Switchable Hi-Z input impedance, how ? - Page 59 - diyAudio
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Old 2nd May 2013, 10:04 AM   #581
AndrewT is online now AndrewT  Scotland
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Tin,
you are still showing the BJT buffer without a DC blocking cap on the input.

Without that necessary cap, anything you attach to the front end will corrupt the BJT biasing you have shown.

Look again at that 2002 pre-amp.
C1, C4, C7, C16, C17, C8, C14, C18, C19, C20 are all DC blocking caps.
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Last edited by AndrewT; 2nd May 2013 at 10:07 AM.
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Old 2nd May 2013, 03:36 PM   #582
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by AndrewT View Post
Tin,
you are still showing the BJT buffer without a DC blocking cap on the input.
yah, I know....but right now Im working on the jfet section only
you may notice I removed output resistors
so the jfet output dc blocking cap would connect directly to the BJT buffer

but that BJT thing is just for fun
output buffer for the final preamp project will different, and have symmetric power supply
unfortunately no time for that right now
but until then I need to stay fit, and on my toes
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Old 5th May 2013, 10:04 AM   #583
tinitus is offline tinitus  Europe
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all wired up, and ready to roll
for the third time
it worked ok yesterday, after a few corrections
even the tone control seemed quite good
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Old 6th May 2013, 01:55 PM   #584
shanx is offline shanx  Canada
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Hi Tinitus, Yup that last layout looks very useable.
I noticed that your cabinet has quite of bit of free space...maybe are you planning to build a complete power amp in there?
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Old 6th May 2013, 07:00 PM   #585
tinitus is offline tinitus  Europe
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hey....it really works
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Old 6th May 2013, 07:07 PM   #586
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by shanx View Post
I noticed that your cabinet has quite of bit of free space...maybe are you planning to build a complete power amp in there?

you just gave me an idea
maybe it would be 'convenient' to have just a small power amp built in
to use just for 'silent practice'
it could also have optional high pass filtered output
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Old 6th May 2013, 09:48 PM   #587
tinitus is offline tinitus  Europe
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next project .... small late night practice 'combo'

from lesson learned here, I would say all that it takes is a single jfet, and a small solid state amp
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Old 8th May 2013, 08:30 AM   #588
tinitus is offline tinitus  Europe
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but since power amp muscle is plenty, and more coming in soon.....

I decided to try with a just hi-z jfet buffer in front of the BJT buffer
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Old 12th June 2013, 03:03 AM   #589
shanx is offline shanx  Canada
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Hi tinnitus,
Any updates for your project? Have not seen any posts for a while. Anyways I hope you keep experimenting and have fun! I am still working on some guitar projects for fet preamps..not quite done yet but getting there.
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Old 12th June 2013, 08:28 PM   #590
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by shanx View Post
Hi tinnitus,
Any updates for your project? Have not seen any posts for a while.
hi shanx
havent changed a thing, and its still rocking

well, one small change
I now use only one channel to drive my 3way speaker

if I continue with 2-channel preamp, the second channel will be for another instrument, like a guitar

Quote:
Originally Posted by tinitus View Post
hey....it really works
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