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Delay/DC Protection for =
Loudspeakers |
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Back=20
to Products
Pricing |
Practically all =
semiconductor=20
power amplifiers are using +/- supplies =
and are=20
DC-coupled to the loudspeaker. This is =
an=20
advantage from the point of view of =
sound=20
quality, but can be dangerous to your =
speakers.=20
If you accidentally insert a low value =
fuse in=20
one of the supply lines and the fuse =
blows, the=20
output is likely to go to the other =
supply=20
voltage and stay there. This DC-voltage =
might=20
cause enough current to flow in the =
speaker to=20
kill it. The same thing can happen if =
one side=20
of the output stage blows up and shorts =
the=20
output to the supply line. This DC =
voltage might=20
damage the speaker, if the fuse doesn't =
blow=20
fast enough.
Power-on and =
power-down can=20
also cause problems with power =
amplifiers. The=20
DC-voltages inside the amplifier are =
changing in=20
discrete steps during the power supply =
ramp-up=20
and ramp-down, causing DC-thumps at the =
output.=20
Although these are normally not harmful =
to the=20
speakers, they are =
annoying.
Inserting a=20
relay between the output of the power =
amp and=20
the speaker can solve both of these =
problems. At=20
power-on the relay is not energized and =
the=20
speaker is not connected to the =
amplifier. After=20
a pre-defined delay the relay turns on =
and the=20
amp is working normally. If there is a =
DC=20
voltage at the output, the relay turns =
off and=20
the speaker is=20
=
disconnected.
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The delay circuit is =
built around=20
the well-known 555 timer, shown as Q4 in =
the=20
schematic. P2+R14 and capacitor C5 define =
the time=20
delay. With the values shown the delay can =
be set=20
between 6 and 12 seconds. I think a =
reasonable=20
delay is about 10 seconds. If you would =
like to=20
allow more time for your power amp to =
settle=20
before the speakers are connected you can =
increase=20
the value of P2+R14 and/or capacitor C5. =
R15 and=20
C6, connected to the trigger pin (pin 2), =
simulate=20
a reset pulse and allow the timer to start =
counting. Pin 5 is decoupled through a =
0.luF/100=20
MKT capacitor.
The relays are =
connected=20
between the output pin 3 and the + supply. =
Because=20
this circuit is meant to be a universal =
one, I am=20
using two relays in parallel, in order to =
cope=20
with high current/high-power amplifiers. =
The relay=20
contacts are connected in anti-parallel to =
minimize rectifying effects. A green LED =
is=20
connected across the relays and can be =
mounted on=20
the front panel to indicate proper =
operation of=20
the power amp.
Q1A and Q1B, a dual, =
FET-input opamp, is configured as a window =
detector. The +input of Q1B is connected =
to a=20
reference voltage of approx. +2V, while =
the -input=20
of Q1A is referenced to -2V. The -input of =
Q1B and=20
the +input of Q1A are connected together =
and form=20
the DC sensing input. As long as the DC =
input is=20
within the "window, defined by the +2V and =
-2V=20
references, both outputs are positive and =
both=20
level-shifting Transistors Q2 and Q3 are=20
off.
When the DC input moves =
outside the=20
"window, i.e. more positive than +2V or =
more=20
negative than -2V, Q1B or Q1A output will =
change=20
to -12V, pulling current through the =
resistor=20
chain R7-R8. Transistor Q2 turns on, which =
turns=20
Q3 on, and pins 6/7 and pin 2 of the timer =
are=20
pulled to zero Volt through D7/D8. This =
resets the=20
timer, and the relays disconnect the =
speaker from=20
the amplifier. When the DC offset is =
removed from=20
the input the opamp output, which was at =
-12V,=20
returns to +l2V and Q2/Q3 turn off. The =
timer is=20
now ready to start counting again and =
after the=20
predefined time delay the relays will =
switch the=20
speaker on.
