18kW, 820W and 220W.
In normal operation, the voltage
across the 0.22W emitter resistor of
Q5 is less than 0.6V. Ignoring the extra
resistors for now, this means that Q6
is biased off and has no effect.
Under fault conditions, the voltage
across the 0.22W resistor increases to
the point that Q6 starts to switch on.
This diverts current from the base of
driver transistor Q4 to the output,
starving the driver of base current.
21
Fig.9: SOA curves for all the output devices you can
use in the Hummingbird, plus load lines for 8W purely
resistive and 45° reactive loads (representing a worst-case
loudspeaker). This shows that all the output devices are
safe for driving such loads with the recommended supply
voltages, except perhaps the TIP35/36, so it’s probably best
to avoid those if possible.
This, in turn, starves the output device
of base drive until the output current
reduces to the point that Q6 is no longer switched on so hard.
This creates a local feedback loop
that limits the output current, thus protecting the output stage. Diode D1 is
included so that the opposing current
protection circuit is not reverse-biased
by heavy output loads.
In the absence of the three extra
resistors, Q6 would switch on at an
output current of about 3A (0.6V across
a 0.22W resistor). This is too early, so
to allow more current, the 820W and
220W resistors form a voltage divider
with a division ratio of 0.21. Thus,
a current of about 13A through the
emitter resistor is required to turn the
over-current protection on.
Without the 18kW resistor, the current limit will be the same regardless
of the output voltage. Adding that
resistor injects more current into the
Fig.10: a similar plot to Fig.9 but this time, the load lines
are for 4W resistive/reactive loads and we’ve eliminated
those output devices that we don’t recommend for driving
4W loudspeakers. All three options are pretty safe; the
MJL3281A/MJL1302A pairing comes pretty close to the
reactive load line, but the SOA protection circuitry is there
to save the day if necessary.
voltage divider formed by the other
two resistors, so that at low output
voltages, more current is injected, and
the current limit kicks in earlier.
This results in the SOA protection
being ‘sloped’ to fit the SOA of the
output devices, and allows more current at high output voltages, because
the voltage across the devices is lower.
Thus they dissipate less power for the
same current.
Output device selection
The pinout of the output devices is
very common. The Hummingbird
delivers the measured performance
with the parts specified, but we have
checked that the amplifier works properly with a range of other output transistors. You do need to change the SOA
protection resistor values, though, as
per Table 1. You also have options for
the driver transistors (Table 2) and VAS
transistor (Table 3).
Construction
All parts are through-hole, and they fit
on the 64 x 88mm, double-sided PCB
coded 01111211, shown in Fig.13 and
availabel from the PE PCB Service. The
parts are closely spaced but not too
tight. We have kept the pad sizes generous to make soldering easier.
Before we continue, we strongly
advise you to use transistors from a
reputable supplier. There are cheap
transistors on internet auction/sale
sites. Do not be tempted by these.
Fakes are prolific, even in surprisingly simple devices. All the devices
recommended for this amplifier are
available at reasonable prices from
major suppliers.
Start by fitting all the small resistors
and diodes – make sure the orientations of the diodes match what’s shown
in Fig.13 and on the PCB silkscreen.
Follow with the trimpot, orienting its
adjustment screw as shown. This is
Table 1 – alternative output transistors
NPN output
SOA protection resistors Comments and limitations
Status
NJW21194G NJW21193G
18kW
820W
220W
Performance as presented
Verified
MJL21194
MJL21193
22kW
750W
220W
Performance as presented; THD <0.001%
at 1kHz with MPSA42 VAS
Verified
FJA4313 or
2SC5242
FJA4213 or
2SA1962
22kW
470W
270W
Limit to 25V AC transformer if driving
difficult 4W loads
Verified
2SC5200
2SA1943
18kW
560W
220W
Performs as specified
Verified
MJL3281A
MJL1302A
18kW
820W
220W
TIP35B/C
TIP36B/C
27kW
1kW
390W
Limit to 25V AC transformer, prefer 8W
load. Surprisingly good performance
Verified
TIP3055
TIP2955
12kW
680W
270W
Limit to 25V AC transformer and 8W load
Not checked
22
PNP output
Not checked
Practical Electronics | December | 2022
Fig.11: this shows all the output transistor SOAs again,
as well as the SOA protection lines (dashed). While
the protection lines are straight, they’re positioned to
stay below the SOA curves in almost all cases, so the
amplifier can’t drive the transistors outside of their SOA
curves. The SOA protection lines for the NJW21193G/
NJW21194G and MJL3281A/MJL1302A are identical
(green dashed line) since, despite being different curves,
they cross over at a critical point.
