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Old 9th April 2004, 08:52 PM   #1
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Default suggest circuit with gain of 60

I need to design an input stage with a total gain of 60. the stage will have a 2:1 output IT so overall gain is 30.

very low output impedance of stage.

The only thing is that I NEED to use tubes that are made today. i.e. 6SN7, ECC99 etc....

Maybe 2 stage DC coupled...

ideas???
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Old 9th April 2004, 09:00 PM   #2
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What do you concern "very low output impedance of stage." ?

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Old 9th April 2004, 10:52 PM   #3
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Hi,

What is the input impedance of the IT you plan to use?

_Same question as Pjotr, actually_

Gain of 60 is easy enough.

Cheers,
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Old 10th April 2004, 09:23 PM   #4
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Old 10th April 2004, 09:50 PM   #5
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A 12AT7 section loaded by a Gary Pimm CCS will give you a "raw" gain of 60. The CCS' mu follower O/P has an impedance well under 1 KOhm. With the step down trafo, the net O/P impedance should be < 200 Ohms.
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Old 10th April 2004, 10:04 PM   #6
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Hi Eli,

Welcome to diyAudio.com!!! Good to see you around here as well! I never did welcome you back over at AA. Even though you had been gone for quite a while.. Glad you're back.

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Old 11th April 2004, 01:14 AM   #7
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Default input stage of mosfet.

mosfet with over 1000pF of input cap.

so thinking like a 12B4 or similar with a 2:1 step down as an IT.

input impedance to the MOSFET less than 500 ohms is good.

Need at least 30v swing on the MOSFET though with a 1v input to tube stage.

kind of want to keep to tubes in current production.
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Old 11th April 2004, 02:16 AM   #8
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The 12AT7 IS in production. The drive capability of a Gary Pimm CCS' mu follower O/P is EXCELLENT. The GP CCS uses FETs.

If you buy 'T7s with tightly matched sections, you can run them in parallel. Parallel sections lower the mu follower O/P impedance in 2 ways. The combined Rp of the 2 parallel sections is 1/2 that of a single section and the doubled current in the CCS lowers the mu follower's O/P impedance too.

I think that a GP CCS will drive that NASTY gate capacitance well.
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Old 11th April 2004, 10:49 AM   #9
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30V into a 1000 pF mosfet input capacitance

Are you building a switching amp?

BTW how do you connect an IT to a Mu follower? You need at least a coupling cap then. So what is the advantage of an IT then?

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Old 11th April 2004, 11:41 AM   #10
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EC/PC86

I don´t think they are in current production, but they´re cheap and easy to find.
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