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Using Arduino => General Electronics => Topic started by: jarrod0987 on Mar 30, 2015, 01:14 pm

Title: Output transistor on - side keeps blowing
Post by: jarrod0987 on Mar 30, 2015, 01:14 pm
What can cause the final Transistor of  the - side of a class B to blow but not the Driver for it or the + side? Can it be bias or perhaps some kind of weird half wave from the pre amp?

I already checked the resistors and they are high tolerance and right on. There is no load atm. This keeps reoccurring. Last 2 times it blew the driver too but not this time.

Q22 is the one that keeps blowing. Please refer to page 13.

Originally, Q22 and it's driver Q20, as well as Q3 were all blown. I replaced Q22 and Q20 because I did not know about Q3 yet. Found no adjust-ability out of R21 so then I discovered Q3 was blown. Replaced that and Q20,22 again. Adjust-ability returned but found base of 1.004 +vdc with R21 all the way counter clockwise. Q20 was only about 1.004 -vdc. Tried to adjust R21 and blew Q20 and 22 again. Replaced em again. Pot all the way CCW again and this time Q22 just blew right away.

Is it safe to just pull out Q20 and Q21 and run it to try to see what is coming in at the base of those?

Been struggling with this one for a while now.
Thank You
Title: Re: Output transistor on - side keeps blowing
Post by: charliesixpack on Mar 30, 2015, 03:28 pm
I don't see a reason why it would be unsafe to remove Q21/Q22 and measure the voltages at the bases.  You have a symmetrical situation where with zero volts across the Q2 differential inputs you should have complementary voltages on the push and pull sections of the circuit (i.e., the base of Q21 is approximately the negative of Q22, the base of Q20 is the negative of Q17, etc.).  You can check for symmetry at all points you can get a probe on and hopefully get some clues.
Title: Re: Output transistor on - side keeps blowing
Post by: Anders53 on Mar 30, 2015, 11:16 pm
Hi -

You should be careful adjusting the bias setting on the pot.
If you get it wrong, you will saturate the drivers Q17/Q20 and then rapidly the output stage Q212/Q22.
Before you even notice then, the weakest end of the pairs will burn out in a rapid thermal run-away.

I suspect that you have a problem in the bias stage Q3/Q18, which are set up as an adjustable Zener diode circuit.
If the pot is aged and worn out, the wiper may loose connection during adjustment, causing the circuit to be biased too high.
Also if capacitor C12 is a dried out electrolytic one, it will no longer dampen rapid changes during adjustment.

Notice the diagram voltages written at Q8/Q19 emitters.
These voltages should be spot on and replicated in the high end emitters Q1/Q16.
If these voltages are not spot on, you should investigate the differentail amplifier Q2 and its emitter circuit.
The two collectors should have equal voltage rops over R12/R13

have you done a basic check on the power supply ?
If one side is out of tolerance, the whole amplifier circuit becomes unbalanced.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Mar 31, 2015, 12:20 am
I don't see a reason why it would be unsafe to remove Q21/Q22 and measure the voltages at the bases.
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.
NO (speaker) load ofcourse... Just a scope on the output.

If the pot is aged and worn out, the wiper may loose connection during adjustment, causing the circuit to be biased too high.
In this amp, as in most amps, bias will be at minimum when the wiper looses connection.

Just short the whole bias circuit. e.g with a jumper/solder blob over C12.
That way you can rule out the bias circuit while fixing the amp.
Without bias, the amp just has a lot of crossover distortion.

Did you use branded transistors. There seem to be a lot of "fakes" around.
Does the amp also blow without a load. With or without input signal.
Do you use 8ohm speakers. IMHO, this amp can't handle 4ohm loads!
120volt rail (2x60) and just a single output pair is typical "home theatre only" stuff.
Leo..



Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 04, 2015, 07:23 am
Lot's of Great help here. I have been doing a lot of reading and YouTube to solve this. What you folks are asking is related to some of that. Let me address your questions as best I can.

Quote
You should be careful adjusting the bias setting on the pot.
If you get it wrong, you will saturate the drivers Q17/Q20 and then rapidly the output stage Q212/Q22.
Before you even notice then, the weakest end of the pairs will burn out in a rapid thermal run-away.
I suspect this is what happened when I tried to adjust the bias the first time. The Positive side (Q17 base) was very close but the negative side (Q20 Base) was way off (1v). After I replaced 21/22 AGAIN they just blew right away, even with the pot all the way CCW like the drawing instructs.

