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This bare board version of the Universal Loudspeaker Protector
can be built into a stereo amplifier to protect the loudspeakers
and prevent switch-on and switch-off thumps.
A universal
loudspeaker protector
for stereo amplifiers
By LEO SIMPSON & BOB FLYNN
This simple circuit is designed to mate with
any stereo amplifier, music system or car sound
system and will protect the loudspeakers from
damage in the case of an amplifier failure. It
could also prevent a fire. It has a turn-on delay
and will eliminate switch-on thumps.
Do you leave your stereo amplifier
or home music system permanently
switched on in standby mode? Do
you realise they could be a fire hazard? If you haven’t thought about this
problem in the past, then this article
is for you.
Most power amplifiers these days
are direct-coupled to the loudspeakers.
This means that there is no output
54 Silicon Chip
coupling capacitor in series with each
loudspeaker terminal. This is true
whether you have a large stereo amplifier which delivers several hundred
watts per channel or a typical home
music system which can be turned
on and controlled by a remote control
handpiece.
This means that if an output transistor goes short circuit or in the case
of smaller home music systems, a
hybrid power amplifier fails, virtually
the full supply rail to that part of the
circuit will be applied to the loudspeaker.
The result is usually a burnt out
loudspeaker voice coil or damaged
suspension system. That’s expensive
to fix but it may not be the end of the
matter. In a worse case, the large DC
current in the voice coil does not burn
it out immediately but allows it to get
red hot so that it sets the speaker cone
on fire. From there, the acetate filling
material in the enclosure and the grille
fabric also catch fire, generating huge
quantities of choking black smoke.
Ultimately, your house may catch
fire too. This is not an imaginary scenario. Stereo systems do fail and they
Fig.1: this is the self-contained version of the Universal Loudspeaker Protector,
intended to be powered from a 9V or 12V DC plugpack. Q1, Q2 and Q3 monitor
the output of channel 1 of the amplifier while Q4, Q5 & Q6 monitor the second
channel. If a high DC offset is detected, the base current to Q7 will be shunted to
deck and this will cause Q8 and the relay to turn off.
can cause house fires. That is why
they should not be left on for long
periods of time, especially if no-one is
present to turn them off in the case of
a fault.
Why does this sort of amplifier fault
cause so much heat in the voice coil
of a loudspeaker? Well, consider a
100W per channel amplifier with ±50V
supply rails and driving loudspeakers
with a voice coil resistance of 6Ω,
a typical value for a speaker with a
nominal impedance of 8Ω.
If one of the amplifier’s output
transistors fails, it will apply almost
the full DC supply rail of 50V to the
loudspeaker. The resulting heat dissipated by the voice coil will be 50V2/6
= 416W! No wonder the voice coil gets
hot and burns out!
Actually, the power dissipation is
generally not as high as that because
the power supply voltage will drop
under such a serious load. If you’re
lucky, the amplifier’s fuses will also
blow before a fire starts, limiting the
damage to just the amplifier and the
victim loudspeaker.
Fire insurance
Now the only safe way to prevent
a major fault occurring while you’re
not listening to your music system is
to turn it off at the wall socket. But
faults can still occur while you are
listening to the system and if you’re
not actually in the room at the time to
turn it off when a major fault occurs,
the results will be costly. So to prevent
damage to your expensive speakers
you need to build the Universal Loudspeaker Protector presented in this
article.
The Universal Loudspeaker Protec-
Advantages Of This New Protector
This is not the first loudspeaker
protector circuit we have published.
The last one was featured in the July
1991 issue of SILICON CHIP. This new
circuit was produced as a result of
development work we have been doing
on a high-power bridge amplifier. The
new circuit is built onto a substantially
smaller PC board and copes with an
amplifier fault condition that would be
ignored by the previous circuit.
By using separate monitoring circuits for each channel of the amplifier,
the ULP can respond to a DC fault
condition in one or both channels
of a stereo amplifier. The previous
Loudspeaker Protector (published July
1991) had only one monitoring circuit
which summed the active lines from
the loudspeakers.
