Blameless-like amp burnt channel need help to prevent that happening again!

[smartx21]

I need some help to prevent my Blame ST from being injured once again since one channel has been burnt after an accidentally TV turn off and on.
Both rail fuses (2.5A), and two transistors (Q9 driver and Q12 output) have died.
Q9 is a BD139-16, and Q12 is a MJ4502 (Q11 is a MJ802). Both transistors lost the B-E junction, due to some overcurrent caused by the "thump". I haven't checked speakers yet.
Since a I am not a designer, could anyone help me adding some devices in order to prevent this happening again?
Thank you very much!
Regards,
Max.

[farjon]

Hi, Max.

I didn`t have this problem in my Blame ST yet. So, I can give only general suggestions.



1- As a VAS protection, I would use (not tested) the D12 diode mentioned here:

Dx Blame ES .... based into the Blameless, i am trying a new amplifier

2- Install catch diodes on Q11 and Q12.

3- Check that the 2K2/2W resistors are installed on power supply outputs. Some people also recomend a kind of catch diodes here.

4- Are you using the DC protection board?

If both rail fuses has died, I think some kind of cross-conduction, when both output transistors conducted simultaneously... what a kind of transient at this TV turn off and on! Can't say who died first and drag the other.



Well, at least avoid turn on-offs with high volume levels sets.

[ingenieus]

The design lacks a VI limiter, but it's an open question whether that would have helped.

[smartx21]

Originally Posted by Sound_Buster

Hi, Max.

I didn`t have this problem in my Blame ST yet. So, I can give only general suggestions.



1- As a VAS protection, I would use (not tested) the D12 diode mentioned here:

Dx Blame ES .... based into the Blameless, i am trying a new amplifier

2- Install catch diodes on Q11 and Q12.

3- Check that the 2K2/2W resistors are installed on power supply outputs. Some people also recomend a kind of catch diodes here.

4- Are you using the DC protection board?

If both rail fuses has died, I think some kind of cross-conduction, when both output transistors conducted simultaneously... what a kind of transient at this TV turn off and on! Can't say who died first and drag the other.



Well, at least avoid turn on-offs with high volume levels sets.

Thank you for your kind support, my friend.
I had been careful most of the time with both amp and TV set volume controls. But that night I was kinda asleep when my 10-year-young son did that deadly operation! I only noticed in the morning what had really happened!

The answer for questions 3 and 4 is YES.

[dadod]

Insert protection resistor of 1k between Q6 collector and the ground.

[farjon]

Originally Posted by Sound_Buster

3- Check that the 2K2/2W resistors are installed on power supply outputs. Some people also recomend a kind of catch diodes here.

More on these diodes here:

Power amp under development

[thaumaturge]

If Q8 opens the outputs will slam to the rails. A stack of two diodes paralelling it shouldn't affect normal operation but might be enough clamp to save some output devices.
Doc