Subwoofer amp problem - s8550 substitute?

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So a few years ago, I built a sub.

I used this amp: Altronics - A2451 150W Active Subwoofer Amplifier

It's been sitting under my computer desk, working fine, until it recently started letting out random thumps and burps. Even when it's in standby. Scares the hell out of me late at night :)

Anyway, I pulled it apart and had a poke around and I think the problem is that the preamp board is only getting -0.75V on the -12V rail. Tracing back and it looks like they are running a zener and S8550 PNP transistor to give -12V from a -55V rail. This seems to be dead.

Not really surprising, as if I'm reading the datasheet correctly the S8550 has a C-E breakdown of -40V max and it's being run at -43V.

Why would they run it from such a high initial voltage you ask? Beats me, it seems to be it's own set of windings on the transformer...

I don't have anything equivalent to this part here, anyone have an idea of a substitute that may work? Preferably with a C-E breakdown of -45V or more.
The other spec which may be relevant is the collector current of 500ma.

I'm a noob when it comes to transistor selection, hoping someone can help me out with something that isn't likely to break again soon.


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Hi Mark
Are the dropping resistor, zener or filter cap also dead?
The actual TO92 transistor could probably be a common 500mA+ type like the original. Perhaps BC640 will have enough gain and a little more capability. 2N5401 should also be fine, but finding stock??

If you only shop retail, you may wind up having to substitute a sub. on a sub. part with a useless net result - try to avoid those sorts of choices that just make matters worse.
There other other common parts +50V Vceo, so you can get what you need through E14, RS etc.
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Hi Ian.
The zener still measures 12V drop when powered up, not sure about the cap, but I'll check that too. I measured the resistance of the rails with the S8550 removed and it's up in the Mohm range for the main board and pre-amp board, so I don't think there is a short.
The bc640 sure looks like a goer. It has less gain, but I think the S8550 was just in a pass configuration thingy, so I'm not sure it will matter? (happy to be corrected here as my ignorance of transistors is pretty profound :) )
The price is right anyway, and easy to get.
Thanks for your help, I'll report back :D


I replaced some 1 watt resistors with 5 watt types, as the PCB under them was discolouring from heat. I replaced the S8550 and S8050 with BC640 and BC639 respectively, and I replaced the nearby small electrolytic caps while I was at it.
Now after having to swap some to-92 legs around (S8x50 are EBC, BC6xx are ECB), installing the transistors the wrong way round (doh!), finding a tiny solder bridge short (*&^#!) and breaking and having to re-solder a connecting ribbon cable (mutter mutter, stupid wire...), it all seems to be working fine :D
By the way, the first datasheet I found for S8050 said it was PNP, it's most definitely NPN, which also caused some grief :mad:

The problem I was having seems to be pretty common with this amp, seems like pretty poor design to me.
Anyway, it's going again, thanks for your help.

Post schematic, this way we can evaluate and help

Otherwise will be a blind man describing what he cannot see.




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Hi Carlos,
Wasn't able to find a schematic, and didn't take the time to work one up of the section with the problem. I was able to follow the power traces back to the faulty transistor and just work from there.
Since it's all working again now, I won't be spending more time on it trying to draw a schematic, especially as the thing is installed back in it's box :p Sorry!

There is a picture with the relevant section in this thread:
which I've also replied in.

In summary, a bad design was using underrated parts, which are prone to failure (1W resistor, S8550/S8050 transistor). I replaced them with higher rated parts and it's working again. Hopefully it will stay working, we will see.

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