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Old 1st September 2013, 08:21 AM   #1
milosus is offline milosus  Serbia
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Default preamplifier with huge amplification

Hello
I need litle help, i am looking for schematic of good driver stage for cathode follower output stage, driver stage must have amplification of 250,300 times..i have tube 6sl7 it will be good some project with that tube but it is not so important

Thank you, best regards!
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Old 1st September 2013, 08:26 AM   #2
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Have a look at the Impasse Pre-amplifier thread.

Although its quite long the schematic appears towards the end of the thread.

ImPasse Preamplifier
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Old 1st September 2013, 10:55 AM   #3
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Quote:
Originally Posted by milosus View Post
Hello
I need litle help, i am looking for schematic of good driver stage for cathode follower output stage, driver stage must have amplification of 250,300 times..i have tube 6sl7 it will be good some project with that tube but it is not so important

Thank you, best regards!
More than 106dB?? Open loop or closed loop?

jan
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Old 1st September 2013, 11:27 AM   #4
SY is offline SY  United States
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Quote:
Originally Posted by milosus View Post
Hello
I need litle help, i am looking for schematic of good driver stage for cathode follower output stage, driver stage must have amplification of 250,300 times..i have tube 6sl7 it will be good some project with that tube but it is not so important
CF output stages are problematic, not just because of the huge swings required (many hundreds of volts), but handling the heater-cathode interface. They can be done, but in practice the tradeoffs (notably higher driver stage distortion) make the exercise futile. You need not just gain, but large signal capability. This means lots of high voltages and driver tubes that can take it.

There's an excellent walk-though of CF amplifier design in Crowhurst's "Understanding Hifi Circuits." He ends up showing that you're best off bootstrapping the driver, which adds distortion that the CF output stage originally reduced. No free lunch.
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Old 1st September 2013, 02:44 PM   #5
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by jan.didden
More than 106dB?? Open loop or closed loop?
300x is 20log300=~50dB !
106dB gives 10^(106/20) ,is over 100k X.
Mona
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Old 1st September 2013, 05:04 PM   #6
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There's an excellent walk-though of CF amplifier design in Crowhurst's "Understanding Hifi Circuits." He ends up showing that you're best off bootstrapping the driver, which adds distortion that the CF output stage originally reduced. No free lunch.
McIntosh used bootstrapping (positive feedback) in their unity coupled O/P stages, for exactly the reason SY stated.

A COSTLY method that might work is to use step up transformer coupling at the O/P stage's I/P. If nothing else, cathode followers present a very light load to the driving circuitry. TANSTAAFL rules again. In addition to the expensive interstage "iron", absolutely top notch O/P transformers are needed too. $$$ Don't even think about an interstage trafo inside a NFB loop! That means the O/P trafo get's no help from NFB.
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Old 1st September 2013, 06:10 PM   #7
milosus is offline milosus  Serbia
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yep 49dB, i know that is complicate but i want to do thatand it must be some way
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Old 1st September 2013, 06:26 PM   #8
milosus is offline milosus  Serbia
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http://www.tubecad.com/2005/July/07/Bootstrapping.png

do you mean this can be work? just one more stage before
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Old 1st September 2013, 06:41 PM   #9
Ketje is offline Ketje  Belgium
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Originally Posted by milosus View Post
http://www.tubecad.com/2005/July/07/Bootstrapping.png

do you mean this can be work? just one more stage before
But that's no PREamp !
To better suggest a possible solution some questions.
What are the max.signal levels
Witch supply voltages do you have
Do you have a tube in mind for the cathode-follower
Or maybe a emitter follower
Any frequency correction
Input impedance
Mona
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Old 1st September 2013, 07:44 PM   #10
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Could you tie the cathode to the heater on a CF driver tube with it's own heater transformer? Provided it could handle the signal voltage? -Fred
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