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Old 30th September 2003, 11:01 AM   #11
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Join Date: May 2002
Location: wheeling illinois
quote
Kevin: Can't gain be increased by lowering the feedback, or would it be too low if reduced from what it currently is?
So there's no way to set up a higher input impedance in this configuration?

Now you are actually starting to think about what you
are trying to do.

Gain is only increased in one of two ways.
For your input circuit either

1) lower the input resistor from 1k to 200 ohms, bad for all sorts
of reasons unless you really have a preamp that likes
this sort of thing (i build preamps that drive 50 ohms)
The 10k to ground resistor may in fact not be low enough
to prevent an open input failure of the fet when power
is applied.

2) increase the feedback resistor to 1 meg. Even worse.
A 1 meg resistor of a voltage rating necessary for this has
significant built in capacitance which changes the frequency
response of the amplifier in significant and audible ways.

This is a high voltage circuit. Many techniques that work
on low voltage power amps will not apply here.
Really, no kidding.
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Old 30th September 2003, 09:12 PM   #12
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Join Date: Jan 2001
Location: Scandinavia
If you decide to do what Kevin suggested you don't (1M feedback resistor), consider putting several units in series to reduce capacitance (but this will increase inductance) or get into som high end high voltage resistors such as thos available from Caddock.

Petter
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Old 30th September 2003, 09:27 PM   #13
Prune is offline Prune  Canada
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Location: Vancouver
Which of course begs the question what capacitance is acceptable anyway. And if I were to find appropriately low capacitance 1M resistors, what would be proper values for the input resistors? Is it possible to set up a configuration that would still allow the use of an attenuator? The lowest from Goldpoint are 10K for the ladder type (though they have kits so one can choose resistors).

And once again, I'm still hoping one of the people here familiar with supersymmetry will help me out with my original question. Depending on the answer I may be wasting my time changing the original schematic (at least Keving seems to think so).
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Old 29th January 2005, 01:21 PM   #14
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Petter, did you ever build this SuSY STAX amp?
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