sjc@angband.UUCP (Steve Correll) (01/11/85)
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!sjcbill@crystal.UUCP (01/12/85)
> > 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). > The Audio Amateur (TAA) is an interesting mag for anyone interested in how to put audio equipment together (with or without taking it apart first -- they do a lot of rebuild articles). I highly recommend it. The Morrey super buffer uses a 531 op amp, I believe. TAA ran Walt Jung's articles on op-amp selection for critical audio uses; he thinks the 531 is a stinker. It also requires an output transistor. I would suggest building a simple non-inverting buffer, with a voltage gain of 3, using an NE5534 (single) or NE5532(dual) op amp -- it directly drives 600 ohm lines, and is very quiet. (Signetics or TI; the 553[24]A version is a low-noise selected version, but the non-A ones are usually about as good) Jameco or DigiKey sells the TI version, I believe. Runs $3 - $5, depending on the usual things. I have several friends who use these for pro-audio stuff, and swear by them (and the '071, below). Walt Jung's Audio IC Op Amp book (Sams) has lots of circuits, but apparently not all of them are very good. Another choice would be the TI bifet op amps, the TL071 family (low-noise), available for around 75cents from Action Electronics (see ads in Radio-Electronics). Need an output transistor, however. -- William Cox Computer Sciences Department University of Wisconsin, Madison WI bill@uwisc ...{ihnp4,seismo,allegra}!uwvax!bill
rcd@opus.UUCP (Dick Dunn) (01/16/85)
> 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. We don't seem to have a predecessor to this article. I'm confused--I don't recall ever seeing audio equipment with a 10 K OUTPUT impedance. Input, yes--fairly common. Did someone get the two confused somewhere along the line? -- Dick Dunn {hao,ucbvax,allegra}!nbires!rcd (303)444-5710 x3086 ...A friend of the devil is a friend of mine.