mjl21193/94 indestructable??


2007-12-26 12:21 am
I have a subwoofer amp I built using 2 pairs of mjl21193/94 transistors, which gets its feed from a line level split to the main amp - the main speakers are powered by a 2ch yamaha amp.
Well today I was seeing how hot the heatsink got under harsh treatment and also making sure no transistors were heating up as the amp is on the sub and gets vibrations - suddenly the output wire to the sub hit the case with a huge spark :hot:

The music stopped and I thought - oh no time to replace the output stage - I looked at the yamaha amp and it was off and strangely my laptop which was feeding the music was also forced off - but after rebooting the computer and turning the yamaha back on the whole setup was unaffected.

My question for the day - how could a short on the speaker line to a subwoofer amp come back through the line level cable and cause the main amp and laptop to suddenly switch off?? it would have to travel in reverse through my valve preamp to get to the laptop - could it have dragged the mains down? the only thing that stayed on was the valve preamp.

Well I guess i'm lucky I don't have to spend a few hours refitting the output stage - but still seems a bit weird.
Sorry, no help from my reply, but I have a similar story. I use to get a static charge build up on me often when I lived in my last apartment. I touched my amp one day to turn the volume knob up that had an aluminum knob and case and got a nasty shock. It turned my amp and my satellite receiver off. I don't know why.


2007-10-10 5:21 pm
The short on the output of the amp probably brought down the AC mains. If you drop below about 95 volts for any length of time some devices powered by switch mode supplies (like computers) will just shut off.

The "free" current limiting on the mains may have been what saved the amp. You might have blown it with stiffer AC mains. 21193's are not completely indestructible (they try) but older types like the 2N6030 would not have survived an event which produced "a big spark".