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Old 18th March 2010, 07:09 PM   #1
exeric is offline exeric  United States
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Default Completely differential amplifier final stage

Hi all. I've been mulling over some of the problems inherent to building amplifiers, both tube and otherwise. I'm a believer in the P-P final stage as it requires much less perfection in an OPT. But I also think that if you "do" create a kind of perfection in a P-P OPT it deserves a final amplification stage that does it justice as far as efficiency and linearity. It would marry the best of both worlds.

I've lurked about these pages for many years absorbing information. It seems to me the kink in the get-a-along is that the P-P output stage isn't traditionally a long tail differential stage that can be used with a CCS. The reason for this is that the final stage is always the limiting factor for power and the slightest move into A2 amplification draws current into the grid circuit on the positive excursion of the input signal. That current draw upsets the balance of current draw between the two final tubes.

I'm just throwing this out there realizing it is a tough nut to crack. I haven't cracked it and am strictly thinking about this for my own audio pleasure. But I'm wondering how one would go about creating a circuit that would compensate for that unbalanced grid current draw? There seem to be so many benefits of this type of topology- including common noise rejection and elimination of DC current through the OPT, and allowing a low quality power supply except perhaps for the servo circuitry -that it is a no brainer to try to implement it. I know there are people who have cracked it. Allen Wright comes to mind. Do any circuit ideas come to mind to those technically oriented out there. Thanks in advance.
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Old 19th March 2010, 12:08 AM   #2
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You could use a separate B+ supply for each side's cathode follower/mosfet follower grid driver. These would float, with negative sides connected to the cathodes. That way no A2 grid current goes thru the CCS tail. Same thing is sometimes done for g2 current to keep it out of the tail.

The simplest way is to just use a cap from each cathode, with a series resistor (or Gyrator) to the main B+ on each side. Each driver draws its B+ current from the top of the caps. The tail CCS then has to provide the usual tail current plus the average DC sum of the driver currents that way.
(the cap supplies the pulsed grid current supply, and the resistor or gyrator keeps tail current add-on to DC)
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Last edited by smoking-amp; 19th March 2010 at 12:13 AM.
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Old 19th March 2010, 03:26 AM   #3
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The whole point of PP is that it DOESN'T need to be class A, hence yielding more power. Putting a CCS in the cathode circuit cripples it, akin to tying two people together and chopping off half their legs, so they have to hop along on the single remaining tied-together pair.

Tim
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Old 19th March 2010, 05:13 AM   #4
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"...that does it justice as far as efficiency and linearity."

I assume you are talking about triodes if you are worried about grid current. A differential(necessarily class A) triode output stage might do well as far as linearity goes, but efficiency will not be good.

Personally, I think you will spend a lot of effort and not have much improvement over an A2 push pull topology. It seems like it would just clean up some even harmonic distortions that there aren't much of and that won't sound that bad anyway. However, sometimes the payoff in learning makes it worth doing crazy things. Don't let me discourage that.
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Old 19th March 2010, 06:28 AM   #5
exeric is offline exeric  United States
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Originally Posted by smoking-amp View Post
You could use a separate B+ supply for each side's cathode follower/mosfet follower grid driver. These would float, with negative sides connected to the cathodes. That way no A2 grid current goes thru the CCS tail. Same thing is sometimes done for g2 current to keep it out of the tail.

The simplest way is to just use a cap from each cathode, with a series resistor (or Gyrator) to the main B+ on each side. Each driver draws its B+ current from the top of the caps. The tail CCS then has to provide the usual tail current plus the average DC sum of the driver currents that way.
(the cap supplies the pulsed grid current supply, and the resistor or gyrator keeps tail current add-on to DC)
Very interesting and creative. I would never have thought of that way of implementing it. I think I'm understanding the basic principle but have a few questions. For the CCS to not have to supply the added DC on peaks wouldn't the gyrator have be supplied from "below" the CCS tail through the gyrator. It seems like otherwise the CCS would have to supply the additional current for large crescendos that last over many cycles. I think I understand that a capacitor could supply instantaneous current and might be attached at the cathode. I'm probably thinking about this wrong as its hard sometimes to visualize these things.

As far as this amp being efficient I have to admit my bad. It won't be efficient at all. It would really be an amp that would serve the same niche that single ended amps serve. It would be pure class A but without the problems that single ended amps have, i.e.

1. SE operates in the middle of the transfer characteristic but all individual tubes differ slightly on either side of the bias point.

2. None of the bad sacrifices that must be made to create a OPT for an SE amp.

3. And it would have the advantages I already said in my first post. (Of course, efficiency not being among them.)

Eric
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Old 19th March 2010, 09:07 AM   #6
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As has been said here, what you are achieving is the best PP output stage. If your the sort of person who can live with the cost benefit analysis of SE then a class A PP stage is just the same - but better.
If you do decide to go down this path then take the opportunity to eliminate the airgap in the OT and so eliminate hysteresis within the core by going for a toroidal output transformer. The benefit comes in low level detail which is otherwise lost at crossover.

Most people are sold on the idea of the extra power achievable through PP and so there are almost no PP amps built with the same attention to detail and considerations applied to SE, I say go for it and live with the advantages and limitations of class A. You will be among a handful of people who have really tapped the full potential of a correctly implemented PP amp. By the way, as far as i am aware the Allen Wright Vacuum State amp doesn't operate in AB2. The other example of a fully class A PP amp is the Amnity.

Direct coupled or Interstage Transformers will give the best that PP has to offer. My current amp is a direct coupled pentode input stage shade 6AS7 differential output stage amp with a peak output of 7watts per channel (for about 120watts input). This is about the top of what can be achieved from PP.

