2A3 SE parafeed with PP-transformer - diyAudio
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Old 8th October 2008, 06:18 AM   #1
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Default 2A3 SE parafeed with PP-transformer

This subject has been up in another thread where it drowned in the very interesting PP anti-triode discussion. This one is purely SE in contrary to what I first thought. As L4 a Hammond 150H/8mA can be used.

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To be able to use a PP-transformer the CT has to be split as the windings are connected with their plus-poles against each other.
This way the currents are equal and opposed to each other so no(toroid) or a very small gap(PP) is needed. R12 should be adjustable as it sets Iq.

For some reason linearity is better with the input connected to Q2s collector. Maybe the CCS works as an anti-triode?

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Old 8th October 2008, 07:07 AM   #2
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About the transformers the PP should be between 10-12kohm. The toroid should have an unloaded secondary of between 5,9-6,5V.

Got second thoughts on version 2. If the parafeed cap is connected to ground instead the amp might work as an anti-triode. What do you say kenpeter ?

Did some quick sims and clipping occurs at the same time but distortion is halved at 1W/8ohm! At 3% the #1 gives 2,1W and #2, 2,9W. It did not work very well with the 6B4G cathode decoupled to ground and the parafeed cap to ground.
But as the results are simmed they are not completely reliable, but they point in some anti-triode direction.

And it is notNFB as the signal out is higher from#2.
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Old 8th October 2008, 10:39 AM   #3
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Was a little to enthusiastic about #2. The sand input is in the region of 500ohm so it will not be a practical solution.
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Old 8th October 2008, 12:29 PM   #4
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Quote:
Was a little to enthusiastic about #2. The sand input is in the region of 500ohm so it will not be a practical solution.
2 silly questions:

If you want to modulate your current source why do you not apply an "Aleph" current source a la Nelson Pass? Btw. , there are transistors with higher beta that you can use instead of BD140.

Secondly, from a theoretical standpoint, if your intention is to cancel the DC bias current in the output transformer, would you not try to bypass the upper half of the primaries with a cap, to remove AC current?

But maybe I am misinterpreting your intentions.
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Old 8th October 2008, 12:58 PM   #5
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Hello Martin,

My only intention is to use a PP(or toroid) as SE OPT as subject shows.

1 Have no clue about it. Please show a schematic. But I think I will skip #1 anyway.

2 There is only problem, it will not work with a bypass cap . But it was a good idea!

As this is an el Cheapo/Simplo SE, the CCS will be made even simpler with only the MOSFET, 3 LEDS and 3 resistors. Like this:


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Old 8th October 2008, 03:46 PM   #6
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Hello Lars,

Re. Aleph current source: All explained in http://www.passdiy.com/articles.htm - Zen Variations 2: The Penultimate Zen's Current Source by Nelson Pass - well worth reading, but then again I am not sure if modulating the current source is a good idea in conjunction with the transformer.

Quote:
2 There is only problem, it will not work with a bypass cap . But it was a good idea!
I do not readily understand why it would not work other than you would off course create a parallel resonant circuit? And yes it would be nice to keep the current constant.
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Old 8th October 2008, 04:01 PM   #7
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Hello again Martin!
I use LTSpice to test all my more or less smart ideas. It is a handy tool that saves a lot of time. Also there is no smoke when you fail !
I tried your suggestion before saying it will not work. The reason it will not is that you short a winding when bypassing it with a cap.
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Old 8th October 2008, 04:31 PM   #8
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Quote:
Originally posted by revintage
Got second thoughts on version 2. If the parafeed cap is connected to ground instead the amp might work as an anti-triode. What do you say kenpeter ?
SE Sand opposing SE Triode in DC balance is not anti-triode.
Where does it sample the Triode to mime an anti-behavior?
Applying arbitrary modulation by some other computation,
even a very linear-izing one, not quite the same...

Yours is a fine circuit, maybe even a great one. I'm only
nitpicking over a topology definition, I don't think it fits.
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Old 8th October 2008, 05:28 PM   #9
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Came to the same conclusion with a little help on the way by Martin. The modulated current source makes things easier for the triode and thereby lowers distortion. Unfortunately there seems to be both stability and HF-problems so I skip it for now.
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Old 8th October 2008, 11:24 PM   #10
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Yes, ZEN's "Aleph" current sink appears to be an anti-FETron.
If somewhat overcomplicated for one thats supposed to be
the heart of simplicity... I still prefer MJK's variant.

If Nelson had only used a Triode in the lower half, instead of
a FETron emulating a Triode, well you can guess the rest...
Another victim of the "Glass Curtain".

I very much doubt he realizes the entire truth of what he's
actually done by externally reconnecting "Mu" in a MOSFET.
Last I exchanged words with Pass, he still believed a Triode
emulator was a cascode of two voltage starved JFETs.

Brilliant, but at the same time, go figure...
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