Problem with st151..

[kibet85]

Hi all..sorry to bother u guys but i have a problem with my st151..its smoking the 680 ohm resistor..wat could be the problem..was testing without first connecting the outputs which are mj15024 and mj15025...my transformer is a 38-0-38 with 15000 uf 63v caps...i have checked and double checked my design..please help..sorry for my bad english..

[jaycee]

You were already told in the ST151 thread... 38VAC from your transformer produces 53V DC power supplies - this is too high for the transistors in the circuit. The transistors T1/T2 and T5 are probably fried and shorted C-E hence all the voltage is being dropped over R11 and making it smoke.

You could probably build the same circuit using MPSA42/92 instead of BC546/556, and MJE340/350 instead of BD139/140. Bear in mind the MPSA's have reverse pinout from the BC's. You might also have to adjust the compensation capacitors.

[kibet85]

Thanks mate..wil do tht..

[AndrewT]

buy the correct transformer for the st151.

[jaycee]

Well yes, there is that too.

ST151 is not really a circuit that is designed for sub use. Sub amps are supposed to be very rugged and have a larger output stage than normal. The ST151 is a hifi amp, and not suited to this task. It is certainly not suitable for higher voltage rails than shown!

Personally I would consider looking at Elliot's P68 amp, or maybe one of Quasi's amps - the N-BIP300 amp would probably work fine with mj15024 transistors in there. You could even change it for a complementary output stage too, if you designed your own PCB.

[kibet85]

'Elliot's P68 amp or the N-BIP300 amp'

i checked out the above amps and they seem ok..the thing is that they are to expensive for me to make and an error would be costly...i also checked out the nmos 200...this seems ok..

[jaycee]

If you can get the mosfets then NMOS200 is your best bet

The reason these designs are expensive is because those extra parts and high spec parts are whats needed to get the stated output power *reliably*.

The NMOS200 is probably best because you can get some good mosfets quite cheap. For example, you could use IRF540 quite easily, and they are pretty cheap.

[AndrewT]

Hi,
the irf540 are 100V devices and should be used on PSU that never exceeds +-50Vdc when supply voltage is at highest worst condition.
Effectively that means +-45Vdc for normal operational use (about 30 to 32Vac depending on transformer regulation).
You can risk a higher voltage but you must accept the unreliability that comes with a decision to use +-53Vdc.

[jaycee]

Well spotted. Oops

There are quite a range of IRF devices that would work though - I think ideal for this would be IRF640. IRF740 also would work, and you quite often find these two used inside CRT monitors.

The recommended IRF840 works of course, but doesn't carry quite as much current.