alternative to protection relay

[coldrunning2002]

To test the amplifier.

1) While the amplifier is playing music, have a test relay dead short the loudspeaker. Damage to amplifier?

2) While the amplifier is playing music with a normal loudspeaker, short out a power mosfet inside the amplifier, gate source & drain all connected together with a relay. Damage to loudspeaker? Damage to amplifier?

3) With the amplifier in test state 2) above, cycle the power to the amplifier and connect a normal loudspeaker. Switch on the amplifier. Amplifier damage? Loudspeaker damage? Awful noise from loudspeaker?

 

[bepowell]

Triac Speaker Protection Crowbar

The only problem with the diac/triac solution is the high breakover voltage of the diac, usually around 30 Volts. I had an amplifier going DC rather slowly and I could hear a nasty noise in the speaker. The speaker clearly would have expired before the DC reached 30V. I have been using two antiparallel BC547s in "negisitor" mode with the bases open circuit. These break down at about 10V. I used 100R to limit the pulses into the triac gate from the LP filter cap. Works a treat protecting the speaker at around +/-11V. I believe there are 8V diacs at about £1 but the result is pretty similar.

Or you could use the age old Diac feeding the gate of a Triac sat across the output. As soon as DC flows in either direction, governed by an R C circuit for a few milliseconds delay, the triac turns on until the supply stops after a supply fuse has blown. It even works when the amplifier produces square wave under over driven conditions.
No contact to weld or arc over!
This has been used on high power amplifiers for years and saved countless speaker systems from frying.

 

[Ian Finch]

....I had an amplifier going DC rather slowly and I could hear a nasty noise in the speaker. The speaker clearly would have expired before the DC reached 30V. I have been using two antiparallel BC547s in "negisitor" mode with the bases open circuit. These break down at about 10V. I used 100R to limit the pulses into the triac gate from the LP filter cap......

Now, that's good to know and a great inspiration for resurrecting some troublesome old kamikaze amps

 

[coldrunning2002]

I did a quick simulation with LT spice for d.c offset detection
Amplifier with +100 Volt and -100 Volt rails
d.c fault window comparator thresholds +/- 2.000 Volts
RC filter R=100K and C=47uF
5Hz 100V p-p loudspeaker test signal = RC Peak ripple voltage 0.637 Volts
Time to detect a short circuit MOSFET at amplifier runtime = 95 milliseconds
There is potential for the 47uF capacitor to develop leakage currents with aging that would stop the circuit working. Need to detect failed MOSFET at startup in less than 1 millisecond and switch off the main power supply.