William
I am learning so much about the aleph X on this thread. But I guess you want to repair the amp so.....
Have you tried to replacing the input pair with your original semiconductors to check if the new input pair is causing the problem. Changing back seems to be the only way to know for sure if they are causing the problem.
Other than that it sounds as though you are just starting to operate a device in its non linear region which can sometimes cause a kink in the curve.Possibly just one semiconductor not quite to spec. Can you check the current accross a few of the 0.33 ohm resistors to see if you get the same problem with each 0.33 ohm resistor/ output device.
Other than that it just seems that the amp is running out of power. Possibly a connection in the power supply?
Don
I am learning so much about the aleph X on this thread. But I guess you want to repair the amp so.....
Have you tried to replacing the input pair with your original semiconductors to check if the new input pair is causing the problem. Changing back seems to be the only way to know for sure if they are causing the problem.
Other than that it sounds as though you are just starting to operate a device in its non linear region which can sometimes cause a kink in the curve.Possibly just one semiconductor not quite to spec. Can you check the current accross a few of the 0.33 ohm resistors to see if you get the same problem with each 0.33 ohm resistor/ output device.
Other than that it just seems that the amp is running out of power. Possibly a connection in the power supply?
Don
Hi,
the output is not the problem, the input pair is. Both amps have the same problem so I don´t think there´s a defect component .
I will try the original pair. I only need to change the drain resistors, Mcmillan resistors, current source resistor and the input pair........
Like I said the problem also exists without a load!
William
the output is not the problem, the input pair is. Both amps have the same problem so I don´t think there´s a defect component .
I will try the original pair. I only need to change the drain resistors, Mcmillan resistors, current source resistor and the input pair........
Like I said the problem also exists without a load!
William
??????????!!!!????
Hello and good morning,
just refitted my 9610, Rdrain 390R, R46/47 10k and another R in the current source.
The good news is that the distortion at higher voltages/ frequencies stays the same meaning that the Jfets are not to blame.
The bad news is that the distortion is still there!
Help!
William
P.S. I won´t kill myself, put back in the Jfets and just don´t listen to 10kHz tones at 26 volts and 8R until I find a sollution or somebody else has a nice idea........
Hello and good morning,
just refitted my 9610, Rdrain 390R, R46/47 10k and another R in the current source.
The good news is that the distortion at higher voltages/ frequencies stays the same meaning that the Jfets are not to blame.
The bad news is that the distortion is still there!
Help!
William
P.S. I won´t kill myself, put back in the Jfets and just don´t listen to 10kHz tones at 26 volts and 8R until I find a sollution or somebody else has a nice idea........
William,
Dumb question: Aren’t the protection transistors kicking in?
Furthermore, I think your diff pair current source has a problem. If you still have a pot in there, try a few different settings while looking at the waveform.
Bear with me, after all that’s been said and done, this is about the only thing I can come up with.
/Hugo
Dumb question: Aren’t the protection transistors kicking in?
Furthermore, I think your diff pair current source has a problem. If you still have a pot in there, try a few different settings while looking at the waveform.
Bear with me, after all that’s been said and done, this is about the only thing I can come up with.
/Hugo
Hi Hugo,
I´ve removed the protection network (R39/40)
no difference
I tried a few difefrent currents through the cs as I changed the drain resistors. I can´t realy play with it as it also changes DC offset.
I think it is a designed in problem as both my amps have it and also Sinuko ahs the same problem. Some others may not have noticed......
It will probably only affect A-Xes with more than 22V power supplies
William
I´ve removed the protection network (R39/40)
no difference
I tried a few difefrent currents through the cs as I changed the drain resistors. I can´t realy play with it as it also changes DC offset.
I think it is a designed in problem as both my amps have it and also Sinuko ahs the same problem. Some others may not have noticed......
