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From: shankar@rnd.gba.nyu.edu (Shankar Bhattachary)
Newsgroups: rec.audio.high-end
Subject: Re: Passive Preamp
Summary: bypassing high-level stages; impedance of pots a possible problem
Message-ID: <3005@uwm.edu>
Date: 21 Mar 90 19:03:51 GMT
References: <9003170016.AA25031@psyche.mit.edu>
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In article <9003170016.AA25031@psyche.mit.edu> jacob@psyche.mit.edu (Jacob Feldman) writes:

>I am also very curious about the by-passing of the gain stage on high
>level sources. 
    [............] (stuff on just having the pots in the path)
>both the PS Audio bypass and an Adcom passive "preamp" I once heard have
>a flat, lifeless quality as well as a very peculiar, unnatural-sounding
>nasal timbre.

Provided you don't need the gain of a high-level stage, I can see only one
problem with bypassing the stage altogether. Driving a power amp directly
off the pots presents the system with a high-impedance source in the
preamp to power amp connection. It becomes important to ensure that cables
are short, or have low overall capacitance, etc.

I don't believe that cable interactions are important for solid state
preamps, apart from obvious issues like avoiding noise pickup, and
keeping connections clean and such. The output impedance of solid state
preamps tends to be negligible.

However, once you drive the power amp directly off the pots, that is no
longer true. The high-level stage of the preamp may offer some buffering,
which may be necessary for long runs of cable, or for higher capacitance
cables. Other than that, any difference between the no-high-level-gain
situation and the high-level-gain-in situation must be alteration of the
signal by the high-level stage.

Any other thoughts out there? There must be.

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Shankar Bhattacharyya, Information Systems, New York University
shankar@rnd.gba.nyu.edu
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