Output transistor on - side keeps blowing

[charliesixpack]

You can't just remove them in THIS amp. Then the whole amp has no DC feedback anymore.
You could remove them, and temp use a ~100-220ohm resistor between B and E.

You still have bias for the input through R17 so I don't see your point.  There is evidently a bias problem and if the DC feedback is causing the problem you will learn something.

[jarrod0987]

Yes I understand about the whole resistor thing but there is not one in the right place for that trick apparently. I too suspect the feedback loop may be broken but not sure where. I just verified Q2 (both of them) are good. I don't know why the Output stage keeps blowing. I don't have any shorts across the rails at this time. I am going to try to pull all the transistors and replace the ones in the power section first and see if the rails are right. Then put in all the ones for the amp section and try again to set the bias.

What about replacing the .1 ohm emitter resistors in the final stage for some 100 ohm ones? I don't know if that would screw up setting the bias at all?

[Wawa]

You still have bias for the input through R17 so I don't see your point.  There is evidently a bias problem and if the DC feedback is causing the problem you will learn something.

Bias is the C12/Q18/Q3 block.
DC feedback is voltage divider R17/R20.
AC (and DC) feedback is R17/R20/R19/C11.
HF (oscillating) feedback is C10.
Don't confuse things.
With the power transistors removed, there is NO link anymore between R42 (drivers out) and R17 (DC feedback in). Without that, the output voltage of the drivers will be all over the place.
Leo..

[Wawa]

jarrod0987.
Try to get the amp running with the power transistors replaced by 220ohm resistors.
You can connect a small speaker THROUGH A 1K RESISTOR to the amp, to hear what's going on.
Low volume, and keep an eye on the temp of the drivers.
Carefully measure everything. A scope makes things a lot easier.
DC (0.0volt) on the out rail is important.
Measure the voltage on the driver bases (+ and - ~1volt).
Before you put the new power transistors in, set the bias to minimum!!!!
Minimum voltage between driver bases, minimum over R42 (maximum pot resistance).
Leave it there untill you're 110% sure the amp is fixed. THEN turn bias up SLOWLY.
An amp without bias just has a small amount of crossover distortion at very low volumes.
Leo..

[charliesixpack]

With the power transistors removed, there is NO link anymore between R42 (drivers out) and R17 (DC feedback in). Without that, the output voltage of the drivers will be all over the place.

The OP asked "is it safe."  What is unsafe?  The input stage Q2 is obviously a differential input with the common mode voltage at GND.  Without the DC feedback you will still maintain symmetry between the differential outputs of Q2 (assuming, of course, you have 0 volts differential input).  You should have equal currents through R12 and R13, Q8 and Q1, Q16 and Q19, R11 and R28, etc.  The bias will not be "all over the place."  The bias will be very well defined.  The bias voltages may be uniformly high or low but if they are all over the place you have a much greater problem.

The question I have is what are the voltages across R12 and R13.  If the bias of the input stage is off nothing will be right.

[Wawa]

The input stage Q2 is obviously a differential input with the common mode voltage at GND.  Without the DC feedback you will still maintain symmetry between the differential outputs of Q2 (assuming, of course, you have 0 volts differential input).

The feedback input of the differential stage is still connected to ground via R20.
But it does not get any feedback from the drivers (R42) when the power transistors have been removed.
So the voltage on R42 might be anywhere between +55 and -55.
The 2x220ohm resistors restore the broken link, so the driver emitters are back to ~+.5 and -.5 volt again.

The bias voltage (over C12) will indeed always be ~2volt.
Leo..

[jarrod0987]

Lot's of good input here. When you say to put in a 220 ohm Resistors where the output transistors are...Do you mean from Base to emitter?
What wattage?
Can I use this trick/values on future AB amps?

Yes I do have a scope BTW. Also...I made sure the voltage at the center line/output jack is 0v when there is no signal being injected. Have had that problem before and toasted a speaker .

Thank You Everyone.

[Paul__B]

Yes I understand about the whole resistor thing but there is not one in the right place for that trick apparently.

Well then, you really need to make a place for them.

What about replacing the .1 ohm emitter resistors in the final stage for some 100 ohm ones? I don't know if that would screw up setting the bias at all?

Sure would!

[Wawa]

When you say to put in a 220 ohm Resistors where the output transistors are...Do you mean from Base to emitter?
What wattage?
Can I use this trick/values on future AB amps?

Yes.

The resistor replaces the B/E diode of the power transistor, so there will be ~0.6volt over that resistor.
That's uhhhh, 0.0016watt. A 1/4watt resistor will do.

Maybe. Some darlington configurations already HAVE that resistor.
Leo..

[TomGeorge]

Hi,
Wouldn't a diode as the B-E junction be better?

Tom....

[jarrod0987]

Good point. I think maybe the problem is it stops adjusting up once it hits .6 or .7 etc. Might be nice if you had a resistor so that if you were set to high you would know? Of course then again, Diode's don't behave like resistors either. I will be happy to see the response.

[Wawa]

Diodes there could be dangerous for the drivers.
Their emiter resistors are now two diodes with 0.4ohm in series.
If you turn up the bias, the drivers have to lift that heavy load.
Leo..

[jarrod0987]

This guy knows his Shiznet


So I can drop in the 220 Ohm resistors and then safely probe,scope, and adjust the bias pot without too much worry? Keeping an eye on the temp of the driver transistors because there will be no heat sink at this point?

What is a safe way to see if the pre amp is putting out some kind of garbage without hooking it up to the power amp? it has a tiny connector and I do know what the pins are. All I can think if is try to set up all the pins and probe the outputs with and without a signal etc?

[Wawa]

1/ Yes. Resistors, and bias to MINIMUM. Leave it there untill the amp is fixed.
Use a variac if you have one.

2/ Cross that bridge when you get there. Fix the main amp first.
Run the main amp without load and without input signal.
Carefully measure all the DC voltages.
Post back if you're not sure about something.
I don't expect problems with the preamp at this stage.
Leo..

[jarrod0987]

I should probably mention at the recommendation of GK I changed about half the op amps in the pre amp due to the gain channel was ok until I turned it up then weird noises happened. Further more before I blew the amp last time I was scoping the output and there was a wierd spike in the second half of the positive alternation. If the Diff amp Q2 (tested good) was working....Must have been on the incoming signal from pre amp?

Also I am trying to really get a strong mental grasp on what is happening with this biasing issue. I get that we are trying to set up the bias of both output transistors so that that they amplify just a tiny bit of the signal that was on the wrong side of the alternation to avoid a dead zone in the middle of the waveform. These close to 0v center parts of the alternations would turn on both transistors at the same time but if biased properly the they cancel partly and leave just the right signal left. Am I right so far?

Just curios, I know .7 is a very popular number for diodes, including the ones in the transistor B - E junction. In your experience, is that the number you will always see? Or are there Audio transistors hanging around that that .6 etc?


I am trying to understand the purpose of D7/11 and D18/19. It seems like D18/19 would never turn on and D7 and 11 I don't get what they do at all.

Also Is Q4 just for phase inversion?

Q1/16 and Q8/19 are just earlier stages of amplification before the drivers and finals yes?

Thanks so much for all the help. I realize these are elementary questions. Most of the video's I find only discuss class A and almost none seem to discuss in depth audio amps of the AB design.