Elbert,
firstly, the voltage drops across your emitter LTP emitter resistors are wrong. Nikosokey's are correct. Your LTP tail current is c. 3.7mA which means each LTP half gets 1.85mA and across 47 Ohms, this is about 87mV.
If I look at the voltages across your emitter resistors in the LTP, is clear the amp is unbalanced, and trying to correct the problem. Are the resistors in the current sources actually 270 Ohms? Double check.
Second issue is around the voltage drop across R14 - you measure 2.97V across 1k = 2.97mA and on R13 you measure 1.07V = c. 1ma for a total of 3.7mA which is about right and confirms the correct current coming out of the T5 current source circuit, which we calculated above at 3.7mA (BUT, its unbalanced, so we need to find out why next)
On the top side, you measure 1.65V and 1.12 V = 2.77V = current source value from T6 of 2.7mA
This is a significant imbalance. Fully complementary designs can easily cater for 5% mismatch in the current sources, but you have a 37% mismatch (T5 is given the correct current, while T6 is 37% lower).
Have a good look at the following:-
Voltage across D2 - is it the same as D1 - assume D1 is correct.
Re-check voltage across R29(?) resistor in the emitter of T6.
Double check that it is actually 270 Ohms
Next, measure the resistance of R21 and then measure the voltage drop across it, and compare it to the same resistor in the collector of T5 (I think its 2.2k?). If this resistor was high for some reason, it would also give this type of problem (this would mean you would see almost the full -ve supply rail across it
First thing on this amp is to get the current sources balanced so that you get the following readings:-
Across each of the LTP emitter resistorts: 85mV +-5mV
Across each of the collector load resistors of the same transistors: 1.85V +-200mV
Across R23 and R27 (VAS emitter resistors) c 1.2V which means the VAS is running at about 35mA on this design
The offset without the servo should be like Nokosokey's amp - about 500mV
firstly, the voltage drops across your emitter LTP emitter resistors are wrong. Nikosokey's are correct. Your LTP tail current is c. 3.7mA which means each LTP half gets 1.85mA and across 47 Ohms, this is about 87mV.
If I look at the voltages across your emitter resistors in the LTP, is clear the amp is unbalanced, and trying to correct the problem. Are the resistors in the current sources actually 270 Ohms? Double check.
Second issue is around the voltage drop across R14 - you measure 2.97V across 1k = 2.97mA and on R13 you measure 1.07V = c. 1ma for a total of 3.7mA which is about right and confirms the correct current coming out of the T5 current source circuit, which we calculated above at 3.7mA (BUT, its unbalanced, so we need to find out why next)
On the top side, you measure 1.65V and 1.12 V = 2.77V = current source value from T6 of 2.7mA
This is a significant imbalance. Fully complementary designs can easily cater for 5% mismatch in the current sources, but you have a 37% mismatch (T5 is given the correct current, while T6 is 37% lower).
Have a good look at the following:-
Voltage across D2 - is it the same as D1 - assume D1 is correct.
Re-check voltage across R29(?) resistor in the emitter of T6.
Double check that it is actually 270 Ohms
Next, measure the resistance of R21 and then measure the voltage drop across it, and compare it to the same resistor in the collector of T5 (I think its 2.2k?). If this resistor was high for some reason, it would also give this type of problem (this would mean you would see almost the full -ve supply rail across it
First thing on this amp is to get the current sources balanced so that you get the following readings:-
Across each of the LTP emitter resistorts: 85mV +-5mV
Across each of the collector load resistors of the same transistors: 1.85V +-200mV
Across R23 and R27 (VAS emitter resistors) c 1.2V which means the VAS is running at about 35mA on this design
The offset without the servo should be like Nokosokey's amp - about 500mV
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Thanks to Nikosokey for providing some usefull voltage references from an amp that is actually working!
I'll see if i can get some pictures taken by the way.
