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Old 19th September 2012, 05:38 PM   #21
Bigun is offline Bigun  Canada
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I'm always on the look out for a current buffer that I can put a triode VAS in front of. Why would I pick this options over a number of alternatives (I'm not trying to be combative, just want to learn why) ?
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Old 19th September 2012, 07:47 PM   #22
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Bigun View Post
Why would I pick this options over a number of alternatives (I'm not trying to be combative, just want to learn why) ?
I certainly wouldn't push any unwilling individual towards this option.
It is undoubtedly an interesting one: it has the ability to produce a result that transcends the crap material it is made of, but the beast isn't easy to tame:
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I've had some problems, one totally unrelated to the topology, a very unusual defect of one of the transistors and the other was a high frequency oscillation (several MHz).
I could trace the origin of this oscillation: it was caused by the wirewound 0.33 ohm emitter resistor.

I made a quick and dirty fix, a 100n ceramic in parallel. Probably not optimal but sufficient to go on with the tests.
This could be incompatible with:
Quote:
If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed.
You have to decide for yourself.....
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Old 19th September 2012, 08:04 PM   #23
CBS240 is offline CBS240  United States
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Quote:
Originally Posted by Bigun View Post
I'm always on the look out for a current buffer that I can put a triode VAS in front of. Why would I pick this options over a number of alternatives (I'm not trying to be combative, just want to learn why) ?
Uh, er... HEC mosfet follower with paralelled output devices and a beefed up bootstrapped driver stage.

I currently am devising something like that using a low gain ECC88 bias at a constant plate current as a balanced VAS, driving 2 HEC follower stages for a balanced output. The EC circuit has enough emitter follower buffers that it has a very high AC input Z, so as not to load the plate.
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Old 20th September 2012, 07:36 AM   #24
Elvee is offline Elvee  Belgium
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J. Broskie also has some nice, proven solutions. They are a safe option, but of course not half as interesting, challenging and effective as an UFO like Unigabuf
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Old 20th September 2012, 04:38 PM   #25
Bigun is offline Bigun  Canada
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but the beast isn't easy to tame
Yeah, it's always difficult not to get attracted to these new ideas, they look like a lot of fun but you are right that I wouldn't necessarily go in this direction if I'm after something relatively foolproof. And at the moment I'm leaning more towards the latter, but I always keep an eye out for the new.

I've not considered HEC. I've not seen it used extensively and am not convinced from what I've read that it offers a real benefit over many other negative feedback topologies. Perhaps it's an idea worthy of renewed interest.
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