►
Fig.12: we deliberately overdrove the amplifier by ►
connecting its output across a load of just 1W and fed it with
a single sinewave pulse. This causes the output transistors
to deliver so much current that it triggers the SOA
protection circuitry. You can see from the bottom trace how
it limits the output voltage/current to protect the transistors.
critical because we need to be able to
set the quiescent current to a minimum
before the module is first powered up.
Next, mount the input and output connectors. We have used parts
with the common 5/5.08mm spacing on these (except the input, a
2.54mm-pitch header).
You should consider how you will
mount the modules before choosing either screw terminals or pluggable connectors. Access to a screw
terminal may be obstructed in some
arrangements, so in that case, use
pluggable connectors.
Now install all the non-polarised
capacitors. Fit the MKT parts close to
the PCB. Ensure you use a 100V-rated
device for the 220pF capacitor.
Follow with the 5A fuses and their
clips. We find it easiest to put the fuses
in their clips and then solder that as
an assembly to the PCB. This ensures
everything is well-aligned.
Fit the electrolytic capacitors next,
noting that they must all be installed
with their + (longer) lead to the left
when the PCB is oriented with the
output devices at the top. Ensure that
you have adequate voltage ratings on
these parts (ie, at least what is specified in the parts list).
Now install the TO-92 transistors. It
is worth finding matched pairs for Q7
and Q8 and also Q9 and Q10, if you
can. To do this, check the hfe figures
of a handful of each type. Select pairs
that have reasonably similar hfe measurements; within 10% is fine. Also,
try selecting pairs that have high hfe
figures compared to the others.
With the BC549 and BC556, an hfe
figure below 100 is cause to throw the
part in the bin, although such a low
reading is rare indeed.
Now is a good time to mount the
remaining resistors. The only ones that
get warm are the 0.22W output stage
emitter resistors, and that’s only when
delivering full-power sinewaves from
the amplifier, which will not happen
with musical material. But it is still
good practice to mount these a few
millimetres proud of the PCB.
The PCB will accept standard 5W
cast resistors, but we liked the look
and fit of some smaller resistors from
Mouser (see the parts list). They need to
have a rating of at least 3W in this application, so 5W is quite conservative.
Making inductor L1
The output inductor is made from
0.8mm enamelled copper wire (ECW)
as follows:
1. Find a mandrel that is a bit over
10mm in diameter and has a slight
chamfer so that once complete, you
can push the inductor off. We used
a large ‘Sharpie’ brand permanent
marker.
2. Put masking tape around this mandrel but ensure the sticky side is
facing outwards.
3. Place a bend in the enamelled copper wire (ECW), 30-40mm from the
end, and wind nine turns onto the
masking tape.
Table 2 – alternative driver transistors
NPN driver
PNP driver
Comments
Status
MJE15032
MJE15033
As specified (MJE15034 and MJE15035 have not been tested but should be similar)
Verified
MJE15030
MJE15031
These perform well with 8W and 6W loads. At 3W, distortion increases faster than
the specified devices, but they are still a fair choice
Verified
TIP31B/C
TIP32B/C
Performs close to specifications. With 3W loads, distortion increases faster than
the specified devices, but they are still a fair choice
Verified
BD139
BD140
Install in opposite orientation (ECB vs BCE pinout). The −16 gain group parts are
the best choice. Limit to 25V AC transformer
Verified
MJE350
MJE340
Install in opposite orientation (ECB vs BCE pinout). Not ideal. Marginal on
maximum current. Limit to 8W and 25V AC transformer
Not
checked
Practical Electronics | December | 2022
23
Table.3 – alternative VAS transistors
NPN VAS
Comment
Status
KSC3505DS
As specified
Verified
BF469
As specified
Verified
BD139
Slightly elevated distortion, but a surprisingly good
Verified
performer – rumour has it that there are many ‘types’
of BD139, so ‘your mileage may vary’.
