I find the solution to this problem.
when increasing the idle current the wiggle disappear but the idle current rise in non-permissible level.
the different internal capacity of the output transistors may cause this.
Νικος
when increasing the idle current the wiggle disappear but the idle current rise in non-permissible level.
the different internal capacity of the output transistors may cause this.
Νικος
Nikosokey,
Hope you are able to solve that issue by changing/ matching output resistors.
But just out of interrest and for the benefit of others that may experience a simmilar issue in the future, how much did you have to increase the idle current in order to make that wiggle go away?
Andrew,
That sine clipping was quite clean looking on the scope, again sorry for the image quality, old scope and lousy camera doesn't give for the sharpest of images.
Regarding the square wave, once I adjusted the output from the generator, the amplified and the signal waveform was identical, just me being a bit paranoid in the beginning there! 🙂
Hope you are able to solve that issue by changing/ matching output resistors.
But just out of interrest and for the benefit of others that may experience a simmilar issue in the future, how much did you have to increase the idle current in order to make that wiggle go away?
Andrew,
That sine clipping was quite clean looking on the scope, again sorry for the image quality, old scope and lousy camera doesn't give for the sharpest of images.
Regarding the square wave, once I adjusted the output from the generator, the amplified and the signal waveform was identical, just me being a bit paranoid in the beginning there! 🙂
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It looks like the sine wave recovery is clean (but I agree with AndrewT that you need to turn the brightness down so we can see it more clearly - the devil is in the detail). What we are looking for is any indication of a.) rail sticking and b.) parasitic oscillations on either the rising edge or falling edge of the clipped waveform.
For the squarewave signal, you should NOT drive the amp with a squarewave so that it 'rails' - ie. slams from one rail into another. A good level for this test is about 50% of the rated output peak voltage swing - so around 20Vpk pk on this amp.
On the squarewave picture above, I'd say you have some capacitve coupling mechanism either into the top output device - so it switched very fast, giving the initial spike, which then subsides, and then the loop takes control and you get the smoother rounded edge.
I am very busy at work for the next few days, but will try to look into this over the week end. A good first place to start is to diable the output stage and close the loop around the VAS and see what waveform you get. If you don't have the spike, it would tend to indicate that the issue is indded in the output stage. If that is the case, there is a way to fix it.
For the squarewave signal, you should NOT drive the amp with a squarewave so that it 'rails' - ie. slams from one rail into another. A good level for this test is about 50% of the rated output peak voltage swing - so around 20Vpk pk on this amp.
On the squarewave picture above, I'd say you have some capacitve coupling mechanism either into the top output device - so it switched very fast, giving the initial spike, which then subsides, and then the loop takes control and you get the smoother rounded edge.
I am very busy at work for the next few days, but will try to look into this over the week end. A good first place to start is to diable the output stage and close the loop around the VAS and see what waveform you get. If you don't have the spike, it would tend to indicate that the issue is indded in the output stage. If that is the case, there is a way to fix it.
Oh, please, don't waste anny attention on those scope-shots!
The sine-wave looked good in real life, and the square wave was simply too much input!
Sorry if I got a bit over-nervous and stirred up the water!
I might have to get back a bit later with some actuall issue, another card is behaving in a mysterious way...
The sine-wave looked good in real life, and the square wave was simply too much input!
Sorry if I got a bit over-nervous and stirred up the water!
I might have to get back a bit later with some actuall issue, another card is behaving in a mysterious way...
So, the saga continues..
I hooked up another amplifier card and measured the output offset.. about 0,5V without DC servo, as expected and what I have been told is a normal value. This one should bea ready to go in to the chassis.
I then hooked up another card, but here the offset without servo was about 1,5V!
I unsoldered T1-4 and did a quick hfe check. There was some room for improvement, so I matched hfe's and soldered back together again. Now the offset was about 1V.
Is this a cause for concern, or could this be considered as a normal variation due to component tollerances etc..?
I hooked up another amplifier card and measured the output offset.. about 0,5V without DC servo, as expected and what I have been told is a normal value. This one should bea ready to go in to the chassis.
I then hooked up another card, but here the offset without servo was about 1,5V!
I unsoldered T1-4 and did a quick hfe check. There was some room for improvement, so I matched hfe's and soldered back together again. Now the offset was about 1V.
