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Old 22nd August 2011, 12:28 PM   #1
diyAudio Member
Join Date: Aug 2006
Location: Copenhagen, Frederiksberg
Default Need amp with 200 kOhm input impedance

Hi ,

Can any of you tell me if there is a solid state amp with more than 200 kOhm input imp. or if there is a design where this parameter can easily be changed?

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Old 22nd August 2011, 01:28 PM   #2
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Join Date: May 2001
Location: Bangalore, India
You can always put a JFET/FET buffer in front of the amp and tune it to whatever input impedance you want. The buffer can be discrete or opamp based.
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Old 23rd August 2011, 08:16 PM   #3
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Join Date: Apr 2011
Location: Pretoria
The easiest way to do this is to use a FET input op-amp in non-inverting mode. For a really high input resistance look at something like the ISL28110/ISL28210. The datasheet give it at about 550 giga-ohm! Slew rate is quite respectable at 23 V/us. You add an extra resistor in parallel with the input with the value you want it your input resistance to be.
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