Quote
I suspect that you have a problem in the bias stage Q3/Q18
Me Too. I found Q3 to be bad but Q18 I had no replacement for at the time. I never tested it but I have a replacement coming now anyways. I got adjustablity at Q17/20 back after replacing Q3.

Quote
Also if capacitor C12 is a dried out electrolytic one, it will no longer dampen rapid changes during adjustment.
Had replaced all caps with brand new ones just in case.


Quote
Notice the diagram voltages written at Q8/Q19 emitters.
These voltages should be spot on and replicated in the high end emitters Q1/Q16.
Will do. I am not 100% clear on the purpose of these transistors except that they seem to be somehow related to the feedback loop?

Quote
If these voltages are not spot on, you should investigate the differential amplifier Q2
I have recently become very suspicious of this section and doing a lot of research on "Long Tail Pairs". I actually only half understood it before but I think I have the whole Idea now. It's a phase inverter also and not just a diff amp. Seems like I have had this go bad in a few amps now. One was spitting out DC on the output jack. I did not check Q2 originally because it was a hard to get 2 in 1 package but I have GK recommended substitutes in the mail now.

Quote
have you done a basic check on the power supply ?
I checked the plus and minus 60 but I forgot the + and -15. Embarrassing. Will check it after all the transistors get replaced. I already paid for them. Going to put them in.

Quote
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.
I realized about the feedback issue after a while. Good tip about the resistors.


Quote
Just short the whole bias circuit. e.g with a jumper/solder blob over C12.
That way you can rule out the bias circuit while fixing the amp.
Without bias, the amp just has a lot of crossover distortion.
Are you sure this is ok??

Quote
Did you use branded transistors. There seem to be a lot of "fakes" around.
I got the first batch from Digikey. I'm getting this batch right from GK themselves.


Quote
Does the amp also blow without a load.
Yes :(
It also blew with an 8 ohm speaker but not for a much longer time and I think that time I turned it up to high for too long.

Thank You All.
Title: Re: Output transistor on - side keeps blowing
Post by: TomGeorge on Apr 04, 2015, 07:35 am
Hi,
A long long long time ago I built an Amp,   Electronics Australia, Twin 25, to be exact.

They had fuses in the two 40Vdc busses that fed the final transistors.
To set it up from scratch, you replaced the fuses with 100R 2W ,I think resistors.

This limited any serious current, but did not impose a great drop on the busses when the bias/quiescent current was set.

This saved lots of blown transistors, as in my case the supplier had supplied four transistors with alternate pin outs, but did not tell the constructor.

You may be worth cutting the 60V bsses and fitting a resistor to limit your current.

Just a suggestion.

Tom...... :)
Title: Re: Output transistor on - side keeps blowing
Post by: Paul__B on Apr 04, 2015, 07:41 am
To set it up from scratch, you replaced the fuses with 100R 2W ,I think resistors.
I think you mean 100R 2W resistive fuses.  :smiley-lol:

(25V-a-side supply?  6 watts at 100 ohms.)
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 04, 2015, 07:43 am
I R to dumb to understand this advice :D I should swap the fuses in the circuit with resistors?
Title: Re: Output transistor on - side keeps blowing
Post by: TomGeorge on Apr 04, 2015, 07:52 am
Hi,
I said it was a long time ago, found the article. 1976.

https://dl.dropboxusercontent.com/u/44941643/Playmaster-Twin-25-v1.pdf

Its 8Meg in size, scanned three part article.
You also had to cut a couple of links to set it up, yes 100R resistors, read, and + and - 30Vdc buss.

Tom... :)
Title: Re: Output transistor on - side keeps blowing
Post by: Paul__B on Apr 04, 2015, 08:00 am
I R to dumb to understand this advice :D I should swap the fuses in the circuit with resistors?
There are fuses on the plus and minus side of the output transistors?
(http://www.eleccircuit.com/wp-content/uploads/2007/05/amp-100w-ocl-2sa1215-600x405.jpg)

Yes, put 47 or 100 ohm resistors there instead whilst you "tune it up".  Cheaper than transistors for starters.

