Mlaen said:
About your Ube's, how do you measure them ?
Hmm, if your amp looks like this, it can't work... where is the 4.7k
from the original schematic gone ?
In this constellation the vbemultiplier can't work !
You have to choose exactly the resistors that are shown in the
schematic, or your area to adjust is completely off...
It might not hurt to replace the 1.8k with a 2k, or the 4.7k with
a 5k, but bigger changes definitely make the vbe-multiplier operate
in useless range.
You have to look at the vbe-multiplier like that the 2 resistors
(1.8k+4.7k) change the 500ohm pot to a 7k pot, but with a "zoomed"
window for adjusting, allowing a more easy/fine tuning.
If you change these resistors, this window is in an area completely
outside the needed voltages.
What is the voltage from base to base for the drivers ?
This needs to be ~2.2volts. (+/- ~0.2volts)
EDIT: Sorry, missed that edl gave you this hint, seems that he
missed that a pnp is used here.
Mike
How to calculate the vbemultiplier:
Let's assume we want 2.2 volts with the pot adjusted to middle.
Across the upper resistor (1.8k) plus half the pot (0.25k) you
have "always" 0.6 volts because of the bc556. (more like 0.63)
This means that the voltagedivider needs to be set to a ratio,
so that on the other/lower half you have 2.2v-0.63 = 1.57v.
1.8k+0.25k for the upper = 2.05k
1.57v/0.63v = 2.49
2.05k * 2.49 = 5.105k minus 0.25k from the pot = 4.85k
for the lower resistor. You see, the 4.7k is quite correct.
Have you already measured the Ube for this bc556 ?
It seems that your problem lies there, without the 4.7k you
should have a voltage of ~0.7volts from base to base in the
drivers, giving a "dead" amp, with no currents through the
emitterresistors.
Mike
Let's assume we want 2.2 volts with the pot adjusted to middle.
Across the upper resistor (1.8k) plus half the pot (0.25k) you
have "always" 0.6 volts because of the bc556. (more like 0.63)
This means that the voltagedivider needs to be set to a ratio,
so that on the other/lower half you have 2.2v-0.63 = 1.57v.
1.8k+0.25k for the upper = 2.05k
1.57v/0.63v = 2.49
2.05k * 2.49 = 5.105k minus 0.25k from the pot = 4.85k
for the lower resistor. You see, the 4.7k is quite correct.
Have you already measured the Ube for this bc556 ?
It seems that your problem lies there, without the 4.7k you
should have a voltage of ~0.7volts from base to base in the
drivers, giving a "dead" amp, with no currents through the
emitterresistors.
Mike
I don't understand what do you mean by how do i measure Ube's?
originl sch:
voltage from base to base Ubb=2.15V
Ube (the one on the heatsik)=0.58V
when I swap 1.8 i 4k7 resistors:
Ubb=0.88V
Ube=0.63V
Ube of drivers is still 3.7V and Ure=4.3V
drivers looks OK when I meassure theirs Hfe and
Could it be possible to burn the drivers by increasing the trimmer too much? At start I have 0V, , when I increase trimmer voltage rizes, and then I can't get it back to 0V)
Here's the PCB (don't mind the capacitor)
originl sch:
voltage from base to base Ubb=2.15V
Ube (the one on the heatsik)=0.58V
when I swap 1.8 i 4k7 resistors:
Ubb=0.88V
Ube=0.63V
Ube of drivers is still 3.7V and Ure=4.3V
drivers looks OK when I meassure theirs Hfe and
Could it be possible to burn the drivers by increasing the trimmer too much? At start I have 0V, , when I increase trimmer voltage rizes, and then I can't get it back to 0V)
Here's the PCB (don't mind the capacitor)
There is a basic problem here. Ube can NEVER be more than a fraction of a volt, UNLESS you have the transistor mounted wrongly of defect, or you use the meter wrongly.
Untill that is solved, everything you "measure" is worthless for anyone.
There is too much going on, changes at the same time.
I maintain my proposal, built it back EXACTLY as it was before, and then start measuring and we will find it eventually.
This way, you'll never get there.
It is pretty useless to start thinking about changing the Vbe multiplier without knowing what the problem really is.
Jan Didden
Untill that is solved, everything you "measure" is worthless for anyone.
There is too much going on, changes at the same time.
I maintain my proposal, built it back EXACTLY as it was before, and then start measuring and we will find it eventually.
This way, you'll never get there.
It is pretty useless to start thinking about changing the Vbe multiplier without knowing what the problem really is.
Jan Didden
Jan, i just wanted to show that the resistors have to be exactly
like in the original schematic... And it helps understanding a circuit
when doing measurings.
Mlaen, you are confusing me... swapped 1.8k and 4.7k ?
I guess, with original sch you meant when built like in schematic ?
In this case Ubb=2.15v and ube of the bc556 with 0.58v are very good. And please keep it original...
Now comes the point, why i ask how you measure:
if Ubb is 2.15v, and output ~0v, it's "difficult" to have the
readings you tell. Is it possible that you reverted powersupply ?
With 4.3 volts over the 120 ohms these should be quite hot ?