Is this a cause for concern, or could this be considered as a normal variation due to component tollerances etc..?
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Another interresting one...
Hooked up a third card, on this the output offset was unstable and the voltage across the output transistor resistors was non-symetrical, i.e. mor current flowing in one output device than the other..
Same excersise, unsolder and check T1-4, match hfe, solder back and hook up.
This time the offset was steady at around 0,2V, something which is lower than the "normal" 0,5V. Current in both output devices were now symmetrical.
Again, should I be concerned about the offset voltage being LOWER than 0,5V, or is this a normal variation???😕
Hooked up a third card, on this the output offset was unstable and the voltage across the output transistor resistors was non-symetrical, i.e. mor current flowing in one output device than the other..
Same excersise, unsolder and check T1-4, match hfe, solder back and hook up.
This time the offset was steady at around 0,2V, something which is lower than the "normal" 0,5V. Current in both output devices were now symmetrical.
Again, should I be concerned about the offset voltage being LOWER than 0,5V, or is this a normal variation???😕
Intuitively, this is allso what I would assume.
But then again, I lack the experience to fully judge the deviations from a "normal", so thanks for the input!
That means that the card with 0,2V offset is allso ready to go in to the chassis! 🙂
This leaves the matter of that other card which has 1V offset.. how can I reduce this further and is it of any true significance??
But then again, I lack the experience to fully judge the deviations from a "normal", so thanks for the input!
That means that the card with 0,2V offset is allso ready to go in to the chassis! 🙂
This leaves the matter of that other card which has 1V offset.. how can I reduce this further and is it of any true significance??
Given that the amp has about 26 dc gain, that translate to
barely 50mV dc output voltage with a cap in the feedback loop,
not an extraordinary value , but still acceptable.
The dc servo should deal with these 1v flawlessly...
barely 50mV dc output voltage with a cap in the feedback loop,
not an extraordinary value , but still acceptable.
The dc servo should deal with these 1v flawlessly...
Well, then I will leave that card as is assuming that the 1V offset is not a sign of something wrong.
Thank you again for your advice Wahab! 🙂
Thank you again for your advice Wahab! 🙂
In view of adding value to the thread as a resource for other crescendo builders, it's time for another update.
Progress has been rather slow lately, and the last thing i did was to blow up an amplifier card by reversing supply rail voltage.. yes, I know...😱
Anyway, the damage immediately obvious was one blown output-transistor and a blown output transistor drain-resistor, the 0,22 Ohm one.
These along with some of the driver transistors were replaced.
Once powered up, I discovered that the voltage across one drain-reistor was twice as high as the other!
On resistor had about 40 mV across, which is OK before adjusting idel current, the other one had over 80 mV across it. What was happening??
After a lot of measurements and checking virtually all the transistors on the card, I was still without a clue..
How could the idle current through one output transistor be twice as high as that of the other??
current leakage through gate? Negative.
Current flowing through the feedback circuit? Negative?
I then measured the Drain-resistors with a multimeter. Allthough the resolution is not good enough for sub-ohm measurements, there was a difference..
I unsoldered the "high current" resistor and measured it at work using a pro milli-ohm meter.
Jupp, 0,426 ohms, no wonder the voltage drop was twice as high over this one!
Where the other resitror blew up completely, this one survived with some sort of internal damage messing up the reistance value.
So the conclusion is: if you measure asymetrical voltages over the 0,22ohm output resistors, check them.
Progress has been rather slow lately, and the last thing i did was to blow up an amplifier card by reversing supply rail voltage.. yes, I know...😱
Anyway, the damage immediately obvious was one blown output-transistor and a blown output transistor drain-resistor, the 0,22 Ohm one.
These along with some of the driver transistors were replaced.
Once powered up, I discovered that the voltage across one drain-reistor was twice as high as the other!
On resistor had about 40 mV across, which is OK before adjusting idel current, the other one had over 80 mV across it. What was happening??
After a lot of measurements and checking virtually all the transistors on the card, I was still without a clue..
How could the idle current through one output transistor be twice as high as that of the other??
current leakage through gate? Negative.
Current flowing through the feedback circuit? Negative?
I then measured the Drain-resistors with a multimeter. Allthough the resolution is not good enough for sub-ohm measurements, there was a difference..
