One cheap speaker protection trick Peavey has used is put a 16-20A triac on the speaker hot output. The gate of the triac is connected to a 4 V diac, a to-92 part that passes current if the voltage on it goes over 4V either way from 0. The bottom of the triac is connected to speaker return. Peavey calls the trigger a bi-directional switch, but farnell calls it a diac. The input of the diac is a RC network consisting of a 47 kohm resistor to the speaker hot, driving a 2.2 uf AC rated cap(non polar electrolytic or 50v ceramic cap). The junction of the R&C drives the diac. This device shorts the output to speaker return if the DC level builds up enough to blow speakers. This is usually caused by a shorted output transistor.
Peavey expected the main AC breaker to protect the output transistors that were not shorted, but his was not good enough on my PV1.3k. When one shorted O.T. set the triac on, 6 of 10 O.T's were shorted out, and the trace was melted on the PCB. At $4.50 each, this was a nuisance. I back installed 25 amp fuses between the rail capacitors and the output transistors. These were 32 V DC rated car fuses. not AC rated ones. When I missed one shorted O.T. the fuse blew on the 95 v rails, so I suppose 32V rated fuses are okay.
Protecting the O.T's from shorted speaker wires is much more difficult. Starting with the CS800x, Peavey installed current transformers on the speaker lines, speaker relays, and a microprocessor to compare current delivered versus Safe Operating Area curves of the transistors. The design is complicated by the varying speaker loads typically installed, including 2 ohm, 4 ohm, and 8 ohm nominal loads. Lower operating voltage (low ohms) allows greater current. This microprocessor can only effectively purchased by an amateur in a burned up amp. See the speaker relay thread currently going on solid state amp thread for discussion of which speaker relay to use for no distortion or failure addition.
Another way to protect the O.T.'s from shorted speaker lines is the archaic single rail (voltage supply) amplifier, that has a capacitor between the O.T.'s and the speaker. Because 3300 uf capacitors (for effective bass) have to be electrolytic by economics, dual rail supplies don't work with this. Crossing zero volts with back to back electrolytic capacitors sounds funny. I tried it, don't bother.
The reviled single rail St120 amp is discussed under "ST120 can be beautiful" started by djoffe. I have one of these with the djoffe bias pcb adder and modern NTE181 O.T's, and also the "TIP" mod that puts 20 pf capacitors across the b-e junctions of fast driver transistors to prevent oscillation. The modified ST120 with output capacitors sounds exactly the same as my CS800s amp as best I can tell, and better than the all tube ST70 with new caps & O.T.'s, and original driver (7199) pcb. djoffe is selling a LM3886 output PCB for the ST120 amp with the output capacitor, but the LM3886 design can't be scaled up from 60 w/channel by paralleling like TO3 O.T's can. The original ST120 had very slow output transistors and crossover distortion due to inadequate bias current at 1 watt, so the magazine reviews in 1966 were terrible.
A fet version of the output capacitor amp is shown at the end of "output capacitors, objective and subjective analysis" thread started by mooly. This is the MJR7-mk4 amp, again with a single pair for FET's out. This could probably be scaled up but nobody has. Higher voltage O.T.'s and rail capacitors cost more, so commercial amp builders don't fool with this design. The way unprotected amps outsell amps with speaker short protection, inhibits amp manufacturers from doing a lot of engineering on this except on the premium amps like the CS800x or protected crown amps.
Of course the ultimate in speaker and amp protection is the tube amp with output transformers and a fuse on the AC input. These are fairly indestructable except by dropping. I've shorted the output with speaker wire whiskers on my ST70, I've taken lightning strikes on the AC line, no damage except the power switch and turn off pop eliminator capacitor.
Peavey expected the main AC breaker to protect the output transistors that were not shorted, but his was not good enough on my PV1.3k. When one shorted O.T. set the triac on, 6 of 10 O.T's were shorted out, and the trace was melted on the PCB. At $4.50 each, this was a nuisance. I back installed 25 amp fuses between the rail capacitors and the output transistors. These were 32 V DC rated car fuses. not AC rated ones. When I missed one shorted O.T. the fuse blew on the 95 v rails, so I suppose 32V rated fuses are okay.
Protecting the O.T's from shorted speaker wires is much more difficult. Starting with the CS800x, Peavey installed current transformers on the speaker lines, speaker relays, and a microprocessor to compare current delivered versus Safe Operating Area curves of the transistors. The design is complicated by the varying speaker loads typically installed, including 2 ohm, 4 ohm, and 8 ohm nominal loads. Lower operating voltage (low ohms) allows greater current. This microprocessor can only effectively purchased by an amateur in a burned up amp. See the speaker relay thread currently going on solid state amp thread for discussion of which speaker relay to use for no distortion or failure addition.
Another way to protect the O.T.'s from shorted speaker lines is the archaic single rail (voltage supply) amplifier, that has a capacitor between the O.T.'s and the speaker. Because 3300 uf capacitors (for effective bass) have to be electrolytic by economics, dual rail supplies don't work with this. Crossing zero volts with back to back electrolytic capacitors sounds funny. I tried it, don't bother.
