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Aug 31 2013

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The Single Super Stage (7) – hybrid – and the “Differential DC Cathode Bias”

.. Hey, I was just wondering, returning back to that concept of a directly coupled differential stage with huge gain.   In the previous representations I keep on getting that strange feeling that the anodes of the first set of tubes will position themselves at arbitrary voltages. This would imply that the cathodes of the next set of tubes would also position themselves at an arbitrary voltage.

But this would in turn mean that the second set of tubes have totally differing operating points, as their anodes are fixed by the cascode to a fixed reference voltage. The result would be a total lack of symmetry and a total mess…

Therefore, I was pondering about that resistor and zener diode feedback from the second set of tubes down to the first set of tubes.

Now I see a problem: this zener + resistor network is obviously not only DC feedback. It also implies some heavy AC feedback.  They go “together” – and this is what I do not like.

I would like this to be DC feedback ONLY.

I would like to have the luxury of deciding about the AC feedback elsewhere, and with no implied restrictions (at least: not THIS restriction).

So what I need to do is to rather “Bias” the first set of tubes in such a manner, so that their anode voltages are equal.  But how do I “bias” a tube, the grid of which is biased by the potential of the commons connection ?

… hmmmm …

How about biasing their CATHODES ?

How about an: Upside-Down “Differential DC Cathode Bias”    (C)    ?

Here is the idea: I transform the Zener + Resistor network into a low-pass filter. We ‘expand’ the concept a little bit and use the following components:

A Zener diode + Resistor + Shunt Capacitor to ground + another resistor.

For each of the tubes within the input stage. Like this:

Upside-Down Differential DC Cathode Bias,  (C) zjj_wwa

Upside-Down “Differential DC Cathode Bias”

The concept works as follows:

Both grids of the input tubes are ground referenced by their respective grid resistors.
Both cathodes of the input tubes are connected to a joint current sink via cathode resistors, constituting an  “AC see-saw” of the Long Tailed Pair. The DC conditions are determined by the joint current sink, the current sources on the anodes, … and the unequal parameters of the individual tubes. As the grids are both grounded, the only way to even out the voltages on the anodes of two “uneven” tubes is to introduce a different voltage on their cathodes.

But, between each respective cathode and it’s LTP degeneration / feedback resistor of the LTP “see-saw”, …  we add yet another a small voltage. A “delta”.  A small additional DC voltage. This delta is our small “correction factor”, referenced to their grids, which are at ground voltage. This corrective DC “delta” voltage – please allow me to call it an “upside-down bias” … is sourced from the cathodes of the second stage tubes, reduced voltage-wise by the zener and by the serial resistor, and FILTERED OUT by a shunt capacitor to ground, whereby the RC constant of this network is way outside the audio range.

Now, lets assume that the RIGHT hand side input tube has a relatively “too high” an anode voltage in comparison to it’s left hand side peer (meaning that the right hand side input tube is conducting less than it should be). That implies that this too high a voltage from the right hand side anode of the input tube is transferred to the grid of the second stage tube on the left hand side of the schematic. This causes this second stage tube on the left to conduct more, and results in a higher cathode voltage under it. This “above average voltage on cathode of left second stage tube” information is conveyed via left zener and resistor to the left shunt capacitor, which converts this information to “DC”. An “above average” DC.  Such a higher “DC” – in the form of “delta” information is then conveyed to the cathode circuit of the left hand side input tube. A higher “cathode” voltage essentially is equivalent to a higher “negative” voltage on the grid (yes, I know, the grid IS grounded and at a fixed voltage). So basically, this has the effect that the left hand side input tube is now forced conduct less. Less current  in terms of it’s DC operating point. But this tube is a part of the input stage’s long tailed pair setup, and is one of a pair of tubes sitting on a constant current sink, .. sinking a CONSTANT current. This implies that the RIGHT hand side input tube, its peer, will now start conducting MORE.

Remember what I said at the beginning of that previous paragraph ?  We started off with an assumption, with a hypothetical problem:  that the RIGHT hand side input tube has a relatively “too high” a voltage on it’s anode and that it is conducting less than it should be. But at the end of the paragraph we see that due to the corrective actions, it starts to conduct more – so that the difference of anode voltages MUST EVEN OUT.

At the same time it seems that all this is happening within the DC domain, and that it does NOT side track the AC signaling of the circuit, as each of the shunt capacitors, that are fed from respective zener diodes, are  separated from each tube by fairly high resistance. The small little delta in the DC voltage hitting the cathodes of the input tubes should keep them DC balanced in such a manner, that their anode voltages “even out”.

Now I think I am happy.

With the even anode voltages of the input tubes, I can reasonably hope that the second stage tubes will be operating at similar operation points, hence providing a reasonably similar drive.

What do you think ?

Oh, …. and just in case that nobody as yet came up with such a crazy, Upside-Down:

“Differential DC Cathode Bias” concept:   (C) zjj_wwa, 2013-08-30, http://hiend-audio.com.

super-stage direct coupled differential hybrid E2

super-stage direct coupled differential hybrid

Actually, now that I think about it, it is well within my best interest to achieve a fairly big time constant of the RC circuit, implying that the higher the value of the resistor, the better. In such case, I suppose that we could skip the zener diodes altogether, and compensate with a much higher value of the serial resistor within the RC network. Better filtering. Better bias.

So, this is the corrected version. No zeners here. Bigger resistor should be more optimal.
AND, obviously, the capacitors should shunt to GROUND, and not to the -12V supply line. That is corrected too.

Regards,

zjj_wwa

P.S.   You may be wondering, … why am I so obsessed by this single super gain stage. It may be a bit more obvious, if you read this following article, of Mr.  Bruno Putzeys:

“The F-Word  -  or, why there is no such thing as too much feedback”
as published by “(c) Linear Audio www.linearaudio.net

There is a Polish translation avalable here of this text. In short, I wish to achieve a very deep FEEDBACK  { the “F-word” :)   },  to achieve unprecedented levels, with the loop gain going as high as 40~45dB, so as to jump PAST the Baxandall’s humpback on the Baxandall’s plot.   Balancing on the verge of stability, I need to come up with a possibly SINGLE, directly coupled stage, with a maximum of a SINGLE coupling capacitor (and associated phase-shift) within the Feedback Loop.  Yes, I am planning a new OTL.  But this time, one that will be based on Super-Triodes and with an immense amount of feedback.  Feedback is GOOD.  There is no such thing as too much feedback.   My target is 45dB within the loop.

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