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Delay/DC Protection for Loudspeakers

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Introduction

Practically all semiconductor
power amplifiers are using +/- supplies and are
DC-coupled to the loudspeaker. This is an
advantage from the point of view of sound
quality, but can be dangerous to your speakers.
If you accidentally insert a low value fuse in
one of the supply lines and the fuse blows, the
output is likely to go to the other supply
voltage and stay there. This DC-voltage might
cause enough current to flow in the speaker to
kill it. The same thing can happen if one side
of the output stage blows up and shorts the
output to the supply line. This DC voltage might
damage the speaker, if the fuse doesn't blow
fast enough.

Power-on and power-down can
also cause problems with power amplifiers. The
DC-voltages inside the amplifier are changing in
discrete steps during the power supply ramp-up
and ramp-down, causing DC-thumps at the output.
Although these are normally not harmful to the
speakers, they are annoying.

Inserting a
relay between the output of the power amp and
the speaker can solve both of these problems. At
power-on the relay is not energized and the
speaker is not connected to the amplifier. After
a pre-defined delay the relay turns on and the
amp is working normally. If there is a DC
voltage at the output, the relay turns off and
the speaker is
disconnected.

Circuit Description

The delay circuit is built around
the well-known 555 timer, shown as Q4 in the
schematic. P2+R14 and capacitor C5 define the time
delay. With the values shown the delay can be set
between 6 and 12 seconds. I think a reasonable
delay is about 10 seconds. If you would like to
allow more time for your power amp to settle
before the speakers are connected you can increase
the value of P2+R14 and/or capacitor C5. R15 and
C6, connected to the trigger pin (pin 2), simulate
a reset pulse and allow the timer to start
counting. Pin 5 is decoupled through a 0.luF/100
MKT capacitor.

The relays are connected
between the output pin 3 and the + supply. Because
this circuit is meant to be a universal one, I am
using two relays in parallel, in order to cope
with high current/high-power amplifiers. The relay
contacts are connected in anti-parallel to
minimize rectifying effects. A green LED is
connected across the relays and can be mounted on
the front panel to indicate proper operation of
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
reference voltage of approx. +2V, while the -input
of Q1A is referenced to -2V. The -input of Q1B and
the +input of Q1A are connected together and form
the DC sensing input. As long as the DC input is
within the "window, defined by the +2V and -2V
references, both outputs are positive and both
level-shifting Transistors Q2 and Q3 are
off.

When the DC input moves outside the
"window, i.e. more positive than +2V or more
negative than -2V, Q1B or Q1A output will change
to -12V, pulling current through the resistor
chain R7-R8. Transistor Q2 turns on, which turns
Q3 on, and pins 6/7 and pin 2 of the timer are
pulled to zero Volt through D7/D8. This resets the
timer, and the relays disconnect the speaker from
the amplifier. When the DC offset is removed from
the input the opamp output, which was at -12V,
returns to +l2V and Q2/Q3 turn off. The timer is
now ready to start counting again and after the
predefined time delay the relays will switch the
speaker on.

The output of your power amp
has audio signals present, which have to be
removed from the input of the DC sensing circuit.
This is done with a single-pole RC network,
consisting of R1 and C1/C2. The -3dB point of this
network has to be selected low enough to avoid
triggering the circuit by low frequency audio
signals. However, it should not be too low,
because it makes the circuit too slow. If you
select C1=1uF, the circuit will activate at 50Hz
with 100W into 8 Ohm. This is not acceptable in
most applications. With 6.8uF I measured full
power down to below l0Hz, without triggering the
circuit. With C1 equal approx. 11uF (two 22uF caps
in series) the -3dB point is at 0.15Hz and full
output is available down to 5Hz. The circuit stays
permanently shot down when you drive the amp with
2Hz at full power. You might want to experiment
with this: if you want to prevent subsonic signals
from reaching your speakers you might want to
increase the -3dB point.

The purpose of Dl
and D2 is two-fold: It prevents the caps from
charging to a very high voltage, which would take
a long time to discharge. More importantly, it is
a protection for the opamp inputs: it prevents the
input voltage to rise above the +l2Volt rail or
below the -12V rail, which would destroy the ICs.
A series resistor R2 limits the input current to
the opamps.

There are two extra DC inputs
to the circuit: S1 and S2. These can be used as
extra logic inputs from other protection circuits.
The logic level is the same as the output of the
opamps: +12V (or no connection) and -12V. With
+12V (or no connection) the circuit is simulating
a normal operation, i.e. the circuit is not
activated. With S1 or S2 equal -12V, Q2 and Q3
turn on instantly (there is no time delay here)
and the speakers are switched off. These inputs
can be used for example for thermal breakers (the
contact of the thermal breaker has to be normally
open and connected to -12V), for circuits testing
for short circuit at the output, for power sensing
circuits, etc.

Power is supplied by two 12V
TO-220 regulators. AC input is 2x12-15V, with
center tap. This can be supplied by the normal
power transformer, or by a separate transformer of
5-10VA
rating.

Connecting the Protection Circuit to
Your Amp.

The board should be physically
close to the output of the amplifier and the
output connector in order to minimize the wire
length. Connect the output of the amp to the pad
marked "AMP" and the output connector to the pad
marked "LS" on the PCB. Use a cable here that
you normally use for output wiring. You can
solder -the cable directly to the pads, or you
can use solder pins, 6.3mm (l/411) Quick
Connects, or screw-type terminal blocks for the
connection. The ground connection stays as it
was before the installation of the
board.

Prepare a twisted pair of AWG
18-20 hook-up wire and connect the output of the
amplifier to the input of the protection
circuit. The ground connection should be taken
from the star-ground of the amplifier if
possible. If these wires have to go close to the
input of the power amp, then it is recommended
to use a shielded cable instead of the twisted
pair. Finally, connect the 2x12-15V AC and the
center tap to the board. Your installation is
now complete. Switch on your power amp and check
that the delay circuit is working. The DC
protection circuit will only work if you have
unwanted DC offset the output. Hopefully, this
will not happen very often.

The
EB-292/132 design is the intellectual property
of Erno Borbely/BORBELY AUDIO. Commercial use or
duplication in any form is not authorized
without license
agreement.

�2001-2007 Borbely Audio

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