I unsoldered the "high current" resistor and measured it at work using a pro milli-ohm meter.
Jupp, 0,426 ohms, no wonder the voltage drop was twice as high over this one!
Where the other resitror blew up completely, this one survived with some sort of internal damage messing up the reistance value.
So the conclusion is: if you measure asymetrical voltages over the 0,22ohm output resistors, check them.
if you don't use a bulb tester after every modification of a mains powered project then, eventually, you will have a time consuming accident.
Well, that was not the end of my problems...
After having installed the card in the chassis and powered up with full supply voltage as opposed to lower voltage from my lab-supply, the voltage across the output resistors jumped to over 80 mVwith the idle current pot at the lowest setting!
removing the card and hokking it up to the lab-supply, I find that adjusting the idle current pot will increase or decrease the idle current, but at full voltage, it is not possible to adjust the idle current down to get a voltage reading between 45-55 mV as specified...
could the tolerances of some of the transistors account for this, or am I looking at a fault here..??
I could allways modify the value of the resistors connected in series (R28-29)with the pot in order to alter the adjustment range, but this would of course not be any good if there is an underlying fault..
Anny suggestions?
You are right Andrew, the bulb tester might well have reduced the consequences of my idiot mistake.
The card was tested and worked perfectly, I was not prepared for the fact that I could make such a F...-up!
It was one of those bad days where I'd been better off keeping my hands off anything more complicated than the coffe-maker...
After having installed the card in the chassis and powered up with full supply voltage as opposed to lower voltage from my lab-supply, the voltage across the output resistors jumped to over 80 mVwith the idle current pot at the lowest setting!
removing the card and hokking it up to the lab-supply, I find that adjusting the idle current pot will increase or decrease the idle current, but at full voltage, it is not possible to adjust the idle current down to get a voltage reading between 45-55 mV as specified...
could the tolerances of some of the transistors account for this, or am I looking at a fault here..??
I could allways modify the value of the resistors connected in series (R28-29)with the pot in order to alter the adjustment range, but this would of course not be any good if there is an underlying fault..
Anny suggestions?
You are right Andrew, the bulb tester might well have reduced the consequences of my idiot mistake.
The card was tested and worked perfectly, I was not prepared for the fact that I could make such a F...-up!
It was one of those bad days where I'd been better off keeping my hands off anything more complicated than the coffe-maker...
Well, I reduced R28 from 270 Ohms to 150 Ohms, and that took the voltage across the output resistors down to 70 mV.
I allso notice that it takes some time for the ofsett voltage to drop to 0 V (approx)
apart from that, everything seems normal..
I even "tested" the thermal compensation resistor T11 by heating it, and that pulled the idle current down so that seems to be working OK.
The obvious thing would be to reduce R28 even further, but I have an uneasy feeling that I'm missing something here..
Anny suggestions?
I allso notice that it takes some time for the ofsett voltage to drop to 0 V (approx)
apart from that, everything seems normal..
I even "tested" the thermal compensation resistor T11 by heating it, and that pulled the idle current down so that seems to be working OK.
The obvious thing would be to reduce R28 even further, but I have an uneasy feeling that I'm missing something here..
Anny suggestions?
I stumbled across an older thread related to the same problem:
http://www.diyaudio.com/forums/solid-state/12885-problems-crescendo-me-need-help-2.html
Here the root cause was identified as Higher current through the current sources caused by the use of green LED's in stead of red ones. The issue was rectified by increasing the value of R20 and R17.
I decided to check against the voltages quoted here:
http://www.diyaudio.com/forums/solid-state/10606-new-adventures-hifi-2.html
In order to see if my idle current issue originated from something similar.
I measured 1,08 V, which I feel is close enough for comfort to the quoted 1,05 V
I then checked Voltages across R27 and 25, and that was allso OK.
Finally, I checked voltage between gates of the output transistors T12 and T13 and got 4,1V, 4V being the quoted value from Elektor.
All these measurements were connected to the amplifier power supply at full operating voltage. DC servo was not connected, but this doesn't have any effect of the idle current wether disconnected or not..
Again, things look perfectly normal except for the fact that idle current is too high!!