Bonsai,
Again, thank you for your answer, but before I go ahead with the checks you prescribe, I would like to re-emphasise that the amp is now modified as per Wahab's instructions:
You should check wether it s the amp itself or the DC servo that
are the source of the misbehaving.
First is to test the amp alone as follow :
-- remove R4 to disconnect the DC servo.
-- replace R2 by a 12K.
-- connect a 220uF cap in serial with R22.
Power on and measure the output dc voltage.
It should be at very low value, since the amp
has now a DC gain of 1 in respect to the input
offset.
If it works this way, then it s the DC servo that
is misbehaving.
If not, then perhaps the amp is oscillating,
in wich case it can be corrected by adding
a compensation that is ABSENT in the original
schematic...
Could this "modification" explain some of the strange voltage imbalances?
Should I remove these modifications and perform further measurements with the servo disconnected??
Would just like to highlight this to make sure that we don't move forward based on some incorrect facts and assumptions..
I'll see if i can get some pictures taken by the way.
Bonsai,
Again, thank you for your answer, but before I go ahead with the checks you prescribe, I would like to re-emphasise that the amp is now modified as per Wahab's instructions:
You should check wether it s the amp itself or the DC servo that
are the source of the misbehaving.
First is to test the amp alone as follow :
-- remove R4 to disconnect the DC servo.
-- replace R2 by a 12K.
-- connect a 220uF cap in serial with R22.
Power on and measure the output dc voltage.
It should be at very low value, since the amp
has now a DC gain of 1 in respect to the input
offset.
If it works this way, then it s the DC servo that
is misbehaving.
If not, then perhaps the amp is oscillating,
in wich case it can be corrected by adding
a compensation that is ABSENT in the original
schematic...
Could this "modification" explain some of the strange voltage imbalances?
Should I remove these modifications and perform further measurements with the servo disconnected??
Would just like to highlight this to make sure that we don't move forward based on some incorrect facts and assumptions..
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No, Wahab's mods were to make the amp feedback network AC coupled and to reduce the value of the input bias resistor in order to minimize output offset. After his mods, you should have had even less offset if your amp was working correctly (assuming the servo was disconnected).
do the measurements I proposed, and then lets take another look.
BTW, I had a thought - the resistors in series with the current source collectors look like 2.2k Ohms - are you sure that in series with T6 collector you don't have a 22k by accident
do the measurements I proposed, and then lets take another look.
BTW, I had a thought - the resistors in series with the current source collectors look like 2.2k Ohms - are you sure that in series with T6 collector you don't have a 22k by accident
Why do builders take on projects in which they are not prepared to invest the time necessary to understand how their project operates?
Are we to be seen a lazy sods who only want music and invest nothing in the process of obtaining that music!
Are we to be seen a lazy sods who only want music and invest nothing in the process of obtaining that music!
I have been at it (very slowly learning solid state analogue audio electronics) for over 40 years. I have invested a lot of time and effort in my hobby.
If someone enters as a newcomer, I see two options:
Learn as you go along finding out how your projects work, or feed off other learning nothing about the hobby.
The question is:
Why do some fall into that second category?
Is it an indication of the way the youth in our World think about collegiate responsibility? Or could it be total selfishness and lack of respect for others?
If someone enters as a newcomer, I see two options:
Learn as you go along finding out how your projects work, or feed off other learning nothing about the hobby.
The question is:
Why do some fall into that second category?
Is it an indication of the way the youth in our World think about collegiate responsibility? Or could it be total selfishness and lack of respect for others?
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Bonsai,
The resistors in the current sources are both 270Ohm spot on.
The voltage across D1 was 1,754 V, and for D2 1,753V, which I guess should be OK.
The voltage across emitter resistor for T5, R17 (270 Ohm) was 1,102V
The voltage across emitter resistor for T6, R20 (270 Ohm) was 1,091V
The voltage across the collector resistor R21 (8,2k) for T6 was 33 V
The voltage across the collector resistor R18 (8,2k) for T5 was 33,2 V
I was not able to measure the resistance of R18 and R21 in situ on the card, but these resistors will be fairly well matched and close to nominal value as I put some effort in to this when assembling the cards. (I hope this answers your doubt regarding 22k vs 2,2k?)