Couldn't find a diagram on Google to show it, but that "Playmaster" design has the fuses only in the collectors of the power transistors, which is preferable for the purpose of calibration.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 04, 2015, 08:25 am
Are you sure this is ok??
Yes.
I have designed/build many power amps, starting with a Quad 303 clone in the early 70s.
That was after several valve amps, and a 600watt guitar amp that blew five from the eight speakers connected to it within 10 minutes :(
And I have fixed a truckload of amps, working as a consumer audio tech for 40+ years.

The advice about replacing the fuses with 100ohm resistors is an old trick to set the bias without blowing up the output stage if you get it wrong. It also is a perfect current sense resistor to measure the bias.
But it won't stop the amp from blowing up if something else is wrong. Transistors die faster than fuses.
It also depends on WHERE the fuses are in the schematic.

Blowing the amp without a load (and input) can mean two things:
The transistors are able to withstand the 60volts idle, but not the 120volt output swing (fake transistors).
Or the bias goes up with voltage swing (bias circuit).
Remember that these transistors have a 100-110watt SOA (safe operating area). That's only 1A at 120volt.
Advice for a non-tech: carefully inspect/measure EVERY part.
And replace every electrolytic cap under 47uF.
Don't ever party with amps like this. This supply voltage needs two or three pairs of output transistors per channel, and a massive heatsink.
Leo..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 04, 2015, 10:49 am
I have re attached the print for convenience. The only fuses I see are in the power rail before the driver and power transistors. This circuit also has some kind of fault protection which frankly doesn't seem to do diddly except on start up.

I bought my replacement transistors from Digi Key and all the caps as well, replaced all the caps.
I don't understand the part about the bias going up etc. Do you mean it is contaminated with AC and rising above it's set level when the positive alternation comes? How can I check for this without blowing my transistors again? it doesn't look like fuses are an option in this case?

I am no audio tech by any means but I would like to be. I have a solid understanding of old school linear power supplies and NPN's set up as class A and that is about it. I'm trying to retrain in switch modes and AB audio amps etc. Learning a lot but still feel like a total newb. Sometimes it's hard to find instructional info on the web because I am so new I don't know the right words to search for etc.

Also...I have been struggling to understand how the bias works in this amp. I don't have a clear procedure in my mind. I'm just trying to make the dc voltages on the drawing match reality and it isn't going well. Maybe someone can point me to a reliable method for this type of biasing somewhere?

Thank You.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 04, 2015, 11:21 am
The bias circuit is probably the easiest part of this amp.
It consists of C12 and every part to the right of it, R18, R22, R23 and pot R21 Q18 and Q3.
It has the function of a "super" zener diode. Current path is Q16 > bias "zener" > Q19.

The zener voltage is the same as the four driver/power transistors's B/E "diodes" need, to just start conducting.
The bias circuit is designed to follow the changing bias voltage of the power stage due to temperature fluctuations (power) as close as possible.

Bias is adjusted by measuring the voltage over the string of emitter resistors from the power stage.
J2-1/J2-2 is the connector for that.
20-40mA (~8-16mV over 0.4ohm) is common. Or follow the designer's recommendation.
Or measure the still working channel, and copy.

A better way is using a scope on the amp's output.
Feed a tiny 1Khz sinewave into the amp, and adjust for just disappearing crossover distortion.

Q11 measures the current in the power stage, and generates a fault condition if this gets too high. 
Hope this helps a bit.
Leo..

P.S. Did you measure the 0.1ohm emitter resistors. The sometimes suffer after a shorted power stage.

Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 04, 2015, 11:28 am
So I get that it is supposed to set up a DC bias for Q17/20. That is correct isn't it? The bases of those Q's should be +1v and -1.2v respectivly if the bias is all proper?

Originaly I was getting no adjustability there and then discovered Q3 was cooked. Replaced it and with R21 all the way CCW Q17 base was at 1.004vdc (with no freq injection btw) and also Q20 was -1.0x vdc. I adjusted R21 until Q20 was -1.2 vdc then a strange buzzing noise and pow, no more output Q's. Replaced em. Put R21 all the way CCW again. Turned it on....Pow...no more output transistors again. This time my plan is to change out every transistor from exact replacements obtained from the manufacturer at twice the price.

I did just verify Q2 is good on both sides as per DCA75 tester. If you have any idea what this means please do tell :(

Thank You
Title: Re: Output transistor on - side keeps blowing
Post by: Anders53 on Apr 04, 2015, 02:01 pm
One of your problems is, that you do a destructive testing, which means you are set back, everytime you are trying to do something, by blown transistors.