Where should I be looking to find an indication of the fault??!😕
Any advice is greatly appreciated.
http://www.diyaudio.com/forums/solid-state/12885-problems-crescendo-me-need-help-2.html
Here the root cause was identified as Higher current through the current sources caused by the use of green LED's in stead of red ones. The issue was rectified by increasing the value of R20 and R17.
I decided to check against the voltages quoted here:
http://www.diyaudio.com/forums/solid-state/10606-new-adventures-hifi-2.html
In order to see if my idle current issue originated from something similar.
I measured 1,08 V, which I feel is close enough for comfort to the quoted 1,05 V
I then checked Voltages across R27 and 25, and that was allso OK.
Finally, I checked voltage between gates of the output transistors T12 and T13 and got 4,1V, 4V being the quoted value from Elektor.
All these measurements were connected to the amplifier power supply at full operating voltage. DC servo was not connected, but this doesn't have any effect of the idle current wether disconnected or not..
Again, things look perfectly normal except for the fact that idle current is too high!!
Where should I be looking to find an indication of the fault??!😕
Any advice is greatly appreciated.
Last thing I did yesterday, was to measure the voltage across Input stage collector resistors R8 R9, and R13 R14
According to elektor, this should be 1,9V +/-10%
This is what I got :
R8= 2,07 V
R9= 1,81 V
R13= 1,91V
R14= 1,98 V
So, the voltage across R13 and R14 looks normal, but R8 and R9 seems to be a bit out..
I measured the voltage across the emitter resistors, and they all had about the same voltage across..
The only explanation I could find for this, was that something was going on in the VAS stage. I checked T7 and T9, found them to bee good, but replaced them nevertheless.
No change.
I have allready changed T8 and T10.
What could be going on here, I'm at a complete loss!!!😕
I've checked / replaced about every semiconductor on this card now, but I just cant seem to find a fault! Could it be that the output transistors (probably comming from another batch than the others I bought) have different properties reulting in a higher idle current?
According to elektor, this should be 1,9V +/-10%
This is what I got :
R8= 2,07 V
R9= 1,81 V
R13= 1,91V
R14= 1,98 V
So, the voltage across R13 and R14 looks normal, but R8 and R9 seems to be a bit out..
I measured the voltage across the emitter resistors, and they all had about the same voltage across..
The only explanation I could find for this, was that something was going on in the VAS stage. I checked T7 and T9, found them to bee good, but replaced them nevertheless.
No change.
I have allready changed T8 and T10.
What could be going on here, I'm at a complete loss!!!😕
I've checked / replaced about every semiconductor on this card now, but I just cant seem to find a fault! Could it be that the output transistors (probably comming from another batch than the others I bought) have different properties reulting in a higher idle current?
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how far back do we look for the schematic?
+-10% is OK, but they should be matched to <1%.
Are your collector resistors matched for value? 1% resistors are OK for loading, but not for checking balance.
eg.
191 vs 198 is a difference of 3.5%, if the resistors are +-1% then that 191 vs 198 represents an imbalance of 3.5%+-1%, i.e. the imbalance could be anywhere between 1.5% and 5.5%.
If the resistors were matched to <0.1% then you can rely on your measurements because effectively you would have 3.5%+-0.1%
Where does this leave you. The best balance of 1.5% in the good pair is still bigger than my recommended <1% matching for balance.
The other is ~14% unbalance.
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Sorry, the schematics are posted on the first page of this thread.
I don't remember the excact values, but considering the time I spent selecting and matching resistors, they are probably well within 1% tollerance from nominal value and even less between similar values on each card I buildt.
So, Assuming there is a unbalance on one side of the differential pair, where could this come from? and could it be related to, or point towards the reason for my problem of not being able to adjust down the idle current?
I don't remember the excact values, but considering the time I spent selecting and matching resistors, they are probably well within 1% tollerance from nominal value and even less between similar values on each card I buildt.
So, Assuming there is a unbalance on one side of the differential pair, where could this come from? and could it be related to, or point towards the reason for my problem of not being able to adjust down the idle current?
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check your LTP balance at r6.7,11,12. These too should be less than 0.1% matching, not +-0.1% tolerance.
The VAS is injecting base current that will make the r8,9,13,14 currents different.
The VAS is injecting base current that will make the r8,9,13,14 currents different.
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