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From: sjc@angband.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: Adding a buffer amp
Message-ID: <38@angband.UUCP>
Date: Fri, 11-Jan-85 13:34:08 EST
Article-I.D.: angband.38
Posted: Fri Jan 11 13:34:08 1985
Date-Received: Sun, 13-Jan-85 08:11:12 EST
Distribution: net
Organization: S-1 Project, LLNL
Lines: 29

If 10k ohm output impedance is typical, I guess the audiophile
agitation about cables may have a technical foundation. A few years
back, IHF added to its preamp-testing standard a test-load designed to
weed out products with high output impedances; apparently many
manufacturers are ignoring it.

At any rate, one article about putting a buffer between a preamp output
and your cables appears in issue 4 of the 1977 Audio Amateur magazine.
Unfortunately, you must buy the entire year of 4 back issues for $10
(Box 176, Peterborough, NH 03458, USA). The same people, wearing a
different hat, also sell for $17.50 a kit KH-8 called the "Morrey Super
Buffer", including parts and board, but not the instructions, which
appear only in the original magazine article (Old Colony Kits, Box 243,
Peterborough, NH 03458; VISA, MC 603 924-6526).

That's kind of expensive for such a simple circuit, particularly if you
want to make several copies of it; an alternative would be to get the
National Semiconductor Audio/Radio Handbook (About $6 from National
Semiconductor, 2900 Semiconductor Drive, Santa Clara, CA 95051 (408)
737-5000), and hunt up your own parts. In addition to discussing RIAA
preamps, tone controls, graphic equalizers, mixers, etc. this book has
one appendix on power supply design and another appendix with basic
op-amp configurations and the equations pertaining to them. Section
2.19 of the 1980 edition talks about driving low-impedance lines, but
it's wild overkill; a simple non-inverting buffer (circuit A4.4 on page
6-12 of my edition) ought to suffice. Enjoy.
-- 
                                                           --Steve Correll
sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc