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Old 5th February 2013, 09:08 PM   #361
tessier is offline tessier  Canada
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Originally Posted by juma View Post
I'd say great. Have a look here:
BF862 Preamp
Hi

Your bf862 preamp are interesting, same topology than you B1 with gain, but I can't find any bf862 here arround. So I will stick to my 2SK170.


Thanx

Paul

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Old 15th February 2013, 03:01 PM   #362
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I am short BC550 on this, Juma. Got ZtX parts and other fancy pants parts like TOshibas and Sanken bjt's. Only question is about Rs for ztx. It doesnt have same gain as BC, from what I see, but most is being used for feedback, so it should work ok, correct.
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Old 15th February 2013, 03:20 PM   #363
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Found some LT SPice models, so will play around a little, but if you know good operating points or suggestions, I am all ears. Already have the boards etched for the BC550, but not big deal to switch.
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Old 15th February 2013, 11:05 PM   #364
juma is offline juma  Germany
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I am short BC550 on this
You obviously know what "this" means, but I don't - this thread is full of different circuits. Please give exact schematic or post number, what parts you got exactly and so on... I'm still not clairvoyant
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Old 15th February 2013, 11:20 PM   #365
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I'm still not clairvoyant
You are getting closer with every post. The circuit is in post 53. Comp Jfet feeding comp Bjt's. I don't have any bc550, so I substituted Ksa1381 and Ksc3503. Found numerous schematics with similar degeneration values for them so ill stick with what you posted. Should be done tonight. You know what I'm thinking
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Old 16th February 2013, 10:34 AM   #366
juma is offline juma  Germany
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The value of R6/R7 can be lower (68R or so) because with 1381/3503 you can afford higher current through output BJTs. Ib will be higher too but it won't have much influence on input JFETs (they should run at Id of 5-7mA so loading them with about 100uA more is not a big deal).
Still, you got to have in mind that this preamp is made to drive light loads (F5) and the Zout is relatively high (about 1k or somewhat less).
All in all you'd be OK - work that pre and I go to practice my third eye, the original two are less and less useful each day...
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Old 16th February 2013, 11:22 AM   #367
tinitus is offline tinitus  Europe
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... circuit is in post 53.
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...
post 53 .... just click on the enhanced marked arrow
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Old 16th February 2013, 12:27 PM   #368
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Ill give it a listen how I have it now and reduce Rs if I like what I hear. It is driving the F5TV2 that will go to my brother, so the low Zo should not be a problem. I had to parallel resistors to get values needed.
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Old 16th February 2013, 03:47 PM   #369
juma is offline juma  Germany
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You might have issues with DC offset because JFETs are not thermally coupled but a coupling cap at the output cures that - it's not for your use anyway...
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Old 16th February 2013, 03:52 PM   #370
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I Have the pot across the Jfet Rs for trimming DC. What would cause the instablity, otherwise? Is it the topology? I suck at all things audio, but bjt's make no sense to me.
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