One possibilty to get such strange readings would be if the amp oscillates,
that can confuse your multimeter...
Mike
like in the original schematic... And it helps understanding a circuit
when doing measurings.
Mlaen, you are confusing me... swapped 1.8k and 4.7k ?
I guess, with original sch you meant when built like in schematic ?
In this case Ubb=2.15v and ube of the bc556 with 0.58v are very good. And please keep it original...
Now comes the point, why i ask how you measure:
if Ubb is 2.15v, and output ~0v, it's "difficult" to have the
readings you tell. Is it possible that you reverted powersupply ?
With 4.3 volts over the 120 ohms these should be quite hot ?
One possibilty to get such strange readings would be if the amp oscillates,
that can confuse your multimeter...
Mike
Forget all I was writing here...I'm an idiot!
Ok, now it's like original sch. but without output transistors (just drivers)
This whole time I was mesuring WRONG!!! Ube of driver transistors is like you said -3.25V!!!!
Emiter of BC556 is ~ +1.1V and collector is ~ -0.9V...Uce ~2.3 V
Voltage on emiter resistors is still 3.7V!
Voltage from base to base of driver transistors is ~4.2V
So what's with this -3.25V, what could be a cause of that?!
MikeB:
0.22R are not connected right now but when everithing is completed they will be under the board!
yes, now the output is like orig.
Transistors are for now connected with short wires until i fix everithing, and after that they will be monted on board!
120R transistora are 5W types so they are not hot at all!
Ok, now it's like original sch. but without output transistors (just drivers)
This whole time I was mesuring WRONG!!! Ube of driver transistors is like you said -3.25V!!!!
Emiter of BC556 is ~ +1.1V and collector is ~ -0.9V...Uce ~2.3 V
Voltage on emiter resistors is still 3.7V!
Voltage from base to base of driver transistors is ~4.2V
So what's with this -3.25V, what could be a cause of that?!
MikeB:
0.22R are not connected right now but when everithing is completed they will be under the board!
yes, now the output is like orig.
Transistors are for now connected with short wires until i fix everithing, and after that they will be monted on board!
120R transistora are 5W types so they are not hot at all!
That's amazing !
Uce for the bc556 is ~2.3v, but right after the 47ohms to the driverbases
you have voltage of 4.2volts ? Thats ~50ma through the driverbases ???
Could you please use a schematic, and draw where you measure
these values ? And try to make as many measurements as possible,
important values are also voltages across resistors, as these show
the actual currents.
Obviously there is a very big error somewhere !
Mike
Uce for the bc556 is ~2.3v, but right after the 47ohms to the driverbases
you have voltage of 4.2volts ? Thats ~50ma through the driverbases ???
Could you please use a schematic, and draw where you measure
these values ? And try to make as many measurements as possible,
important values are also voltages across resistors, as these show
the actual currents.
Obviously there is a very big error somewhere !
Mike
Hmm, 2.5a to rms gives 3.5a, times 2 because of halfwaves gives 7a,
at maxoutput of ~28volts you can drive 4ohms without problems.
I think these 2.5a fuses should be adequate, with a fullrange sinewave
into 4ohms they don't blow.
But you're right, if amp is driven hard into clipping, they might blow.
Mike
at maxoutput of ~28volts you can drive 4ohms without problems.
I think these 2.5a fuses should be adequate, with a fullrange sinewave
into 4ohms they don't blow.
But you're right, if amp is driven hard into clipping, they might blow.
Mike
Ahh, that's better !
But, if sims would show me these voltages, it will be wiped from harddisk immediately...
Is it possible that your 5watts-120ohm resistors are completely wirewound ?
In this case it's very possible that the drivertransistors are oscillating like hell, producing these crazy voltages.
Please turn the pot around to the 4.7k, measure again, and if
the 2.73volts reduce to at least 2 volts, you were hunting a
phantom, an error produced by installing these 120ohms.
If you'd like, replace these resistors with normal standard
resistors, i think you have some 100ohms left. They won't burn up,
as you get a drop of ~0.6volts across these. (~3.6mW)
If you have doublechecked everything, especially pinouts and other
connections, i suggest that you remove these 120ohms, and reinstall
the outputtransistors, after you have turned the pot to the 4.7k.
This gives minimum bias. At the actual bias you would get very high current.
Then, while measuring across the 0.22ohms, turn the pot slowly until you read ~10mv, or at max 25mv.
If this works, everything is fine and the amp should play, eventually your other drivertransistors were defect.
If it still doesn't work, verify that your 0.22ohms are not the same wirewound "crab". (unlikely)
All voltages you showed except after drivertransistors are "perfect",
except a hfe-mismatch between the drivers, but that's not causing these problems.
If it all doesnt work, measure again, and show the voltages !
Mike
But, if sims would show me these voltages, it will be wiped from harddisk immediately...
Is it possible that your 5watts-120ohm resistors are completely wirewound ?
In this case it's very possible that the drivertransistors are oscillating like hell, producing these crazy voltages.
Please turn the pot around to the 4.7k, measure again, and if
the 2.73volts reduce to at least 2 volts, you were hunting a
phantom, an error produced by installing these 120ohms.