MPSA42
Pinout is different. Measured THD <0.001% at 1kHz
with MJL21193/4 output transistors. More negative
rail ‘sticking’ than KSC3505DS, but not excessive
Verified
Fig.13: building the Hummingbird
is straightforward; fit the
components to the PCB as shown
here. Watch the orientations of all
diodes, transistors and electrolytic
capacitors. For the TO-220 and
TO-126 devices, the metal tabs
face as shown here (if your TO-126
device lacks a metal tab, it would
typically be opposite the side
with writing on it). Don’t forget
that if any of your transistors are
substitutes for the recommended
devices, they will have different
part codes than those shown here
– see Tables 1-3.
4. Put a few drops of super glue on
the ECW. Don’t worry if it gets
on the masking tape, but you
probably don’t want to get it on
your mandrel!
5. Give this a minute to set, then
wind another layer on top of the
first nine turns. You might only be
able to get eight more in; that is OK.
Add more superglue and again let it
to set.
6. Add the final winding of nine turns
over that and glue again.
7. Push the inductor off the mandrel.
Don’t be scared to give it a solid push.
8. Tease the masking tape from inside
the inductor; we used long-nose
pliers. Then we added some extra
super glue.
9. Trim the ends, scrape the enamel
off them and mount it to the PCB
above the 4.7W resistor as shown.
Finishing construction
Now fit the remaining transistors: solder
Q2, Q4, Q12 and Q14 directly to the PCB.
The BD139, NJW21193 and NJW21194
devices that mount on the main heatsink
(Q5, Q9 and Q11) come last.
Before proceeding, check your
mounting arrangements and ensure
that you load these at the right height
for mounting on the main heatsink. The
best way to mount these transistors to
the heatsink is with the insulating kits
and machine screws. Bend their leads
to fit the board and then solder them.
If you can, tap the heatsink to accept
the screws, otherwise, drill through
between fins and use long screws/nuts.
24
Adjustment and testing
It is critical that the bias adjusting
potentiometer is set to maximum resistance so that the initial bias current is
very low. Do this by turning it clockwise a minimum of 20 turns. Check
with a multimeter that there is close
to 500W between the cathode (striped
end) of diode D3 and the right-hand
end of the 390W resistor, just to the
left of Q11.
Do this now – if you forget, you
might blow the fuses when you power
it up, and fuses aren’t always fast
enough to protect semiconductors.
You can do some initial testing
without mounting the amplifier to a
heatsink. This test will check that the
amplifier is operational. Remove the
5A fuses from the board and install
the test (blown) M205 fuses with 10W
5W resistors soldered across them.
We refer to these as ‘safety resistors’.
Connect a voltmeter between the
output and ground, set to a 200V range
(or similar). Connect another voltmeter across one of the 10W resistors, set
to a 20V range or similar. If you only
have one meter, run an initial check
monitoring the output voltage only.
With the input to the module disconnected, apply power. Anything
over about ±15V is fine. If you can, set
the current limit on the power supply
to about 100mA.
Check that the output voltage settles
to 0V ±50mV. We built 14 test units,
and all were within that range. Also
check that the voltage across the 10W
safety resistor is less than 1V.
If either of these tests fail, immediately power it off and check for
the cause.
Have you got the bias pot set at
the right end of its travel? Are all the
capacitors in the right way around? Do
you have a signal feeding
the input? If so, disconnect it. Are all the transistors and diodes in the
right places and the right
way around? Check that
those output devices are
in the right spot!
Fig.14: route the wiring
to each module like
this to ensure you get
the stated performance.
Current flowing through
these wires will cause
magnetic fields, which
affect the operation
of components on the
amplifier. Routing the
cables this way keeps
those magnetic field
strengths low. Once
you’ve run them, use
cable ties and cable
clamp to hold them
in place and keep
everything neat.
