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Old 12th July 2001, 05:03 AM   #1
plp is offline plp
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Join Date: Jul 2001
Excuse my obvious ignorance, but recently I have come accross a number of 2stage single-ended designs which somewhat mimic the basic concept of the Aleph.

The important difference between such designs is what appears to be an active bias element (Q104 / R109 /R110 on the Aleph 3 schematic)connected between -V and the signal from the diff pair to the gates of the IRF244's.

I have no intention of building an Aleph as they do not satisfy my requirements at this time, but I am curious to know why this biasing element is used in the Aleph? What advantages does it offer over more simplier designs? Is it because the Aleph 3 uses all NPN devices in the output, while others use PNP/NPN?

Could this be the reason many commercial reviewers have claimed a significantly regressed midrange from Aleph amplifiers.

I know the answer is going to be really obvious and I'm going to feel like a fool for asking, but I can't see the woods for the trees on this one. Can somebody enlighten me please?
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Old 12th July 2001, 05:51 AM   #2
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There is no such thing as ignorance my friend plp.
Who searches finds, as they say.
First of all in the output we donīt habe bipolar transistors. They are mosfets and they are called N-channel or p-channel. Since it is not a complimentary output stage working in class B or AB it doesnīt use complimentary devices. The alephs are working in class A and single ended topology. One mosfet amplifies and the other is its current source. In the aleph amps though this current source is not a normal one. Itīs based on a Pass Labs patent and it is called active current source so the current through it is not steady but rises when more power is needed. This means these amps have more efficiency than normal single ended amps.
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Old 12th July 2001, 08:18 PM   #3
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The transistor you reference is a current limiter,
which protects the negative half of the output stage
from a short circuit at the load. It does not
conduct in normal operation.

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Old 16th July 2001, 06:29 AM   #4
plp is offline plp
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Join Date: Jul 2001
A quick note of thanks to Nelson Pass for answering my query. It was very much appreciated. Having only recently looked at the Aleph design, this was one area were curiosity got the better of me.

Also thanks to promitheus for his advice, also much appreciated. There are a number of hobbiest active current sourced se designs in existence these days - possibly inspired by Nelson Pass' patent documents - but only Nelson Pass uses NPN devices as the current sources. I'm not saying one is right and the other wrong, more just curiosity as to the differences in topology.

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Old 17th July 2001, 05:52 AM   #5
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Itīs unbelievable Mr. Pass is so kind to help out. I really feel happy and surprised each time I read something new from him. Mr. Pass thanks.
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