Solve that problem first.
Fuses and resistors have been mentioned as current limiters.

Why not try a bulb ?
It wont blow up and it will limit the current, and have a nice side effect of giving you a faint visual alarm light.
This is my preferred method of fault finding 12V electric circuits in cars.

Donno what your local power is, if you are in a US based location, you would have easy acces to 120V bulbs.
But 240V bulbs can be used as well.
A 120V 40W bulb in each power side would limit the current to something around 100mA.
240V 60W bulbs will give you an even lower maximum current.

I fear, that you might have the infameous problem of a self oscillating amplifier, which would exactly explain a buzzing sound.
Such a problem is normally due to a broken feedback loop, even a hairthin crack in a pcb copper track or a bad solder joint.
Title: Re: Output transistor on - side keeps blowing
Post by: charliesixpack on Apr 04, 2015, 02:34 pm
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.
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 04, 2015, 03:14 pm
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?
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 04, 2015, 11:28 pm
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..
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 04, 2015, 11:45 pm
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..

Title: Re: Output transistor on - side keeps blowing
Post by: charliesixpack on Apr 05, 2015, 12:32 am
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.

Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 05, 2015, 01:10 am
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..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 05, 2015, 06:02 am
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.
Title: Re: Output transistor on - side keeps blowing
Post by: Paul__B on Apr 05, 2015, 06:21 am
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!
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 05, 2015, 08:49 am
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..
Title: Re: Output transistor on - side keeps blowing
Post by: TomGeorge on Apr 05, 2015, 08:54 am
Hi,
Wouldn't a diode as the B-E junction be better????

Tom.... :)
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 05, 2015, 10:42 am
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.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 05, 2015, 11:01 am
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..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 05, 2015, 11:14 am
This guy knows his Shiznet :D


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?
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 05, 2015, 11:54 am
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..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 05, 2015, 03:12 pm
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.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 05, 2015, 11:58 pm
HF oscillations can indeed do damage. Evidence of that can usually be found at the amp's output.
R49/R50/R51 usually release their precious smoke.

I think you're starting to understand the bias circuit.
Give the four B/E diodes (drivers and power transistors) a certain pre-voltage, so they already start to work a bit.
Then a small (music) signal does not have to overcome that B/E diode threshold first.

The voltage that a diode just starts conducting is ~0.5volt, but higher with more current. And temperature dependant.
0.65volt is average, and should be used for calculations.

D18/19 is easy to understand. They have the same function as the kickback diode over a relay.
The speaker/crosover connected to the amp is an inductive load. Spikes generated by the speaker are dumped onto the power rails by these diodes.

D7/11 seem to be "soft clip" diodes. e.g. the base of Q17 can't be driven any higher than the volt-drop over R12.

Q4 is a level shifter. Takes things from +57volt to -57volt.

Q1/16 and Q8/19 are the voltage amplifiers. The power stage is the current amplifier.
Leo..


 


Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 06, 2015, 06:20 am
So the final driver and output stages have no voltage gain? Only current gain? I thought that for a while but eventually convinced myself they had some voltage gain as well. Also there for current gain of course. Is this wrong?

Also for "Soft clipping" is this just for circuit protection when adjusting the bias in case a person went to high?

Thank You
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 06, 2015, 06:46 am
Power amps usually have a voltage stage, and a current stage.
But there are also "compound" designs (rare).

The output (current) stage in this amp has indeed no voltage gain.
The bias block/driver bases are pushed/pulled to almost the supply voltages by the voltage stage.
The drivers/power transistors just follow this with more "balls".

Soft clipping has nothing to do with bias.
It is not possible to have a higher voltage out of the amp than the rail voltage (+60v/-60v).
If you try, it "clips" the tops from your signal.
When you turn up the voume more, your sine wave will turn into a square wave.
Clipping has sharp edges.
That distortion adds lots of high frequencies that are not pleasant to hear and kill tweeters.
Soft clip is a way to stop the amp from reaching 100%.
Sort of braking before you hit the brick wall.
Leo..