So, the Diodes and then the Base voltages of T5 and T6 looks OK.
Despite of this, there is a difference in the current that flows from T5 and T6.
Could a severe missmatch between these two transistors cause this??
The reason I started on this project in particular, was that it was (seemingly) well docummented and presented in an elektor Magazine article, and the anticipation was that with some effort of etching cards and soldering the components in place as described, I could assemble some nice Hi-fi amplifiers for a relatively moderate cost compåared to buying something equivalent new.
Had I known that I would run in to such problems ,which evidently calls for more knowledge and experience than I have, I would never have started this in the first place.
But now that I have invested time and money in this, my only two options are to find the fault or to throw everything out.
The resistors in the current sources are both 270Ohm spot on.
The voltage across D1 was 1,754 V, and for D2 1,753V, which I guess should be OK.
The voltage across emitter resistor for T5, R17 (270 Ohm) was 1,102V
The voltage across emitter resistor for T6, R20 (270 Ohm) was 1,091V
The voltage across the collector resistor R21 (8,2k) for T6 was 33 V
The voltage across the collector resistor R18 (8,2k) for T5 was 33,2 V
I was not able to measure the resistance of R18 and R21 in situ on the card, but these resistors will be fairly well matched and close to nominal value as I put some effort in to this when assembling the cards. (I hope this answers your doubt regarding 22k vs 2,2k?)
So, the Diodes and then the Base voltages of T5 and T6 looks OK.
Despite of this, there is a difference in the current that flows from T5 and T6.
Could a severe missmatch between these two transistors cause this??
The reason I started on this project in particular, was that it was (seemingly) well docummented and presented in an elektor Magazine article, and the anticipation was that with some effort of etching cards and soldering the components in place as described, I could assemble some nice Hi-fi amplifiers for a relatively moderate cost compåared to buying something equivalent new.
Had I known that I would run in to such problems ,which evidently calls for more knowledge and experience than I have, I would never have started this in the first place.
But now that I have invested time and money in this, my only two options are to find the fault or to throw everything out.
Ok, thank you! Unfortunately, the amplifier cards are allready made, so any improvements you have come up with are unfortunately something I won't be able to benefit from.. 🙁
From want of anything better to do, I decided to check the Gate-source voltage of the output mosfets.. T12 measured -2,11V and T13 1,98V.
This was done without DC servo, but with the 220Uf cap removed and R2 restored to its original value. The output offset was about -8V
To get -8V DC on the output, I understand that T12, which feeds from the positive rail, must be conducting more than T13 connected to the negative rail.
As the gate-source voltage is higher on T12, it should conduct more.
Does the asymmetry in gate voltages have abearing on the -8V offset?
If so, the output transistors are behaving as they should,but being fed the wrong votage on the gate. This would be nice, as it would save me from scrapping a lot of expensive output transistors.
Any thougths??
Just desperately trying to elliminate uncertainties here...😕
This was done without DC servo, but with the 220Uf cap removed and R2 restored to its original value. The output offset was about -8V
To get -8V DC on the output, I understand that T12, which feeds from the positive rail, must be conducting more than T13 connected to the negative rail.
As the gate-source voltage is higher on T12, it should conduct more.
Does the asymmetry in gate voltages have abearing on the -8V offset?
If so, the output transistors are behaving as they should,but being fed the wrong votage on the gate. This would be nice, as it would save me from scrapping a lot of expensive output transistors.
Any thougths??
Just desperately trying to elliminate uncertainties here...😕
Elbert
You have made significant progress. So don't go scrapping anything just yet. Hang in there.
"So, the Diodes and then the Base voltages of T5 and T6 looks OK.
Despite of this, there is a difference in the current that flows from T5 and T6.