If you'd like, replace these resistors with normal standard
resistors, i think you have some 100ohms left. They won't burn up,
as you get a drop of ~0.6volts across these. (~3.6mW)
If you have doublechecked everything, especially pinouts and other
connections, i suggest that you remove these 120ohms, and reinstall
the outputtransistors, after you have turned the pot to the 4.7k.
This gives minimum bias. At the actual bias you would get very high current.
Then, while measuring across the 0.22ohms, turn the pot slowly until you read ~10mv, or at max 25mv.
If this works, everything is fine and the amp should play, eventually your other drivertransistors were defect.
If it still doesn't work, verify that your 0.22ohms are not the same wirewound "crab". (unlikely)
All voltages you showed except after drivertransistors are "perfect",
except a hfe-mismatch between the drivers, but that's not causing these problems.
If it all doesnt work, measure again, and show the voltages !
Mike
When I turn the trimmer to 4k7 resistor Ubb=2.1V!
I replaced emiter resistors with 150R 1% metal film (that's the closest I have) and it worked for a few minutes... I measured Ube=0.6V (of driver tranzistors) and 0.56V over emiter resistors...
But aftfer a few minutes, it's like before, Ube= -3.7V, etc...
Resistors that I used are not wirewound...at least I didn't ordered them...what is visual difference?
I replaced emiter resistors with 150R 1% metal film (that's the closest I have) and it worked for a few minutes... I measured Ube=0.6V (of driver tranzistors) and 0.56V over emiter resistors...
But aftfer a few minutes, it's like before, Ube= -3.7V, etc...
Resistors that I used are not wirewound...at least I didn't ordered them...what is visual difference?
Okay, you could increase the 1.8k to 2k... But 2.1 volts is ok for now...
I think your 120ohms are wirewound, it's not always visible at once.
It's possible that because of some warmup your amp went into
some oscillation again, but maybe you should ignore that, as behaviour
changes when outputtransistors are present.
The values you measured are very fine, Ube=0.6v and the Ure of 0.56v
are perfectly the expected values.
Just go on and reinstall the outputdevices, if these are not defect,
everything should work !
It's now obvious that oscillations fooled your measurements...
Mike
I think your 120ohms are wirewound, it's not always visible at once.
It's possible that because of some warmup your amp went into
some oscillation again, but maybe you should ignore that, as behaviour
changes when outputtransistors are present.
The values you measured are very fine, Ube=0.6v and the Ure of 0.56v
are perfectly the expected values.
Just go on and reinstall the outputdevices, if these are not defect,
everything should work !
It's now obvious that oscillations fooled your measurements...
Mike
I connected output transistors and everithing else...and not all went well....
In series with PS I have 33R resistors (only 5W) and when I turn it on there were 28V on them, so current was ~850mA...since pot is turned to 4k7 res. I don't think this is OK...
But just before I smelled somethig is wrong I measured Ube=0.6V of driver transistors, ,so at least this is ok...
Maybe output transistors are oscilatin now since his emiter resistors are are visualy the same as those 120R that were used on drivers?
In series with PS I have 33R resistors (only 5W) and when I turn it on there were 28V on them, so current was ~850mA...since pot is turned to 4k7 res. I don't think this is OK...
But just before I smelled somethig is wrong I measured Ube=0.6V of driver transistors, ,so at least this is ok...
Maybe output transistors are oscilatin now since his emiter resistors are are visualy the same as those 120R that were used on drivers?
Mlaen said:
Did you include the 10ohms-100nF zobel at the output, to lower the chances of oscillation? Also, is there a (dummy) load?
What do you mean that the Re is now 120? Isn't it .22?
Another precaution is to connect your meter across the Vbe multiplier for the bias, switch on and then quickly turn the pot for min bias. You can even temporarily short the bias generator. The idea is to get the amp up and running and then go look at the bias.
Jan Didden
Yes, possible that these are also wirewound...
Have you measured Ubb at drivers AND outputtransistors ?
It's possible that everything is ok, but that the vbe-multiplier
does not match your outputdevices.
To check if the current is drawn because of oscillation, you can
install a 100nf in paralell to the 220ohms that is connected between
the emitters of the drivers. If current reduces because of this change,
you are dealing with oscillation. This cap reduces crossconduction
that typically appear if the outputstage puts out high frequency.
I think, with these resistors you can turn on safely the amp for
a few seconds, just enough to get some readings.
Another question, are you using the mj15003/4 as outputdevices ?
Mike
Have you measured Ubb at drivers AND outputtransistors ?
It's possible that everything is ok, but that the vbe-multiplier
does not match your outputdevices.
To check if the current is drawn because of oscillation, you can
install a 100nf in paralell to the 220ohms that is connected between
the emitters of the drivers. If current reduces because of this change,
you are dealing with oscillation. This cap reduces crossconduction
that typically appear if the outputstage puts out high frequency.
I think, with these resistors you can turn on safely the amp for
a few seconds, just enough to get some readings.
Another question, are you using the mj15003/4 as outputdevices ?
Mike
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