 
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 06, 2015, 07:04 am
Quote
Soft clipping has nothing to do with bias.
It is not possible to have a higher voltage out of the amp than the rail voltage (+60v/-60v).
If you try, it "clips" the tops from your signal.
When you turn up the volume more, your sine wave will turn into a square wave.
Clipping has sharp edges.
That distortion adds lots of high frequencies that are not pleasant to hear and kill tweeters.
Soft clip is a way to stop the amp from reaching 100%.
Sort of braking before you hit the brick wall.
Leo..
I guess I can't understand how you would get a situation where you had a higher voltage then the rails because there is no place where it is available to  get it form? Is it so that the voltage from the pre amp signal doesn't add on top of the rail voltage and give you rail + signal voltage?

Also...Does this 220 ohm resistor trick work on power mosfets (Gate to source) as well? I have a finicky switch mode that keeps blowing it's outputs while I'm trying to adjust the feedback pots etc.

Thank You
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 06, 2015, 07:22 am
The output from a preamp is normally a few volts top/top.
The voltage stage of the main amp has a fixed gain (R17:R19/20), so if you feed more than a few volts into the main amp, the voltage stage will clip (at about +55v/-55v).

Mosfets and SMPSs is a different kettle of fish.
Amplifier tricks don't apply there.
Post a picture/schematic, and I will see if I can help.
Leo..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 06, 2015, 07:58 am
Ipod Camera is shit. I have a different thread for that issue. I get back to that problem. This one is more important. Thank you for all your explanations and tricks. If there is something I can do for you please let me know.

Thank You
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 13, 2015, 10:48 am
The Saga Continues....

Replaced every single transistor on the board with the exact parts right from GK except for Q2 which was a sub they shipped me. I have them installed right. I am getting exactly the same readings as before so I don't believe it was the cause of the problem anyways.

So here is the testing scenario:

No tone, No load, no Final output Transistors (Q21,Q22). I have 220 ohm resistors installed B - E in there place.

My rail voltages for + and - 60vdc are actualy +58 and -58.1 on my multimeter which is not NIST calibrated. + and -15vdc is reading +13.8vdc and -14vdc.

Is this going to be a problem? I  have a nice digital Tek O-Scope that is probably way more accurate but for the life of me I don;t dare hook it to anywhere but cold ground which I can't find a good spot on this board. All I see is the earth ground going straight to the chassis. I see no place to clamp onto the neutral with this board. Thought about putting something on somewhere. Last time i tried to use Hot ground and that was a mistake :( Thought it would work like a volt meter but I forgot the probe is earth grounded. Should I just lift the ground on the scope with a wall adapter and use hot ground? Isolation transformers are expensive and I can't afford it right now.


Any ways...Moving on...

With R21 adjusted all the way down I am supposed to be able to set up +1.0vdc on Q17 and -1.2vdc on Q20.

However....Q17 is already at +1.030vdc and that is already to high (But probably ok?)
Q20 is -1.103vdc.
If I adjust R21 to make Q20 -1.2ish then Q17 becomes +1.1ish etc.
What should I do here?
Thought I would see if the fuse R42 is blown causing the emitters to float. Haven't found it yet. Measuring between the 2 emitters reads 472 ohms. That is not right but not open either.

Update: Yep, found it. It's wide open. Going to replace and hopefully Bias will be right.
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 13, 2015, 12:15 pm
So I need FR25-100. It is 100 ohm 1/4 watt @ 1% tolerance.
Not finding it at Digikey or mouser.

How about this one?

http://www.digikey.com/product-detail/en/MR106100R00AAE66/MR106-100-.05-ND/957473 (http://www.digikey.com/product-detail/en/MR106100R00AAE66/MR106-100-.05-ND/957473)

Man those are expensive.

Can I just use a regular 1/4 watt 100 ohm precision resistor or is that a bad idea?

Thank You
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 13, 2015, 12:46 pm
Hi again.
Seems you might have found the problem. R42.
Tolerance is not that important there, but fuseable is.
If you can't find an affordable fuseable resistor, use a standard cheap 1/4watt metalfilm resistor.
Mount it a few milimeters off the board, so when it burns it can't do any harm.
Keep that bias DOWN untill you're sure everything is ok.
Leo..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 13, 2015, 01:22 pm
OK so I had an Idea that I could buy 1 fusible one and then get some cheaper metal film ones just for setting it up. Put the real one in at the end?

Can this Fuse being open cause my bias range issue? I have 100 ohm resistors but I don't know what kind. They just the plain tan ones. Also tolerance is probably gold.

