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Amp up FAOW #2
Amp up FAOW #2
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Old 16th May 2019, 02:44 AM   #1
Jenghis is offline Jenghis
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Default Amp up FAOW #2

Hi everyone,

I've been looking at single ended solid state designs, and Michael Rothacher's FAOW Variation #2 caught my attention: http://www.firstwatt.com/pdf/art_faow_v2.pdf

I think it's a beautifully simple design and I like the idea of a transformer coupled output to give it a more tube like sound.

Unfortunately, even with my small listening space and modest volume levels, 1 watt won't be enough. Could the bias current be raised to give more clean output power? Assuming of course the power supply can handle it. I've looked at the data sheet for the IXTH6N50D2, and it looks to me like 70 ma bias current is pretty conservative.

I assume higher bias current would also demand a bigger output transformer, like Edcor's CXSE25-3K.

Am I thinking on the right track or is more power too much to ask for this circuit?
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Old 16th May 2019, 05:31 AM   #2
juanitox is offline juanitox  France
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you will get surely more power this transformer at double the current bias , a bypass capacitor on P1 will help with the gain and for no feedbach too .
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Old 16th May 2019, 05:41 AM   #3
Zen Mod is offline Zen Mod  Serbia
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Amp up FAOW #2
build ACA with lower Iq and that's it
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Old 16th May 2019, 05:47 AM   #4
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Quote:
Originally Posted by juanitox View Post
you will get surely more power this transformer at double the current bias , a bypass capacitor on P1 will help with the gain and for no feedbach too .
Thanks juanitox, I hadn't thought of a cathode bypass (drain bypass?) capacitor.

Pushing the bias up to 140 ma would certainly increase output power, but I'm concerned that the MOSFET's distortion may increase too much with it.
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Old 16th May 2019, 05:48 AM   #5
juanitox is offline juanitox  France
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Quote:
Originally Posted by Zen Mod View Post
build ACA with lower Iq and that's it
but then it will sound less "tubey" my ACA doesn't sound like my single ended VFet or even my Aleph mini .
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Old 16th May 2019, 05:49 AM   #6
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Originally Posted by juanitox View Post
but then it will sound less "tubey" my ACA doesn't sound like my single ended VFet or even my Aleph mini .
My thoughts exactly. The appeal for me is the output transformer.
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Old 16th May 2019, 06:00 AM   #7
Zen Mod is offline Zen Mod  Serbia
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Amp up FAOW #2
in that case don't do black art ...... define output power you need , then calculate Iq , choose output xformer by that

if you're not able to do later , at least write here first condition
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Old 16th May 2019, 11:36 PM   #8
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Originally Posted by Zen Mod View Post
in that case don't do black art ...... define output power you need , then calculate Iq , choose output xformer by that

if you're not able to do later , at least write here first condition
Right, it might be easier to start from scratch rather than trying to modify this design.
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Old 17th May 2019, 05:46 AM   #9
juanitox is offline juanitox  France
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this design works with all kind of device as it is like an autobias SET. i have build one with a sony VFET and it sounded very smooth.

just put a Fuse on top of the output transformer to avoid any damage with accidental bad biasing
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Old 17th May 2019, 02:33 PM   #10
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Amp up FAOW #2
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Right, it might be easier to start from scratch rather than trying to modify this design.
it's rather easy doing whatever (scaling up) , but first one need to define goals ...
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