SS output protect circuit

[dhart67]

just repaired an old Sunn Colisium-300 bass amp, (thanks to what I've learned from many of you on this forum). had shorted output transistors, open resistors, and shorted transistor in power amp board. was interested in any suggestions on best way to prevent this from happening again. this old amp doesn't have an output to speaker protect circuit. has anyone added such? been researching this found several approaches using relays or FETs. what would be best? thanks

[52 Bill]

Are you looking to protect the amp or the speakers from future damage?

[Mick Bailey]

A transistor can decide to short on its own. In that scenario you can only realistically protect the speakers with an existing amp circuit. By the time there's DC on the output it's too late to save the amp. Ashdown uses fuses in the speaker output. I've had these blow and save the speakers, or blow and save the amp. Or just blow. Tricky balance to get the rating and type just right, but easy to do. Maybe there are circuit breakers that could do the same thing but not be a nuisance.

[Leo_Gnardo]

Have a look at Peavey's crowbar circuit. They used it on their classic CS800 and CS400 amps, and probably others besides. If it sensed a large DC offset, switched a triac into conduct mode, throwing a short circuit across the amp's output. Of course that's a self destruct event for the triac, but would save speakers from being destroyed and maybe even catching fire.

[Enzo]

Or consider that the amp lasted 40 some years as it was. At this point I am not sure adding a bunch of stuff will help.

[olddawg]

In my experience most output damage is caused by bad load conditions. I used to get frustrated with people blowing up their amps due to bad cables. So I would just mount a 2ohm large(ish) ceramic resistor in series with the positive output terminal of each channel. They lost negligible power and got some protection. It was especially effective on Yamahas.

[dhart67]

thanks to all for your sugggestions. will do more research. looking to protect both amp and speakers.

[teemuk]

The amp already features a short circuit protection. It's a crude VI-limiter -type circuit, which steers drive current away from the driver transistors when high-enough output current is detected. The problem of these crude circuits is that they can not reliably handle a -sustained- short circuit condition, so with anything but momentary short circuits of the load the amp will likely fail due to significantly increased power dissipation of the output devices. The VI-limiter can only help some, best scenario is to get rid of the short before it starts to cause damage. Basically, all these tend to require a relay-based control to disconnect the load (or something similar) when a sustained short circuit condition is detected. Muting also helps: Without output signal there is no short circuit ccurrent either.

You can likely modify the VI-limiter so that when the transistors in it activate they also activate circuitry to disconnect load, mute input signal, turn off input stage of that PA, etc.

The VI-limiter circuit will not prevent DC in the output, but if you google search you will likely find several schematics how to implement output DC and short circuit protections to transistorized power amps, usually with delayed muting feature (so that the amp doesn't "pop" annoyingly when you turn it on and off). Heck, I wouldn't be surprised if someone even sells such stuff in a pre-designed kit.