have an SPDT contact that puts the
speaker to ground as we can see in the
circuit. This arrangement will take the
arcing to ground and not through the
speaker, forcing the fuse to blow.
Of course, there are relays designed
for large DC voltages and currents, but
usually those are expensive. There are
relays with magnets placed close to
contacts that will break the arcing, or
with the contacts sealed in an inert gas;
relays with contacts with a higher melting
point (like tungsten) doubled by silver
contacts for better conductivity; or
multiple contacts in series, etc.
Or, we can use two fast solid-state
relays (fairly expensive) controlled by the
circuit presented here to disconnect both
DC rails when DC voltage is present at
the output of the power amp. All these
could be employed, but they will end up
doing the same thing as the G2R relays
For drawing the schematics, I use
VISIO Technical. I did the PCB (printed
circuit board) design on double-sided
FR4 with 2 oz of copper for higher
current capability. The software used is
Pad2Pad and can be downloaded from
the Pad2Pad website
An external trace of 1.00 mm width
with 2 oz copper can carry 3A. On my
board, the high current traces are wider
and doubled as you can see in Figure 3
and Figure 4.
Following the same principle, an AC
detecting circuit can be inserted (as
shown in Figure 5) for applications where
there is not 12V DC that falls fast enough to disconnect
the load before the output of the amplifier goes through
the sudden large voltage swing that can stress the
speakers and the ears.
The TLP222A is a solid-state relay (SSR) used in series
with the 12V supply. It stays open ( 2.0 ohms max) if there
is AC voltage at the input of the DF10M bridge rectifier.
Per its datasheet, the time taken by the SSR to open and
cut the voltage to the protection circuit is maximum 0.5
ms. The maximum release time of the G2R relay is 5 ms.
For different AC voltages, the value for the 2K ohm
resistor must be changed to limit the current through the
LED inside the SSR to a typical 7. 5 mA. In the datasheet,
the recommended range for the current for the LED is
from 5 mA to 25 mA. The test I did was for 12V AC and
the current-limiting resistor was 2K ohm.
This circuit will protect speakers in case of amplifier
January 2018 39
■ FIGURE 3. PCB artwork, top and bottom views.
■ FIGURE 4. Photo of PCB, top and bottom.