diy tube buffered passive preamp

Posted on Feb 7, 2014

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

diy tube buffered passive preamp
Click here to download the full size of the above Circuit.

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...

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