It will probably only affect A-Xes with more than 22V power supplies
William
Well,
let´s see it realistic. As long as it sounds good (because normally you will never see such high amplitudes at those frequencies) and you don´t have to give it away for a magazine test no one will notice......
But when somebody has an idea I´m open for this. Shouldn´t it be possible to simulate this?
William
let´s see it realistic. As long as it sounds good (because normally you will never see such high amplitudes at those frequencies) and you don´t have to give it away for a magazine test no one will notice......
But when somebody has an idea I´m open for this. Shouldn´t it be possible to simulate this?
William
Hi,
I must correct myself. The distortion is not exactly the same. It starts a bit later (at a somewhat highr voltage) than with the Jfets.
I suppose this is because there´s a bit more open loop gain and thus a bit more available feedback?
William
P.S. I think I will go and wash my car as normal people do on saturdays (must be living in Germany for too long.....)
I must correct myself. The distortion is not exactly the same. It starts a bit later (at a somewhat highr voltage) than with the Jfets.
I suppose this is because there´s a bit more open loop gain and thus a bit more available feedback?
William
P.S. I think I will go and wash my car as normal people do on saturdays (must be living in Germany for too long.....)
wuffwaff said:
I don't think so, mine is 17,5v. It might not affect the original one with 15v.
At this point, I am running out of ideas also. May be we should try to find similarities between the three identified circuits that are presenting this behavior and we should also find a circuit that works fine. Anyway, I also think that a lot of people are having this behavior without knowing.
Any happy user wants to be the first in checking whether his amplifier is presenting this behavior or not
?Xavier
William,
This is my reasoning and it seems to fit what you described.
Your AX should have an open loop gain of about 300 at 8 ohm (and more at no load), meaning that you have NFB of a factor of 30.
At an output of 30V pk-pk, your input is 3V pk-pk, and I expect you to have 0.1V pk-pk across the gate of the diff pair.
Assuming that you really have poor linearity (i.e. lots of 2nd harmonics) at the diff pair, and 10% of that 0.1V appears as common mode voltage at the source-junction (or CCS out) of the diff pair, you should not see more than 10mV there.
But you were seeing somthing like 0.15V, and I suspect that this can only be common mode being fed to the loop via the McMillan resistors.
This large common mode voltage should not be there in the first place (at least not such large amplitude).
Because it is common mode (my hypothesis), the diff pair will do nothing to that and just pass it on.
This probably happens at no or light load, because you have a slightly excessive HF gain.
This is damped when the load increases (or load impedance decreases, e.g. at 4 ohm).
If the above hypothesis (and I stress that) is true, then the following may be the cure :
Connect the two outputs using 2x 20k resistors (as in the McMillan resistors).
Instead of feeding the node mid-way of the 2 resistors directly to the diff pair common source, connect this to the common source via a single, third resistor of say 5k.
Add a cap (say 22uF, best non-polar) to the junction of the 3 resistors and tie the other end to ground.
This way, you still feed back common mode error (absolute DC) at low frequencies, but prevent any HF interaction between the diff pair and the output (grounded by the cap).
I admit I have not tried because I do not have the problem, but the experiment is simple enough to do.
You can probably get away with a small 22uF100V electrolytic for the purpose of the trial.
Hope you get what I mean.
Patrick
This is my reasoning and it seems to fit what you described.
Your AX should have an open loop gain of about 300 at 8 ohm (and more at no load), meaning that you have NFB of a factor of 30.
At an output of 30V pk-pk, your input is 3V pk-pk, and I expect you to have 0.1V pk-pk across the gate of the diff pair.
Assuming that you really have poor linearity (i.e. lots of 2nd harmonics) at the diff pair, and 10% of that 0.1V appears as common mode voltage at the source-junction (or CCS out) of the diff pair, you should not see more than 10mV there.
But you were seeing somthing like 0.15V, and I suspect that this can only be common mode being fed to the loop via the McMillan resistors.
This large common mode voltage should not be there in the first place (at least not such large amplitude).
