BUL45 power transistors ok for audio?

[Alex R]

I am attaching a datasheet for the BUL45 NPN power transistor. These are TO-220 devices that will handle high current and high voltage. They are cheap and readily available. However they are advertised as good for switch-mode power supplies. Their power handling abilities and price seem to suggest that they would be good to stock for driver replacements - but does anyone know a reason why they should not be used for audio?

[Enzo]

They are good for high current and high voltage, yes, but that is intended to be in brief intermittent pulses, not audio. Look down the data sheet to Safe Operating Area. Note the part can be used at various cuurents and voltages, but the curve tells you how to use both together. The curve of interest is the one marked DC. The part is OK with 5 amps current, but only up to about 15v. By the time you get up to 50v, the allowed current is down to 0.7 amp. And at 100v, it is only half of that much even. Looking further, you CAN draw 10A through the part at 100v, but you can only do it for 50 microseconds.

The venerable TIP41C and TIP42C are 6A 100v parts, NPN and PNP respectively, and Allied currently lists them at 60 cents and 44 cents each. Need higher voltage for drivers? Try these from Allied:
MJE15030 NPN 8A 150V 94 cents
MJE15031 PNP 8A 150V 94 cents
or a bit more voltage:
MJE15032/33 NPN/PNP 8A 250v about $1.05 each

Those are US types, Asian numbers like these are used as drivers in comercial gear:
(Prices are from MCM)
2SA1306 PNP 160v 1.5A $1.70
2SC3298 NPN 160v 1.5A $2.29
2SA1011 PNP 180v 1.5A $0.62
2SC2344 NPN 180v 1.5A $1.23

I don't know how ceap you are finding these parts, but I think real audio parts are not overly expensive. Cetrtainly there are others than those I mentioned, I just spotted those in my parts drawers.

Having said that, you might get away with it in your application.

[Alex R]

- a really useful list which I have printed out... thanks Enzo. Of course I was seduced by the headline voltage and current figures and did not trouble to look at the curves.

Prices for parts like those are not too different in the UK, from firms like Nikko if RS are too dear. RS however do free delivery if you have a business account with them & don't seem to mind if I order six low-value packets in a day etc. Must be because they make a big margin but free next morning delivery for orders put in up to 8pm generally gives me what I need, also they do bulk discounts.

If anyone in the UK doesn't know of this firm I can recommend them as suppliers from stock of more hard-to-find parts at ok prices:

www.dalbani.co.uk

[Steve Conner]

How about the MJE340 and MJE350? Douglas Self and Rod Elliot used them as drivers in their audio amp designs.

[Alex R]

How about the MJE340 and MJE350? Douglas Self and Rod Elliot used them as drivers in their audio amp designs.

Thanks Steve, but these need to be 2A to replace 2SA958. I got some MJE15032/3 to make sure. This amp uses MJE340/50 in its earlier stages, not very well heatsinked possibly, just those U-shaped sheet steel thingies. I've taken in a pair of nice old amps from a theatre - JBL/Urei 6260s. One was fine, but this one has shorted/opened most of the negative side of one channel - from the output back though the current limiter into the voltage amplifier stage. Keep thinking I've fixed it, but I'd have been better off replacing every semiconductor in the channel, I keep finding something else shorted. If it goes on much longer I'll be asking poor Enzo again...

[Enzo]

There are tons of alternatives. I stuck to TO220, since that was what his proposed part was.

You didn't metion you were replacing something, I thought you were building.

2SA958 is a 200v 2A 30w part, so from my list, the 15032 is probably the best choice.

In a SS amp, add the rails together, and that is the max voltage a transistotr will see. SO if there are +/-50v rails, then xstrs have to handle 100v, so a 150v part should be good. COnsider that when subbing.

[Alex R]

It's 70v a side, so it will need the 15032/33, yes. I have them on order.

In looking for subs I'm used to looking at max voltage and current rather than curves and statements about intended use, and will be looking at the latter in future. However the BUL45 looked so different from the rest in max Vce and Ie figures that I thought there must be a catch, hence my question. Thanks again for the explanation.

[Enzo]

SOA is an important consideration in a power semi. That is why your 250w amplifier does not have one pair of MJ15024/25 in it, it will have several of each. Even though they are 250v 16A parts - They are 250v OR 16A, not both.

There aer guys who build stuff with switching transistors just as you would have. They often get away with it. But xstrs designed for switch mode are not made for steady current.

But if a data sheet talks about audio versus industrial control or something, then it is not so important