If the amplifier in question failed
simultaneously in both channels, it
is possible that one channel would
produce a positive DC fault and the
other channel a negative DC fault. If
a common sensing circuit was used,
these two fault conditions would effectively cancel each other out and
the Loudspeaker Protector would fail
to operate.
Is it possible for both channels of an
amplifier to fail at once? And with oppo-
site faults in both channels? Definitely!
It is certainly possible although
we admit that it is unlikely with conventional stereo amplifiers. However,
where a stereo amplifier is driving a
single loudspeaker in bridge mode, it
is highly likely.
In most bridged amplifiers, one
channel gets its signal from the output
of the second channel. So if the second
channel fails and its output goes high,
the first channel will have its output
forced low. So the fault condition will
exist in both channels and both channels must be sensed separately, as
in the ULP.
April 1997 55
Fig.2: this version of the Universal Loudspeaker Protector is identical with that
shown in Fig.1 except that it derives its power from the amplifier’s DC supply
via regulator transistor Q9.
Fig.3: this version of the
Universal Loudspeaker
Protector is mainly intended
for protecting speakers
connected to bridged output
amplifiers in cars.
tor (ULP) will continually monitor the
DC conditions at the outputs of your
stereo amplifier. If a fault occurs, the
ULP will operate a relay to disconnect
the loudspeakers.
As a bonus, the ULP has a delay at
switch-on and if it is built into a stereo
amplifier, it will prevent switch-on
thumps from the loudspeakers.
Three versions
We are describing three versions
of the ULP. One is self-contained and
56 Silicon Chip
powered with a 9V or 12V DC plug–
pack. The second is intended to be
built into a stereo amplifier and has
its own on-board regulator. The third
version is intended for bridged ampli
fiers in cars. We’ll talk about these two
latter versions later in this article.
Fig.1 shows the complete circuit
diagram of the self-contained version.
Let’s talk about how Q1, Q2 & Q3
monitor the active output terminal of
an amplifier. The active signal is fed
via a two-stage low pass filter network
consisting of three 22kΩ resistors and
two 47µF NP (non-polarised) electrolytic capaci
tors. This filter network
effectively removes any audio frequen
cies and ensures that only DC signals
are fed to the following transistors.
This is necessary because we don’t
want normal audio signals to trip the
ULP in any way.
Now let’s see how the three transistors operate together. The line from
the low pass filter is connected to the
emitter of transistor Q1 and the base of
The self-contained version of the Universal Loudspeaker Protector is housed in
a plastic case and powered from a 9V or 12V DC plugpack. Note the resistor in
series with the DC power socket. This is only required if a 12V DC plugpack is
used (see text).
transistor Q3. In effect, Q1 monitors for
negative DC signals while Q3 monitors
for positive DC signals.
If a positive DC signal of more than
0.6V is present, Q3 will turn on. Similarly, if a negative DC signal of more
than 0.6V is present, the emitter of Q1
will be pulled below its base and so Q1
will turn on and turn on Q2. Both Q2
and Q3 have a common 56kΩ load resistor (R1) and this normally feeds base
current to Q7. Q7 feeds base current
to Q8 and so both of these transistors
and the relay are on.
However, when either Q1 or Q3 turn
on, the base current for Q7 is shunted
to deck and so Q7, Q8 and the relay
are turned off, disconnecting the
speakers.
The same working principle applies
to the monitoring of the second amplifier channel, with Q4 sensing negative
DC signals and Q6 sensing positive
DC signals. Q5 & Q6 share the same
common 56kΩ load resistor as Q2 &
Q3. So if either of these transistors are
turned on by fault voltages, they will
also rob Q7 of base current and cause
Q8 and the relay to turn off.
Arc protection
When the relay operates to discon-
nect the loudspeakers, the moving
contacts are shorted to the loudspeaker
ground lines via the “unused” contacts. This has been done because if a
large DC voltage (say more than 30V)
appears at the amplifier outputs, the
resulting high current can cause an
arc across the relay con
tacts. Until
that arc is extinguished, the loud-
speaker is still being subjected to the
high current and the possibility of
damage.