The reviled single rail St120 amp is discussed under "ST120 can be beautiful" started by djoffe. I have one of these with the djoffe bias pcb adder and modern NTE181 O.T's, and also the "TIP" mod that puts 20 pf capacitors across the b-e junctions of fast driver transistors to prevent oscillation. The modified ST120 with output capacitors sounds exactly the same as my CS800s amp as best I can tell, and better than the all tube ST70 with new caps & O.T.'s, and original driver (7199) pcb. djoffe is selling a LM3886 output PCB for the ST120 amp with the output capacitor, but the LM3886 design can't be scaled up from 60 w/channel by paralleling like TO3 O.T's can. The original ST120 had very slow output transistors and crossover distortion due to inadequate bias current at 1 watt, so the magazine reviews in 1966 were terrible.
A fet version of the output capacitor amp is shown at the end of "output capacitors, objective and subjective analysis" thread started by mooly. This is the MJR7-mk4 amp, again with a single pair for FET's out. This could probably be scaled up but nobody has. Higher voltage O.T.'s and rail capacitors cost more, so commercial amp builders don't fool with this design. The way unprotected amps outsell amps with speaker short protection, inhibits amp manufacturers from doing a lot of engineering on this except on the premium amps like the CS800x or protected crown amps.
Of course the ultimate in speaker and amp protection is the tube amp with output transformers and a fuse on the AC input. These are fairly indestructable except by dropping. I've shorted the output with speaker wire whiskers on my ST70, I've taken lightning strikes on the AC line, no damage except the power switch and turn off pop eliminator capacitor.
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Understand what the protection is for, what its purpose is.
That traic across thge output is known as a "crowbar" circuit. In the presence of DC on the output, it turns on and shorts the output to ground. This is not done to protect anything in the amp. it is there to prevent the speakers from catching on fire. Seriously. When the crowbar fires, it generally gives its life in the process. And that dead short can easily further stress any surviving outputs.
The main breaker is not there to protect output devices. Transistors can be shorted out a LOT faster than any breaker can trip. The breaker, like that triac, is there to prevent the amplifier catching fire and potentially burning down the building. Any protection of the internal parts is just a happy bonus.
That traic across thge output is known as a "crowbar" circuit. In the presence of DC on the output, it turns on and shorts the output to ground. This is not done to protect anything in the amp. it is there to prevent the speakers from catching on fire. Seriously. When the crowbar fires, it generally gives its life in the process. And that dead short can easily further stress any surviving outputs.
The main breaker is not there to protect output devices. Transistors can be shorted out a LOT faster than any breaker can trip. The breaker, like that triac, is there to prevent the amplifier catching fire and potentially burning down the building. Any protection of the internal parts is just a happy bonus.
I would think that the output crowbar is there to protect the speaker array.
Peavey probably want to avoid consequential claims.
This works for domestic type system where the amp fuses blow and the music stops.
I doubt any PA user would find that acceptable.
The PA amp system must keep playing, or at least, as many channels keep playing as possible while the errant amp/s are temporarily limited, until the fault is cleared.
Peavey probably want to avoid consequential claims.
This works for domestic type system where the amp fuses blow and the music stops.
I doubt any PA user would find that acceptable.
The PA amp system must keep playing, or at least, as many channels keep playing as possible while the errant amp/s are temporarily limited, until the fault is cleared.
Most are worried about protecting the speakers in the event of an amplifier malfunction. The speakers/speaker arrays usually cost a heck of a lot more than the amplifiers do. When setting up a system the first thing that comes out is my impedance meter far before anything else does. One must first make sure the load isn't going to be a problem. One must not rely on the speaker manufacturers nominal load because the load changes with frequency changes. I have seen speakers rated at 8 ohms go to 4 or less in the low frequencies and all this must be considered when setting up any system.
When one must absolutely make sure the audio doesn't go down then one must rely on monitoring cards that will switch the amplifier or mixer in the event of a malfunction. This does require the use of a signal generating card. The signal is fed thru the system and to make this simple if the signal doesn't make it thru the mixer a back up mixer is switched in and the same goes for all the power amplifiers.
Want the system to work? Don't buy junk speakers, amplifiers, and related components.
Back to the protection circuit .... there is no one single way for 100% protection. The crowbar works well. DC protection circuits with relay work well. Have seen instances where they fail and the result will be destroyed speakers/drivers.
The 9440 Altec amplifier was designed very well for its time. If there was a failure the amplifier would automatically reduce its power output to 40% and keep on running. Haven't seen anything of a current design do the same thing. Loosing a bunch of output transistors still managed to put the amplifier on the sideline.
When one must absolutely make sure the audio doesn't go down then one must rely on monitoring cards that will switch the amplifier or mixer in the event of a malfunction. This does require the use of a signal generating card. The signal is fed thru the system and to make this simple if the signal doesn't make it thru the mixer a back up mixer is switched in and the same goes for all the power amplifiers.
Want the system to work? Don't buy junk speakers, amplifiers, and related components.
Back to the protection circuit .... there is no one single way for 100% protection. The crowbar works well. DC protection circuits with relay work well. Have seen instances where they fail and the result will be destroyed speakers/drivers.
The 9440 Altec amplifier was designed very well for its time. If there was a failure the amplifier would automatically reduce its power output to 40% and keep on running. Haven't seen anything of a current design do the same thing. Loosing a bunch of output transistors still managed to put the amplifier on the sideline.
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