The voltages from the supply being ever so slightly low is not a problem either right?

BTW it hides under a heat sing so I can't give it too much height off the board.

How about this one? It's 5% instead of 1% but Fusible 100 Ohm 1/4 Watt.

Thank You.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 13, 2015, 10:28 pm
I woud certainly put it in there to try.
And buy the proper one before you close the case.

Without that resistor, with the same bias setting, bias current in the output stage is higher.
That could have blown the output stage.

Don't worry about the supply voltages being lower.
They depend on mains power fluctuations.

!!! Lower that bias all the way BEFORE you put the power transistors in !!!
!!! Leave it on low while testing the amp !!!
Adjust after you have replaced the resistor with the original one.
Leo..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 15, 2015, 12:45 am
Quote
[/!!! Lower that bias all the way BEFORE you put the power transistors in !!!
!!! Leave it on low while testing the amp !!!
Adjust after you have replaced the resistor with the original one.
Leo..quote]

It's been all the way down. I just crept it up .1v for a few seconds to see what effect it would have on the - side bias. I put it back down again. I'm not going to turn it up until i figure out why the Bias is already so high on the positive side without having it turned up.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 15, 2015, 02:40 am
Bias voltage is measured ACROSS C12. And finally, when the power transistors are in, over the 0.4ohm emiter resistors.
There is NO positive bias or negative bias.

Output OFFSET (on the speaker terminal) is also important. Has to be as low as possible, but 50mv is ok.

If the base of the driver needs 1 or 1.5volt to do that, so be it.
Leo..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 18, 2015, 12:18 pm
Quote
Bias voltage is measured OVER C12
I am sorry but I don't understand this statement. Do you mean across C12? Like how you would measure the voltage drop across a resistor etc? If so what should it be?


I replaced R42 which was blown today. It had very little effect on the voltage at the base of Q17. Now 1.045v (should be 1.0) and Q20 is -1.106v (should be -1.2v). If I turn up Q20 to -1.2 I will get 1.145v at Q17 which is already too high as per the drawing. (These measured with bias all the way down and no test tone or load etc.

Upon further investigation I have found that R34 is not fusable resistor anymore but a precision one.
I don't know if that is a big deal or not.

R9 is out of spec. 850 ohms instead of the 1000.

Voltage at R34 should be -1.55 and at R30 -1.0 but both read 2.1v
R29 should be -0.38v but is reading 1.4 volts. Tired now. will try to understand what is happening in the AM.
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 18, 2015, 12:39 pm
Changed to across..

The voltage between the two driver bases won't change much when you replace R42.
But the current through the drivers will.

Look upon C12 (and all the bias the parts attached to it) as an elevator.
It can rise up and down, almost to the 60th floor and 60 floors down.
But the height of the elevator always stays the same.

A fusable resistor is just a normal resistor that burns up in a controlled way when something is wrong.
Up to you to use fusables or not.


R9 should be 1k. I assume you measured that disconnected, not in circuit.
A wrong value here could upset the balance of the amp.
Leo..




 
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 18, 2015, 12:58 pm
Quote
The voltage between the two driver bases won't change much when you replace R42.
But the current through the drivers will
Understood.

Quote
Look upon C12 (and all the bias the parts attached to it) as an elevator.
It can rise up and down, almost to the 60th floor and 60 floors down.
But the height of the elevator always stays the same.
OK, But what voltage should I see across C12? How do i know when I have it set right?

Quote
A fusable resistor is just a normal resistor that burns up in a controlled way when something is wrong.
Up to you to use fusables or not.
Understood. Just wanted to know if fusible was important here or just a cost saver.

Quote
R9 should be 1k. I assume you measured that disconnected, not in circuit.
A wrong value here could upset the balance of the amp.
I measured it just in circuit for an initial reading. going to pull it and test it right tomorrow.


Somewhere I think I was advised the voltages at the bases of the transistors listed in the drawing needed to be exact and so shouldn't the R29,30, and 34 voltages drops etc. Is that right? They are all high.

Thank You

Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 18, 2015, 10:42 pm
OK, But what voltage should I see across C12? How do i know when I have it set right?
The voltage across C12 should be ~~~2volt.
That voltage is there to put the power stage (Q17,20,21,22) on the edge of conducting.
That threshold point is temperature dependent (power/music).
So the bias circuit measures the temp, and adjust that voltage.
That variation in voltage is not huge, but even the slightest change makes a big difference to the power stage current.
How do you know it's right? Bias current causes a volt drop across the four 0.1ohm resistors.
Set the bias to 20-40mA when the amp is fixed. Explained in previous posts.

