The Aleph controlled current source is a brilliantly simple and effective circuit which provides the active load in the output stage of several Pass designs. The quiescent current in the output stage is set by a single sense resistor and is controlled by a bipolar transistor in a feedback loop. The current is modulated by feeding an audio signal into the base of the control transistor.
The collector load of this transistor is a conventional bootstrap type, with the mid point of two resistors, in series, being fed from the output via a large electrolytic capacitor.
At low to moderate signal levels, the bootstrap node nicely tracks the amplifier output and the gate voltage of the active load mosfet will be a bit higher than the threshold voltage at around 4.5 volts +/- a few hundred mV ac signal. As such, the active load mosfet is operating normally in the saturation region.
If you examine the output voltage, and the gate voltage of the active load mosfet at higher signal levels, you can see that as the amplifier output swings up towards the positive supply rail, Vds falls towards zero whilst Vgs rises way above 4.5 volts.
During these peaks, the active load mosfet is pushed into the linear region and behaves like a resistor of a few milli Ohms. The transitions between normal operation and the lnear region are abrupt and result in instability which takes 100 µS or so to subside. Relative to the 20V nominal mid point, the distortion in the output is noticeable at a positive swing of about 16 volts. (Whether it is audible is another question!?)
A 5.6V zener diode clamp placed across the bipolar transistor prevents the instability:
[IMGDEAD]http://www.pmeweb.co.uk/Audio/images/ZenClipComp_Vo.gif[/IMGDEAD]
The circuit, with zener clamp shown in red:
[IMGDEAD]http://www.pmeweb.co.uk/Audio/images/ZenV4Clamp.gif[/IMGDEAD]
These observations have been with regard to Zen V4 and LTPZen, see thread http://www.diyaudio.com/forums/showthread.php?s=&threadid=9459
but should be applicable to other Aleph CSS designs.
Maybe, the amplifier should not be driven so hard, however, occasional transients may be hard to avoid and the modification is both simple and effective.
For further details: http://www.pmeweb.co.uk/Audio/Aleph.html
The collector load of this transistor is a conventional bootstrap type, with the mid point of two resistors, in series, being fed from the output via a large electrolytic capacitor.
At low to moderate signal levels, the bootstrap node nicely tracks the amplifier output and the gate voltage of the active load mosfet will be a bit higher than the threshold voltage at around 4.5 volts +/- a few hundred mV ac signal. As such, the active load mosfet is operating normally in the saturation region.
If you examine the output voltage, and the gate voltage of the active load mosfet at higher signal levels, you can see that as the amplifier output swings up towards the positive supply rail, Vds falls towards zero whilst Vgs rises way above 4.5 volts.
During these peaks, the active load mosfet is pushed into the linear region and behaves like a resistor of a few milli Ohms. The transitions between normal operation and the lnear region are abrupt and result in instability which takes 100 µS or so to subside. Relative to the 20V nominal mid point, the distortion in the output is noticeable at a positive swing of about 16 volts. (Whether it is audible is another question!?)
A 5.6V zener diode clamp placed across the bipolar transistor prevents the instability:
[IMGDEAD]http://www.pmeweb.co.uk/Audio/images/ZenClipComp_Vo.gif[/IMGDEAD]
The circuit, with zener clamp shown in red:
[IMGDEAD]http://www.pmeweb.co.uk/Audio/images/ZenV4Clamp.gif[/IMGDEAD]
These observations have been with regard to Zen V4 and LTPZen, see thread http://www.diyaudio.com/forums/showthread.php?s=&threadid=9459
but should be applicable to other Aleph CSS designs.
Maybe, the amplifier should not be driven so hard, however, occasional transients may be hard to avoid and the modification is both simple and effective.
For further details: http://www.pmeweb.co.uk/Audio/Aleph.html