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Aug 06 2013

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The Single Super Stage ..

The Single, Super-Voltage-Amplifying Differential Stage  … with a twist …

... with vertical feedbacks

… with vertical feedbacks


super-stage 1

with cross bridge “Van Scoyoc” style feedback

super-stage 3 williamson

The humble (but genius) Williamson Input stage … multiply is all by TWO  in order to get a “differential image” …


Have you ever had nightmares regarding the total phase shift within your amplifiers signal path, prior to closing the negative feedback loop?   Or maybe a dream of reducing the number of coupling capacitors within the signal path ?  Or maybe you had dreams of a “Super Voltage Amplifying Stage, that is SINGLE,  very linear, with a low output impedance, and oh, by the way, did I mention that it is fully symmetrical and with a very local, hence very fast, local feedback ?

Well, if I were lazy, I would simply provide you with the links as below, and then say:   “Go build yourself an OTL, the kind of like the Atma-Sphere, but with a twist … and be done with it.



But that would be too easy.   Let’s just say that I am orbiting around this, as above, as well as around a few other concepts, so as to come up with a totally elegant OTL solution …

Now, after digging through that patent stuff, which could be basically cut down to a short description as a:

  • Differential Amplifier, of the Long Tailed Pair type,
  • a Cascode in the middle level, separating the voltage variations from the top tubes from the anode of the bottom tube, which sees essentially a local “current source”
  • an active load at the top, similar to those in SRPP,
  • Outputs taken from the cathode, in a low impedance mu-follower fashion.
  • An option of local cross-branch negative feedback options, a la Van Scoyoc, when looking at it
  • An option of vertical negative feedback, with the output “modulating” the voltage reference of the underlying cascode.

Ok, so we have a very sweet solution here.  BUT, but , but, … The gain of the whole stage is only as good as the maximum theoretical gain of the bottom tube, operating under constant anode voltage conditions. (current source loaded).

What if we “expand” this concept a little bit ?

Do you remember that venerable, almost boring, as some of you will say, input stage of the Williamson amplifier and/or it’s clones ?    It was a directly coupled input gain stage, common cathode, whose anode is working directly into the grid of the next stage, which is elevated in DC terms, so as to make it possible for a direct galvanical connection between the two.

As the second tube must be at approx. 90V on the grid, the cathode of L2 would normally be connected do a big resistor,  so as to “lose” that 100V or so from the cathode to ground.

Now … imagine that we “merge” the two concepts, and create the following:

  • A Differential Amplifier, of the Long Tailed Pair type, consisting of the L1 (L1′) tube of the Williamson amplifier.  We load the pair of tubes with an active, transistor based current sink, operated from say -12VDC.
  • As we would normally “do” in a Williamson amplifier, we go from the anodes of these L1/L1′ tubes to the grids of L2 / L2′ tubes.   The cathodes of these tubes are configured as an independent long tailed pair, at an offset of approx. 100VDC,  with the current sink done on a normal pentode, like the 6BH6  or the EF86.
  • a Cascode in the middle level, separating the voltage variations from the (very) top tubes from the anodes of the L2 and L2′  tubes, therefore the L2 and L2′ in essence see a local “current source”.
  • an active load at the very top, above the cascodes, similar to those in SRPP,
  • Outputs taken from the cathode, in a low impedance mu-follower fashion.
  • An option of local cross-branch negative feedback options, a la Van Scoyoc, when looking at it
  • An option of vertical negative feedback, with the output “modulating” the voltage reference of the underlying cascode.

Ok, so what is the big deal ?   Well think about it.   Especially, if you want to build an OTL amplifier.

You have a high gain differential pair of tubes, like the 12ax7  (mu=100), operating from a pair of anode resistors,or better: current sources, hence mu is close to max = 100;   You have a DC coupling to a second differential stage, which just so happens that it is also a 12ax7 pair, with mu=100, and they are directly under their respective cascodes.

In essence, you get a gain stage of compound mu = 100 x 100 = 10000, or realistically speaking 7000.  Let’s be reasonable now.  We use the cross bridged negative local feedback scenario, a la Van Scoyoc, to hammer it down a little and at the same time in return obtain a fair amount of linearity of this SINGLE stage.

Did I just say “SINGLE” stage ?   Oh, yes, It just so happens that this IS a single stage, more or less. No capacitors in-between.   Just like in the “Williamson” tubes L1 and L2, but everything “differential-ized”, so to speak. No phase-shift up to the very output signal capacitors differential pair. By the time we reach the first set of capacitors, we are equipped with a highly linear open loop voltage gain that can go into the “thousands”, a high amplitude output voltage signal, with low output impedance and absolutely symmetrical / differential.

I see that you are starting to “feel” where I am getting at with all of this.  In essence, we have a potential zero-phase-shift super stage, with super gain, with super linearity (at our discretion,depending on the level of LOCAL differential negative cross-bridge feedback !), with low output impedance, and with an amazing potential with respect to the possible output voltage swing … In essence, a single stage that is capable of directly driving a set of OTL output tubes. Or traditional, transformer loaded output tubes.

Now, imagine what that super-gain could do to the output impedance of a pair of OTL tubes driving your speaker systems directly, with no intermediary transformer…   Obviously we could close the feedback loop, and use, lets say, maybe a few thousand factor for the sole purpose of hammering down the output impedance, into a range of mere tenths of an ohm, or even less.

I think that we need a drawing here, before we carry on ….

Till the next time.

I am dreaming of a single super gain stage, which is directly driving the pair (s) of output triodes of my next generation OTL.   Super-Triodes, actually, but that is YET a different story.  :)



P.S.  …. and Oh!   by the way, did I mention that some of the tubes are NOT tubes … but rather high voltage depletion mode Mosfets ?


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