augmented cathode follower?

[smoking-amp]

I've been reading thru the copy Jose sent. These look like what we would call unity gain buffers these days. I'm sure some similar versions have shown up on the TubeCad journal, maybe the tube Buffers article.

I seem to recall that Ross MacDonald wrote some articles on pre-distortion too. Think I saw them listed in the RDH4. I'll bet the two amplifier references (1 & 2) on the last page of the Augmented CF paper are interesting designs.

Don

[smoking-amp]

I just looked on Ross Macdonald's web site! (Wonder if he is still around! He has journal papers published in 2007!)

"Active-Error Feedback and its Application to a Specific Driver Circuit," Proc. I.R.E. 43, 808-813, July (1955). Theoretical and experimental. See also: Instruments and Automation 28, 1938, November (1955). Also: Letter-to-the-editor, Wireless World 79, 295, June (1973).
"A Multi-Loop, Self-Balancing Power Amplifier," I.R.E. Trans. on Audio AU-3, 92-107, July-August (1955). Experimental. See also Instruments and Automation 29, 288, February (1956). I.R.E. P.G.A. Senior Paper Award, 1957.


"Nonlinear Distortion Reduction by Complementary Distortion," I.R.E. Trans. on Audio AU-7, 128-133, September-October (1959). Theoretical.
"Reply to Comments on 'Nonlinear Distortion Reduction by Complementary Distortion'," I.R.E. Trans. on Audio AU-8, 104-105, May-June (1960).
"More on Nonlinear Distortion Correction," I.R.E. Trans. on Audio AU-9, 103-105 July-August (196l). See also: dB, 13, 2,4, November (1979); 14, 6,8, April (1980); and 14, 6,7, December (1980).

[ashok]

korneluk ,

Can I have a copy of the paper"Some augmented cathode follower circuits" please ?
Thanks.

amadhavan(at)eth(dot)net

[korneluk]

Originally posted by ashok
Can I have a copy of the paper"Some augmented cathode follower circuits" please ?
...
amadhavan(at)eth(dot)net

No problem - can you get me a job at the ETH?

-- josé k.

[ashok]

Thanks.
Not sure if you know who ETH is.
First it was a small private ISP. Then it was taken over by a larger company called VSNL and now is owned by a private company called TATA ! Same guys who are buying steel plants and car companies around the globe !

[tubelab.com]

I have read and reread this paper, and I have come to the conclusion that I must build an "acf-2" or an "acf-3" or both, only using somewhat BIGGER tubes. Then I must "test" it in true Tubelab style. As I am leaving town in a few days my "testing" will be confined to the simulation world for now.

[mashaffer]

Sorry guys, but I missed how one clamps the plate in proportion to the cathode (fixed K-P voltage).

mike

[SY]

Stack another triode on top of the CF and drive its grid with the output, level-shifted so that the CF has enough plate-to-cathode voltage to operate. I'll see if I can find a Tektronix schematic that shows how it's done- it was a common trick for scope design back in the '50s and '60s.

edit: Here's a quick conceptual sketch

[tubelab.com]

A normal cathode follower has a very high PSRR to plate voltage variations, meaning that the plate voltage can move around a lot without causing a serious change in the output as long as there is enough voltage for the tube to work. In a normal circuit the plate voltage must be high enough to so that the largest signals may pass unclipped. This means that there is usually a fairly large voltage across the tube. The plate voltage is usually fixed, and the cathode voltage varies with the signal voltage. The parameters of the tube vary slightly with the applied voltage causing small distortions of the signal.

Now suppose we could devise a circuit that moved the plate voltage in step with the signal voltage. Tha K-P voltage in the cathode follower would remain constant over all signal conditions eliminating these small distortions. Most agree that the small distortions created in the small signal cathode followers seen in typical audio designs are far below the distortion created in the output stages. The added complexity needed to "modulate" the plate voltage is probably not worth the effort.

But, what if we are designing a cathode follower output stage? Now we have the opportunity to design an output stage with virtually no distortion. Fixing the K-P voltage offers us another benefit. It is now possible to greatly reduce the power dissipated in the output tube.

How do we do this? The simplest way is to add another follower (tube or mosfet) between the output tube and the power supply and drive it with a shifted (zener diodes) version of the drive signal. It is also possible to design a complex SMPS do do the same job. I have built several versions of these circuits and they do seem to offer some serious improvements in an output stage at the expense of circuit complexity. I plan to post some of my circuits on my web site when I return from an impending trip, and have some time to document them. I am enclosing one of my simulation schematics to illustrate the technique.

This thread has made me aware of work done along these lines back in 1957 by J. Ross Macdonald. His work is centered on low power applications with very high input impedance (many megohms) and low output impedance (5 ohms) with very low distortion, all accomplished with 12AX7's and 5687's. These attributes are accomplished with multiple cathode followers and a feedback loop. I believe that this work can be used to create a very serious cathode follower output stage.

Read the Macdonald paper and decide if the added complexity of this circuit can be of use to you. The "cathode follower" that I am building contains 5 tube sections.

[Tony]

hi,

what about cathode followers with constant current sinks in place of the cathode resistors?

anybody played with them?