Somewhere I think I was advised the voltages at the bases of the transistors listed in the drawing needed to be exact and so shouldn't the R29,30, and 34 voltages drops etc. Is that right? They are all high.
The whole voltage stage is a balancing act.
Start measuring voltage across R12/R13 (~1.55volt).
Q4 converts that into a current (0.5mA).
That is converted into a voltage across R34 > R30 > R29.
! the -1.55v listed next to R34 is the voltage across D13 and R34 !
Leo..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 19, 2015, 05:51 am
Quote
The voltage across C12 should be ~~~2volt.
I have 2.151v. Will that be acceptable? The numbers on the drawing would give 2.2v.

Also...My understanding of bias is that it is DC and you measure it as DC with 1khz test signal or anything.

I am a little confused because I thought this was an AB class amplifier and it's whole benefit was that there was no current through the emitters of the final stage when there was no signal? I Guess .5ma is still basically nothing. I will try to make this adjustment tonight as it seems like there isn't much wrong now. I do have to check that 1K resistor out though.

Thank You


UPDATE: So I checked that 1K resistor. It's fine out of circuit. I rebuilt the amp.

 I was to scared to try to turn it on without the heat sink on because I have blown it so much. I can't get to the temporary 220 Ohm resistors when the heat sink is installed. I really am unclear weather those various voltages stated on the drawing must be exact or not. At any rate I have checked every single component on the board now and replaced a few bad things since the last time I tried to adjust anything. In fact...I could get any adjustments out of R21 because I did not know Q3 was bad last time around. I also replaced that burned out fuse which was probably causing the problem with the outputs blowing. I know I'm not supposed to put the outputs back in until I'm sure it's right. Normally I would put a signal in and look at the resistors I installed but I'm to chicken to do it with no heat sink and I can't get to them with one. I felt pretty confident it should be proper now that I checked every component and replaced every E cap and Transistor. So I decided to go for it. I put the outputs in (Brand new). Turned bias down, no load, no signal, and powered up. No fuse blowing this time :) No Weird noise.

 I measured across J2. Got 0.000V. I started to creep up R21 super slow. Nothing, Nothing, Nothing.....I chickened out and shut it down. Last time I turned it up it blew. I know I changed that fuse but I still didn't feel safe doing it. Then I remembered the factory had used that black nail polish to mark/hold the pot. I decided it wasn't that much further away from where I chickened out at so I decided to try it right that. 0.001mv. Aha....so very very slowly I got it up to where it's just flickering the meter between 0.004 and 0.005mv which should be .005mv exactly (on my non calibrated meter).

 Most references and Wawa say .008 -.020mv. Manufacturer says .005v. I'm going to go with the safer number. My plan is to let it run a while and monitor the heat sink. So far it seems room temp or slightly higher. Going to use my laser temp probe on it :) Then I will try plunging in the preamp and see what happens. Then I will try a small 1khz Signal and try to look for crossover distortion. Then I will try testing it at a higher power and let it go for a bit and make sure there are no overheating or other distortion issues. Will update then.
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 19, 2015, 09:07 am
Another Q regarding this amp. I am using the grey rubberized thermal covers on my output transistors instead of grease, couldn't get mica in that size and I hate it anyways. I just read an article that says silicone thermal pads are useless at high power. This is a 100w amp. Am I using the wrong thermal product?

What is a normal temp range for the heat sink?
I have been running it for a while with no signal and pre amp. Ambient is about 67F(19C) and the warmer (driver) side of the heat sink is 86F(30.1C) but of course I am not making the amp do anything right now.

Also...When I scope it I have been using the ground side of the output for the ground clip on my oscope. I tried using earth ground but it was really noisy. However, all the articles say use earth ground. Am I doing that wrong too?
Title: Re: Output transistor on - side keeps blowing
Post by: Wawa on Apr 19, 2015, 09:43 am
Don't go by the bias voltage. Only the result (current) matters.

Dangerous to use analogies, but here you go.
Bias is like the idle RPM of a car. Low, but ready to take off quickly: Class AB
Turning the car off while waiting for a trafic light: Class B

Bias current is measured WITHOUT input signal.
Re-adjust the bias after 20 minutes of warm-up time.