By shorting the moving contacts of
the relay to the speaker ground lines,
the arc current is diverted and the
amplifier fuses will blow if the arc
still persists.
The fact that this Universal Loudspeaker Protector can be used with
high power amplifiers which can produce very large output currents means
that a heavy duty relay must be used.
The one specified has DPDT (double
The amplifier and loudspeaker
connections are run to the selfcontained unit via a terminal
block at one end of the case.
April 1997 57
Fig.4: use this diagram when wiring the self-contained version of the ULP. The missing components
at the lefthand side of the PC board are for other versions.
Fig.5: this is the wiring diagram for the built-in version of the circuit, as shown
in Fig.2. Note that the external resistor RY is only required if the amplifier’s DC
voltage supply is above 40V.
discharged and no base current can
flow via 56kΩ resistor R1. C1 then
charges via 220kΩ resistor R3 and
eventually sufficient voltage is present
to allow resistor R1 to turn on transistor Q7. This turns on Q8 and the relay
and so the loudspeakers are connected
to the amplifier. This delay is several
seconds and it allows the voltages
within the amplifier to stabilise, so
when the speakers are connected, no
thumps are heard.
When power is removed from the
ULP circuit, the relay disconnects the
speakers almost immediately, preventing turn-off thumps.
Note that this “thump” protection is
only available if the ULP is powered
from the supply rails of the amplifier, as in Figs.2 & 5. If it is built as a
self-contained unit and powered from
a DC plugpack, the thump protection
will not be provided.
Construction
pole, double throw; changeover) contacts rated at 10 amps.
Power supply
As noted above, we are presenting
three versions of this circuit. The first
version, intended as a self-contained
unit to be used with any amplifier or
music system, can be powered with a
9V or 12VDC plugpack.
The second version, presented as a
PC board to be built into a stereo amplifier, can derive its supply from the
positive amplifier DC supply rail and
this can range from +30 to +75V DC.
Its circuit diagram is shown in Fig.2.
In this case, the amplifier’s supply
rail is fed to transistor Q9 and associ58 Silicon Chip
ated components and these operate to
provide a regulated +12V supply for
the relay and other transistors.
The third version, intended for
bridged amplifiers in cars, takes its
supply directly from the 12V battery
line. Its circuit is shown in Fig.3.
Turn-on delay
So far we have described the main
function of the ULP which is to prevent loudspeaker burnouts. The minor
function, mentioned above, is to prevent thumps from the loudspeakers
when the amplifier is turned on. This
is achieved with resistors R1 & R3 and
capacitor C1.
When power is first applied, C1 is
Let’s now describe the construction
of the self-contained version. All the
parts are mounted on a PC board coded
01104971 and the wiring diagram is
shown in Fig.4. As you can see, some
parts are missing from one end of the
board. These are for the on-board regulator (Q9, etc) which are used only
in the in-amplifier version.
Fit the PC pins first and then the
resistors. The four 47µF electrolytic
capacitors can go in either way around
since they are the non-polarised (NP)
type. The 100µF capacitor is polarised
and must be inserted the correct way
around.
The eight transistors and the diode
can be inserted next. Check that you
insert the correct type in each position
and make sure that each is oriented exactly as shown in the wiring
diagram.
Don’t forget to install the wire link,
LK1. This has been provided to enable a thermal cutout to operate the
circuit but this feature is not used
here.
Finally, the relay can be installed.
We mounted ours by soldering short
lengths of stout tinned copper wire
to each relay pin. These wire leads
are then pushed through the relay
mounting holes on the board and then
soldered. We understand that some kitset suppliers may provide a PC board
with slotted holes so that the tinned
copper wire may not be necessary.
With the board complete, it’s time
to install it in the plastic case. You
may elect to use a different case from
our prototype; as long as everything
fits, the case size and shape are unimportant.
