O.H. Schade(1938) meets MOSFET - diyAudio
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Old 14th May 2009, 08:37 AM   #1
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Default O.H. Schade(1938) meets MOSFET

Lately there has been quite a few threads covering the work of OH Schade 1938, http://www.clarisonus.com/Archives/T...er%20Tubes.pdf . He showed how a tetrode could get triode characteristics with the help of what he called "inverse-voltage feedback". While working as a triode the efficiency of the tetrode was still kept.

On the left is a tube SE in accordance to his suggestion. It has the same output impedance and distortion characteristics as if it had been triode-strapped but twice the output power! On the right the output device is substituted by a common MOSFET that according to Schades suggestion is feedbacked ca 20dB.

Seeing any similarity with a Zen?

By the use of an IT there is a better chance of getting much better HF-bandwith. The example sims with a -3dB point in the region of 100kHz.
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Old 14th May 2009, 08:41 AM   #2
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A MOSFET has pentode-like characteristics.

With shunt-feedback(inverse-voltage feedback) applied they change to triode-like.

But this old news for the most of you !
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Old 14th May 2009, 03:19 PM   #3
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Very Excellent.

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Old 15th May 2009, 10:51 AM   #4
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Default Letīs Schade the 2SK170

So what happens if we adapt what Schade taught us to a JFET as buffer?

Works like wonders !

Zout below 200ohms, together with low THD and acceptable bandwith (-3dB at well over 200kHz). Zin chosen to 10kohm. With 22kohm/91k bandwith is 100kHz.

But donīt try to a volume-pot at the input! Must be fed from a lowZ source.

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Old 15th May 2009, 11:06 AM   #5
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Very interesting buffer concept, but off course it inverts phase, so where you need this ... and it also looks pretty similar to the buffer in the alephono MC stage
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Old 15th May 2009, 11:18 AM   #6
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Quote:
but off course it inverts phase
Is this a problem? If so, just shift plus and minus on your speaker terminals .
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Old 15th May 2009, 12:21 PM   #7
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Not a problem at all (as long as you keep track of the number of times you change phase in your system and do not switch to and fro inverting / non-inverting stages), and it should be useful where you have a requirement to revert phase ....
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Old 15th May 2009, 06:49 PM   #8
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Default Re: Letīs Schade the 2SK170

Quote:
Originally posted by revintage
Zout below 200ohms, together with low THD and acceptable bandwith (-3dB at well over 200kHz). Zin chosen to 10kohm. With 22kohm/91k bandwith is 100kHz.

But donīt try to a volume-pot at the input! Must be fed from a lowZ source.
You don't need R10. This circuit works to provide a virtual ground at the
Gate of the JFET, so the input Z is 10 Kohm as shown. Actually you can drive
it with a range of source impedances, and adding a pot as high as 25K should
work fine.

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Old 16th May 2009, 12:02 AM   #9
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HighZ lowZ, don't matter. As long as its stable with frequency.
It just becomes part of the feedback divider...

There's a nice photo of a curve trace for a similar circuit:
http://www.radiomuseum.org/forum/the...ate_triode.htm

The only difference, Trioderizer gate is battery biased.
A battery could last a long time, if its only biasing gate...
I imagine you would want to bypass with a good cap???

something weird with that link. You have to find it the hard way...

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Old 16th May 2009, 03:40 AM   #10
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Very interesting!

Wachara C.
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