Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: osc!jgk@ames.arc.nasa.gov (Joe Keane)
Newsgroups: rec.audio.high-end
Subject: Re: Low-Z Input Amp Question
Message-ID: <6043@uwm.edu>
Date: 31 Aug 90 12:57:05 GMT
Sender: news@uwm.edu
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Approved: tjk@csd4.csd.uwm.edu

Getting low-noise performance with such a low impedance is tricky.  A good
low-noise op-amp has an input voltage noise density of around 3 nV/Hz^.5.
With a very-low-noise matched transistor pair you can cut that in half.  This
is still about an order of magnitude above the thermal noise of a 2 ohm
resistance.  Plus your feedback resistors generate a lot of noise unless they
have very low resistance, which causes other problems.  So you can see why
people want to use a transformer.