Could a severe missmatch between these two transistors cause this??"
No, because you are measuring the correct voltage across the resitors feeding the LTP emitter circuits. Its clear from the further readings that the imbalance is caused because the front end is trying to correct a problem further down the line - i.e. in the VAS stage or the output stage.
If the total current being supplied by T6 is 3.7mA, but the total current going into R8 and R9 is only about 2.7mA (beacuase we know they are 1k resistors and we mneasured the volt drop across them), where is the other 1mA going? It can only be going into the base of T7. Check T7 again, and also check the MJE350. Try reading the voltage on the emitter of the MJE350- if it is correct, I'd replace T7. Again, no need for matching, as long as it is the same type of device.
Transistors do fail in some strange ways, and I've seen this type of failure before.
If this proves not to be the case, the next step is to see if we can prove definitively that the front and and the VAS are working correctly.
Desolder R32 and R33 (gate stopper resistors connected to the output devices) where they connect to T11, and then solder the free ends of these resistors to the sources of T12 and T13 (output devices. This disconnects the output stage, and turns these devices OFF.
Next, desolder R23 (?) the 12k feedback resistor at the output side, and then solder the free end to the drain of T11 (or the collector of the MJE350). I can't see the circuit reference clearly.
Measure your supply current. It should be about 35mA. If its much above this, you have a problem with your output devices.
Now measure the voltage at the drain of T11. It should read 0V +-500mV
If it indeed does read this, then we should be able to assume the entire front end is ok. Your problem is in the output stage.
Try these things and lets see what you get.
You have made significant progress. So don't go scrapping anything just yet. Hang in there.
"So, the Diodes and then the Base voltages of T5 and T6 looks OK.
Despite of this, there is a difference in the current that flows from T5 and T6.
Could a severe missmatch between these two transistors cause this??"
No, because you are measuring the correct voltage across the resitors feeding the LTP emitter circuits. Its clear from the further readings that the imbalance is caused because the front end is trying to correct a problem further down the line - i.e. in the VAS stage or the output stage.
If the total current being supplied by T6 is 3.7mA, but the total current going into R8 and R9 is only about 2.7mA (beacuase we know they are 1k resistors and we mneasured the volt drop across them), where is the other 1mA going? It can only be going into the base of T7. Check T7 again, and also check the MJE350. Try reading the voltage on the emitter of the MJE350- if it is correct, I'd replace T7. Again, no need for matching, as long as it is the same type of device.
Transistors do fail in some strange ways, and I've seen this type of failure before.
If this proves not to be the case, the next step is to see if we can prove definitively that the front and and the VAS are working correctly.
Desolder R32 and R33 (gate stopper resistors connected to the output devices) where they connect to T11, and then solder the free ends of these resistors to the sources of T12 and T13 (output devices. This disconnects the output stage, and turns these devices OFF.
Next, desolder R23 (?) the 12k feedback resistor at the output side, and then solder the free end to the drain of T11 (or the collector of the MJE350). I can't see the circuit reference clearly.
Measure your supply current. It should be about 35mA. If its much above this, you have a problem with your output devices.
Now measure the voltage at the drain of T11. It should read 0V +-500mV
If it indeed does read this, then we should be able to assume the entire front end is ok. Your problem is in the output stage.
Try these things and lets see what you get.
Elbert,
after an adjustment, the DC offset voltage will almost exclusively depend on thermal balance in the input stage, provided there`s no functional disorder anywhere, like the oppressive disease called hyperthermionic oscillation, usually striking amplifying devices that are being exposed to uncharitable thermal abuse.
To me, the output transistors seem to be fine.
after an adjustment, the DC offset voltage will almost exclusively depend on thermal balance in the input stage, provided there`s no functional disorder anywhere, like the oppressive disease called hyperthermionic oscillation, usually striking amplifying devices that are being exposed to uncharitable thermal abuse.
To me, the output transistors seem to be fine.