The output of your =
power amp=20
has audio signals present, which have to =
be=20
removed from the input of the DC sensing =
circuit.=20
This is done with a single-pole RC =
network,=20
consisting of R1 and C1/C2. The -3dB point =
of this=20
network has to be selected low enough to =
avoid=20
triggering the circuit by low frequency =
audio=20
signals. However, it should not be too =
low,=20
because it makes the circuit too slow. If =
you=20
select C1=3D1uF, the circuit will activate =
at 50Hz=20
with 100W into 8 Ohm. This is not =
acceptable in=20
most applications. With 6.8uF I measured =
full=20
power down to below l0Hz, without =
triggering the=20
circuit. With C1 equal approx. 11uF (two =
22uF caps=20
in series) the -3dB point is at 0.15Hz and =
full=20
output is available down to 5Hz. The =
circuit stays=20
permanently shot down when you drive the =
amp with=20
2Hz at full power. You might want to =
experiment=20
with this: if you want to prevent subsonic =
signals=20
from reaching your speakers you might want =
to=20
increase the -3dB point.
The =
purpose of Dl=20
and D2 is two-fold: It prevents the caps =
from=20
charging to a very high voltage, which =
would take=20
a long time to discharge. More =
importantly, it is=20
a protection for the opamp inputs: it =
prevents the=20
input voltage to rise above the +l2Volt =
rail or=20
below the -12V rail, which would destroy =
the ICs.=20
A series resistor R2 limits the input =
current to=20
the opamps.
There are two extra DC =
inputs=20
to the circuit: S1 and S2. These can be =
used as=20
extra logic inputs from other protection =
circuits.=20
The logic level is the same as the output =
of the=20
opamps: +12V (or no connection) and -12V. =
With=20
+12V (or no connection) the circuit is =
simulating=20
a normal operation, i.e. the circuit is =
not=20
activated. With S1 or S2 equal -12V, Q2 =
and Q3=20
turn on instantly (there is no time delay =
here)=20
and the speakers are switched off. These =
inputs=20
can be used for example for thermal =
breakers (the=20
contact of the thermal breaker has to be =
normally=20
open and connected to -12V), for circuits =
testing=20
for short circuit at the output, for power =
sensing=20
circuits, etc.
Power is supplied by =
two 12V=20
TO-220 regulators. AC input is 2x12-15V, =
with=20
center tap. This can be supplied by the =
normal=20
power transformer, or by a separate =
transformer of=20
5-10VA=20
=
rating.
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Connecting the Protection =
Circuit to=20
Your Amp.
The board should be =
physically=20
close to the output of the amplifier and =
the=20
output connector in order to minimize =
the wire=20
length. Connect the output of the amp to =
the pad=20
marked "AMP" and the output connector to =
the pad=20
marked "LS" on the PCB. Use a cable here =
that=20
you normally use for output wiring. You =
can=20
solder -the cable directly to the pads, =
or you=20
can use solder pins, 6.3mm (l/411) Quick =
Connects, or screw-type terminal blocks =
for the=20
connection. The ground connection stays =
as it=20
was before the installation of the=20
board.
Prepare a twisted pair of =
AWG=20
18-20 hook-up wire and connect the =
output of the=20
amplifier to the input of the protection =
circuit. The ground connection should be =
taken=20
from the star-ground of the amplifier if =
possible. If these wires have to go =
close to the=20
input of the power amp, then it is =
recommended=20
to use a shielded cable instead of the =
twisted=20
pair. Finally, connect the 2x12-15V AC =
and the=20
center tap to the board. Your =
installation is=20
now complete. Switch on your power amp =
and check=20
that the delay circuit is working. The =
DC=20
protection circuit will only work if you =
have=20
unwanted DC offset the output. =
Hopefully, this=20
will not happen very =
often.
The=20
EB-292/132 design is the intellectual =
property=20
of Erno Borbely/BORBELY AUDIO. =
Commercial use or=20
duplication in any form is not =
authorized=20
without license=20
=
agreement.
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