You will need to drill a hole at one
end of the case to take the DC socket
for the plugpack. At the other end you
will need to mount a six-way insulated terminal block and drill holes for
wires to run inside the case. Install the
PARTS LIST
Self-contained version
1 plastic case, 150 x 80 x 60mm
1 PC board, code 01104971,
107mm x 55mm
1 9V or 12VDC 150mA plugpack
with 2.1mm DC plug
1 2.1mm DC socket
10 PC board pins
1 Relay DPDT 10A 240VAC, 12V
coil <at> 75mA, Jaycar SY-4065
or similar
6 3mm x 20mm screws
6 3mm nuts
4 6mm spacers
4 adhesive rubber feet
Semiconductors
5 BC547 NPN (Q1,Q3,Q4,Q6)
2 BC557 PNP transistors (Q2,Q5)
1 BC327 PNP transistor (Q8)
1 1N4004 silicon diode (D1)
largest terminal block you can obtain
which will fit. The larger ones have
larger wiring holes which makes it
easier to connect the speaker wires,
Capacitors
1 470µF 16VW electrolytic
1 100µF 16VW electrolytic
4 47µF 50VW NP electrolytic
Resistors (1%, 0,25W)
1 220kΩ
2 22kΩ 1W
2 56kΩ
1 2.2kΩ
4 22kΩ
1 39Ω 0.5W (RX)
Extra parts for built-in version
1 BD649 NPN transistor (Q9)
1 13V 0.5W or 1W zener diode
(ZD1)
1 100µF/100VW electrolytic
capacitor
1 2.7kΩ 1W resistor
1 220Ω 5W wirewound (RY; see
text)
1 U-shaped TO-220 heatsink
(Altronics Cat H-0502 or equiv).
particularly if you are using heavygauge cables.
Note that we have shown a resistor
in series with the supply from the
April 1997 59
Bridged Amplifiers In Car Audio Systems
Fig.6: this is the wiring
diagram for the bridged
version of the ULP, as
shown in Fig.3.
M
ANY HIGH-POWERED amplifiers in cars operate in bridged
mode and they are often run at high
power for extended periods. When they
fail, the speakers are just as likely to be
damaged as the speakers in a home
stereo system. And the possibility of a
fire is just as high. So to protect valuable loudspeakers in cars, the ULP is
a wise investment.
You will need one ULP for each stereo
amplifier and one for each bridged
amplifier. In each case, the ULP can
be powered directly from the +12V
battery line.
The circuit for this bridged amplifier
version is shown in Fig.3 while the
wiring diagram is shown in Fig.6.
plugpack. It is marked RX on Fig.4. If
you use a 9V plugpack, this resistor
should not be necessary. However,
the unloaded voltage of a typical 12V
DC plugpack can easily be +15V or
even higher and that could cause an
increase in power dissipation in the
relay. Therefore, the series resistor is
necessary. We suggest that RX be a 39Ω
0.5W resistor. If the plugpack voltage is
higher still, increase RX to 47Ω.
Testing
When all the wiring is complete, it
is time for a power test. Do not connect any wires from your speakers or
amplifier at this stage. Just connect
the plugpack and apply power. The
relay should close after a short delay
of about two seconds. If that happens,
you are almost home and hosed.
Next, you can simulate a fault condition with a 6V or 9V battery (or even
two 1.5V cells in series). Connect the
battery across each of the inputs in
turn, first with one polarity and then
the other. In each case, the relay should
immediately open and then close as
soon as the battery is removed.
60 Silicon Chip
Fig.7: here is the full size etching pattern for the PC board.
If you strike trouble, switch off and
check the circuit for errors. Normally,
you can expect the unit to work as soon
as you switch it on so now it should
be merely a matter of wiring the unit
in series with your loudspeakers and
then you can rest easy.
Fig.5 shows the wiring of the builtin version. This is the same as for Fig.4
except that the regulator components
involving transistor Q9 are included.
Note that Q9 is mounted on a U-shaped
heatsink.
In addition, if the amplifier’s DC
supply is above 40V, it will be necessary to connect an external 5W wire
wound resistor (RY) in series with the
collector of Q9. This resistor is shown
on Fig.5 and a table of values is shown
on Fig.2. For example, if the amplifier’s
DC supply is around 60V, resistor RY
SC
should be 220Ω 5W.
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