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Old 21st December 2007, 02:35 AM   #1
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Default augmented cathode follower?

Years ago, Glass Audio ran an article that mentioned a series of complex cathode followers called augmented cathode followers. Kind of an instrumentation amplifier, perhaps, very high performance in certain parameters.

They were pretty interesting but relatively more complicated - maybe 3-4 tubes at least two power supplies etc, but impressive performance on paper.

Has anyone here ever built/listened to one for audio?

Thanks

Murray
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Old 21st December 2007, 02:53 AM   #2
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Yes, I've played with them. I think that with a properly designed cathode follower, these are generally not necessary. Other may (and do, notably Allen Wright) disagree. The idea is to fix the cathode-plate voltage so that the grid capacitance isn't modulated.

A properly designed and appropriately used cathode follower will give an impressive performance on the test bench. And listening backs that up, at least in my living room.
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Old 21st December 2007, 03:45 AM   #3
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I have a ccs loaded 6922 cathode follower in front of my gainclone .
It certainly doesn't degrade the signal in any way compared to a signal fed directly to the gainclone. On the other hand it improves the interface to the source and some sounds are even better than a direct connection.
I can't imagine how it could be improved upon audibly by making it more complex. However it could possibly make an improvement on measurements.
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Old 21st December 2007, 01:39 PM   #4
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Quote:
Years ago, Glass Audio ran an article that mentioned a series of complex cathode followers called augmented cathode followers.
Does anyone know what issue this article was in? I have been through my stack of old Glass Audio magazines and can't find it. I don't have every issue though. I would like to read this article.

I have been experimenting with using cathode followers as output stages. Fixing the plate to cathode voltage seems to remove most of the distortion components created in the tube. I believe that it has to do with removing the voltage related parameter shifts inherent in the tubes, but I am still experimenting.

I have built some pretty complex cathode follower designs including one using DSP and SMPS technology. It recently won a prize in a Circuit Cellar design contest, not bad for a tube amp in a high tech embedded systems magazine!

Why would you want to do this? How about 20+ watts at 1% distortion from a single ended triode in class A with a plate efficiency near 50 %. The signal path is all tube. I am still tweaking the design, but it will be on my web site early next year.
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Old 21st December 2007, 10:31 PM   #5
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TubeLab - if I find either the GA mag or the photocopy of the Journal referenced I'll post it. I honestly cannot find 75% of what I look for on any given day due to the accumulation of priceless krap that has just been doubly densified (another made up verb?) to give up half the basement for a family room.

I am on a quest to procure walking room, and by unloading some things I'll be able to dig down to another layer of a previous civilization. I know what color the envelope is (manila!) and the postmark/sender's address. I just have to find the box it's in.


Other Questioner, regarding advantages of the ACF variations, I think I recall 6-7 figure bandwidth, much much lower output impedance (I don't know if it got into op amp territory), and distortion on the order of 20 ppm (apparently conserving zeroes...on the fly, is that 0.002%?)

Happy Holidays all,

Murray
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Old 21st December 2007, 10:44 PM   #6
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Folks,

I have the original IEEE article on "Some augmented cathode follower circuits" by MacDonald published in 1957. Email me for a copy.

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Old 21st December 2007, 11:26 PM   #7
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Yeah, MacDonald that was it!

What do you organized people DO with all your spare time ?

I DID find my 5867's, or was it 5687 (I have both, but can tell them apart :O) )

Thanks for coming to the rescue.

Happy Holidays

Murray
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Old 21st December 2007, 11:48 PM   #8
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multi-volti:

Don't panic looking for anything. I got the IEEE journal publication from Jose. The described technique of the ACF-2 circuit is somewhat similar to what I am doing. I have built a circuit with 6336's driving a 600 ohm OPT. In one itteration I am using stacked triodes like Macdonald shows. In another I replace the top tube (V4) with a mosfet, and in yet another version I replace V4 with a DSP controlled agile SMPS. This allows V3 to operate with a constant 50 to 100 volts across it, and the SMPS (replacing V4) dissipates very little power. The total efficiency is in the 50% range (20 watts dissipated for 20 watts output), and distortion is under 1%. It just has this pesky problem of blowing up occasionally, nothing serious.

I am still in the experimenting stages, but I have built, measured, and listened to some unique amplifiers that exhibited some very impressive measurements for SE tube gear and sounded good too. I plan to post it all in the coming months on my web site.

I thought that I had found something new, but it has been said that all of the possible vacuum tube circuits have been invented already. Maybe this is true, but all of the possible embodiements and applications have not been developed yet.

I too have magazines, papers, and even circuits that I built dating back to the late 1960's. Finding things has become more and more difficult as the years and the "stuff" pile up. I know that I have some more Glass Audio issues in another box somewhere, I just don't have the time to dig for them right now.
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Old 21st December 2007, 11:51 PM   #9
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Quote:
What do you organized people DO with all your spare time
We only have the appearance of organization.

Quote:
I DID find my 5867's, or was it 5687 (I have both, but can tell them apart :O)
Isn't one of those a big RF triode similar to a 3-500Z? I have some of them around here somewhere, but no suitable OPT's.
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Old 22nd December 2007, 12:03 AM   #10
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Quote:
Originally posted by multi-volti
What do you organized people DO with all your spare time ?
You should see my office, my workbench, and my 3-car garage...

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