NEVER run the amp, or adjust bias, when the power transistors are not mounted on a heatsink !!!
Thermal runaway kills the power stage very quickly.

AFAIK, silicone pads are ok. Just use no, or very little thermal grease.

Ok to use the speaker ground for measurements.

Heatsink temp should be a result of power, not bias.
That is, for a class AB amp.
Leo..
Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 19, 2015, 10:03 am
Quote
Don't go by the bias voltage. Only the result (current) matters.
I measured the x4 .1ohm resistors and I know they are right on spec. SO I ran the calculations and this is below 20ma which would be 8mv. I just want to be safe and not blow it up until I can get it on the scope.

I just got done testing it for temperature stability and idle and also have measured only 16mv between ground and output prongs on the output jack. It seems to stay 4mv bias until it warms up a bit. Once it stabalizes I get about 5mv.

Quote
Bias is like the idle RPM of a car. Low, but ready to take off quickly: Class AB
Turning the car off while waiting for a trafic light: Class B
I thought this was the other way around?

I remember watching a vid that showed class B as having both transistors on when the signal was close to Ov which was wasteful and inefficient. SO they added diodes (in theis case the BE diode's of the preceding stage) which fixed the problem? I wish I could find the link to that vid now but of course I can't.

Quote
Bias current is measured WITHOUT input signal.
Re-adjust the bias after 20 minutes of warm-up time.
Will do that right now. I was pretty sure it was without 1khz but what does it hurt to ask. It is confusing in the drawing because they put the bias procedure right next to a note about test conditions etc. I know Bias is DC preliminary set up before the signal comes. Just wanted to make sure.

Quote
NEVER run the amp, or adjust bias, when the power transistors are not mounted on a heatsink !!!
Thermal runaway kills the power stage very quickly.
No of course not. What I meant was that I didn't want to run it with the heat sink off while I had the 220 resistors in instead. Do you count drivers as Power transistors? I thought you meant the final output transistors.

So far I plugged in the pre amp and no explosions. Going to let it run for 20 then check the bias as you suggested, then time for the scope and a signal.

Thank You so much for all your help. I know a lot of these questions are amateurish but it has been a very long time and I am used to class A amps and logic gates. This AB audio stuff is more different then I thought.

Thank You
P.S. Is there any good books or DVD's on this subject. All I can find are a few design books. I would like repair books. Anything come to mind?

Title: Re: Output transistor on - side keeps blowing
Post by: jarrod0987 on Apr 19, 2015, 11:10 am
So far it seems fixed, I scoped it, I can see no cross over distortion at all. All the knobs seem to do what they should. Running it at mediocre volume for a while and keep an eye on the heat sink temp.

Thank You for all your input. I learned a ton. I wish I knew where to start reading to get all that knowledge of yours. YouTube and Amazon don;t seem to go real deep in Guitar amp subject. My customer/Friend is going to be ecstatic. He bought this thing broken 7 years Ago (I think) just because of the name on it. Now he gets to have it work. Too bad It needs a new speaker too :(

Thank You
Title: Re: Output transistor on - side keeps blowing
Post by: Ghosty08 on Sep 20, 2017, 11:36 pm
This was a great post, l leaning a lot, thanks to you guys,.
I have a question.... in this case the amp has only 1 transistor per side,  for testing reason you replace the power transistor by a 200 ohms resistor right?.... Now if you have more than 1 transistor 2, 3, 4, etc per side, do i have to replace for testing reason all the transistors by 1,2,3, etc 200 ohms resistors,  that means per every transistor 1 resistor?..

thanks in advance.
F.c.
Title: Re: Output transistor on - side keeps blowing
Post by: MarkT on Sep 21, 2017, 12:21 pm
This thread is over two years old, its a big ask to expect people to read 50 odd postings from years ago
to understand the context to your question...  Either start a new thread explaining the hardware and
software afresh, or point us to which postings in this thread are relevant to your question.
Title: Re: Output transistor on - side keeps blowing
Post by: polymorph on Sep 21, 2017, 06:10 pm
I agree, ask your question in its own thread.
Title: Re: Output transistor on - side keeps blowing
Post by: Ghosty08 on Sep 22, 2017, 01:25 am
Ok guys, thanks i will