Because it is common mode (my hypothesis), the diff pair will do nothing to that and just pass it on.
This probably happens at no or light load, because you have a slightly excessive HF gain.
This is damped when the load increases (or load impedance decreases, e.g. at 4 ohm).
If the above hypothesis (and I stress that) is true, then the following may be the cure :
Connect the two outputs using 2x 20k resistors (as in the McMillan resistors).
Instead of feeding the node mid-way of the 2 resistors directly to the diff pair common source, connect this to the common source via a single, third resistor of say 5k.
Add a cap (say 22uF, best non-polar) to the junction of the 3 resistors and tie the other end to ground.
This way, you still feed back common mode error (absolute DC) at low frequencies, but prevent any HF interaction between the diff pair and the output (grounded by the cap).
I admit I have not tried because I do not have the problem, but the experiment is simple enough to do.
You can probably get away with a small 22uF100V electrolytic for the purpose of the trial.
Hope you get what I mean.
Patrick
Sim
William,
I remember struggling with similar trouble (simulation first, and
later with the amp). See original Aleph-X thread. Fred AKA .....
did help back then in his special way.
Now, to reproduce the results was a little bit difficult.
But to make a long story short, I used source degeneration of
the input pair as the solution.
I have been able to reproduce the behaviour (to some extent)
of your amp in a ALEPH 3 sim.
See attached document.
Sorry for the short answer.
I hope this helps a little.
William,
I remember struggling with similar trouble (simulation first, and
later with the amp). See original Aleph-X thread. Fred AKA .....
did help back then in his special way.
Now, to reproduce the results was a little bit difficult.
But to make a long story short, I used source degeneration of
the input pair as the solution.
I have been able to reproduce the behaviour (to some extent)
of your amp in a ALEPH 3 sim.
See attached document.
Sorry for the short answer.
I hope this helps a little.
Hi William,
I tried with a variac to change the rail voltage, at lower voltage it seems to be better (as you suggested)..(but I need more tests)
Paolo
I would cancel my post here http://www.diyaudio.com/forums/showthread.php?postid=1172456#post1172456
I tried with a variac to change the rail voltage, at lower voltage it seems to be better (as you suggested)..(but I need more tests)
Paolo
I would cancel my post here http://www.diyaudio.com/forums/showthread.php?postid=1172456#post1172456
> appropriate valued resistor between drains in diff pair?
A resistor between drains acts as an attenuator. This reduces gain at all frequencies, just like adding source degeneration. While it might cure the problem, it also affects overall performance (i.e. amount of negative feedback). You might like this, or you might not.
There are at least 2 more ways to reduce gain at HF (for both diff & common mode) :
1) Put a (polystyrol) cap, optionally in series with a resistor between the diff pair drains. This attenuates any HF signal coming out of the diff pair.
2) Add a Sobel network (e.g. 10R, 0.1uF) at each output to ground, (or just one between the outputs). This adds load at HF.
But if my hypothesis is correct, then trying to break the common mode feedback loop (due to the McMillan resistors) at HF would just cure the problem without changing the diff mode performance (which again might or might not be better or worse).
Patrick
A resistor between drains acts as an attenuator. This reduces gain at all frequencies, just like adding source degeneration. While it might cure the problem, it also affects overall performance (i.e. amount of negative feedback). You might like this, or you might not.
There are at least 2 more ways to reduce gain at HF (for both diff & common mode) :
1) Put a (polystyrol) cap, optionally in series with a resistor between the diff pair drains. This attenuates any HF signal coming out of the diff pair.
2) Add a Sobel network (e.g. 10R, 0.1uF) at each output to ground, (or just one between the outputs). This adds load at HF.
But if my hypothesis is correct, then trying to break the common mode feedback loop (due to the McMillan resistors) at HF would just cure the problem without changing the diff mode performance (which again might or might not be better or worse).
Patrick
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