Ever since reading Corey Greenburg's Stereophile article in the early 90's about building a buffered passive preamp, I have wanted to do just that. There are a lot of things to recommend the design -- it is generally simple, there really is no longer a need for gain in a preamp, and since there is little to the circuitry it doesn't tend to muck with the sound as much as an active preamp. Further, by sufficiently lowering output impedence of a circuit and supplying enough current to drive cables, it does not suffer the same problems as passive designs. As such, it seems to solve the many of the problems inherent in both active and passive preamps without causing too many additional problems of is own.

The problem that I encountered in doing such a build from the Stereophile article is that not only is the monolithic buffer used there not readilly available, but the successor to it is also not available (they are available on the NOS market, however, if one is really interested.) Further, of the currently available buffer chips, all of them (like the Burr Brown buf634 or the Intersil HA3-5002) seem to have something about them that make them poor choices for this type of projetct -- prmarily that they just don't sound that good.

This leaves a couple of additional options: The first is to use an opamp that is stable at unity gain. Opamps, however, suffer from the same problems as existing monolithic buffers (that they sound bad). Further, the best opamps are generally not stable at unity gain anyway, and thus, this would become an active preamp. The two remaining options, however, are both good. These are to use a discrete solid state design, or to use a tube design like a cathode follower. For this project I chose to use the cathode follower.

Cathode followers are much maligned, but at the same time they crop up in a lot of places. Despite the bad press, they do a good job of lowering the output impedence of a circuit, and really don't sound as bad as their reputation suggests. As has been said elsewhere, you don't listen to the reputation.

This project is just getting started, so for now it is only a circuit, and a PCB design. When, and if, it gets built, I'll post more. For now, here is the basic schematic.

This is really just the circuit from the Morgan Jones book with the addition of R1 for safety. The same circuit can be used for a plethora of tubes, including some commonly used audio tubes as the 12AX7, 6DJ8, and the 6SN7. I decided to design a printed circuit board (PCB) around the 9 pin miniature tube variety. This was mostly because I happen to have several 6922's and several 7308's around, and I thought I might put them to good use. These tubes are dual triodes which means that a single tube can be used as a stereo buffer.

Here is what the PCB looks like.

The last revision of this was done 08/03/2005.

You can download the files to have these boards made from here. (notice that this design is copyrighted and hereby licensed under a creative commons license. It may be used and redistributed for personal, non-commercial, purposes, and you must always credit the original designer. This design is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Indeed, this design may use lethal voltages. Do not try to build it without sufficient experience to deal with them.) You will need software from ExpressPCB for the board. This company will produce 3 boards for $51.

This design has not been built of protyped. You should do a thorough check for errors on your own as I assume no responsibility for mistakes -- and there most assuredly are a few.

Anyway, you'll notice that the schematic and the PCB do not contain parts values. This is because the PCB is a general purpose board and can be used with a nuimber of tubes that will change these values. What this means is that you will need to figure them out yourself. The resistors are sized for 2W Kiwamee carbons because I like them. The caps are sized for 1uF and 4.7 uF Solens. Other axial film caps should work. Note that C1 stands on its end with the second lead connecting to the pad above it. C2 is mounted off the board with the second lead connecting to the pad near R6.

When and if this project moves forward, I'll post more ...