Practical Electronics | December | 2022
Parts List – Hummingbird (for one amplifier)
1 double-sided PCB coded 01111211, 64 x 88mm, avavaiable from the PE PCB Service
1 split rail power supply delivering ±20V to ±40V DC (eg, 15-28V AC mains transformer, bridge rectifier, filter
capacitors, mains socket, mains-rated wiring, heatshrink tubing etc) – see Fig.15
3 2-way 5/5.08mm pitch mini terminal blocks (CON1, CON3, CON4)
1 2-way polarised/locking pin header (CON2)
4 M205 fuse clips (F1, F2)
2 5A 5mm ceramic fuses (F1, F2)
Altronics kit
1 1m length of 0.8mm diameter enamelled copper wire (L1)
Australian company Altronics offer a kit for this
1 500W vertical or side-adjust multi-turn trimpot (VR1)
project, code K5158, at around £35 per module
2 TO-3P insulating kits (washers and bushes)
PLUS p&p – see: www.altronics.com.au
1 TO-126 insulating kit (washer and bush)
3 M3 x 25mm panhead machine screws
3 flat washers to suit M3 screws
3 crinkle washers to suit M3 screws
3 M3 hex nuts
2 blown 5mm fuses (for testing, or purposefully blow 100mA fuses)
1 heatsink, type depending on intended application (we used one Altronics H0545 for six modules)
1 small tube of superglue
1 5cm length of masking tape
Semiconductors
5 BC556 65V 100mA PNP transistors, TO-92 (Q1, Q3, Q7, Q8, Q10)
1 MJE350 300V 500mA PNP transistor, TO-126 (Q2) [Altronics Z1127, Jaycar ZT2260]
1 MJE15032G or MJE15034G 250V/350V 8A NPN transistor, TO-220 (Q4) [element14 9556621, Digi-Key
MJE15034GOS-ND, Mouser 863-MJE15032G]
1 NJW21194G or MJL21194 250V 16A NPN transistor, TO-3P (Q5) [Jaycar ZT2228, element14 2535656, Digi-Key
NJW21194GOS-ND, Mouser 863-NJW21194G]
3 BC546 65V 100mA NPN transistors, TO-92 (Q6, Q13, Q17)
1 BD139 80V 1A NPN transistor, TO-126 (Q9) [Altronics Z1068, Jaycar ZT2189]
1 NJW21193G or MJL21193 250V 16A PNP transistor, TO-3P (Q11) [Jaycar ZT2227, element14 9555781, Digi-Key
NJW21193GOS-ND, Mouser 863-NJW21193G]
1 MJE15033G or MJE15035G 250V/350V 8A PNP transistor, TO-220 (Q12) [element14 9556630, Digi-Key
MJE15035GOS-ND, Mouser 863-MJE15033G]
1 KSC3503DS 300V 100mA NPN transistor, TO-126 (Q14) [element14 2453955, Digi-Key KSC3503DS-ND, Mouser
512-KSC3503DS]
2 BC549 30V 100mA NPN transistors (Q15, Q16)
3 1N4148 75V 250mA small signal diodes (D1-D3)
Capacitors
1 220μF 25V electrolytic [Altronics R5144, Jaycar RE6324]
2 100μF 50V 105°C electrolytic [Altronics R4827, Jaycar RE6346]
2 47μF 50V low-ESR electrolytic [Altronics R6107, Jaycar RE6344]
1 10μF 50V low-ESR electrolytic [Altronics R6067, Jaycar RE6075]
1 10μF 50V non-polarised electrolytic [Altronics R6560, Jaycar RY6810]
1 220nF 63V MKT [Altronics R3029B, Jaycar RM7145]
5 100nF 63V MKT [Altronics R3025B, Jaycar RM7125]
1 22nF 63V MKT [Altronics R3017B, Jaycar RM7085]
1 1nF 63V MKT [Altronics R3001B, Jaycar RM7010]
1 220pF 100V NP0/C0G ceramic [eg, element14 2860112, Digi-Key 445-173535-1-ND, Mouser
810-FG28C0G2A221JNT6]
Resistors (all 1/4W+ 1% metal film axial unless otherwise stated)
1 220kW
5 100W 0.5W or 0.6W 1% metal film
2 22kW 1 82W
2 18kW
2 68W
2 3.9kW 2 47W 0.5W or 0.6W 1% metal film
3 2.2kW
1 39W
1 1.2kW 1 15W 1W
2 820W
1 10W
1 390W 2 10W 5W 10% (for testing)
4 220W
1 4.7W 1W
2 0.22W 5W 5% [element14 1735119, Digi-Key BC3440CT-ND, Mouser 594-AC050002207JAC00]
Is your power supply delivering
both positive and negative rails, and
do you have the ground connected?