Shoog
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Old 19th March 2010, 03:27 PM   #7
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Originally Posted by exeric View Post
Very interesting and creative. I would never have thought of that way of implementing it. I think I'm understanding the basic principle but have a few questions. For the CCS to not have to supply the added DC on peaks wouldn't the gyrator have be supplied from "below" the CCS tail through the gyrator. It seems like otherwise the CCS would have to supply the additional current for large crescendos that last over many cycles. I think I understand that a capacitor could supply instantaneous current and might be attached at the cathode. I'm probably thinking about this wrong as its hard sometimes to visualize these things.
Eric
To keep grid current out of the cathode circuit requires that the gyrator supply current be returned to the cathode of the output tube. Here is a SE circuit to illustrate but you could do the same for each side of the P-P driver.

Smoking-amp's suggestion to use a cap returned to the output cathode instead of a whole isolated supply might be perfectly fine if the cap has a chance to recharge between grid current peaks. That's a clever solution that might work well for grid current that only flows during part of the cycle. If you have say a 50mA sine shaped peak that is say 20% duty cycle then you only need to supply on average about 7 or 8 mA over the driver idle current to keep the cap from discharging.

If you do use isolated supplies they can be dead simple because the gyrator filters out the noise and ripple. Mine use a tiny toroid, a bridge rectifier, and a single cap.

I'm planning to convert my 300B P-P class A monoblocks to from choke direct coupled drivers to gyrator direct coupled + A2 using this circuit. I might try the cap approach just to see how it works.

Cheers,

Michael

PS even though the extra grid current is returned to the output cathode, when you drive a tube into class A2 there is somewhat more asymmetry to the load line. IOW, there is more output swing when driving the grid (+) than when driving the grid (-). Your tail CCS will not allow this and force symmetry. There will be somewhat less output power than without the CCS tail and the harmonic signature will be different. But I guess that's what you're going for. It will be interesting to hear about your resullts.
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Last edited by Michael Koster; 19th March 2010 at 03:36 PM. Reason: ps
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Old 19th March 2010, 05:54 PM   #8
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Michael's approach looks excellent. Can use a cheap floating power supply, Gyrator keeps out the noise, and the floating connection to the cathode keeps current out of the "tail".
--------------------------------
RE:
"For the CCS to not have to supply the added DC on peaks wouldn't the gyrator have be supplied from "below" the CCS tail through the gyrator. It seems like otherwise the CCS would have to supply the additional current for large crescendos that last over many cycles."

The non-floating pwr supply gyrator version smoothes out the current draw thru the tail to near DC (and being class A mode, it will have to draw enough steady current to cover the peaks pulled from the cap) so the tail CCS just needs up rating to cover it. The gyrator does have a time constant, which will interact with long crescendos, so the floating PS gyrator, like Michael's, is the best way (the long time constant then only affects the voltage on the driver drains then).
--------------------------------
RE: Tim:
"Putting a CCS in the cathode circuit cripples it, akin to tying two people together and chopping off half their legs, so they have to hop along on the single remaining tied-together pair."

and: Shoog:
"Most people are sold on the idea of the extra power achievable through PP and so there are almost no PP amps built with the same attention to detail and considerations applied to SE, I say go for it ..."

Tim has a point here, in that differential mode becomes more distorting at odd harmonics at large power levels (output stages), it does work well for small signal input stages. And efficiency goes to He.. with class A. Some fixup is possible with an optimum tail impedance to null the 3rd harmonic. Efficiency suffers with a CCS tail too, an inductor tail could remedy a little of that.

Shoog has a point here on class A P-P and attention to detail and avoiding crossover dist. But at a considerable cost in efficiency. Class AB1 can do quite well at avoiding crossover if done well (local Fdbks). The SS designers also do have a potent local Fdbk solution for avoiding crossover dist. in AB mode using "Hawksford error correction", which is never seen in tube designs (but was originally patented for a tube amp: Llewellyn). Not that hard to do really.
-------------------------------

For Class A2 or AB2, I would prefer avoiding operating in both the grid current and non grid current regions, where another crossover effect occurs. Keep an eye out for scaled g2,g1/Mu drive or whatever...drive threads coming soon. Interest is developing in this new approach, which can stay in grid current mode continuously, efficiently, safely and linearly. Even has enhanced power output capablility.

Don
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Last edited by smoking-amp; 19th March 2010 at 06:14 PM.
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Old 21st March 2010, 02:27 AM   #9
exeric is offline exeric  United States
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Originally Posted by smoking-amp View Post
For Class A2 or AB2, I would prefer avoiding operating in both the grid current and non grid current regions, where another crossover effect occurs. Keep an eye out for scaled g2,g1/Mu drive or whatever...drive threads coming soon. Interest is developing in this new approach, which can stay in grid current mode continuously, efficiently, safely and linearly. Even has enhanced power output capablility.

Don
I am intrigued by this idea, and have no idea how it would be implemented. I will hold fire on a fully balanced amp until I hear further. Perhaps you can give us a heads up on this thread as well as when another thread is started on this subject for those of us who don't keep up that much, but who get emails updates on threads we've already contributed to. Thanks much.

Eric
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Old 21st March 2010, 03:19 AM   #10
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Here is a start:

Posted new P-P power amp design

LS50/GU50 Both Grid Drive

Suppresor Grid used for Feedback?

stay tuned
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Last edited by smoking-amp; 21st March 2010 at 03:29 AM.
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