Devastated with amplifier.

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#21
Hi K-amps. Interesting, how many of those transistors do you have available?

The next amplifier design will be using 20 outputs per channel. The new power supply will facilitate 750W on 4ohms and I want the output stage to be formidable and stable, with plenty of damping and very safe SOA.

I will use a form of current limiting on my amplifier between the filter caps and the amp during adjustment so that if anything happens, the caps can't dump all of their current into the amplifier and toast it.

I'll also be incorporating a crowbar into the design so that if any excessive things should happen, I can have the power fuse blown in an instant.
 
#25
Here's a tip with Vbe multipliers.

With reference to a BD139 (NPN), wire a resistor from collector to base, typically 1K5. Then wire in the pot between base and emitter.

This way, if the trimpot goes open circuit, which is common, then the transistor turns on hard, pulling bias voltage to around 0.1 volts, turning OFF the output stage.

If you wire the trimpot into the collector/base circuit, if it goes open circuit (which is probably very likely what happened, these are often poor quality components), the transistor switches off completely and the complete rail to rail voltage appears across the two halves of the output stage. This turns it on big time, biblical would appear to be a most appropriate word, and the output stage goes thermonuclear.........

You haven't been through the mill until you've tip toed through the silicon crematorium, and recognise EVERY tombstone.

Cheers,

Hugh
 
#26
Duo said:
Well, I have discovered why the amplifier blew up so well.

My Vbe multiplier had something I missed. The way it was implemented, if I turned the pot a certain amount, the Vbe transistor lost bias, allowing full current to flow right across the output stage as the amplifier shorted out.

The voltage breakdown of the transistors was not reached in this situation, just too much current flowed.

My power supply was far more than enough to fry all those transistors.

Either way, I'm changing the design so almost no matter what happens to the bias pot, it will not make the amp short out. And it'll be limited so that the output stage will only go up to a certain current so no accidents happen.
Click to expand...

This can be problem with the potmeter. I allways use the pot between the emitter and base of the Vbe multiplier transistor. If the potmeter goes wrong (no contact) the bias falls zero.
If You apply the pot between the base and collector, You can kill the output stage...

sajti
 
#27
AKSA said:
Here's a tip with Vbe multipliers.

With reference to a BD139 (NPN), wire a resistor from collector to base, typically 1K5. Then wire in the pot between base and emitter.

This way, if the trimpot goes open circuit, which is common, then the transistor turns on hard, pulling bias voltage to around 0.1 volts, turning OFF the output stage.

If you wire the trimpot into the collector/base circuit, if it goes open circuit (which is probably very likely what happened, these are often poor quality components), the transistor switches off completely and the complete rail to rail voltage appears across the two halves of the output stage. This turns it on big time, biblical would appear to be a most appropriate word, and the output stage goes thermonuclear.........

You haven't been through the mill until you've tip toed through the silicon crematorium, and recognise EVERY tombstone.

Cheers,

Hugh
Click to expand...


You won!😉

sajti
 
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