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Old 24th September 2018, 09:37 AM   #211
juma is offline juma  Germany
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Quote:
Originally Posted by augustoriodosul View Post
... possible to build a simple Cubie(Cubiezilla?) with a jfet pair input (k170-K366 / J74-J108), VAS bc550/560 and output Lfet K58/J62 power between 15 and 20WRMS in 8 ohms, class AB...
Sure it is possible.

What is the Idss of your JFETs at Vds=20V ?
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Old 24th September 2018, 11:55 AM   #212
augustoriodosul is offline augustoriodosul  Brazil
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Quote:
Originally Posted by juma View Post
Sure it is possible.

What is the Idss of your JFETs at Vds=20V ?
Hi Mr. Juma and all: sorry, I'm newbie to audio and electronics. So, only I like this a lot. My request for help is because I have tracked several threads about VSSA and it seems to be positive opinion that the input is JFET (detailing and low noise), the BC550 / 560 due to low noise and the LFET output because they were designed to play audio, very close to the sound of tube. So I figured that 20 WRMS / 8Ohm is good power for multiple Xover active systems and special amplifier for each speaker. I also want to try changing this amp to transconductance (current). It looks a lot like tube sound, of course, much cheaper to build. Can you help anyway, please?
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Old 24th September 2018, 03:30 PM   #213
juma is offline juma  Germany
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Upping the output power of Cubie2 to 20W@8R is not a big deal - just a few values need changing. That is why I asked you about Idss of your JFETs.


The rest of your wishes/ideas/assumptions show lack of experience and I think you should go at it slowly, step by step, until you really know what you want.


Maybe you should start with something more popular, something that many others have already built....
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Old 24th September 2018, 07:01 PM   #214
augustoriodosul is offline augustoriodosul  Brazil
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Dear Juma: I appreciate your help. I leave transconductance matter aside. I think I mixed two different subjects. But I insist: Could a new Cubie simpler, modeled with only 6 semiconductors: two jfets input, pair BC550/560 VAS and an output with K1058 and J62, in class AB, powered with 25VDC close to 20W @ 8ohms? Thank you again.
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Old 24th September 2018, 10:10 PM   #215
juma is offline juma  Germany
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It is possible (6 transistors only) but it won't be Cubie2 any more and I don't want to do it. Of course, you are free to follow your wish, but please use some other name for that project.
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Old 24th September 2018, 10:36 PM   #216
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Quote:
Originally Posted by augustoriodosul View Post
Dear Juma: I appreciate your help. I leave transconductance matter aside. I think I mixed two different subjects. But I insist: Could a new Cubie simpler, modeled with only 6 semiconductors: two jfets input, pair BC550/560 VAS and an output with K1058 and J62, in class AB, powered with 25VDC close to 20W @ 8ohms? Thank you again.
Maybe beta22 by amb.org is a bit like what you are describing...?
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Old 24th September 2018, 10:52 PM   #217
augustoriodosul is offline augustoriodosul  Brazil
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Sorry, I do not know how to design amplifiers, I can only build from schematic of the designers. Mr Juma, once again, thank you for your guidance and your patience in trying to help me. Dear Mr. pidesd, thank you for your suggestion. I'll check.
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Old 25th September 2018, 09:20 AM   #218
juma is offline juma  Germany
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Nice 6-transistor variation of this idea can be done with Curl-ish symmetrical folded cascode as transconductance gain stage (with variable OLG) plus source-follower output stage, all nicely wrapped in controlling loop.

It can be done with common-source output stage too, with one or two sets of power supply rails, with or without feedback loop (local or global, low or high Z), etc. - it can be varied in many ways so no values are shown, just a somewhat simplified diagram.
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Last edited by juma; 25th September 2018 at 09:23 AM.
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Old 25th September 2018, 09:28 AM   #219
kasey197 is offline kasey197
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just for fun - how would this look like with common drain OPS ? Just move the earth ?
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Old 25th September 2018, 10:54 AM   #220
juma is offline juma  Germany
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Source follower is common drain connection - exactly as shown.
You probably mean common source OPS, where output is taken from the drains, to provide voltage and current gain and you also probably mean Transnova-like OPS. There are few other ways to do it - a matter of preference...
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