HI Elbert
If you measure -8 volt pin 6 OP177 It makes sense because you have offset problem.
Counting the -8volt output is another problem.
Step by step verified the correct placement of parts .
Have you connected the wrong cable bias?
Do it this :
(member "tiefbassuebertr "post #4)
In this way will not damage the mosfet and do the measurements for offset with certainty.
For checking this stage alone, remove the MOSFET T12 and T13. Make a short connection between drain and source of T11 (T11 = Vbe multiplier).
Disconnect the right side of NFB resistor 12 K (R23) and connect it with the short connection between drain and source of T11.
I have done all this.
And before putting the 550 and 560 (T1T2T3T4) measures 2,7 volt offset OUTPUT and 8 volt on pin 6 OP177.
I put bc 550c and 560c On Semi T1T2T3T4 matched .
and the result :500mv output offset without op177 , when i put the OP177 on board then 0V Output . on pin 6 OP177 I measure 1,7 volt .
R6=87 mv
R7= 88mv
R11=87mv
R12=88mv
If you measure -8 volt pin 6 OP177 It makes sense because you have offset problem.
Counting the -8volt output is another problem.
Step by step verified the correct placement of parts .
Have you connected the wrong cable bias?
Do it this :
(member "tiefbassuebertr "post #4)
In this way will not damage the mosfet and do the measurements for offset with certainty.
For checking this stage alone, remove the MOSFET T12 and T13. Make a short connection between drain and source of T11 (T11 = Vbe multiplier).
Disconnect the right side of NFB resistor 12 K (R23) and connect it with the short connection between drain and source of T11.
I have done all this.
And before putting the 550 and 560 (T1T2T3T4) measures 2,7 volt offset OUTPUT and 8 volt on pin 6 OP177.
I put bc 550c and 560c On Semi T1T2T3T4 matched .
and the result :500mv output offset without op177 , when i put the OP177 on board then 0V Output . on pin 6 OP177 I measure 1,7 volt .
R6=87 mv
R7= 88mv
R11=87mv
R12=88mv
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Hi,
have you tried to monitor the output voltage of the amplifier and the output voltage of the servo opamp at start up? The first few hundred milliseconds could be very interesting.
Bonsai,
Believe it or not, I actually follow your reasoning here, once presented to me it makes very good sense. Too bad Im too see such things my self! 😀 (one day perhaps...)
Anyway, I'll definitively do the checks you recommend, That 1 mA has to go somewhere, and If it gets gulped up by the base of T7 something might be strange.
Nikos,
Just for my benefit here again, before you soldered in matched BC 560 and 550 transistors, did you have the same offset problem as I have?
Alll components are double and tripple checked, and I've had the exactly same fault on two cards now (I have 6 all in all, not tested the forur last), so I suspect it is a batch of poor semiconductors rather than a random one-off misstake.
Believe it or not, I actually follow your reasoning here, once presented to me it makes very good sense. Too bad Im too see such things my self! 😀 (one day perhaps...)
Anyway, I'll definitively do the checks you recommend, That 1 mA has to go somewhere, and If it gets gulped up by the base of T7 something might be strange.
Nikos,
Just for my benefit here again, before you soldered in matched BC 560 and 550 transistors, did you have the same offset problem as I have?
Alll components are double and tripple checked, and I've had the exactly same fault on two cards now (I have 6 all in all, not tested the forur last), so I suspect it is a batch of poor semiconductors rather than a random one-off misstake.
Andrew,
I haven't monitored this very closely, but with the servo connected, the output voltage would surge to around -8V, and then decline to arround -1V.
At that point the servo output was at full swing, i.e. around 18V
So it seems the servo was working, but the imbalance was just way too much!
I haven't monitored this very closely, but with the servo connected, the output voltage would surge to around -8V, and then decline to arround -1V.
At that point the servo output was at full swing, i.e. around 18V
So it seems the servo was working, but the imbalance was just way too much!
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