BUV20 good or bad opportunity ?

[altec9440]

hello,

a closing shop have old buv20 from ST or Moto. Not saw much amplifiers with this transistor. What would be wise, leave or buy ?

http://www.farnell.com/datasheets/685063.pdf

 

[MarcelvdG]

It is primarily meant as a switching power transistor (that's what the U in the Pro-Electron code stands for). For class-AB amplifiers, the lack of information on second breakdown is an issue and for class-D the relatively low switching speed could be a problem.

 

[maton00]

Better than a 2N3055/MJ802 for sure they can be used in place of those and can sink a lot of current too

 

[JeffYoung]

I'm led to believe they work well in Naim amplifier circuits, but I haven't tried them yet.

They are dual die, for what that's worth. (They're also commonly counterfeited, often with smaller pins.)

 

[wg_ski]

Depends on what they cost. I buy off-beat transistors all the time, if the price is right. After popping one open to see if they are real, of course.

 

[altec9440]

$5 piece. It's old stock from a well known shop.

 

[davidsrsb]

Using a dual die device in a linear amplifier means that you have to assume one of the die is hogging all of the current for SOA calculation

 

[altec9440]

So not really atractive for audio use due to dual die.

Thanks friends for yours advices.

 

[JeffYoung]

Their SOA chart takes that into account. I suspect they're using resistance wire to do the bonding or something, as the kink in the SOA is there, but isn't all that dramatic:

$5 is a great price if they're real. Can you get some pictures?

Cheers,
Jeff.

 

[MarcelvdG]

Where did you find that SOAR chart? The datasheet that altec9440 linked to has no SOAR chart at all.

 

[JeffYoung]

Motorola's datasheet.

Cheers,
Jeff.

 

[wg_ski]

$5 piece. It's old stock from a well known shop.

Not too bad. I paid the same for MJ11032 and 33 darlingtons from Skycraft. Same 50 amps, and 30 volt SOA breakpoint. They actually share current reasonably well at high current and low voltage. If they didn’t you wouldn’t have any gain at 50 amps. The sharing breaks down at high voltages, hence the relatively poor SOA. All switching transistors, even single dies, do this. The reason the SOA falls off in the first place is because the current isnt uniformly distributed within even a single die. Same phenomenon, really.

I wouldn’t call them “useless” for audio - just not good for the typical application of running class AB 4 ohms on +/-50 to 70 volts. Where lower voltages are used and high currents are needed (no beta droop at 20 amps, for instance) , they could be better than spending two MJL3281. Lower package count, need fewer emitter resistors, and of course the metal case.

 

[wg_ski]

And I still have a couple of Je Ne Sais Quoi TO-3’s in the 50 amp package that are only labeled “K70” for type number. Anyone know if these are similar to the BUV20/21 or closer to the 2N5686? I’ve put a few of them into high current series regulators over the years, never anything more critical.

 

[davidsrsb]

The SOA graph shows that in a typical +/- ~50V rail amplifier, that you only have about 1A@50V for reactive loads
This part is only useful at supply rails of about 30V

 

[MarcelvdG]

In any case, what I wrote in post 2 is not correct: they are suitable for quasi-complementary class-B and class-AB amplifiers, under the limitation that David mentioned.

 

[Mark Tillotson]

The dies aren't even mounted close together making the thermal coupling worse than it might be - the size of a hot-spot compared to the extent of the die(s) is key. Hot spots get smaller at higher power densities and with lower thermal conductivity, higher voltage allows hot-spots to reach higher ultimate power densities. You want the natural hot-spot size to be bigger than the die to keep roughly even dissipation across the die.


As a switching device the power density is at its greatest when the voltage is smallest, so hotspot development is much less severe.

 

[JeffYoung]

The SOA graph shows that in a typical +/- ~50V rail amplifier, that you only have about 1A@50V for reactive loads
This part is only useful at supply rails of about 30V

The Naim circuit is +/-40V, which gives you 4A on the SOA. That's about the same as the MJL3281 (4.5A). Those are at DC and 1Hz, so both will do better with a music signal.

I sure wish I knew what the BDY58 SOA curve looked like....

 

[Vrystaat]

Guy's I have a couple of bux21 trannies, seems to be 8MHZ devices,will they be usable for an amp ?

 

[MarcelvdG]

Yes, as long as it is a quasi-complementary amplifier and the supply voltage is not too high. The limiting factor is second breakdown with reactive loads, just like for the BUV20.

 

[savvas]

I've used BUX20 in Naim circuit (Avondale actually) and they sounded marvelous. Both BUV20 and BUX20 have a meatier sound than standard MJ15003, they would be my choice for Jeff's new boards so go for it.