Can someone please explain Buffered Passive for Dummies?

[Shadowfax]

I get that a passive Pre is pretty much a volume control between the source and amp. My VAC pre has a Buffered Passive mode that uses only the 12ax7s and bypasses the 12au7s, but that is where my understanding ends. What are they using the tube to do if not gain? I suppose if they used the tube for gain, it would no longer be passive.

Is ther a simple way to explain this mode of my Pre? In normal and high gain modes, it uses both sets of tubes.

Thanks!

[octadyndude]

Not sure if this helps much
If you're only considering gain factor, the 12AX7 family falls like this:
12AX7 - 100
5751 - 70
12AT7 - 60
12AY7 - 45
12AV7 - 41
12AU7 - 19
I have been told with the 12au7 being a low voltage gain it has a very high current gain. I can only guess it is because of the output transformer design. The Octave V70 and V110 uses a single 12ax7 for an input tube and a pair 12at7's for splitter/driver tubes. The V80 uses a single 12at7 for an input and a pair of 12au7's for driver/splitter tubes. The output transformers in the V80 is better but not sure why.

[Shadowfax]

Well I thought there would be no gain used or how can it be called passive. So what is the Buffered action if not gain? Are they using the tube to do something else?

[jerryz1963]

a "buffer" is a layman's term for cathode-follower. This circuit topology means the output is taken off the cathode instead of off the anode. when the output is taken off the cathode, there is no VOLTAGE GAIN. NO VOLTAGE GAIN. It "buffers" the stage preceding it. the idea is that the buffer will not present a load to the stage driving it. That is a very good thing for sound quality. Generally speaking, in a good design, the first stages will be gain stages, then a buffer, then the power stage. Of course, in a preamp, there is no power stage.

Passive means there are no ACTIVE devices (tubes and transistors are active devices) in the signal path.

A buffer does not impart tonal qualities. Thus, a buffer can be any active device, even a transistor and, if properly biased, no one will know it is there except the builder/designer.

A buffer's purpose is (usually) two-fold. take the signal from the preceding stage and pass it on to the next stage without loading down the preceding stage AND providing sufficient CURRENT to drive the stage following the buffer.

[joeinid]

Welcome to the forum forum Jerry, thank you for joining.

[Speedskater]

It's just an oxymoron.

= = = = = = = = = = =
A buffer is far from the only circuit that does not impart tonal qualities.

[audio.bill]

a "buffer" is a layman's term for cathode-follower. This circuit topology means the output is taken off the cathode instead of off the anode. when the output is taken off the cathode, there is no VOLTAGE GAIN. NO VOLTAGE GAIN. It "buffers" the stage preceding it. the idea is that the buffer will not present a load to the stage driving it. That is a very good thing for sound quality. Generally speaking, in a good design, the first stages will be gain stages, then a buffer, then the power stage. Of course, in a preamp, there is no power stage.

Passive means there are no ACTIVE devices (tubes and transistors are active devices) in the signal path.

A buffer does not impart tonal qualities. Thus, a buffer can be any active device, even a transistor and, if properly biased, no one will know it is there except the builder/designer.

A buffer's purpose is (usually) two-fold. take the signal from the preceding stage and pass it on to the next stage without loading down the preceding stage AND providing sufficient CURRENT to drive the stage following the buffer.

That is an excellent and informative explanation, welcome to AudioShark! The only point that I would respectfully question is that "A buffer does not impart tonal qualities." There are tube buffers which are sold with the specific claim to impart some of that "tube magic" into either a solid state system or a digital front end. Even though their active stage may not provide any gain it is my understanding that they can still influence the resultant sound quality and a system's relative tonal balance. I suppose it's possible that it's actually other aspects of the sound which are changed with tube buffers without affecting the tonal qualities specifically. An interesting topic for discussion!

[jerryz1963]

manufacturers especially make what I consider gross exaggerations to sell their products. a properly biased cathode follower, source follower, etc, is pretty much transparent. Studies were done, double blind listening tests. They started with one buffer, nobody could tell. then 2 buffers, nobody could tell. Then 3 buffers and nobody could tell. They kept adding buffers until a difference was noted. I forget the exact number of buffers in series that were added, but it was at least 10. I think it took like 16 buffers before anybody could tell.
i would consider it a marketing ploy if somebody were to claim a cathode follower was imparting "tube magic" into the circuit. Again a buffer WILL improve the sound quality by not loading down the stage driving it and by providing plenty of current for the tube being driven. But that's not the same thing as saying it is imparting tube magic. My own experiences in testing and designing have led me to believe that not loading down the preceding stage is most of the benefit unless one is using screen drive (G2 drive).
I should mention that buffering SHOULD be done in a passive as potentiometers have capacitance, and that capacitance can short the high frequencies to ground. Max Robinson on "Fun With Tubes," has a very readable and easy to understand dissertation on the topic. You do not want potentiometers in series without an active device (gain stage or buffer) in between them. the cumulative capacitance will shunt the highs, cause a high frequency roll-off