Aunt Corey's Homemade Buffered Passive Preamplifier Mo' Better Mods, Page 3
I had originally wired up my preamp with Straight Wire LSI-Encore, an excellent reasonably priced cable. But now I was in the throes of Modkateer Fever; mere excellence wasn't good enough anymore.
I've always liked the various Kimber Kables (the AC cord on my preamp is a Kimber Power Kord), so I got hold of some of their silver KCAG interconnect; you know, that cool-looking white stuff with the three braided wires and no shield. Inside the preamp, I used it just like it's used for interconnect: one wire for signal, the other two tied together for ground. I soldered the two ground wires to the RCA jacks' ground lugs, and left the other ends floating (not connected), in order to isolate the channels more effectively. Ever seen those graphs in Stereophile's preamp reviews that show a 6dB/octave decrease in channel separation above a certain frequency? That's due to capacitive coupling between the channels, and grounding the signal wires like this helps minimize this interaction.
So how did it sound? Stereophile's "Recommended Components" section describes the KCAG as "astonishingly transparent"; after rewiring my preamp with it, I'd have to say that "astonishingly KICK ASS" is more like it! Rewired with the silver Kimber, the sound just came alive, with a further reduction in high-end grain that let the preamp really open up on top. The gains in clarity, image outline, and focus were outstanding, well worth the trouble of rewiring. Ray Kimber says he'll sell unterminated lengths of KCAG through his dealers; I was so impressed with the sound of the KCAG inside my preamp, I now use it as the main interconnect throughout my system (footnote 7). Being unshielded, the KCAG is a bit more system-dependent than fully shielded cable; try it at home before you lay out that long green. If you stick your ear to the tweeters and hear Tito Puente, your system needs a good shielded interconnect. Or you're insane.
Psychedelic dual regulation, that's where it's at
Hoo boy, have I saved the best for last, ssslllllrrrrssssmmmmkk! (footnote 8). After putting the Penny & Giles volume pot back in, after installing the high-speed rectifiers, after rewiring the innards with Kimber KCAG, I was seriously digging the new clarity shining out of my preamp. It was certainly no slouch before the mods, but the sum total of all these tweaks made for a very real and significant improvement to the sound, and I was finally satisfied that I'd wrung all the performance out of the design that I could.
John Curl called me with what turned out to be the most important mod I ended up making to Aunt Corey's Homemade Buffered Passive Preamp; seems he'd read my original article, and it reminded him of when he built the piece of gear that put him on the high-end map in the first place: the Mark Levinson JC-1 preamp. He (and everyone else at the time) thought it was just about as good as it got, but one of his friends told him that he got much better sound by using two JC-1s, used as "mono" preamps for each channel. John tried it, and sure enough, the soundstaging got better! After he went back and looked at the circuit, John found that because both channels were fed by the same power-supply regulators, the two channels "talked" to each other; what happened in the left channel affected the right, and vice versa. By giving each channel its own dedicated regulation stage, John was able to isolate the two channels sufficiently to where the JC-1 sounded as good in stereo as two did in mono.
"Dual regulation," John advised me. "That's where it's at!"
Well, when John Curl talks, I listen. After fishing out a couple more Linear Technology LT1033/1085 regulators from my parts drawer (footnote 9), I completely rewired my entire power supply; using single solid-core legs of AudioQuest's Type 4 speaker cable for the DC rails, I basically duplicated everything after the rectifiers. In other words, where before each of the two rectifier bridges fed a single regulation stage, now it feeds two. Connect the input of the second positive regulator to the positive terminal of C1; that of the new negative regulator to the negative terminal of C8. Once you finish wiring the supply up, each BUF-03 should be powered by its own pair of LT1033/1085 regulators. Go on, spoil the li'l buffers.
I really tried to lay the wiring out neatly this time; no more Squiddly Diddly retardo mess. And in a nod to the Chicken Littles who wrote all those letters about my use of 18V DC rails in the original article (footnote 10), I sighed and brought them down a bit to a safe and happy ±15V DC by changing the value of R2 and R4 in the original schematic (Vol.14 No.11, p.101) from 12.9k to 10.5k. Real men run their BUF-03s at max supply voltage, but I guess such a brazen display of machismo is unwelcome in this time of male rebirthing and Wildman gatherings. I need a hug.
Because the current demand on each LT regulator is halved when you go for dual regulation, I lowered the value of the bleeder resistors across each supply rail to ground, R7 and R8, to 750 ohms/0.5W. This gives an additional 20mA current drain per regulator; the LT chips need to work a bit before they get their mojos workin', and the new resistor values keep the regulators nice and warm.
Footnote 7: Interestingly, even though KCAG also has a hot reputation as a digital data cable, most people I know simply use a length of regular KCAG for this, unaware that Kimber sells a dedicated digital version of the KCAG that's wired differently! While the cable itself is identical for both versions, the audio interconnect uses one leg for signal and two for ground, and the digital link uses two legs for signal and one for ground. Which way sounds better, Jack?
Footnote 8: Sound of licking chops in a wolf-like manner.
Footnote 9: The two top drawers of my desk, home also to my old set of num-chuks, a collection of erotic finger puppets I got in Mexico, and a genuine red plastic Oscar Meyer wiener-whistle, which still toots after all these years. I love these drawers; every time I open them, I take a trip down memory lane.
Footnote 10: See "Letters," Vol.15 No.2, February '92, pp.15-20.---JA