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passive + Active Aleph L?
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Old 31st October 2005, 03:45 AM   #1
Ipanema is offline Ipanema  Malaysia
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Default passive + Active Aleph L?

According to this website : http://www.high-endaudio.com/RC-Linestages.html on Aleph L

"The most original feature of this preamplifier, never duplicated or imitated by anyone else, including Pass Labs themselves, is its automatic Passive/Active signal routing. If the audio system doesn't need more gain than the signal source provides, the Pass will not further amplify the signal. This is achieved by a unique volume control."

I try to look at Aleph L Service manual, but can't find the circuitry which can do the passive to Active mode change. Any idea?

This product also claim to have passive to active changeover feature :

http://www.ciaudio.com/ PA1 preamp

Does anyone know how this is done? To get the best of both passive and active in on pre.


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Old 31st October 2005, 03:54 AM   #2
GRollins is offline GRollins  United States
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You're trying too hard.
There's no 'circuitry' involved. That would negate the advantage. Relax and take a closer look at the way the volume control itself is wired.

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Old 31st October 2005, 07:38 AM   #3
Ipanema is offline Ipanema  Malaysia
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Pls see the attachement of simplified Aleph L diagram . Is this correct?

When volume is set beyong 3 o'clock, output is taken in between Rf1 and Rf2 and does the gain is determine by Rf1/Rin? Does Rf2 contribute to the output impedance of the opamp since it is in series with the opamp?

Pls advise.

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Old 31st October 2005, 07:43 PM   #4
Nelson Pass is offline Nelson Pass  United States
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passive + Active Aleph L?
No, that is not correct. Imagine a non-inverting gain stage
driving the top of an attentuating ladder (selector switch with
resistors between the pins). The bottom of the ladder goes to
ground, and the wiper goes to output. Ordinary output volume
control, right?

Now take the input to the gain stage and also connect it to one
of the pins toward the middle. Too high, and the system
will oscillate without a load (or maybe even with one). Now
you have an output attenuator where it starts out at the CCW
bottom in "passive" mode, at one point the output is directly
connected to the input, and above that, the system has active

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Old 1st November 2005, 01:47 PM   #5
SRMcGee is offline SRMcGee  United States
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passive + Active Aleph L?
Not surprisingly, you folks are operating at a level far beyond the understanding of this newbie. Let me ask a simple question: has anyone implemented a clone of the attenuator in Mr. Pass's Aleph P? I am considering building an Aleph P using Kristijan's PCBs, but they do not incorporate volume control or source selection. One option is to try the Dantimax products, but I don't know whether they measure up sonically to the Aleph P. If there is another option available to beginners like me, I'd love to hear about it.

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