Funny and odd - My DBs run so hot it melted the solder....
Mar 2, 2007 at 11:36 AM Thread Starter Post #1 of 8

kin0kin

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So I made a balanced diamond buffer amp and the DBs were biased to 44mV as I have usually done with my MH and PPA build....now the interesting thing is...the DBs actually melted the solder...I realised this when I was touching the MJE2x3, and they moved....but the buffer still runs fine. It really surprised me as I know it gets hot...but never till it melted the solder :lol: havent encoutered that in my previous builds at all and I've been using the same solder. I'm reluctant in lowering the bias
tongue.gif
will consider slapping some ram heatsink on them to reduce the heat....

bdb2.jpg

bdb1.jpg


Perhaps I placed them too close to each other?
 
Mar 2, 2007 at 12:27 PM Post #3 of 8

kin0kin

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Ok, it seems like I've got some very cranky transistors this time. I let them warm up again and started finding out the reason. Apparently I didnt let them warm up enough earlier....they were probably hitting over 100mV when the solder melted. I'd them warmed up for at least 30 mins now and the bias voltage going across the transistors is still very high...I've got my stock pot set to max resistance now and the voltage going across is around 60-76mV. They worked fine...sounds very good too...no wonder it sounded so darn good earlier
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I need to get some pots with higher resistance.....
 
Mar 2, 2007 at 2:37 PM Post #4 of 8

firefox360

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Well thats a first... I've never heard of something overheating so much that it would melt the solder. I'de expect the chips to fry before they even get to that point. What solder are you using?
 
Mar 2, 2007 at 3:23 PM Post #5 of 8

error401

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Quote:

Originally Posted by kin0kin /img/forum/go_quote.gif
Ok, it seems like I've got some very cranky transistors this time. I let them warm up again and started finding out the reason. Apparently I didnt let them warm up enough earlier....they were probably hitting over 100mV when the solder melted. I'd them warmed up for at least 30 mins now and the bias voltage going across the transistors is still very high...I've got my stock pot set to max resistance now and the voltage going across is around 60-76mV. They worked fine...sounds very good too...no wonder it sounded so darn good earlier
biggrin.gif
I need to get some pots with higher resistance.....



Wow, that is pretty impressive. A thermal runaway situation isn't usually something you walk away from
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.

I think I'll stick to NTC devices in the output stage myself :p
 
Mar 2, 2007 at 6:03 PM Post #6 of 8

kin0kin

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The solder I used was the common ratshack silver solder. Works good for me.

I wasn too comfortable with the 35mA bias and went back to my workshop and begin swapping output resistors to 5.7ohm....now..this is even more awkward...I managed to change all the resistors for the right channels buffer....but I ran out of resistors for the left channels...I totally forgotten that I need 16 resistors...but I had in mind only 8 because (1 board)...Now my left channel is stuck with 2.7ohm and right channel with 5.7 ohm. I biased them the right channel to 20+mA, and funnily..when I take the measurement for the left channel buffers now....they averaged at 27mA for about 30 mins. Do transistors need to "burn-in" ?
wink.gif
I'm starting to think that I either have some very odd transistors, or these are really "overclockable"
 
Mar 2, 2007 at 6:43 PM Post #7 of 8

error401

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Quote:

Originally Posted by kin0kin /img/forum/go_quote.gif
The solder I used was the common ratshack silver solder. Works good for me.

I wasn too comfortable with the 35mA bias and went back to my workshop and begin swapping output resistors to 5.7ohm....now..this is even more awkward...I managed to change all the resistors for the right channels buffer....but I ran out of resistors for the left channels...I totally forgotten that I need 16 resistors...but I had in mind only 8 because (1 board)...Now my left channel is stuck with 2.7ohm and right channel with 5.7 ohm. I biased them the right channel to 20+mA, and funnily..when I take the measurement for the left channel buffers now....they averaged at 27mA for about 30 mins. Do transistors need to "burn-in" ?
wink.gif
I'm starting to think that I either have some very odd transistors, or these are really "overclockable"



Transistors shouldn't need to burn in, but they do need to warm up. BJT transistors are fairly susceptible to thermal runaway. As the temperature increases, the internal resistance decreases, more current flows, increasing the gain. The increased gain causes more dissipation in the transistor, making it hotter, which decreases the internal resistance further etc. If your heatsinks are not sufficient to remove the heat and the transistor can just keep getting hotter, it will eventually destroy itself (if there's no external current limit or internal protection). Because of this it's very important to watch the bias current as the transistors warm up. You'll need to tweak it quite a few times before you get it to stabilize on the correct current because of the temperature dependence.
 
Mar 2, 2007 at 6:50 PM Post #8 of 8

kin0kin

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Yup, im aware of it's heat constrain and pitfall. I've been keeping my eyes on them quite frequently now just to see what's the temperature that they'd eventually settle down and stabilize.
 

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