Aunt Corey's Homemade Buffered Passive Preamplifier Page 3

I have only one itchin' desire: let me stand next to your wire
More than any other part of the preamp, here's where I say, "Knock yo' bad se'f out," because nobody agrees on wire! Whether 'tis nobler to use solid-core or multi-strand, copper or silver, twisted pair or coax, it's ALL UP TO YOU. I wired up my preamp with the twisted-pair inner conductors of Straight Wire LSI-Encore; with the white leg used for signal and the red leg tied to ground at the RCA jacks but left floating at the circuit board, isolation between conductors is higher than if I'd simply used single legs for all the hookup. Again, though, use whatever you like; most cable manufacturers will sell you a few feet of unterminated cable for you to use as hookup wire. And remember to ask the manufacturer the best way to tin the wire; some, like Straight Wire and Cardas, have each individual strand insulated in a high-temp varnish or Urethane, and these need special care during tinning.

Don't want to spend the money? Wanna wire this thing up NOW? My wire spy, a well-known cable designer who I'll call "Deep Dielectric," recommended to me yet another low-cost, good-sounding Radio Shack product: their 24ga solid-core "Rainbow Wire," part #278-857. This two-conductor ribbon cable uses good-quality copper solid-core and polyethylene insulation; just separate the black and white wires and twist them into a twisted pair, again tying the black to ground at the jacks but leaving it floating at the circuit. I tried the "Rainbow Wire" in the circuit, and it sounds real good; at 10 cents a foot, how can you lose?

I built my own "hi-fi" AC cord using a length of Kimber speaker cable and a heavy-duty AC plug, and it definitely improves the sound. You can build one like mine, or buy a ready-made AC cord from Ray Kimber which, unlike mine, sports a cool-man textile-weave outer jacket to protect the wire from nicks and other damage. Whichever route you take, spend some time listening to the preamp with the cord plugged in both ways; I found the correct AC polarity to be very audible with this preamp, and an incorrect plug orientation will result in shut-in highs and a reduction in soundstage width. And warts.

El cajon esta muy cheesy
As for the chassis, I used a pretty cheesy-looking aluminum "BUD box" I got at a local Surplus Hut which I swear gets all its salespeople from a prison work-release program. But their prices are right, and I can usually barter with cartons of Marlboros and girlie mags. And although I don't usually care what my gear looks like, the bare aluminum box was just too "Junior's First Electronics Project"-looking, so I stuck a black 19" rack panel on the front to hide the preamp's cheesy origins. Again, whatever you want to use is fine, so long as there's enough room for the circuit.

The supply's the thing
I already beat this dead horsie like Chuck Barris's gong in my PLC roundup elsewhere in this issue, so I won't get the soapbox out again. I'll just say that the quality of the power supply is so important, especially in such a simple circuit as this one, that if you aren't willing to take the time to build something a little better than a couple of 7815/7915s and some cheap 'lytics, don't even bother with this project; all the potential of this design will be wasted if you couple it with a mediocre power supply. In his column in the Four/90 issue of Audio Amateur (footnote 5), Gary Galo details a high-quality power supply, based on Linear Technology regulator chips and designed by Walt Jung and Rich Markell, that works extremely well in this preamp. I've included the schematic here, along with the suppliers for all the parts. I used AudioQuest Type 2 speaker wire stripped and tinned with Wonder Solder for the power-supply rails and ground, but any good heavy wire will work here; just be sure you guard it against oxidation by tinning it.

The bleeder resistors, R7 and 8, are there to ensure that the supply draws the minimum 10mA current it needs to work properly. They also make it possible to build and check out the power supply without having to have it hooked up to the buffer circuitry.

Switch it; switch it good
I found a really good, inexpensive switch to use: the Electroswitch model 900A. It's got silver-plated brass contacts and six positions, and it's held up to over six months of constant twiddling. I'm told this is the switch The Mod Squad used to use in their Line Drive, and after using it for a while I know why they switched switches: it feels rather cheap. Even though it sounds great, the 900A feels like a lightweight, not at all like the Heavy Duty Judys on a lot of the high-dollar gear. If you must have a switch that feels like a mains breaker, there are plenty available, but they may not sound as good as the Electroswitch. And if you don't mind a separate toggle switch for each component, C&K makes a really good-sounding DPDT switch called the 7203 which also sports silver contacts and is widely used throughout the high end by Theta, ARC, and others.

Star grounding is alive and well and living in Paris, TX
One feature I've always incorporated into my self-built gear is star grounding; every point in the circuit that needs to tie to ground (RCA jacks, the pot, circuit ground, etc.) is taken via its own separate ground wire to a single point, where they're all connected and either tied to the chassis or left floating. Many designers feel that star grounding contributes to a more open, transparent sound, and I agree. Whether or not you tie the single ground point to the chassis is up to you; connected, the circuit is better shielded from RFI, but some designers like Mod Squad prefer the sound when the ground isn't connected to the chassis. Knowing as I do just how much RFI is swarming around at all times, I tie my chassis to ground, and it sounds fine.



Footnote 5: P.O. Box 576, Peterborough, NH 03458-0576. Tel: (603) 924-9464. Fax: (603) 924-9467. This issue is available as part of the complete 1990 set for $26 including S&H. Although some of its contributors come across as ham-radio geeks, Audio Amateur [now called Audio Electronics---Ed.] is always full of interesting DIY projects for those inclined to roll their own; AA also carries advertisements from a large number of parts suppliers. A one-year subscription (four issues) costs $20 and is worth it for the Classifieds section alone if you're a vintage-vulture.
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