Setting the bias
This requires the amplifier to be
mounted to a heatsink with appropriate insulators for the output devices
Practical Electronics | December | 2022
and Vbe multiplier transistor. Power
it up and adjust the bias by turning
potentiometer VR1’s screw anticlockwise while watching the voltage across
the 10W resistor. Nothing will happen
for quite a few turns; then, the bias current will rapidly increase. Adjust this
to achieve 500mV across the resistor.
Allow this to settle and readjust. It
will take a while to settle; depending
on your mounting arrangement this
should be done with the full supply
voltage applied (ie, the final voltages
you intend to use).
Re-install the 5A fuses, and you are
ready to go. You can check the bias
25
The Amplifier can be cleanly mounted to
a 75mm heatsink as shown above. The
SOA protection resistors are missing as
we wanted to compare the performance
with and without them. After which you
can daisy-chain them together to form a
larger system such as a six channel setup
shown adjacent. This setup was mounted
in a 2U rack case.
later by measuring the voltage across
the 0.22W resistors; you should see
10mV across each. If you’re mounting multiple modules on a heatsink
sideways as we did, the side-adjust
style trimpot specified makes this
quite easy.
Installation
To minimise distortion to the levels
presented requires careful attention to
layout and the power supply wiring.
Our recommended wiring layout per
module is shown in Fig.14, and the
recommended power supply configuration is shown in Fig.15.
The wiring from the main supply
capacitors should have the positive,
negative and ground wires twisted
together. The output should fold back
toward the output devices, run parallel to the 0.22W output resistors, then
follow the power wires.
The output wire should follow the
power wires back past the power supply and pick up a ground wire, minimising the loop area created, then run
as a pair from there to the speaker terminals (see above).
Ensure that the power supply has a
‘star earth point’ from which you connect to the input ground, the amplifier ground and the speaker output
ground. Also check that the way you
connect the rectifier and its ground
connection to the capacitors does
not inject noise onto your star earth
point. Connect the input shielded
cable screen to the star point.
Make sure all connections are
secure and have low resistance; poor
connections can easily double the
distortion levels, or more. We found
26
this measuring a batch of modules
we built to verify our results; we had
to tighten the connections to achieve
consistent results.
Getting the most out of it
We expect this module to find use
where a small, low distortion, rugged
and reasonably priced multi-channel
amplifier is required. As these modules will fit on a 75mm heatsink, many
of them can be mounted vertically in
a 2U rack case.
Our original application for this
amplifier was to provide six channels
for a stereo system using three-way
loudspeakers with active crossovers.
With two channels for subwoofers,
two for mid-range two for tweeters,
we expect the maximum continuous
power to be 60W on each subwoofer
channel, possibly half this for the mid
and a tiny fraction of this on the high.
As a result, a power supply based
on a 300VA transformer will be more
than enough for all six channels.
Even a 160VA transformer might
cut it if you don’t plan on driving it
especially hard. If your application
calls for high power levels, there are
more appropriate options, such as the
SC200 Amplifier Module (see PE, January to Match 2018). You could use
a pair of those for the low-frequency
channels and the Hummingbird for
the others.
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
Fig.15: we’ve left the power supply for the Hummingbird somewhat open-ended, as
it has pretty standard requirements. It just needs split DC rails without too much
ripple, somewhere between ±20V and ±40V. The configuration above will produce
around ±34V, which is right in the sweet spot and uses commonly available parts.
Make sure your filter capacitors have a high enough voltage rating (above the
highest expected peak DC voltage) and enough capacitance to ‘hold up’ the supply
between 100Hz recharge pulses at the maximum sustained output power you’re
expecting. Generally, you will need at least a few thousand microfarads per rail;
ideally, at least 10,000μF per rail for multiple amplifier modules.
Practical Electronics | December | 2022
